ASME Conference Presenter Attendance Policy and Archival Proceedings

2012;():i. doi:10.1115/HT2012-NS1.

This online compilation of papers from the ASME 2012 Heat Transfer Summer Conference (HT2012) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Heat Transfer in Energy Systems

2012;():1-12. doi:10.1115/HT2012-58007.

In this study, the heat transfer coolant utilized in the heat exchanger is a molten salt, which transfers thermal energy to water (steam) for power production by a supercritical Rankine (25MPa) or subcritical Rankine (17MPa) cycle. Molten salts are excellent coolants, with 25% higher volumetric heat capacity than pressurized water, and nearly five times that of liquid sodium. The greater heat capacity of molten salts results in more compact components like pumps and heat exchangers. However, the use of a molten salt provides potential materials compatibility issues. After studying a variety of individual molten salt mixtures, chlorides and fluorides have been given the most serious consideration because of their heat transport and transfer characteristics.

In this study thermal designs of conventional (shell and tube), and compact (printed circuit) heat exchangers are carried out and compared for a given thermal duty. There are a couple of main issues that need to be addressed before this technology could be commercialized. The main issue is with the material compatibility of molten salts (especially fluoride salts) and secondarily, with the pressure difference across the heat exchanger. The heat exchanger’s primary side pressure is nearly atmospheric and the secondary side (power production) is pressurized to about 25MPa for supercritical cycle and 17MPa for subcritical cycle. Further in the analysis, the comparison of both the cycles will be carried out with recommendations.

Commentary by Dr. Valentin Fuster
2012;():13-20. doi:10.1115/HT2012-58008.

We report a novel optofluidic solar concentration system based on electrowetting. With two immiscible fluids (water and silicone oil) in a transparent cell, we can actively control the orientation of the water-silicone oil interface via electrowetting. The naturally-formed meniscus between the two liquids can function as an optical prism and hence a beam deflector. With 1wt% KCl and 1wt% SDS (Sodium Dodecyl Sulfate) added into the DI water, the orientation of the water-silicone oil interface has been successfully modulated between 0° and 26° that can deflect and steer sunlight within the incidence angle of 0°–15°. Without any mechanical moving parts, this dynamic liquid prism allows the device to adaptively track both the daily and seasonal changes of the sun’s orbit, i.e., dual-axis tracking. An integrated dual-axis tracker and solar concentrator can be constructed from the optofluidic beam deflector in combination with a fixed optical condenser (Fresnel lens). The beam deflector consists of liquid prism arrays and electrowetting modifies the orientation of each individual liquid prism in order to steer the deflected beam normally towards the Fresnel lens as the incident sunlight beam shifts. Therefore, electrowetting tracking can adaptively focus sunlight on a concentrating photovoltaic (CPV) cell sitting on the focus of the Fresnel lens as the sun moves. This approach can potentially reduce capital costs for CPV and increases operational efficiency by eliminating the power consumption of mechanical tracking. Importantly, the elimination of bulky tracking hardware and quiet operation will allow extensive residential deployment of concentrated solar power. In comparison with traditional silicon-based photovoltaic (PV) solar cells, the electrowetting-based self-tracking technology will generate ∼70% more green energy with a 50% cost reduction.

Commentary by Dr. Valentin Fuster
2012;():21-28. doi:10.1115/HT2012-58039.

Desiccant Indirect Evaporative Cooling is a good alternative to conventional vapor compression systems to meet new economic, environmental, and regulatory challenges. The advanced desiccant cooling systems through the Maisotsenko Cycle (M-Cycle) discussed here have the potential to phase out the use of CFC refrigerants, reduce energy-operating costs and peak power demands, meet new ventilation rate standards and improve indoor air quality.

The M-Cycle combines the thermodynamic processes of heat exchange and evaporative cooling in a unique indirect evaporative cooler resulting in product temperatures that approach the dew point temperature (not the wet bulb temperature). This cycle utilizes the enthalpy difference between air, at its dew point temperature, and air saturated at a higher temperature. This enthalpy difference or potential energy is used to reject the heat from the higher temperature air stream [1–3].

The first time the M-Cycle technology was proven was in 1984. Currently Coolerado Corporation produces several air conditioners (commercial, residential, solar and hybrid) relying only on the M-Cycle. The National Renewable Energy Lab (NREL) tested Coolerado’s air conditioners documenting that they are up to 80% more efficient than traditional systems [10]. The M-Cycle has been investigated extensively in different countries for unusual applications because it can be used for many applications for producing cooling, power system performance improvement, distilled water production, heat recovery processes and others [see Refs. 4–10, 13–18]. This paper describes the basic M-Cycle and advances by coupling the M-Cycle with a desiccant system.

Commentary by Dr. Valentin Fuster
2012;():29-34. doi:10.1115/HT2012-58066.

An extension of the so-called maximum ecological power cycle introduced by Angulo-Brown for the endoreversible maximum power problem of Curzon-Ahlborn is presented here for the purpose of comparing heat transfer requirements versus power generated by a conceptual power system with LNG re-gasification. This re-gasification concept is being considered as an alternative solution to LNG re-gasification using sea water heat exchangers at LNG receiving terminals. Analytical results of maximum power cycles and cascades are made easier to obtain by using the optimum heat conductance allocation rules obtained in previous work; they are expressed in terms of the heat source and sink temperature ratio, internal irreversibility factor, and heat source mean temperature parameters. These results are then applied to maximum power cycles and cascades for the purpose of integrating power production and LNG re-gasification.

Commentary by Dr. Valentin Fuster
2012;():35-41. doi:10.1115/HT2012-58071.

Measuring internal heat transfer coefficients in solid matrices has been a challenge in the design of Compact Heat Exchangers (CHXs). Steady-state methods have proven to be infeasible, and, although numerous transient methods have been explored, it is our opinion that none have distinguished themselves in terms of convenience and accuracy. In the present study, a new and unique combined experimental and computational technique for determining the internal heat transfer coefficient within randomly stacked woven-screen matrices is presented and results are obtained for air as the working fluid. To obtain the local heat transfer coefficient the solid phase matrix is subjected to a uniform step change in heat generation rate via induction heating, while the fluid flows through under steady state flow conditions. The transient fluid phase temperature response between the inlet and outlet of the sample is measured. The heat transfer coefficient is determined by comparing the results of a numerical simulation based on Volume Averaging Theory (VAT) with the experimental results. The local heat transfer coefficient is defined from VAT in terms of several lower-scale integral and differential terms present in the averaged thermal transport equations. Obtaining the heat transfer coefficient experimentally provides closure to the general VAT thermal energy equations. Several matrices were selected for this experimental study and the results are presented in terms of Nusselt number over a Reynolds number range of about 100 to 400. The characteristic length scale in the dimensionless numbers is the porous media hydraulic diameter derived from the VAT-based governing equations. It is proposed that this new method can provide a convenient and accurate tool for CHX designers.

Commentary by Dr. Valentin Fuster
2012;():43-49. doi:10.1115/HT2012-58075.

A shroud and baffle configuration is used to passively increase heat transfer in a thermal store. The shroud and baffle are used to create a vena contracta near the surface of the heat exchanger which will speed up the flow locally and thereby increasing heat transfer. The goal of this study is to investigate the geometry of the shroud in optimizing heat transfer by locally increasing the velocity near the surface of the heat exchanger. Two-dimensional transient simulations are conducted. The immersed heat exchanger is modeled as an isothermal cylinder which is situated at the top of a solar thermal storage tank containing water (Pr = 3) with adiabatic walls. The shroud and baffle are modeled as adiabatic and the geometry of the shroud and baffle are parametrically varied. Nusselt numbers are obtained for a range of Rayleigh numbers, 105 ≤ RaD ≤ 107 in order to determine optimal shroud and baffle configurations.

Commentary by Dr. Valentin Fuster
2012;():51-54. doi:10.1115/HT2012-58076.

Using molten salts as thermal energy storage (TES) in concentrated solar power (CSP) system has several benefits. Molten salts are thermally stable up to very high temperatures (over 500 ° C). This can extend the operational capability of CSP system and eventually improve the overall system efficiency. Molten salts typically have lower vapor pressure (less mechanical stress) and cheaper than conventional TES materials (mineral oil, fatty acid, etc.). However, the usage of molten salts as TES is limited due to their low thermo-physical properties (e.g., Cp is less than 2 J/g°C, k is less than 1W/mK). Nanomaterials are nanoparticle dispersions in a solid matrix or a solvent. They have been reported for their large enhancement in thermo-physical properties. It is expected that well dispersed nanoparticles can significantly enhance thermo-physical properties of molten salt materials. In this study, a molten salt nanomaterial will be synthesized by dispersing inorganic nanoparticles into a molten salt. Heat capacity measurement will be performed using a modulated differential scanning calorimeter (MDSC). Material characterization analyses will be performed using electron microscopes (SEM / TEM). The utility of the molten salt nanomateial as TES in CSP will be explored.

Commentary by Dr. Valentin Fuster
2012;():55-66. doi:10.1115/HT2012-58077.

Liquid desiccant systems are emerging as promising alternatives to achieve humidity control in a variety of applications with high latent loads and low humidity requirements. Their advantage lies on their ability to handle the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a study of the performance of a counter flow internally heated liquid desiccant regenerator. A tubular heat exchanger is proposed as the internally heated element of the regenerator and water as the heating fluid. The desiccant solution is sprayed into the internally heated regenerator from the top and flows down by gravity. At the same time, ambient air is blown from the bottom, counter-flowing with the desiccant solution. The desiccant is in direct contact with the air, allowing for heat and mass transfer. The water, flowing inside the tubes of the regenerator, provides the necessary heat for regeneration. A heat and mass transfer theoretical model has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Experimental data from previous literature have been used to validate the model. Excellent agreement has been found between experimental tests and the theoretical model, with the deviation not exceeding ±6.1%. Following the validation of the mathematical model, the dominating effects on the desorption process have been discussed in detail. The three most commonly used liquid desiccant solutions (LiCl, LiBr, CaCl2) and two different flows (DDU: water downward – desiccant downward – air upward, UDU: water upward – desiccant downward – air upward) have been also evaluated against each other. Considering the flow analysis, the type of flow does not affect the regeneration capacity as much as the type of the desiccant solution. It has been concluded that high regeneration rate can be achieved under DDU flow (water downward – desiccant downward – air upward), low solution concentration, high air inlet temperature, high solution inlet temperature, low air inlet humidity ratio and CaCl2 as the desiccant solution.

Commentary by Dr. Valentin Fuster
2012;():67-77. doi:10.1115/HT2012-58129.

In this present work, we attempt to maximize the electrical power generation by optimizing Thermoelectric Generator (TEG) geometry for a prescribed TEG volume. A plate-fin heat exchanger topology is assumed and consideration is given to additional pressure drops associated with the fins placed in the exhaust flow path; and the cross-sectional changes across TEG inlet-exit ports. The thermal profile in the TEG is computed such that different thermoelectric modules may be placed where they provide the maximum Seebeck coefficient and hence provide the maximum power output. Multiple filled skutterudites are employed in the higher temperature regions and Bismuth Telluride modules are used at lower temperature regions of the TEG. Separate calculations are performed for cases where the available maximum number of TEMs is limited for cost optimization.

Commentary by Dr. Valentin Fuster
2012;():79-88. doi:10.1115/HT2012-58130.

Mass and heat transfer are experimentally investigated in a discoidal and unshrouded rotor-stator cavity where an air jet passes through the stator and impinges on the rotor center. Using a jet impingement is a way to bring fresh air inside the air gap and to increase shear stresses and so heat transfer over the rotor. This study focuses on comparisons between heat transfer coefficients and velocity fields obtained inside the air gap for the case of a dimensionless spacing interval G = 0.02 and a low aspect ratio for the jet e/D = 0.25. Two jet Reynolds numbers ranging from 16000 to 42000 and three rotational Reynolds numbers between 30000 and 516000 are considered. Mass transfers are investigated by Particle Image Velocimetry technique while the radial distribution of heat transfer coefficients over the rotor is obtained using a thick wall method and temperature measurements by infrared thermography.

Commentary by Dr. Valentin Fuster
2012;():89-96. doi:10.1115/HT2012-58146.

Steam Generator (SG) is one of the most important pieces of equipment in High Temperature Gas-cooled Reactor (HTGR). It requires high reliability in a very critical working condition. The thermal analysis of HTGR SG and its uncertainty becomes very important. Large thermal non-uniformity and the resulting high temperature will damage the structure. The SG of HTGR is very different from the boilers of conventional thermal power plant. The heat transfer almost all contributes to convection (counter flow pattern) but not radiation. One dimensional (1D) and two dimensional (2D) codes were developed for the thermal analysis of the SG of HTGR. The 1D code is able to calculate the overall performance. It solves the one dimensional equations for both the shell and tube side. The 2D code is designed to analyze the temperature non-uniformity in the SG. Two dimensional Reynolds-Averaged Navier Stokes equations are solved for the shell side, one dimensional equations are solved for the tube side. The thermal mixing effect in the shell side tube bundle can be included. The thermal deviations caused by secondary side flow rate uncertainty and manufacturing tolerance of tube helical diameter are analyzed. The results show that radiation only contributes to about 0.6 percent of the total thermal power. Secondary flow rate fluctuation of 1% causes an outlet steam temperature variation of 3 °C. Heat transfer tube helical diameter tolerance of 1mm causes an outlet steam temperature deviation of 4 °C.

Commentary by Dr. Valentin Fuster
2012;():97-110. doi:10.1115/HT2012-58149.

Solar thermal cracking of methane produces two valuable products; hydrogen gas and solid carbon, both of which can be used as a fuel and as a commodity. During the course of this two-phase phenomenon, carbon particles tend to deposit on solar reactor window, wall, and exit. Especially when they accumulate at the reactor exit, agglomeration of these particles completely blocks the exit. Therefore, this problem has been the major issue preventing solar cracking reactors from running continuously. To address this problem, a cyclone solar reactor was designed to enhance the residence time and make carbon particles fly in circles in the reactor instead of moving towards the exit all together at a time. In order to better understand and explain the flow dynamics inside the solar cyclone reactor, a prototype reactor was manufactured to test the concept and to analyze the flow via Particle Imagining Velocimetry (PIV). In this paper, design steps of this new solar reactor concept are given and a brief summary of the CFD simulations incorporating discrete ordinate radiation model (DO), species transport with volumetric reactions, and discrete phase model (DPM) for particles are presented. Then experiments focusing on the PIV analysis are described. To understand the flow evolution along the vortex line, several images in axial direction along the vortex line were captured. The results showed that when the main flow is increased by 25%, the vertical velocity components became larger. It was also observed that the vertical vortices along the vortex line showed stronger interaction with outward fluid in the core region, which implied the horizontal twisting motion dominated the region due to the main flow, which could trapped the particles in the reactor for longer time. Furthermore, when the main flow was increased by 50%, the flow was a cyclone-dominated structure. During the vertical evolution along the vortex line, more vortices emerged between the wall region and core region, implying the energy was transfer from order to disorder. In summary, by appropriate selection of parameters, the concept of aero-shielded solar cyclone reactor can be an attractive option to overcome the problem of carbon particle deposition at the reactor walls and exit.

Commentary by Dr. Valentin Fuster
2012;():111-120. doi:10.1115/HT2012-58161.

An inevitable consequence of the second law of thermodynamics is that any electric power plant that operating on the closed Rankine cycle must reject approximately 60% to 70% of the heat that is added to the cycle through the condenser to the ambient environment in order to complete the cycle. The temperature of the waste heat exiting power plants, while too low for electric power generation, is often suitable for other purposes such as heating greenhouses and aquaculture facilities, particularly those that reject this waste heat directly to the atmosphere via cooling towers.

Few facilities currently exist that utilize waste heat from power plants on a relatively large scale. The challenges are institutional and economic, not technical. Most electric utilities see little benefit to themselves in waste heat utilization. The cost of delivering hot water to the waste heat user can be significant compared to the benefit to the end user. However, this paper presents a new concept for utilizing waste heat that significantly reduces the cost of delivering waste heat by an amalgamation of users and provides a significant benefit to the power plant by reducing the heat sink temperature, thus increasing the efficiency of the turbine cycle and increasing the electrical output.

A dedicated piping system was provided in the original design of the Tennessee Valley Authority’s (TVA) Watts Bar Nuclear Plant (WBNP) to utilize the turbine cycle waste heat and a portion of the nuclear plant reservation was dedicated for that purpose. However, when Unit 2 of the plant was not completed as scheduled, plans for the waste heat energy park (WHEP) were shelved in the early 1980’s. As this author supervised the engineering of that project, it will be used in the proposed paper to illustrate how the new concept may be applied.

Commentary by Dr. Valentin Fuster
2012;():121-127. doi:10.1115/HT2012-58167.

Top Down–Bottom Up plane blinds have become popular in recent times. These are blinds that can be both raised at the bottom and lowered at the top. Such blinds have the potential to reduce energy consumption in buildings by allowing the controlled use of sunlight to illuminate the building (daylighting) and/or the use passive solar room heating while still providing shade from the direct sunlight and privacy for the occupants. The effect of such blind systems on the convective heat transfer rate from the window to the room to which it is exposed has not been extensively studied. The purpose of the present work therefore was to numerically investigate the effect of the blind openings with Top Down-Bottom Up plane blinds on the convective heat transfer from the window. The cases where the flow over the window-blind system is laminar or turbulent have been considered. The present study, as is the case in many previous window heat transfer studies, considers only the convective heat transfer from the window. In the present study the mean flow has been assumed to be steady and two-dimensional. The Boussinesq approach has been used and it has been assumed that the “window” is at a uniform temperature. The solution has been obtained by numerically solving the governing equations using the commercial CFD solver, FLUENT©. The standard k-epsilon turbulence model with full account being taken of the effect of the buoyancy forces has been used. Results have been obtained for a Prandtl number of 0.74, which is essentially the value for air. The effects of the top and bottom dimensionless blind openings, of the dimensionless depth to which the window is recessed and of the Rayleigh number on the Nusselt number have been studied.

Topics: Convection
Commentary by Dr. Valentin Fuster
2012;():129-133. doi:10.1115/HT2012-58173.

A battery pack prototype has been designed and built to evaluate various air cooling concepts for the thermal management of Li-ion batteries. The heat generation from the Li-Ion batteries was simulated with electrical heat generation devices, with the same dimensions as the Li-Ion battery (200mm × 150mm × 12mm). Each battery imitator generates up to 15W of heat. There are 20 temperature probes placed uniformly on the surface of the battery imitator, which can measure temperatures in the range from −40 °C to +120 °C. The experimental chamber has 2 battery imitators. All temperatures are recorded using a PC based DAQ system. We can measure the average surface temperature of the imitator, temperature distribution on each surface and temperature distributions of the chamber. The pack which holds the battery imitators is built as a crate, with adjustable gap (varies from 2mm to 5mm) between the imitators for air flow channel studies. The total system flow rate and the inlet flow temperature are controlled during the test. The cooling channel with various heat transfer enhancing devices can be installed between the imitators to investigate the cooling performance. The pack is thermally isolated, which prevents heat transfer from it to the surroundings. The flow device can provide the air flow rate in the gap of up to 5m/s velocity and air temperature in the range from −30 °C to +50 °C. Test results are compared with computational modeling of the test configurations. The present test set up will be used for future tests to develop and validate new cooling concepts, such as surface conditions or heat pipes.

Commentary by Dr. Valentin Fuster
2012;():135-143. doi:10.1115/HT2012-58183.

Plasmon resonance in nanoscale metallic structures has shown its ability to concentrate electromagnetic energy into subwavelength volumes [1–3]. Metal nanostructures exhibit a high extinction coefficient in VIS and NIR spectrum due to their large absorption and scattering cross sections corresponding to their surface plasmon resonance [4]. Hence, they can serve as an attractive candidate for solar energy harvesting material. Nanofluids have been proven to increase the efficiency of the photothermal energy conversion process in direct solar absorption collectors (DAC) [5, 6]. Early work has evaluated the extinction coefficient impacts on DAC [7]. The present work extends this with a quantitative comparison between core-shell nanoparticle suspensions and solid-metal nanosphere suspensions in a DAC. Ultimately, this study seeks a better understanding of how to best utilize the plasmon resonance effect to maximize the efficiency of nanofluid-based DACs or other volumetric heating systems.

Commentary by Dr. Valentin Fuster
2012;():145-150. doi:10.1115/HT2012-58188.

The optical properties of nanoparticle suspensions in liquids have garnered significant interest recently for their potential use in applications ranging from biomedical imaging to solar energy harvesting. Although previous investigations have provided useful insight into the spectral properties of such suspensions they have been primarily limited to experimental investigation at room temperature. As these suspensions will be used in systems with significant variations in temperature it is important to understand the effects of temperature on the optical properties. Furthermore, the primary spectrometric technique used has considered only transmittance and not the effects of scattering. Here we investigate the effects of temperature on the response of TiO2, Ag, and Au nanoparticle suspensions in water utilizing an integrating sphere and heated cuvette. Additionally the responses are analytically modeled to understand the individual contributions from variations in particle size, volume fraction, and surrounding medium temperature-dependent refractive index changes.

Commentary by Dr. Valentin Fuster
2012;():151-155. doi:10.1115/HT2012-58197.

Air Sourced Heat pump Hot Water Heaters (ASHPWH) are integrated in buildings as a multitask system to act as both a water heater and air cooler with a lower carbon footprint. The cost effectiveness of the ASHPWH systems relative to the conventional water-heaters is a challenging issue which is researched in this project. A refined heat pump was integrated into a water heater tank and tested using the Australian testing standard AS/NZS 5125. Experiments included measurement of flow rate, temperature and pressure of water, air and refrigerant (R410a) to determine Coefficient of Performance (COP) and cost-effectiveness of the ASHPWH relative to the current 310L-ASHPWH and a standard electric water heater of similar size. The leaving water location of the condenser was also studied to reduce scalding / to destroy Legionella bacteria, to improve mixing and the delivery of hot water to the storage-tank, to enhance heat transfer stratification, buoyancy, or pumping. The relative position of the water from the storage-tank was varied and coefficient of performance (COP) of the Heat-pump and effectiveness of the water heater were determined. Experiments included measuring water, air and refrigerants flow-rates and temperatures and power consumed in compressor. This research in progress will recommend on the conditions this method is reasonable. The operational cost of 310L was 38% cheaper than the standard electric. The design (claimed) heat pump would operate 52% cheaper than the electric water heater. Carbon tax was not included but it is estimated that carbon footprint of the tested heat pump and the Standard 310L - ASHPWH were lower due to lower consumption of 40% and 33% electricity respectively relative to the electric water heater.

Topics: Heat pumps , Water
Commentary by Dr. Valentin Fuster
2012;():157-163. doi:10.1115/HT2012-58200.

Extensive research work on modeling the simultaneous heat and mass transfer taking place in a direct expansion (DX) cooling coil which is a key element in a DX air conditioning (A/C) system has been carried out. A steady-state model (coil model) has been developed for evaluating the equipment sensible heat ratio (SHR) and the total cooling capacity (TCC) of the DX cooling coil in an experimental variable speed DX A/C system under different operating conditions based on known evaporating temperature and the air flow rate passing through the cooling coil. On the other hand, a previous mathematical model (system model) for the complete experimental DX A/C system has also been developed. The evaporating temperature of the DX A/C system may be predicated using this system model, and can then be used as an input to the coil model. Therefore, by combining the coil model and the system model, a new steady-state model for the DX A/C system, which links the compressor and fan speeds on the input side with the Equipment SHR and TCC on the output side, will be developed. The new model will be experimentally validated, and the validated model will be used as a base to develop a model-based controller for a variable speed DX A/C system to achieve the simultaneous control of indoor air temperature and humidity.

Commentary by Dr. Valentin Fuster
2012;():165-171. doi:10.1115/HT2012-58202.

Further improving energy efficiency in internal combustion (IC) engines is the research-topic for a large number of investigators. This project covers the integration of an IC-based cogeneration with a vapor-absorption chiller. Heat from the cogen is fed to the generator of the absorption chiller. It is to improve performance and efficiency of internal combustion engines for use in motor vehicles.

Less than one third of the total energy in a given amount of fuel is transferred into the rotational motion that drives vehicles forward. The remainder is either used to overcome friction and drive mechanical devices such as the alternator and water pump or is rejected as heat through engine cooling and exhausted gas. The heat rejection from the exhaust is due to the Carnot law and it is low grade heat that cannot be efficiently converted to work in the engine and thus, requires a second system for energy recovery. The two forms of rejected heat each account for approximately a third of the available energy.

Traditional methods of improving performance such as increasing capacity and air intake through forced induction also results in more fuel being consumed. Improving fuel efficiency meanwhile has resulted from running the engines leaner and downsizing engines. However the current methods have not taken advantage of the energy supply available in the exhaust stream.

Harnessing the thermal waste energy in the exhaust can be achieved through integration of a heat exchanger in order to power a vapor absorption cycle for the purpose of cooling intake air. This cooler, denser charge improves both volumetric and mechanical efficiency of engines with the outcome being improved performance, better fuel economy and lower emissions.

Recommendations for future work and other applications will be provided based on the analyzed results contained in the body of this paper.

Commentary by Dr. Valentin Fuster
2012;():173-177. doi:10.1115/HT2012-58203.

Carbon nanotube is a promising material for thermal-management of micro devices because of its high intrinsic thermal conductivity. However, most bulk nanotubes show very low thermal conductivity due to the high thermal contact resistance. There are very few reliable experimental data for the contact issue of nanotubes. This paper uses three kinds of multi-walled carbon nanotubes; pristine, thermally-oxidized, and acidized nanotube. Each has unique nanoscale structure in their outermost surface. We measured thermal conductivity of their pellets and simultaneously conducted computational analysis treating random network model of spherocylinders. By comparing both results, thermal contact resistances between nanotubes are estimated and the effect of defected structure is discussed. The reliability of our method is also successfully confirmed compared with reported data using individual nanotubes.

Commentary by Dr. Valentin Fuster
2012;():179-185. doi:10.1115/HT2012-58222.

Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelength to increase energy conversion efficiency. In this work we report the design and analysis of a STPV using a high-fidelity 2D axisymmetric thermal-electrical hybrid model that includes thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The radiative spectra of the absorber and emitter are engineered by using two-dimensional periodic square array of cylindrical holes on a tantalum (Ta) substrate. The optimal solar concentration and resulting temperature are determined by considering the energy losses associated with re-emission at the absorber, low energy (below band gap) emission at the emitter, and carrier thermalization/recombination in the PV cell. The modeling results suggest that the overall efficiency of a realistic planar STPV consisting of Ta PhCs and existing InGaAsSb PV cells with a filter can be as high as ∼8%. The use of high performance PhCs allows us to simplify the system layout and operate STPVs at a significantly lower optical concentration level and operating temperature compared with STPVs using metallic cavity receivers. This work shows the importance of photon engineering for the development of high efficiency STPVs and offers design guidelines for both the PhC absorber/emitter and the overall system.

Commentary by Dr. Valentin Fuster
2012;():187-191. doi:10.1115/HT2012-58224.

The present study aimed to improve a model to predict thermal performance of a heat pipe heat exchanger (HPHX) as pre-heater of the absorption system. The prediction was performed by utilizing thermal resistance and ε-NTU method. Numerical modeling used an effective thermal conductivity (keff) without using measured data. Temperature profiles around heat pipes of the HPHX were predicted and compared with measured data. They agreed within the maximum deviation of 16 %. Thermal performances of the HPHX were predicted with the measured data within approximately 4 %. Calculated heat transfer coefficients for the evaporator of the heat pipe showed the similar with the values predicted by the literature correlation, while those inside the condenser of the heat pipe showed larger than the values predicted by the literature correlation.

Commentary by Dr. Valentin Fuster
2012;():193-201. doi:10.1115/HT2012-58232.

A Computational Fluid Dynamics (CFD) study has been reported on the eccentric annuli with a wide range of radius ratios (α = 0.5, 0.65, 0.8 and 0.95) and dimensionless eccentricity values (0.0, 0.3, 0.5 and 0.7) representing small gap ratios. All the geometric cases are investigated either by imposing a constant heat flux or peripherally varying heat flux on the inner wall. The narrow zone in the annular channel has been observed to have encountered drastic variations in hydrodynamic (velocity and Darcy friction factor) and thermal (temperature and Nusselt number) characteristics. The velocity in the narrow gap increases by 17% for values of α ranging from 0.5 to 0.65, and decreases thereafter. For a typical eccentricity value of 0.7, the mass flux in the narrow gap zone decreases by 75% and Darcy friction factor by 23%. The maximum temperature at the inner cylinder surface is found to increase by 188% at this eccentricity for constant heat flux case and 140% for varying heat flux.

Commentary by Dr. Valentin Fuster
2012;():203-211. doi:10.1115/HT2012-58236.

A non-isothermal medium is modeled using the multilayer approach in which the continuous temperature distribution in a one-dimensional system as modeled as being piecewise constant. This has been shown to provide accurate results for a surprisingly small number of layers. Analysis is performed on a non-isothermal gray medium to attempt to characterize the ways in which the errors introduced by the multilayer modeling change with various physical parameters namely, the optical thickness and the temperature or emissive power gradient.

A demonstration is made of how the multi-source k-distribution method is capable of evaluating the heat flux within a one-dimensional system with piecewise constant temperature distribution with line-by-line accuracy with a significant decrease in computational expense. The k-distribution method for treating the spectral properties of an absorbing-emitting medium represents a powerful alternative to line-by-line calculations by reducing the number of RTE evaluations from the order of a million to the order of ten without any significant loss of accuracy. For problems where an appropriate reference temperature can be defined, the k-distribution method is formally exact. However, when no appropriate reference temperature can be defined, the method results in errors. The multi-source k-distribution method extends the k-distribution method to problems with piecewise constant temperature and optical properties.

Commentary by Dr. Valentin Fuster
2012;():213-222. doi:10.1115/HT2012-58243.

This paper presents an innovative high solar fraction plant, operating since December 2011, which provides heating and cooling in an office building (426.6m2) in Athens. The design solar fraction accounts for 80%. The plant design includes solar thermal collectors (149.5m2), a cylindrical underground thermal energy storage (58m3), an absorption cooling machine (35kW), a cooling tower and a heat pump. The overall control strategy is presented focusing on the charging of the thermal storage and on the solar cooling operation. As solar collectors operate throughout the whole year and is the main heat source, an energy efficient control has to be implemented. In this plant the concept of critical radiation is adapted accompanied by variable mass flow rate. Regarding the absorption chiller the control strategy is based on the characteristic equation ΔΔt. The chiller operation will be controlled by the inlet cooling water temperature to the chiller. Initial measurements of the storage charging and heating operation are presented. The successful integration of the heat pump to the solar system is also indicated by the initial measurements.

Commentary by Dr. Valentin Fuster
2012;():223-231. doi:10.1115/HT2012-58245.

During reactivity initiated accidents in a core of a nuclear reactor, a power excursion occurs on some fuel rods. The consequent rapid boiling is a matter of study for the nuclear power plants safety evaluation, because of the risk for rod-clad failure. In order to better understand the influence of power excursions and to characterise the phases of the rapid boiling phenomenon, an experimental set-up has been built at the Institut de Mécanique des Fluides de Toulouse (IMFT). The test section is a semi annulus. The inner half cylinder is made of a stainless steel foil, heated by Joule effect. Its temperature is measured by an infrared camera filming the backside of the foil, coupled with a high speed camera for the visualization of the phenomena. Measurements were made when a square current signal is applied to the foil. They showed the influence of the supplied power and of the wall temperature increase rate during boiling.

Commentary by Dr. Valentin Fuster
2012;():233-240. doi:10.1115/HT2012-58271.

In this paper, we study the limits of light trapping for amorphous silicon thin film solar cells using surface metallic gratings. Adopting a method used recently by Sheng et al. [31], arbitrarily shaped periodic surface textures described by Fourier series with limited terms are considered, and global inverse optimization techniques such as Simulated Annealing are used to adjust the structural variations of the unknown texture to yield maximum light trapping. The optimization is done with respect to two objective functions: enhancement in the number of absorbed photons and, maximal spectral absorptivity enhancement. We show that compared with the rectangular structures previously studied, curved structures result in additional waveguide modes and more broadband enhancement in absorptivity of silicon. An overall improvement of over 60% is achievable in the number of absorbed photons for polarized incident sunlight using the shape functions we will describe. We compare the results with conventional Lambertian limit of light trapping [1] and with the more recent theoretical limits of Yu et al. [30] and Sheng et al. [31] for thin films. We show that at near-infrared ranges, absorptivity enhancements remarkably higher than those results can be achieved using the proposed structures and inverse optimization.

Commentary by Dr. Valentin Fuster
2012;():241-248. doi:10.1115/HT2012-58276.

Storage of thermal energy is of practical interest for concentrated solar power (CSP) applications. Thermal energy storage (TES) for CSP systems has been accomplished with the use of liquid eutectic mixtures of KNO3 and NaNO3 that are exchanged between a cold and hot tank with appropriate heat exchangers in between. While such two tank systems primarily rely on the sensible heat of the fluid to store thermal energy the storage system that utilizes latent heat can significantly be smaller, more effective and less expensive. The current work is focused on TES at higher temperatures using encapsulated phase change material (EPCM) as NaNO3 encapsulated by stainless steel that is applicable to Rankine and other power generation cycles. We present here two dimensional transient heat transfer analysis for the NaNO3 encapsulated in a cylindrical shaped capsule for charging (storing thermal energy) and discharging (retrieving thermal energy) process. Energy stored and retrieved are in both sensible and latent heat form. Simulations are conducted for both vertically and horizontally placed cylindrical rods to investigate the effect of gravity on the charging and discharging process for various diameters of rods. It has been found that heat transfer into/from EPCM rods is not posing any problem for 6–8 hours storage/retrieval of thermal energy.

Commentary by Dr. Valentin Fuster
2012;():249-258. doi:10.1115/HT2012-58281.

One of the major impediments of current energy applications is the availability of an economical and reliable heat transfer fluid. Such applications include concentrated solar power, gas processing, petrochemicals, nuclear, and other high-temperature processes. Organic heat transfer fluids currently in use have limitations approaching 390°C, and other salt-based fluids have rather high freezing temperatures. Ternary nitrate salts have the potential to operate at high temperatures while maintaining low freezing temperatures. Mixtures of various concentrations of LiNO3-NaNO3-KNO3 salts and their properties have been investigated. For various LiNO3-NaNO3-KNO3 compositions, specific heat, latent heat, and viscosity are reported at various temperatures. Phase diagrams have also been predicted for the LiNO3-NaNO3-KNO3, CsNO3-NaNO3-KNO3, and CsNO3-LiNO3-KNO3 systems using mathematical modeling and the results are encouraging. The results presented in this work are expected to make a significant impact on the development of economical and practical ternary nitrate mixtures in energy applications.

Topics: Heat transfer , Fluids
Commentary by Dr. Valentin Fuster
2012;():259-265. doi:10.1115/HT2012-58284.

Phase change materials (PCMs) use latent heat to store a large amount of thermal energy over a narrow temperature range. While PCMs are commonly used for thermal storage applications, they may also be used to dampen large pulsed heat loads, which are commonly generated by high-power electronics and direct-energy weapons. During a pulse, the PCM absorbs some of the large heat load, and between pulses the heat is dissipated to a cooling system, which minimizes the instantaneous heat load applied to the cooling system, reducing its physical size and power consumption.

To minimize the size of a PCM heat exchanger, a simple computational model that can capture the transient thermal response of a flat plate PCM heat exchanger in a vapor compression cooling system with a pulsed heat load was developed. Using this model, the effect of PCM thermal conductivity, melt temperature, and latent heat on the size of the PCM heat exchanger was studied. PCM thermal conductivity and melt temperature had the greatest impact on the PCM heat exchanger size. The ideal PCM heat exchanger would contain relatively high thermal conductivity PCM with a melt temperature close to the desired heat source temperature.

Commentary by Dr. Valentin Fuster
2012;():267-272. doi:10.1115/HT2012-58291.

Over the last decade nanofluids, colloidal suspensions of nanoparticles (∼5–100nm) in a base fluid have created excitement, as well as controversy, due to the reported enhanced thermal properties. Most of the research in the past has focused on the thermal characteristics of nanofluids or their performance in micro systems and/or in simple fluid geometries. The objective of this study is to investigate heat transfer performance of nanofluids in an industrial type heat exchanger. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop in a laboratory scale Plate Heat Exchanger (PHE) using nanofluids with that of water. SiO2-water nanofluids consisting of 20±2 nm diameter particles at three different particle mass concentrations of 1%, 3% and 5% are used as the working fluid. The experimental setup consists of the nanofluids in the hot stream and tap water in the cold stream. In addition, pressure drop across the heat exchanger inlet and outlet is also measured to estimate the flow performance of nanofluids. The results show a consistent increase in the total heat transfer coefficient of the heat exchanger for the nanofluids concentrations tested. However, the pressure drop in the hot (nanofluids) flow line also increases that effect can substantially limit the applicability of nanofluids in a PHE.

Commentary by Dr. Valentin Fuster
2012;():273-278. doi:10.1115/HT2012-58302.

The Modular High Temperature Gas-Cooled Reactor (MHTGR) has been chosen as a reference design for the Next Generation Nuclear Plant (NGNP) project. This reactor consists of concentric stacks of graphite blocks containing embedded fuel elements. Helium will be used as the coolant and will flow through designed coolant channels interspaced axially within the graphite blocks as well as in the gaps separating the blocks (called the bypass flow). A key phenomenon that may lead to localized hot spots in the reactor is the degradation of heat transfer effects in the bypass flow due to geometry distortions. Geometry distortions are the result of the graphite blocks being irradiated with energetic neutrons as well as coefficient of thermal expansion effects due to temperature changes. Idaho State University is studying heat transfer within the bypass flow and is developing an experiment to study the deterioration of heat transfer in the bypass flow stemming from these geometry distortions. Experimental data gathered from this project will be used to benchmark numerical codes used in the design and safety analysis of the MHTGR.

Baseline MHTGR operating conditions are for a system pressure of approximately 7 MPa and a helium exit temperature from the reactor of approximately 850 °C. In place of using helium at these extreme conditions, it is our desire to perform the experiments with air entering the experiment at atmospheric pressure and temperature. Additionally, it is desirable to have an open-air system as opposed to a closed helium system. In order to quantify the impacts on temperature increase as well as pressure drop, a scaling analysis will be performed to compare the respective values from both helium and air. Important non-dimensional parameters, such as Reynolds number, non-dimensional heat flux, the acceleration factor, and non-dimensional buoyancy, will be matched for the various conditions in order to provide a similitude between the helium and air. These factors cannot be matched all at once, except by using actual conditions. The range of Reynolds number will be chosen to ensure an operating regime from the purely laminar to completely turbulent. This paper presents the results of this scaling analysis.

Commentary by Dr. Valentin Fuster
2012;():279-285. doi:10.1115/HT2012-58309.

There is a growing need to develop technology to harness previously untapped sources of energy and waste heat is one source of energy that provides significant potential to increase the efficiency of overall energy use in multiple applications. Waste heat as a source of energy supply has relatively low availability and a storage system is required to efficiently utilize the energy. One method for harvesting this waste heat is through the use of phase change material (PCM) as a thermal energy storage (TES) medium.

This work examines the operation of a thermal energy storage device that collects and stores heat in a PCM. The TES relies on the phase change of icosane wax, which possesses a large latent heat of fusion and high thermal storage capacity. However, the icosane wax typically has low thermal conductivity. This work focuses specifically on enhancing the thermal conductivity of the sink by incorporating an additive possessing high thermal conductivity without significantly reducing the storage capacity (volume) of the TES. The performances of both the TES as well as TES with enhanced conductivity are monitored to validate operation.

Three different major experiment sets were performed; one contained only icosane wax as the PCM, a second incorporated a copper foam mesh along with the Icosane to increase the thermal conductivity of the working fluid. Finally, the third comprised of a novel copper matrix with increased surface area and thus, better conductivity for the PCM. The power absorbed by the PCM and the thermal storage potential for each of these tests was also studied in these experiments.

The novel copper matrix showed good promise in greatly increasing the thermal conductivity of the system from 0.49 W/mK in the icosane-only test to 3.90 W/mK. A 200% increase in the power absorbed by the device was also achieved with the copper matrix conductivity enhancer. Results indicate an improvement of average thermal conductivity by a factor of 8 due to this work. A steady state power absorbed of 0.9 kW/m2 was achieved for the preliminary test of unmodified icosane. A similar steady state value of 0.96 kW/m2 was achieved with the copper mesh enhancer. The maximum power absorbed was achieved with the novel copper matrix at 2.76 kW/m2.

Commentary by Dr. Valentin Fuster
2012;():287-296. doi:10.1115/HT2012-58323.

The 3-step sulphur-iodine-based thermochemical cycle for splitting water is considered. The high-temperature step consists of the evaporation, decomposition, and reduction of H2SO4 to SO2 using concentrated solar process heat. This step is followed by the Bunsen reaction and HI decomposition. The solar reactor concepts proposed are based on a shell-and-tube heat exchanger filled with catalytic packed beds and on a porous ceramic foam to directly absorb solar radiation and act as reaction site. The design, modeling, and optimization of the solar reactor using complex porous structures relies on the accurate determination of their effective heat and mass transport properties. Accordingly, a multi-scale approach is applied. Ceramic foam samples are scanned using high-resolution X-ray tomography to obtain their exact 3D geometrical configuration, which in turn is used in direct pore-level simulations for the determination of the morphological and effective heat/mass transport properties. These are incorporated in a volume-averaged (continuum) model of the solar reactor. Model validation is accomplished by comparing numerically simulated and experimentally measured temperatures in a 1 kW reactor prototype tested in a solar furnace. The model is further applied to analyze the influence of foam properties, reactor geometry, and operational conditions on the reactor performance.

Commentary by Dr. Valentin Fuster
2012;():297-302. doi:10.1115/HT2012-58331.

Forced convection heat transfer of supercritical carbon dioxide in circular horizontal tube, d = 8.7 mm, at relatively high Reynolds number (2×104 < Re < 105) is investigated. Experiments are carried out at two mass flow rates of 0.011 and 0.014 kg/s, for fluid inlet temperatures from 20 to 70°C, system pressures from 75 to 90 bar and constant heat flux of 20 kW/m2. Averaged heat transfer coefficients at several locations are obtained to investigate the influence of the fluid bulk temperature and pressure, respectively, on the forced convection heat transfer in the tube. The, the experimental results are then compared with a widely used empirical correlation. The results indicate that the effect of buoyancy on the heat transfer coefficient cannot be ignored in the near-critical and pseudocritical regions of fluid in this flow geometry. This dependency is believed to be due to the extreme dependence of fluid properties to temperature and pressure in this region.

Commentary by Dr. Valentin Fuster
2012;():303-309. doi:10.1115/HT2012-58345.

Experimental investigations were carried out to characterize forced convection behavior of Nanoparticle Enhanced Ionic Liquids (NEILs). 1-butyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl} imide ([C4mim][NTf2]) was used as the base ionic liquid (IL) with 0.5% (weight%) loading of Al2O3 nanoparticles. Flow experiments were conducted in a circular tube in the laminar flow regime. Convection results from IL without nanoparticles were used as the base line data for comparison with convection results with NEIL. Viscosity and thermal conductivity of the NEIL and base IL were also measured. NEIL displayed superior thermal performance compared to the base IL. An average of 13% enhancement in heat transfer coefficient was found for the NEIL compared with that of the base IL. Probable reasons of these enhancements are discussed in the paper.

Commentary by Dr. Valentin Fuster
2012;():311-321. doi:10.1115/HT2012-58349.

A problem with managing the electric grid is the variability of wind-generated electricity, particularly when it represents a significant fraction of the total electric power. One solution for smoothing the variability in wind production is energy storage. Unlike conventional thinking of using electric energy storage, this paper discusses the opportunity to use ice-storage for air conditioning in large commercial buildings to provide the smoothing (i.e., balancing) services. When the wind generation is high so that excess electricity is available, ice is made. When wind generation ceases and the grid demand exceeds generation capacity, the air conditioning supplied by chillers is shut down and cooling is provided by the stored ice.

Exploring thermal energy storage for the wind balancing was motivated by the observation that thermal energy storage using ice is an order of magnitude less expensive than electric energy storage. This paper addresses the technical challenges and economic opportunities of operating a chiller/storage system to balance the wind production for the grid and well as meeting the building cooling needs. If ice storage is to be an effective approach for wind balancing then the system must be optimized in a way that ensures that both the commercial building owner and the electric distribution system profit.

The paper describes the features of a combined ice storage/chiller system operation and details the numerical approach to achieve an optimal strategy that minimizes operating costs. Because of the high variability of wind electricity generation, the time discretization of the optimization had to be of the order of 5 minutes which increaes the size of the optimization problem significantly. The optimization problem is non-linear when a real time pricing rate structure to the commercial building owner is considered. This paper discusses the results of exploring the optimal control strategies under various conditions.

Commentary by Dr. Valentin Fuster
2012;():323-330. doi:10.1115/HT2012-58353.

The feasibility of several alternatives to long-haul truck idling are investigated. Battery Powered systems (BPS) where batteries are charged off the alternator is considered. Moreover, BPSs with various battery types are compared to determine which one would have the greatest impact on the fuel consumption and overall performance. In addition, applicability of thermal energy storage (TES) is studied as a means of cooling instead of a standard compressor air conditioner. Fuel cell powered systems (FCS) are investigated to replace batteries as a means of energy storage source on the truck. It is concluded that the most feasible method for truck idling reduction is BPS featuring lithium ion batteries while, BPS with lead-acid batteries is the cheapest solution.

Commentary by Dr. Valentin Fuster
2012;():331-337. doi:10.1115/HT2012-58361.

Micro-scale coolers have a wide range of potential application areas, such as cooling for chip- and board-level electronics, sensors and radio frequency systems. Miniature devices operating on the Stirling cycle are an attractive potential choice due to the high efficiencies realized for macroscale Stirling machines. A new micro-scale Stirling cooler system composed of arrays of silicon MEMS cooling elements has been designed. In this paper, we use computational tools to analyze the porosity-dependence of the pressure and heat transfer performance in the regenerator. For laminar flow in the micro-scale regenerator, the optimal porosity is in a range of 0.85∼0.9 based on maximizing the system coefficient of performance (COP). The system’s thermal performance was then predicted considering compressible flow and heat transfer with a large deformed mesh in COMSOL. The Arbitrary Lagrangian-Eulerian (ALE) technique was used to handle the deformed geometry and the moving boundary. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model was used to replace the pillars in the model, allowing for numerical predictions of full-element geometry. Parametric studies of the design demonstrate the effect of the operating frequency on the cooling capacity and the COP of the system.

Commentary by Dr. Valentin Fuster
2012;():339-345. doi:10.1115/HT2012-58365.

Accurate knowledge of the instrument lineshape (ILS) of Fourier transform infrared (FTIR) spectrometers is required for proper measurement analysis. An ILS is instrument- and spectrally-dependent and can substantially deviate from the ideal ILS. The ILS of a low-resolution FTIR has been determined for spectral resolutions of 1, 2, 4, 8, 16, and 32 cm−1. Each ILS was recovered with the retrieval software LINEFIT. Using the recovered ILS, transmissivity spectra of carbon monoxide (CO) and carbon dioxide (CO2) were calculated using the HITEMP2010 spectral database. These spectra were then compared with (1) idealized spectra (calculated using HITEMP2010 and the ideal lineshape) and/or (2) measured spectra from the FTIR to analyze both the accuracy of LINEFIT and the systematic error that can result from assuming the ILS to be ideal.

Commentary by Dr. Valentin Fuster
2012;():347-352. doi:10.1115/HT2012-58366.

Recently, it has become possible to conduct line-by-line (LBL) accurate radiative heat transfer calculations in spectrally highly nongray combustion systems using the Monte Carlo method. LBL accuracy, in principle, adds little to the computational load as compared to gray calculations. However, when employing the Monte Carlo method, choosing appropriate emission wave numbers for statistical photon bundles can be numerically expensive. In this paper, a new scheme for wave number selection is proposed, significantly decreasing CPU requirements compared to previous work. The accuracy of the new method is established and its time requirements are compared against the previous method.

Commentary by Dr. Valentin Fuster
2012;():353-356. doi:10.1115/HT2012-58402.

Copper nanoparticles were incorporated with Carbon Nanotube (CNT)/polymer nanocomposites in order to enhance the thermopower by enlarging the energy gap between the Fermi level and the mean of differential electrical conductivity. The thermopower was increased as ∼ 2 times with 1 vol% of copper nanoparticles. The effects of copper concentration on the electrical and thermal properties of the composites were studied.

Commentary by Dr. Valentin Fuster
2012;():357-361. doi:10.1115/HT2012-58404.

Heating cables inserted in metal conduit that is embedded in concrete are used in frost heave prevention of liquefied natural gas storage tanks (LNG tanks). The subfreezing temperature of the tank can cause the soil below and around it to freeze. This phenomenon causes heaving of the soil and damage to the foundation of the tank. This study investigates both the worst heat transfer scenario, where the heating cable is positioned in the center of and not in direct contact with the conduit, and the best heat transfer scenario, where cable is positioned on the bottom of and in direct contact with the conduit, for the purpose of preventing frost heave of the tank. Experiments are carried out to evaluate the cable power output and sheath temperature under a variety of conduit temperatures and applied voltages. A coupled thermal-electric-fluids numerical model is developed as well in Ansys-CFX to predict the cable power output and its temperature distribution. The numerical model is calibrated and the predicted cable power output and cable sheath temperature are compared with the experimental data. The numerical predictions demonstrated good agreement with experimental data. The heat transfer mechanism between cable and conduit involves thermal conduction, convection and radiation.

Topics: Cables , Heating
Commentary by Dr. Valentin Fuster
2012;():363-372. doi:10.1115/HT2012-58416.

This paper reports a numerical study on the thermal radiative transport in a cloud of dry water particles. Dry water is a water-in-air inverse foam which consists of micrometer-sized water droplets encapsulated by hydrophobic fumed-silica nanoparticles. The radiative properties of this novel material were estimated using the Mie theory for coated spheres. The radiative transport equation (RTE) was solved for a one-dimensional geometry using the discrete ordinates method. The effects of silica particle and water droplet size as well as the volume fraction of dry water particles on reducing radiative heat transfer were studied numerically. The results were compared with respect to two limiting cases: (i) system with no particles and (ii) silica particles with no water. The results showed that dry water reduced the local radiative heat flux as much as 20% more than that by silica particles alone. Additionally, reduction of the diameter of dry water particles from 75 to 25 μm reduced the radiative heat flux by 17%. Finally, parametric analysis showed that increasing the volume fraction of dry water by 10 times decreased the radiative heat flux by about 30% at the receiver end.

Commentary by Dr. Valentin Fuster
2012;():373-380. doi:10.1115/HT2012-58432.

Cooling poses as a major challenge in the IT industry because recent trends have led to more compact and energy intensive microprocessors. Typically microprocessors in current consumer devices and state-of-the-art data centers are cooled using relatively bulky air cooled heat sinks. The large size heat sinks are required due to the poor thermophysical properties of air. In order to compensate for the poor thermal properties of air, it is typical to use chillers to pre-cool the air below the ambient temperature before feeding it to the heat sinks. Operating the chillers requires additional power input thereby making the cooling process more expensive. The growing cooling demand of electronic components will, however, render these cooling techniques insufficient. Direct application of liquid-cooling on chip level using directly attached manifold microchannel heat sinks reduces conductive and convective resistances, resulting in the reduction of the thermal gradient needed to remove heat. Water is an inexpensive, nontoxic and widely available liquid coolant. Therefore, switching from air to water as coolant enables a much higher coolant inlet temperature without in any way compromising the cooling performance. In addition, it eliminates the need for chillers and allows the thermal energy to be reused. All these improvements lead to higher thermal efficiency and open up the possibility to perform electronic cooling with higher exergetic efficiency. The current work explores this concept using measurements and exergetic analyses of a manifold microchannel heat sink and a small scale, first of its kind, hot water cooled data center prototype. Through the measurements on the heat sink, it is demonstrated that the heat load in the state-of-the-art microprocessor chips can be removed using hot water with inlet temperature of 60°C. Using hot water as coolant results in high coolant exergy content at the heat sink outlet. This facilitates recovering the energy typically wasted as heat in data centers, and can therefore result in data centers with minimal carbon footprint. The measurements on both the heat sink and the data center prototype strongly attest to this concept. Reuse strategies such as space heating and adsorption based refrigeration were tested as potential means to use the waste heat from data centers in different climates. Application-specific definitions of the value of waste heat were formulated as economic measures to evaluate potential benefits of various reuse strategies.

Commentary by Dr. Valentin Fuster
2012;():381-387. doi:10.1115/HT2012-58455.

Energy recovery from vehicle engine exhaust has attracted considerable interest recently. Key parameters associated with the engine exhaust, including temperature, mass flow rate, maximum extractable energy, and optimum location for energy extraction all factor strongly into the materials research and device design for waste heat recovery. This review paper compiles available data in literature on the vehicle engine exhaust resources for several different vehicles, and under various operational conditions. Three vehicles types, namely, mid-size sedans, light duty trucks, and heavy duty trucks, have been considered, and the driving cycles including Federal Test Procedure (FTP) series, Highway Fuel Economy Test (HFET) and New European Driving cycle (NEDC) are considered in this review. The results show the average temperatures at highway driving cycle and city driving cycle remain in the ∼500–650°C and ∼200–400 °C range, respectively. The mass flow rate varies significantly with vehicle size. The available thermal power calculated based on the collected data is 3–10 kW.

Commentary by Dr. Valentin Fuster
2012;():389-394. doi:10.1115/HT2012-58466.

The use of direct energy conversion devices to convert waste heat into useful power has been the focus of extensive research for many years. Optimization of the performance of power harvesting systems has led to various models of the irreversibilities occurring in these devices. The majority of these models are based on dimensionless parameter referred to as the irreversibility factor to account for the entropy generating effects occurring within a generic heat engine. The purpose of this paper is to describe the use of the exergetic or second law efficiency to characterize these non-ideal effects and to describe how this approach may be used in the design and optimization of power harvesting or waste heat recovery systems. The use of a thermoelectric generator in a power harvesting system is considered to illustrate the proposed model.

Commentary by Dr. Valentin Fuster
2012;():395-403. doi:10.1115/HT2012-58487.

Charging of electric double layer capacitors (EDLCs) may cause significant heat generation. The resulting elevated temperatures lead to shortened cell life and increased self-discharge rates and cell pressure. Better understanding and accurate modeling of the fundamental physical phenomena involved are needed for developing thermal management strategies and for designing and optimizing the next generation of EDLCs. Existing thermal models of EDLCs rely on experimentally measured heat generation rates or cell electrical resistances. This makes them unsuitable for assessing new and untested designs. The present study aims to develop a physical model accounting for the dominant transport phenomena taking place in EDLCs. It accounts for the presence of the Stern layer, finite ion size, ion diffusion, and Joule heating. It solves the modified Poisson-Nernst-Planck model with a Stern layer and the heat diffusion equation. A dimensional analysis was performed and six dimensionless parameters governing electrodiffusion coupled with heat transfer were identified and physically interpreted. The scaling analysis was successfully validated numerically.

Commentary by Dr. Valentin Fuster
2012;():405-420. doi:10.1115/HT2012-58492.

The search for a clean and green locomotive propulsion system is gaining importance due to the increased cost of imported oil and the requirement to meet the higher EPA standards for reduced emission of greenhouse gases and pollutants. A hybrid diesel engine and battery locomotive with regenerative braking system is one such potential electric propulsion system that has been under consideration by railroad industries around the world. Lithium ion batteries are considered as one of the leading types as compared to the other batteries for the battery systems to be employed in electric vehicles (EVs) or hybrid electric vehicles (HEVs). Some of the major challenges with the full-scale commercial use of batteries for electric or hybrid vehicles are the requirement of high energy density, compatibility with high charge and discharge rates for different load cycles while maintaining high performance, and prevention of any thermal runaway conditions.

The objective of this research is to develop a computer simulation model for coupled electrochemical and thermal analysis and characterization of a lithium ion battery performance subject to a range of charge and discharge loading, and thermal environmental conditions. The electrochemical model includes species and charge transport through the liquid and solid phases of electrode and electrolyte layers along with electrode kinetics. The thermal model includes a number of heat generation components such as reversible, irreversible and ohmic heating, and heat dissipation by conduction through layers of battery cell and convection from the surface. Simulation results show sensitivity of charge and discharge rates on the electrochemical performance and thermal conditions of the battery. Variation in the voltage loss due to reaction and ohmic irreversibilities are observed when the battery is subjected to different discharge and charge rates, and thermal conditions. The cell temperature distributions for different load cycles and boundary conditions indicate the need for cooling the cell in order to avoid thermal run-away. The model developed helps in gaining a good insight of the complex processes and can form a platform for identifying materials for enhanced battery performance and thermal management system for EVs and HEVs.

Commentary by Dr. Valentin Fuster
2012;():421-432. doi:10.1115/HT2012-58514.

This paper presents an analysis of new heat-transfer correlations developed for SuperCritical Water (SCW) and SuperCritical Carbon Dioxide (SCCO2) flowing upward in vertical bare tubes. Previous studies have shown that existing correlations deviate significantly from experimental Heat-Transfer-Coefficient (HTC) values, especially, within the pseudocritical range for both fluids.

Therefore, new empirical correlations based on the following approaches in terms of characteristic temperature were developed: 1) bulk-fluid-temperature approach (SCW); 2) wall-temperature approach (SCW and SCCO2); and 3) film-temperature approach (SCW). Analysis showed that for SCW the best correlations, i.e., the most accurate ones, are based on the bulk-fluid- and wall-temperature approaches. Calculated wall temperatures according to these new correlations were within ±15% and HTC values were within ±25% for analysed datasets. For SCCO2, the new correlation was within ±20% for wall temperatures and within ±30% for HTC values.

The proposed correlations can be used for (1) calculations of SCW and SCCO2 heat-transfer in SuperCritical (SC) steam generators / heat exchangers; (2) preliminary heat-transfer calculations in reactors fuel channels as a conservative approach; (3) future comparisons with other independent datasets and with bundle data; (4) verification of computer codes for thermalhydraulics; and (5) verification of scaling parameters between SCW, SCCO2 and other SC fluids.

Commentary by Dr. Valentin Fuster
2012;():433-440. doi:10.1115/HT2012-58535.

The thermal vapor compressor (TVC) is an essential part that governs the overall process in the MED (Multiple-Effect Distillation) -TVC system. The flow and heat transfer in the TVC is very complex due to the strong compressibility, non-equilibrium phase change and supersonic turbulent flow of the stream. So in improving the performance of an ejector system, an investigation of the characteristics for the heat transfer and high speed flow in inside the ejector is often required. In the present study, the supersonic steam flow with non-equilibrium phase change and condensation shock was numerical studied based on the computational fluid dynamics (CFD) method. The special phenomena in the supersonic steam flow in the nozzle were investigated. The effects of various operating pressures and temperatures on the nozzle performance were investigated. The research explored the effect of steam pressure, temperature, supercooling level and super-saturation ratio on the onset of the nucleation and the intensity of condensation shock in the supersonic steam flow.

Commentary by Dr. Valentin Fuster
2012;():441-445. doi:10.1115/HT2012-58568.

High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyrolysis of organometallic compound method. The growth process of Bi2Te3 nanocrystals was revealed by transmission electron microscopy (TEM) images. Samples synthesized at the temperature of 35°C show a dominant morphology of 0D nanoparticle or 1D nanorod, while samples synthesized at the temperature above 75°C show a dominant morphology of 2D nanoplate. Phonon vibrational behavior was investigated by Raman spectroscopy. 2D nanoplates show similar Raman features to few-quintuple thick Bi2Te3 layers, while 0D and 1D nanostructures show a blueshifted A1g2 mode and a much stronger A1u mode. This is the report about Raman spectra obtained on small Bi2Te3 nanoparticle and nanorod whose size is within the strong quantum confinement region.

Commentary by Dr. Valentin Fuster
2012;():447-452. doi:10.1115/HT2012-58593.

Exhaust manifolds are used in many areas, such as turbocharged diesel engines. The purpose of an exhaust manifold is to direct the hot gases from the combustion chamber to the turbocharger or an acceptable exhaust outlet. The high gas temperatures in the exhaust manifolds lead to very high thermal stresses. This research focuses on the integration of computational fluid dynamics (CFD), Finite Element Analysis (FEA), and virtual reality visualization to analyze and visualize thermal stresses of manifolds for better understanding and better decision-making for design, troubleshooting, and optimization of manifolds. CFD is used to obtain the temperature distribution in the exhaust manifold. Using the temperature data FEA is then performed to determine the thermal stresses in the exhaust manifold. Using the results of the FEA analysis a 3-d virtual model is built to visualize the thermal stresses in the exhaust manifold. Methodologies have been developed for such integration and applied to industrial manifolds. Results will be presented in detail in the paper.

Commentary by Dr. Valentin Fuster
2012;():453-457. doi:10.1115/HT2012-58603.

Ceramic foams have widely been used as radiation receivers and heat exchangers, with the excellent characteristics of radiation absorption and heat transfer. In this paper, experimental investigations, on the characteristics of the radiation absorption and conversion, the heat transport of coupled radiation with conduction, the convection heat transfer between the gas phase and solid phase in the ceramic foams, were performed. The influences of the physical parameters of the ceramic foams were analyzed, on the radiation absorption, energy conversion and heat transport in tne ceramic foams. The results show: For the ceramic foams used as radiation receiver, the structure of the pores, the air flow velocity, and the parameters of the geometry have a strong effect on the temperature distribution, the radiation absorption, the energy conversion and the heat transport in the ceramic foams.

Commentary by Dr. Valentin Fuster
2012;():459-474. doi:10.1115/HT2012-58608.

The diffusion absorption refrigeration (DAR) cycle can provide refrigeration in remote locations using waste-heat or other low-grade-thermal input. Unlike conventional absorption systems, the DAR cycle receives no mechanical input, so all flows must be driven by passive mechanisms. Further, a third inert gas is employed to allow refrigerant expansion since conventional throttling devices impart large pressure drops. Thus, DAR absorber design is challenging due to increased mass transfer resistance from the inert gas, multiple outlet flow paths for the inert gas and solution, and limited (passive) external cooling. In the present study, a detailed, coupled heat and mass transfer model is developed for a counter-flow serpentine-tube DAR absorber. The model is applied to the analysis of an absorber for a small-scale refrigeration system with a 36 W cooling capacity. Studies are conducted to investigate the effect of key configuration and operational parameters on absorber performance, and guidelines are provided for component and system design.

Commentary by Dr. Valentin Fuster
2012;():475-479. doi:10.1115/HT2012-58615.

The vapor cycle refrigeration system is used in aircraft cooling. The changes of the ambient air properties will affect the condenser performance of the vapor cycle refrigeration system. The air-side characteristics between the flow and pressure drop at low pressure conditions must be taken into account, especially when it is used in the aircraft. Through the three tests of the condenser fan, the condenser, and the condenser module, the common property model of the air-side flow and pressure head/drop for each part is established and experimentally verified. The results show that the flow and pressure head or drop relationship of each unit under different pressures of 101.3kPa, 79.5kPa, 61.7kPa, and 47.2kPa is simply deduced by means of the ground test. From this study, the operation points of the condenser module under low ambient pressure conditions can be derived as well.

Commentary by Dr. Valentin Fuster

Theory and Fundamental Research

2012;():481-490. doi:10.1115/HT2012-58054.

The frost growth process in low temperature air-coolers of a refrigeration system is described by a complex set of partial differential equations which do not lend themselves to direct analytical solution. Although various forms of lumped and distributed models have been developed, they are often difficult to work with because of their complexity. Recent numerical solution schemes, either for the governing equations themselves or for simplified approximations of them, have been growing in popularity as computing power increases. Such solution schemes can yield excellent results when compared to experiments, but the time required to construct and solve them is great and good computing resources are needed.

Owing to the difficulty in using both approximate and numerical solutions, there is a continuing demand for correlations that are capable of predicting frost layer thickness with reasonable accuracy over a wide range of conditions. Many correlations have been presented, but have not met with good results outside narrow ranges of conditions specific to certain types of experimental apparatuses. It is thus desirable to obtain a more general correlation based on the consideration of the physical behavior during frost growth as opposed to the pure regression fits common to most current correlations. The correlation detailed herein is presented as a potential solution, as care has been given to account for the physical behavior of the frost layer during growth.

A particular functional form is chosen to mimic the solutions to other physical processes that exhibit similar behavior, such as the velocity profile in growing boundary layers and transient heat conduction. Significant parameters during frost growth are included in the form of dimensionless variables and placed in the correlation according to their overall influence on the growth process. Remaining variances in the growth profile, resulting from the impossibility of capturing exact behavior with a correlation, are accounted for by strategic inclusion of experimental coefficients.

The correlation is developed based on transient frost thickness data obtained using high accuracy visual methods for validation. The model is then extended to include data from many different sources, which allows for the coverage of wider ranges of conditions and eliminates design specifics of the apparatus as a factor in prediction. The tradeoff for such generality is a slight reduction in accuracy over the narrower range of experimental conditions used initially. Results show that the correlation is able to match frost growth trends with good accuracy across a wide spectrum of conditions. Results compare favorably with the predictions of other correlations.

Commentary by Dr. Valentin Fuster
2012;():491-507. doi:10.1115/HT2012-58061.

We report the results of experiments using high resolution imaging of transient frost melting to obtain quantitative information on frost thickness and porosity. A highly controlled test chamber and surface are described along with visual and digital methods extract quantitative data. When the frost layer approaches the fully permeated state and melting proceeds to liquid droplets, we report a digital method for analysis of droplet size and geometry. For each of the measurements, an error analysis is presented. We find that the measurement techniques employed in the present study provide faster data acquisition and higher accuracy than traditional approaches. The present data are utilized successfully in a companion paper in connection with an empirically based model of frost growth.

Commentary by Dr. Valentin Fuster
2012;():509-515. doi:10.1115/HT2012-58080.

Droplets on engineered surface roughness exhibits superhydrophobic properties. Condensate droplets in Cassie state may have larger contact angles and lower dynamic hysteresis to gain superior mobility and high condensation heat transfer coefficient. In this article, research investigations have been focused on visualization of dropwise condensation in high aspect ratio (HAR) roughness structures. Both single and hierarchical HAR structures are developed for visualization of dropwise condensation in an open moisture environment. Experimental visualization is performed though combining high-speed Sensi-Cam and Nikon microscope to zoom in condensation area or look through a large condensed droplet. Experimental images indicate that condensate droplets co-exist in both Wenzel and Cassie states within the single HAR roughness structure. Condensation site density reduces with increase of the roughness structure depth due to air/non-condensable gas (NCG) barrier. In contrast, only Cassie state droplets are observed on and in the hierarchical HAR structures. Less condensation droplets in deep structure may be attributed to the reduced surface energy that inhibits the generation of condensation sites. Under droplet visualization denotes that large droplets hold in Cassie state on both the HAR roughness structures. Inner droplet flow driven surface tension may play a critical role in heat transfer of the dropwise condensation.

Commentary by Dr. Valentin Fuster
2012;():517-523. doi:10.1115/HT2012-58082.

The effect of a perturbation in the diameter of a circular microchannel on critical heat flux (CHF) is explored across a variety of predictive CHF correlations, and an optimum rate of diameter expansion is sought. It is demonstrated that a nonzero diameter expansion necessarily improves performance under several criteria for critical heat flux, and an optimum expansion parameter exists for many of these criteria. CHF relations are seen to follow a few distinct types, and those relations which contemplate effects which may directly influence CHF, such as pressure and phase velocity, tend to better reflect the experimentally demonstrated effect of the perturbation on the diameter of the microchannel.

Commentary by Dr. Valentin Fuster
2012;():525-531. doi:10.1115/HT2012-58114.

This paper reports on experimental and numerical investigations of electrically powered MEMS structures operated under different gas pressure and electrical power conditions. The structures studied are boron-doped single crystal silicon-on-insulator (SOI) microbridges that are heated by an electrical current. The microbridges are 85 μm wide, 125 μm tall and 5.5 mm long and lie 2 μm above the substrate. The impact of the narrow gap in the gas phase thermal transport is evaluated by operating the devices under various nitrogen gas pressure conditions, ranging from 625 Torr to ∼1 mTorr — spanning the continuum to noncontinuum gas heat transfer regimes. Raman thermometry is used to obtain spatially-resolved temperature measurements along the length of the device under the various operating conditions. The large dopant concentration (∼4 × 1019 cm−3) within the active silicon layer is found to affect the Raman spectrum used for thermometry via Fano-type interactions, resulting in an asymmetric Raman line shape. With large Raman peak asymmetries, use of the Raman line width as the temperature metric is less reliable as it shows decreased sensitivity to temperature. However, the asymmetry itself, when considered as a fitting parameter, was found to be a reliable indicator of sample temperature. The measured device temperatures are compared to finite element simulations of the structures. Noncontinuum gas phase heat transfer effects are incorporated into the continuum simulations via temperature discontinuities at the solid-gas interface, provided by a model developed from noncontinuum simulation results. Additionally, the impact of the large dopant concentrations is incorporated into the thermal models via a modified thermal conductivity model which considers impurity scattering effects on thermal transport. The simulation and experimental results show reasonable agreement.

Commentary by Dr. Valentin Fuster
2012;():533-539. doi:10.1115/HT2012-58125.

To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications. It is commonly understood that the micro-fin enhances heat transfer in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of heat transfer from laminar to transition and eventually the turbulent regions. Furthermore, it is difficult to find the information about the effect of inlet configuration on the micro-fin tube heat transfer in the open literature. Therefore, more in-depth study for the micro-fin tube heat transfer characteristics is necessary.

In this study, heat transfer was measured in a single test section fitted with a micro-fin tube and compared with the data of a plain tube. Both of the tubes with the same internal diameter of 14.9 mm were tested under the uniform wall heat flux boundary condition. Three inlet configurations, re-entrant, square-edged, and bell-mouth, were used in this study. The entire experiment covered the Reynolds number range between 1100 and 23,000.

From the heat transfer results, the transition from laminar to turbulent region for the plain and micro-fin tubes was clearly established. For both of the plain and micro-fin tubes in the laminar region, buoyancy effects and heat transfer magnitudes were comparable. It was also observed that for the micro-fin tube heat transfer enhancement initiated in the transition region and lasted through the turbulent region.

For both of the plain and micro-fin tubes, the transition from laminar to turbulent region was found to be inlet dependent. For bell-mouth inlet, it was obvious that the micro-fin tube had an earlier transition than the plain tube. Furthermore, for both of the plain and micro-fin tubes with bell-mouth inlet, an unusual behavior of the local heat transfer coefficient was observed in the transition and turbulent regions.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2012;():541-547. doi:10.1115/HT2012-58152.

In this paper, a mathematical model is developed, which can be used to predict the evaporation and fluid flow in thin film region and the analytical solutions are obtained for the dimensionless heat flux distribution, the maximum heat flux, the location and thickness of the thin film corresponding to the maximum heat flux and the total heat transfer rate per unit width along the meniscus. The numerical results show the effects of the superheat on the profile of the thin film, the heat flux and its maximum value, in addition, the thickness of the thin film and the thermal resistance ratio when the heat flux reaches its maximum value. Finally, the heat transfer capacity of the thin film evaporation is compared with references.

Commentary by Dr. Valentin Fuster
2012;():549-557. doi:10.1115/HT2012-58185.

Aerosolized metal nanoparticles have numerous important applications in materials science, but their functionality depends strongly on their size. Very recently, time-resolved laser-induced incandescence (TiRe-LII) has been investigated as a technique to size aerosolized metal nanoparticles, but this procedure requires an accurate model of the heat transfer through which laser-energized particles re-equilibrate with the bath gas. This paper presents a model for laser-energized molybdenum nanoparticles, which is then applied to analyze experimental TiRe-LII measurements made on molybdenum nanoparticles formed by photolysis of Mo(CO)6 in helium, argon, nitrogen, and carbon dioxide. While it is possible to estimate the size distribution width, recovering nanoparticle sizes from TiRe-LII data requires independent knowledge of the thermal accommodation coefficient.

Commentary by Dr. Valentin Fuster
2012;():559-566. doi:10.1115/HT2012-58283.

The thermal transport at the carbon nanotube (CNT) interfaces such as CNT-oxide and CNT-CNT junctions can significantly impact the device performance and reliability of nanotube network based thin-film transistors. For an example, the high electrical and thermal resistance at CNT junctions can cause hot spots, inefficient heat removal or even breakdown of CNTs. This paper presents a molecular dynamics based computational study of the heat dissipation at CNT-CNT junctions supported on silicon dioxide substrate. The breakdown of CNTs at high power densities and heat dissipation mechanism at CNT-CNT-oxide junctions is analyzed for different contact structures. It has been observed that at similar power densities the temperature in hanging CNTs can be hundreds of degree higher and can reach to breakdown temperature compared to the CNTs well-contacted with the substrate. The energy transfer in different frequency bands across the CNT-CNT-oxide contact was investigated using spectral energy density method. The lower CNT in the supported CNT-CNT junctions blocks the direct transport of high frequency phonons of top CNT to the oxide substrate.

Commentary by Dr. Valentin Fuster
2012;():567-577. doi:10.1115/HT2012-58328.

This paper presents a model for analysis of heat and momentum transport in a novel spiraling radial inflow heat sink. The design provides uniform heat removal at high heat flux rates making this design attractive for electronics cooling applications or for heat removal from concentrating solar photovoltaic systems. The design consists of swirling flow in a microchannel between two concentric disk surfaces with flow accelerating inwards and then exiting at the center. The geometry of the flow path can further be varied to increase heat transfer and allow for greater surface temperature control. The governing flow and energy equations are solved using the integral method to achieve solutions for both the constant wall heat flux and constant wall temperature conditions. The model is used to explore the parametric effects of inlet flow and passage geometry, including constant and varying channel sizes. Case studies are examined for liquid water as the working fluid rejecting heat fluxes up to 100 W/cm2 while keeping surfaces below 80 °C.

Commentary by Dr. Valentin Fuster
2012;():579-587. doi:10.1115/HT2012-58350.

Heat transfer across nanoscale metal/dielectric multilayers involves multiple thermal conduction mechanisms. Electron or phonon interface scattering can augment the thermal conductivity anisotropy in multilayer composites. Weak electron-phonon coupling and quasi-ballistic phonon transport normal to the metal film further increase the anisotropy for metal-dielectric multilayers with period shorter than the relevant free paths. This paper models these physical mechanisms using an approximate thermal resistor network with support from the Boltzmann transport equation. We measure the in- and cross-plane thermal conductivity of a Mo/Si (2.8 nm/4.1 nm) multilayer as 15.4 and 1.2 W/mK, respectively, which agree with the proposed theoretical model. This work introduces a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

Commentary by Dr. Valentin Fuster
2012;():589-594. doi:10.1115/HT2012-58354.

Evaporation of a nanoscale meniscus on a nano-heater array surface via pulsed heating is simulated using molecular dynamics. The nano-heaters, each of width 2.56 nm, are evenly spaced on the 28.22 nm wide surface. The temperature of the nano-heaters is increased for a short time period in regular intervals to mimic pulse heating. The simulation results show that the non-evaporating film breaks during the early stages of evaporation due to the pulse heating (unlike a previous simulation performed in absence of pulse heating where non-evaporating film forms). Thus, heat transfer rates can be significantly increased during bubble nucleation and growth by using nano-heater arrays with pulsed heating as it breaks the formation of non-evaporating film allowing the surrounding cooler liquid to come in contact with the surface enhancing heat transfer.

Commentary by Dr. Valentin Fuster
2012;():595-600. doi:10.1115/HT2012-58358.

Measurements of the internal heat transfer coefficient in complex surfaces such as random fiber matrices are difficult and often done using the so-called single blow transient test method. In such a method the inlet fluid temperature is perturbed, and measurement of the transient fluid temperature response at the outlet allows the heat transfer coefficient to be determined. Obtaining an accurate heat transfer coefficient for a sample, using this method, relies on developing an accurate model for the thermal phenomena taking place. Existing models employed appear to be too simple and lack rigor in their derivation. A model based on Volume Averaging Theory (VAT) is believed to alleviate such problems. A precise expression for the local heat transfer coefficient has previously been rigorously derived from the microscale governing equations. This expression provides a clear definition of the transport coefficient that is being measured. Nusselt number results for several random fiber matrices are obtained for Reynolds numbers between 5 and 70, and are compared to existing correlations. The dimensionless numbers used are scaled with the simple “universal” porous media length scale. It is found that this new combined experimental and computational method is effective in determining the local convective heat transfer coefficient in complex porous structures. Moreover, the experimental apparatus and VAT-based model may be used in other samples of complex morphology provided the porosity and specific surface area can be determined.

Topics: Fibers , Modeling
Commentary by Dr. Valentin Fuster
2012;():601-607. doi:10.1115/HT2012-58445.

We determine how the natural distributions of phonon-phonon and phonon-boundary scattering free paths affect the prediction of thermal conductivity for thin films, nanowires, and porous nanofilms. Using Monte Carlo sampling, the effective mean free path for each phonon mode is calculated using a Poisson distribution for the phonon-phonon free path and assuming an equal probability of the phonon originating at any point in the nanostructure. We find our predictions to be consistent with an analytical result for the in-plane direction in the thin films, as opposed to the Matthiessen rule, which leads to an under-prediction by up to 10%. Furthermore, we are able to use our approach to predict the thermal conductivities of complex nanostructures, where correct application of the Matthiessen rule is challenging.

Commentary by Dr. Valentin Fuster
2012;():609-615. doi:10.1115/HT2012-58451.

This study investigates the suitability of the known Berkovsky-Polevikov correlations, used for predicting the wall-averaged Nusselt number, Nuav, of “wide” enclosures heated from the side and filled with a fluid undergoing natural convection, to predict the heat transfer coefficient inside a nonhomogeneous enclosure heated from the side, filled with uniformly distributed, disconnected and conducting solid objects also saturated with a fluid undergoing natural convection. Hence, defining γ = RaHPr/(0.2+Pr), a correlation in the form of Nuav = AγB is investigated for curve fitting numerical simulation results. The numerical results are obtained by simulating the heat transfer process of the two distinct constituents, namely the fluid and the solid, within the enclosure using the finite-volume method with appropriate conservation equations and compatibility conditions at their interfaces. The right wall of the enclosure is maintained at temperature lower than that of the left wall, with the horizontal top and bottom surfaces of the enclosure assumed to be adiabatic. Results for 1, 4, 9, 16, and 36, evenly distributed square solid blocks are presented. Appropriate numerical values for coefficients A and B are determined and presented for the utilization of the corresponding Berkovsky-Polevikov correlations. Good correlation is obtained when the Rayleigh number is high (≥107), as to yield distinct boundary layers inside the enclosure.

Commentary by Dr. Valentin Fuster
2012;():617-624. doi:10.1115/HT2012-58459.

This paper investigates anharmonic phonon dispersion relations measured directly from molecular dynamics simulations at finite temperatures and pressure. The measured dynamical matrix and resulting anharmonic dispersion relations do not require an a-priori analytical expression regarding the strength of anharmonic processes. Therefore, no assumptions concerning the degree of anharmonicity are made beyond specifying an interatomic potential. We calculate phonon properties pertinent to thermal transport in graphene. Specifically, we demonstrate the calculation of phonon dispersion relations and group velocities over the entire Brillouin Zone, as well as the branch-dependent contribution to specific heat capacity and ballistic thermal conductance. We highlight the capabilities of this technique to lend fundamental insight into the anharmonic characteristics of phonon-mediated transport. Finally, we discuss how anharmonic phonon dispersion relations may be used to evaluate the differences in phonon properties between various interatomic potentials commonly used in the simulation of phonon-mediated thermal transport.

Commentary by Dr. Valentin Fuster
2012;():625-633. doi:10.1115/HT2012-58505.

Employing nanofluids is an innovative way to enhance heat transfer in cooling system of internal combustion engine. However, the local flow enhancement due to the adding of nanoparticles, which is one of the key mechanisms behind heat transfer enhancement in nanofluids, still lacks a microscale-level understanding. The aim of this work was to study the microscopic mechanism for local flow enhancement in nanofluids by molecular dynamics (MD) simulation. Local flow characteristics of nanofluids were simulated by MD method and statistically analyzed, and the microscopic mechanism for local flow enhancement was discussed. The MD simulation results revealed that the microscopic mechanism for local flow enhancement in nanofluids is mainly because the irregular movements of nanoparticles, including rotational and translational motions, enhance momentum exchange between fluid molecules and cause disturbance of base fluid. And therefore flow of nanofluids would be more active, which is better for heat transfer. The present work suggests the microscopic mechanism of local flow enhancement in nanofluids, which is the basis of understanding heat transfer enhancement of nanofluids and further application of them in cooling system of internal combustion engine.

Commentary by Dr. Valentin Fuster
2012;():635-641. doi:10.1115/HT2012-58508.

This paper performs molecular dynamics simulations on flow and heat transfer process of nanofluids containing spherical nanoparticles with various diameters (2–6 nm). Instantaneous rotational velocity components of nanoparticles in a flow field with and without a temperature difference are outputted and compared. Number density method is used to examine the thickness of absorption layer. And by equally dividing the fluid into 60 fluid layers, temperature distributions of nanofluids and base fluid are examined. It was found that rotational speed of nanoparticle decreases with an increasing diameter. By applying temperature difference rotational speed of nanoparticles are generally increased. The rotational speeds of nanoparticles are generally about 1E9 rad/s. the rotation of nanoparticles is attributed to Brownian motion due to their nanoscale size. The diameter of nanoparticles has little effect on the thickness of the absorption layer, and the thickness of absorption layer is about 0.8 nm. By comparing temperature distributions of nanofluids and base fluid, it was found that the internal temperature difference in nanofluids is less than that of base fluid. And according the temperature gradient in nanofluids near the solid wall will be larger, which is better for heat transfer. This phenomenon is attributed to the fast-rotating nanoparticles accompanied by the absorption layer of liquid atoms. The present work examines the rotation of nanoparticles and absorption layer, which is the basis of understanding heat transfer mechanism in nanofluids and proposing mathematical description for the transfer process.

Commentary by Dr. Valentin Fuster
2012;():643-650. doi:10.1115/HT2012-58524.

Heat transfer between contacting surfaces is a key factor in the thermal behaviour of engineering components in turbomachinery and various other areas of technology. Thermal contact conductance (TCC) is a parameter that quantifies this heat flow. An ongoing challenge in measuring and modelling TCC is the different length scales of surface topography exhibited on real components. Manufacturing techniques such as turning and fly cutting introduce repeatable surface deviations of medium wavelength which are the focus of this study.

An instrumented split tube with in-line washers, loaded under carefully controlled conditions, was used to measure the TCC of washers made of PE16. Fly cutting was used in this study to introduce a repeatable lay typifying a range of manufacturing techniques. Experimental determination of TCC for (i) flat contacting washers and (ii) contact between flat washers and fly cut washers with repeatable lay is reported. As expected the latter case gives rise to significantly lower TCC than the former. A 2D finite element model is described which models the elastic-plastic deformation of a representative machining induced contact line. Using the TCC data for flat contacting surfaces, FEA is used to calculate the reduced TCC for the machined case. Predicted values of TCC assuming plane stress and plane strain are compared with experimental data.

Commentary by Dr. Valentin Fuster
2012;():651-658. doi:10.1115/HT2012-58528.

In the current work, two-dimensional spectral element simulations are used to investigate the heat transfer and fan power performance of the developing regions of finite-length, grooved channel passage arrays, including the accelerating and decelerating flows entering and exiting the arrays. The performance of the grooved channel arrays is compared with that of flat passage arrays with the same average wall center-to-center spacing for Reynolds numbers ranging from 1000 to 3000. The simulations show that unsteadiness develops after a number of groove lengths and results in enhanced heat transfer. The unsteadiness improves the overall heat transfer compared with a flat passage array of equal average channel height by a factor of 1.46 at Re = 1000 and a factor of 2.75 at Re = 3000. The grooves also cause an increase in the required fan power by a factor of 8.56 at Re = 1000 and a factor of 18.10 at Re = 3000. Since past simulations have shown that three-dimensional simulations are necessary to accurately predict heat transfer and fan power performance in transversely grooved passages, the current two-dimensional results will be used as a starting point for a three-dimensional model that will ultimately be used to predict heat transfer and friction factor performance in developing grooved channel flows.

Commentary by Dr. Valentin Fuster
2012;():659-670. doi:10.1115/HT2012-58554.

In this work, molecular dynamics (MD) simulations are performed to predict the lattice thermal conductivity of PbTe bulk and nanowires. The thermal conductivity of PbTe bulk is first studied in the temperature range 300–800 K. Excellent agreement with experiments is found in the entire temperature range when a small vacancy concentration is taken into consideration. By studying various configurations of vacancies, it is found that the thermal conductivity in PbTe bulk is more sensitive to the concentration rather than the type and distribution of vacancies. Spectral phonon relaxation times and mean free paths in PbTe bulk are obtained using the spectral energy density (SED) approach. It is revealed that the majority of thermal conductivity in PbTe is contributed by acoustic phonon modes with mean free paths below 100 nm. The spectral results at elevated temperatures indicate molecular scale feature sizes (less than 10 nm) are needed to achieve low thermal conductivity for PbTe. Simulations on PbTe nanowires with diameters up to 12 nm show moderate reduction in thermal conductivity as compared to bulk, depending on diameter, surface conditions and temperature.

Commentary by Dr. Valentin Fuster
2012;():671-679. doi:10.1115/HT2012-58565.

Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in CdSe QD, EQD and LQD, which are of the same diameter but an increasing length along c axis. Our work shows that both the length and system temperature have a strong impact on the electronic properties and electron relaxation dynamics of the CdSe QRs. Higher frequency phonons are excited and scattered with electrons at higher temperatures. The band gap shows a negative dependence on the temperature. The band gap decreases and the electron and hole density of states increase with increasing the length. However, not all the properties studied here vary with the length in a straight way. The band gap shows a stronger negative temperature dependence for the EQD than the QD and LQD. The electron-phonon couples stronger in the EQD than the QD and LQD. The hot electron relaxation proceeds faster in the EQD than the QD and LQD. Furthermore, the hot electron decay rate varies linearly with the average electron density of states and this linear relationship can be well described by the Fermi’s golden rule and of practical use in predicting the hot electron decay rate with the knowledge of the average NA coupling and electron density of states.

Commentary by Dr. Valentin Fuster
2012;():681-685. doi:10.1115/HT2012-58592.

The statistical rate theory (SRT) approach is derived and applied to predict the evaporation conditions of an ethanol droplet. In this study, a series of experiments of ethanol droplet evaporation at steady state have been conducted in an evaporation chamber. A temperature discontinuity is found across the liquid-vapor interface at the centerline during steady-state evaporation. The interfacial liquid temperature, the interfacial vapor temperature, the radius of droplet, and the average evaporation flux are used to predict the vapor-phase pressure. The predicted pressure is found to agree with the measured value. It is suggested that the SRT approach can predict the experimental evaporation conditions. As a factor in the SRT approach, the droplet radii are evaluated numerically. It is found that the radius with less than the micron scale significantly affects the vapor-phase pressure.

Topics: Drops , Evaporation , Ethanol
Commentary by Dr. Valentin Fuster

Aerospace Heat Transfer

2012;():687-695. doi:10.1115/HT2012-58053.

This study compares several band models for calculating the thermal radiation from the plume of a simulated rocket motor. First, a non-scattering plume is considered allowing one to use the line-of-sight integration (LOS). The results for three band models are compared. Next we study a scattering plume. The radiative heat transfer equation is solved numerically using the axisymmetric finite volume method (FV). Detailed analysis of the numerical scheme of the FV method demonstrated that it has a large numerical diffusion term which, although formally of the first order, in practice can cause large errors in calculating thermal signatures of hot jets. The numerical scheme was hence modified using what we call the “quasi-Cartesian” approach. We demonstrate that for a non-scattering media in the first order the numerical diffusion term vanishes and the modified scheme formally reduces to the integration along the line of sight resulting in a very good agreement between the FV and LOS predictions. The new method is then used to calculate radiation from a scattering plume and the results for two band models are compared.

Commentary by Dr. Valentin Fuster
2012;():697-706. doi:10.1115/HT2012-58122.

This study is dedicated to an innovative heatshield aeroshape intended to be the AEROFAST martian aerocapture demonstrator. The analysis of three representative Mars aerocapture trajectory points (maximum heat flux, maximum dynamic pressure and exit point) results in building a partial AeroThermo-DataBase including both convective and radiative heat flux evaluations carried out assuming CO2 chemical non equilibrium.

Commentary by Dr. Valentin Fuster
2012;():707-716. doi:10.1115/HT2012-58237.

The present study concerns the investigation of external natural convection driven around a Montgolfiere scientific balloon. Ground measurements are carried out on a small scale heated model located within an enclosure. Non intrusive planar Particle Image Velocimetry and thermocouple sensors are used to characterize both dynamical behavior of the flow generated around the balloon and heat transfers on its surface. It is shown that a relative good agreement is reached on wall heat transfers with simplified axisymmetrical 2D RANS simulations. However significant discrepancies exist regarding the analysis of the flow topology and dynamic quantities in comparison with the experimental data. The proposed numerical rebuilding is thus completed with a 3D Delayed Detached Eddy Simulation (DDES) in order to overcome conventional RANS approach inability to represent low frequency thermal plume instabilities.

Commentary by Dr. Valentin Fuster

Gas Turbine Heat Transfer

2012;():717-726. doi:10.1115/HT2012-58023.

The present study uses a novel transient liquid crystal technique to measure heat transfer on a rotating, radially outward coolant channel with jet impingement and a crossflow outlet condition. The jet impingement cooling scheme is studied on the leading and trailing sides of a gas turbine internal coolant channel with the jet impingement target surface oriented normal to the direction of rotation. Several aspects of jet impingement are studied under rotating conditions: effect of increasing Rotation number (Ro = 0–0.003), effect of jet inclination angle (90° and 70° from the vertical), and effect of jet-to-target surface distance (H/d = 1, 3, and 5). Heat transfer measurements are obtained on the target surface using the transient liquid crystal technique. All configurations studied have a constant jet-to-jet spacing, P/d = 5. The spacing between the two adjacent rows is P/d = 3. Corresponding flow measurements are taken from stationary conditions. Results show that rotation does not change the heat transfer magnitudes and distributions greatly compared to the stationary results for all H/d and jet orientation cases. As x/d increases, stationary H/d = 5 heat transfer results show a steady decrease, where effectiveness of the jets diminishes. As x/d increases for H/d = 3, the maximum and minimum heat transfer values dampen to a steady constant average value. As x/d increases for H/d = 1, the heat transfer begins very low then steadily increases for higher x/d.

Commentary by Dr. Valentin Fuster
2012;():727-735. doi:10.1115/HT2012-58100.

A two pass stationary square duct with rib turbulators subjected to sand ingestion is studied using Large Eddy Simulations (LES). Each pass has ribs on two opposite walls and aligned normal to the main flow direction. The rib pitch to rib height (P/e) is 9.28, the rib height to channel hydraulic diameter (e/Dh) is 0.0625 and calculations have been carried out for a bulk Reynolds number of 25,000. Particle sizes in the range 0.5–25 μm are considered, with the same size distribution as found in Arizona Road Dust (medium). Large Eddy Simulation (LES) with wall-model is used to model the flow and sand particles are modeled using a discrete Lagrangian framework. 220,000 particles are injected at the inlet and perfectly elastic collisions with the wall are considered. Results quantify the distribution of particle impingement density on all surfaces. Highest particle impingement density is found in the first quarter section of the second pass after the 180° turn, where the recorded impingement is more than twice that of any other region. It is also found that the average particle impingement per pitch is 28% higher in the second pass than the first pass. Results show lower particle tendency to hit the region immediately behind the rib in the first pass compared to the second pass where particle impingement is more uniform in the region between two ribs. The smooth walls do not show much particle impingement except the wall in second pass where the flow impinges after the turn. The rib face facing the flow is by far is the most susceptible to impingement and hence deposition and erosion. The results of this simulation were also compared with results obtained from experiments conducted on an identical two pass geometry with Arizona Road Dust particles. The particle impingement pattern is recorded by using a sticky tape on all surfaces to capture the particles. The numerical predictions showed good qualitative agreement with experimental measurements. These results help identifying the damage prone areas in the internal cooling passages of a turbine blade under the influence of sand ingestion. This information can help modify the geometry of the blade or location of film cooling holes to avoid hole blockage and degradation of heat transfer at the walls.

Topics: Cooling , Sands , Ducts
Commentary by Dr. Valentin Fuster
2012;():737-743. doi:10.1115/HT2012-58131.

Adiabatic film cooling effectiveness contours are obtained experimentally with the use of temperature sensitive paint on low thermal conductivity full coverage film cooled surfaces. The effects of blowing ratio, surface angle and hole spacing are observed by testing four full coverage arrays composed of cylindrical staggered holes all compounded at 45°, which parametrically vary the inclination angle, 30° and 45°, and the spacing of the holes, 14.5 and 19.8 diameters. Local film cooling effectiveness is obtained throughout these largely spaced arrays over up to 23 rows for the 19.8 spacing array and 30 rows for the 14.5 spacing array. The coolant takes several rows to merge and begin to interact with lateral holes at these large spacings, however; at downstream rows the film builds and provides high effectiveness in the gaps between injection. At low blowing, each individual jet throughout the entire array can be seen in the effectiveness profiles. At higher blowing rates, the profile is far more uniform due to jets spreading as they reattach with the cooled wall. Laterally averaged values of effectiveness easily approach 0.3 in most cases with some, high blowing low spacing, even reaching 0.5.

Topics: Film cooling
Commentary by Dr. Valentin Fuster
2012;():745-755. doi:10.1115/HT2012-58135.

Film cooling performance of a tripod hole anti-vortex geometry is evaluated on cascade vane pressure and suction surfaces with steady-state IR (infrared thermography) technique and compared to a baseline cylindrical hole geometry performance. The base geometry is a simple cylindrical hole design inclined at 30° from the surface with pitch-to-diameter ratio of 3.0. The tripod hole geometry, also called an anti-vortex design, is where two side holes, also of the same diameter, branch out from the root at 15° angle. The pitch-to-diameter ratio between the main holes for this design is 6.0. Two secondary fluids — air and carbon-dioxide — were used to study the effects of coolant-to-mainstream density ratio (DR = 0.95 and 1.45) on film cooling effectiveness. Several blowing ratios in the range 0.5 –4.0 were investigated independently at the two density ratios. Results show that the tripod hole design clearly provides higher film cooling effectiveness the baseline case with overall reduced coolant usage on both pressure and suction side of the airfoil. Additional testing was also conducted to measure the aerodynamic effects of injecting coolant through the cylindrical and tripod hole designs. Results show that the coolant issued from a tripod hole design has a significantly smaller effect on the overall aerodynamic performance of the vane.

Topics: Film cooling
Commentary by Dr. Valentin Fuster
2012;():757-767. doi:10.1115/HT2012-58144.

Adiabatic film-cooling effectiveness is examined systematically on a typical high pressure turbine blade by varying three critical flow parameters: coolant blowing ratio, coolant-to-mainstream density ratio, and freestream turbulence intensity. Three average coolant blowing ratios 1.0, 1.5, and 2.0; three coolant density ratios 1.0, 1.5, and 2.0; two turbulence intensities 4.2% and 10.5%, are chosen for this study. Conduction-free pressure sensitive paint (PSP) technique is used to measure film-cooling effectiveness. Three foreign gases — N2 for low density, CO2 for medium density, and a mixture of SF6 and Argon for high density are selected to study the effect of coolant density. The test blade features axial shaped holes on the suction side and pressure side, and 3 rows of 30° radial-angle cylindrical holes around the leading edge region. The inlet and the exit Mach number are 0.27 and 0.44, respectively. Reynolds number based on the exit velocity and blade axial chord length is 750,000. Results reveal that the PSP is a powerful technique capable of producing clear and detailed film effectiveness contours with diverse foreign gases. As blowing ratio exceeds the optimum value, it induces more mixing of coolant and mainstream. Thus film-cooling effectiveness reduces. Greater coolant-to-mainstream density ratio results in lower coolant-to-mainstream momentum and prevents coolant to lift-off; as a result, film-cooling increases. Higher freestream turbulence causes effectiveness to drop everywhere except in the region downstream of suction side. Results are also correlated with momentum flux ratio and compared with previous studies. It shows that compound shaped hole has the greatest optimum momentum flux ratio, and then followed by axial shaped hole, compound cylindrical hole, and axial cylindrical hole.

Commentary by Dr. Valentin Fuster
2012;():769-777. doi:10.1115/HT2012-58178.

The paper presents an experimental study on heat transfer coefficients in a straight rectangular channel containing continuous transverse ribs. The ribs were located on one side of the channel which is heated with a uniform heat flux. Three values of the rib pitch-to-height ratio (10, 20 and 30) were considered, with the Reynolds number, based on the channel hydraulic diameter, ranging from 57,000 to 127,000. The studied geometry is relevant for hot internal structures in aircraft engines. The steady state, liquid crystal thermography technique was used to obtain detailed heat transfer coefficients in the inter-rib surface regions. The main purpose of this study was to investigate the heat transfer behavior between the first repeated ribs, i.e., in the regions where the flow and thermal fields are not yet periodically fully developed.

Commentary by Dr. Valentin Fuster
2012;():779-789. doi:10.1115/HT2012-58219.

Film cooling experiments were run at the High-Speed Cascade Wind-Tunnel of the University of the Federal Armed Forces Munich. The investigations were carried out on a linear cascade of aerodynamically highly loaded turbine blades. The main targets of the tests were to assess the film cooling effectiveness and the heat transfer in zones with main flow separation. The cascade was designed to have a large zone with flow separation on the pressure side starting at the leading edge and reaching up to approximately half of the axial chord. Film cooling is provided on the pressure side at the front part of the blade in order to reduce flow separation and to provide effective film cooling inside the separation bubble and at the reattachment zone.

The studies comprise the measurement of adiabatic film cooling effectiveness and heat transfer coefficient for the different cascades under a set of different Mach and Reynolds numbers at engine relevant levels. Periodic wakes, generated by bars moving upstream of the cascade, simulate the rotor stator interaction. The results show that film cooling is detrimental for cooling the surface inside the flow separation zone, whereas after reattachment improved overall film cooling effectiveness is obtained with periodic unsteady inflow compared to the cases with homogeneous inflow.

Commentary by Dr. Valentin Fuster
2012;():791-800. doi:10.1115/HT2012-58260.

Accurate prediction of ribbed duct flow and heat transfer is of importance to the gas turbine industry. Detailed heat transfer in a two pass stationary square duct with rib turbulators is studied using wall modeled Large Eddy Simulations (WMLES). Each pass has ribs on two opposite walls and aligned normal to the main flow direction. The rib pitch to rib height (P/e) is 9.28, the rib height to channel hydraulic diameter (e/Dh) is 0.0625 and calculations have been carried out for a bulk Reynolds number of 25,000. The present study validates the use of WMLES for predicting flow and heat transfer with published data on similar geometries. The calculations predict the major flow features with reasonable accuracy especially distribution of mean and turbulent quantities in the developing, fully developed and 180° bend region. It is found that the mean flow and turbulent quantities do not become fully developed until the flow passes the fifth rib of the duct. Results show that the heat transfer augmentation is higher in the second pass after the 180° turn compared to the first pass. Local heat transfer comparisons show that the heat transfer augmentation shifts towards the outside smooth wall in the second pass after the 180° turn. In addition to primary flow effects, secondary flow impingement on the smooth walls is found to develop by the fifth rib, while it continues to evolve downstream of the sixth rib. Results show the local and average distribution of Nusselt numbers normalized with classical Dittus and Boelter correlation.

Commentary by Dr. Valentin Fuster
2012;():801-808. doi:10.1115/HT2012-58327.

The cooling systems found in gas turbine blades and combustor liners often employ the use of convective cooling channels. The heat flux along the wall of an internal channel is typically computed using Newton’s Law of Cooling requiring knowledge of the local heat transfer coefficient and fluid bulk temperature at that location. Experimentally, the measurement of a heat transfer coefficient and the associated driving temperature difference is often difficult. By tracking the surface temperature response to a change in the fluid temperature during a transient experiment, sufficient data can be obtained to determine both the fluid temperature and heat transfer coefficient using inverse methods provided an appropriate mathematical model of the surface temperature response is employed. This procedure avoids the difficult measurement and tedious calculation for the determination of the fluid bulk temperature.

To validate the technique, experiments were conducted for the ‘sudden’ heating of a flat plate in a small wind tunnel. The fluid temperature rise was measured with a ‘rapid’ response thermocouple while both thermocouple and thermo-chromic liquid crystal surface temperature history data were obtained on the surface of the plate. Results indicate that for data sets containing sufficient transient surface temperature history, the method can accurately measure heat transfer coefficients and the asymptotic temperature of the fluid temperature rise (within 5% to 10% depending on the surface temperature model used).

In many situations the fluid temperature rise is not a ‘step change’. For example, for axial positions far downstream in a channel the assumption introduces bias due to the cooling along the channel walls upstream. The bias generated by the data analysis in the heat transfer coefficient and fluid temperature is examined and compared to more appropriate models describing the temperature rise. A similar situation was simulated in the tunnel and the results analyzed.

Commentary by Dr. Valentin Fuster
2012;():809-818. doi:10.1115/HT2012-58340.

This paper presents forced convection studies on a flat plate and NACA 0010 section airfoil surfaces. The surface temperatures of the models at given stations are measured with K-type thermocouples and used to assess the heat transfer characteristics of the models. The model surfaces are subjected to constant heat fluxes of 1.45kW/m2 (for the flat plate) and 0.60kW/m2 (for the airfoil) using KH Kapton flexible heaters. The temperature readings are then utilized to determine the heat transfer coefficients on the surfaces over a range of Reynolds numbers, from which Nusselt number correlations are deduced. The experiments were conducted in an open circuit wind tunnel, powered by a 37 kW motor, capable of generating air velocities of up to about 41 m/s in the 24 square-inch test section. For the flat plate, the Nusselt number correlations obtained agree well with what is reported in literature. The plate length (0.25m) used for the experiment was just enough for the initiation of turbulent thermal boundary layer at 35.60 m/s air speed. The flow phenomena and Nusselt number correlations on the NACA 0010 airfoil surface are also evaluated and found to fit correlations similar to that of a cylinder in cross flow for laminar case. However, turbulence on the airfoil surface has a significant influence on the average Nusselt number relation. Correlations for the laminar and turbulent flow regimes have been presented using a modified Hilpert and Churchill correlation for a cylinder in crossflow.

Topics: Flat plates
Commentary by Dr. Valentin Fuster
2012;():819-825. doi:10.1115/HT2012-58343.

Heat transfer augmentation values are obtained experimentally with the use of temperature sensitive paint on constant flux heaters attached to full coverage film cooled surfaces as a function of blowing ratio. The effects of blowing ratio, surface angle and hole spacing are observed by testing four full coverage arrays of round staggered holes, all compounded at 45°, which parametrically vary the inclination angle, 30° and 45°, and the spacing of the holes, 14.5 and 19.8 diameters. Local heat transfer augmentation is obtained throughout these largely spaced arrays over 20 rows for the 19.8 spacing array and 30 rows for the 14.5 spacing array. The first five to six rows show low heat transfer enhancement between holes with peaks in augmentation occurring directly downstream of the hole. Heat transfer enhancement is seen to be close to unity at the leading edge of the arrays. Laterally averaged values of heat transfer augmentation increase every row, leveling out to values between 20 and 30% augmentation with peaks reaching the 40% mark.

Commentary by Dr. Valentin Fuster
2012;():827-839. doi:10.1115/HT2012-58360.

The effect of rotation in a leading edge, two-pass channel is experimentally investigated in this study. Cooling air, traveling radially outward, is supplied to a smooth, square channel. The coolant turns 180°, and travels radially inward through a semi-circular, smooth channel. This semi-circular passage models a cooling channel located in the leading edge region of a modern turbine airfoil. For the radially outward flow in the square channel, the coolant Reynolds number is varied from 10000–40000. The rotational speed of the channel varies from 0–500 rpm, the rotation number in the first-pass channel varies from 0 to 1.2, and as a result, the buoyancy parameter can exceed 5.0 under the given flow conditions. Due to a slight reduction in the hydraulic diameter, the semi-circular, second-pass experiences Reynolds numbers ranging from 10300–41000, rotation numbers varying up to 1.0, and buoyancy numbers exceeding 4.0. With both passes of the serpentine passage instrumented, the effect of rotation on heat transfer with both radially outward and radially inward flow can be characterized under high rotation and buoyancy numbers. The channel is oriented 90° to the direction of rotation, and the Nusselt numbers on both the leading and trailing surfaces deviate from those measured in a non-rotating channel. The degree of separation between the leading and trailing surfaces depends on the position within the passage; near the inlet of the channel, the effect of rotation is minimal as the heat transfer coefficients are more strongly influenced by the entrance geometry into the channel. Moving downstream of the entrance, the effect of rotation increases. With the current channel geometry, all regions in the second pass of the channel experience heat transfer enhancement with rotation, and the separation between the leading and trailing surfaces is reduced compared to the first pass.

Commentary by Dr. Valentin Fuster
2012;():841-850. doi:10.1115/HT2012-58410.

Jet impingement is often used to efficiently cool the leading edge of modern turbine airfoils. This investigation employs cylindrical jets with varying edge conditions and inlet flow conditions to obtain detailed Nusselt number distributions on a leading edge model of a turbine airfoil. Jet Reynolds numbers of 13600 and 27200 are investigated. For each test, a set mass flow rate is supplied to the test section; the radial supply flow is then bypassed to achieve the desired jet Reynolds numbers. The results are compared to baseline tests with equivalent jet Reynolds numbers and no radial bypass. Three inlet and exit conditions are investigated for the cylindrical jets: a square edge, a partially filleted edge, and a fully filleted edge. The ratio of the fillet radius to hole diameter (r/djet) is set at 0.25 and 0.667 for the partially and fully filleted holes, respectively. The relative jet – to – jet spacing (s/djet) is maintained at 8, the jet – to – target surface spacing (z/djet) is maintained at 4, the jet – to – target surface curvature (D/djet) is maintained at 5.33, and the relative jet length (t/djet) is maintained at 1.33. Results indicate the amount of bypass flow can significantly change the shape of the stagnation region as well as the magnitude of the Nusselt numbers obtained on the cylinder. Similarly, the relative size of the fillet further influences the enhancement (or degradation) of the Nusselt numbers on the target surface.

Topics: Heat transfer , Jets
Commentary by Dr. Valentin Fuster
2012;():851-862. doi:10.1115/HT2012-58519.

The present study is geared towards quantifying the effects of imposed thermal boundary condition in cooling channel applications. In this regard, tests are conducted in a generic passage, with evenly distributed rib type perturbators at 90°, with a 30% passage blockage ratio and pitch-to-height ratio of 10. Uniform heat-flux is imposed on the external side of the slab which provides Biot number and solid-to-fluid thermal conductivity ratio around 1 and 600 respectively. Through infrared thermometry measurements over the wetted surface and via an energy balance within the solid, conjugate heat transfer coefficients are calculated over a single rib-pitch. The local heat extraction is demonstrated to be a strong function of the conduction effects, observed more dominantly in the rib vicinity. Moreover, the aero-thermal effects are investigated by comparing the findings with analogous aerodynamic literature, enabling heat transfer distributions to be associated with distinct flow structures. Furthermore, the results are contrasted with the iso-heat-flux wetted boundary condition test case. Neglecting the thermal boundary condition dependence, and thus the true thermal history of the boundary layer, is demonstrated to produce large errors in heat transfer predictions.

Commentary by Dr. Valentin Fuster
2012;():863-871. doi:10.1115/HT2012-58531.

A lattice structure for internal cooling with coolant bleeds is investigated experimentally. The lattice configuration provides a serpentine complex flow passage where the flow takes multiple twists and turns with impingement before exiting the coolant flow channel. The combination of impingement, tortuous flow path and turbulators are expected to provide high heat transfer coefficients. In this study, measurements of heat transfer coefficient and total pressure drop were performed for a constant cross-section lattice geometry with bleed holes at the end of the passage as the flow exits. A transient liquid crystal technique was used for the measurements. Stationary tests were performed for four Reynolds number (5500<Re<22000) in a lattice structure with two inlet channels. The data indicated high heat transfer coefficients at locations corresponding to the impingement sites (with the peak Nu/Nu0 ranging from 8–9 at the lowest Re and 3–4 at the highest Re). The spanwise-averaged Nu/Nu0 ratios showed a rapid asymptotic development and shows constant values beyond about 10 sub-channel hydraulic diameters. Channel averaged Nu/Nu0 values are obtained in the range 2.25–3.1. Pressure drop measurements were made, and are combined with the Nu/Nu0 values to produce a TPF. These values are in the range of 1–1.6 with the higher values exceeding TPF’s of turbulated and pin-fin channels.

Commentary by Dr. Valentin Fuster
2012;():873-879. doi:10.1115/HT2012-58550.

A comparison study between axial and radial swirler performance in a gas turbine can combustor was conducted by investigating the correlation between combustor flow field geometry and convective heat transfer at cold flow conditions for Reynolds numbers of 50,000 and 80,000. Flow velocities were measured using Particle Image Velocimetry (PIV) along the center axial plane and radial cross sections of the flow. It was observed that both swirlers produced a strong rotating flow with a reverse flow core. The axial swirler induced larger recirculation zones at both the backside wall and the central area as the flow exits the swirler, and created a much more uniform rotational velocity distribution. The radial swirler however, produced greater rotational velocity as well as a thicker and higher velocity reverse flow core. Wall heat transfer and temperature measurements were also taken. Peak heat transfer regions directly correspond to the location of the flow as it exits each swirler and impinges on the combustor liner wall.

Commentary by Dr. Valentin Fuster

Transport Phenomena in Materials Processing and Manufacturing

2012;():881-885. doi:10.1115/HT2012-58030.

Aluminum nitride (AlN) is a wide bandgap semiconductor material of wide interest. Aluminum nitride films are typically grown using metalorganic chemical vapor deposition. In this process, group-III and group-V precursors, namely tri-methyl-aluminum and ammonia, are injected into a reactor. Subsequently, these reactants react both in the gas-phase as well as at the surface to deposit an epitaxial layer of AlN on a hot substrate (wafer). It has been experimentally observed that AlN nanoparticles are formed in the gas-phase during this process. Although these particles are formed in the vicinity of the hot substrate, they tend to stay away from the hot substrate and are observed to deposit on the cold walls of the reactor. This is undesirable since the particles do not contribute to the growth of the AlN film, and end up damaging the walls of the reactor. In this computational study, the trajectories of the AlN particles are simulated with the goal to understand the mechanisms responsible for their motion. A three-dimensional (3D) Lagrangian Brownian dynamics simulator based on the Langevin equation is first developed. It is then coupled with a 3D computational fluid dynamics solver to simulate the background flow, and the chemical reactions responsible for AlN particle formation. The combined model is first validated, and then exercised for the problem at hand. It is found that thermophoretic forces are primarily responsible for driving the particles away from the hot substrate and depositing them on the cold reactor walls.

Commentary by Dr. Valentin Fuster
2012;():887-895. doi:10.1115/HT2012-58038.

Melting, vaporization and resolidification in a gold thin film subject to multiple femtosecond laser pulses are numerically studied in the framework of the two-temperature model. The solid-liquid phase change is modeled using kinetics controlled model that allows the interfacial temperature deviates from the melting point. The kinetics controlled model also allows superheating in the solid phase during melting and undercooling in the liquid phase during resolidification. Superheating of the liquid phase caused by nonequilibrium evaporation of the liquid phase is modeled by adopting the wave hypothesis, instead of Clausius-Clapeyron equation. Melting depth, ablation depth, and maximum temperature in both liquid and solid are investigated and the result is compared with that from Clausius-Clapeyron equation based vaporization model. The vaporization wave model predicts a much higher vaporization speed which leads to a deeper ablation depth. The relationship between laser processing parameters, including pulse separation time and pulse number, and phase change effect are also studied. It is found that longer separation time and larger pulse number will cause lower maximum temperature within the gold film, as well as lower depths of melting and ablation.

Topics: Lasers , Melting
Commentary by Dr. Valentin Fuster
2012;():897-898. doi:10.1115/HT2012-58046.

In this paper, a critical point model with three Lorentzian terms for interband transition was proposed for dielectric permittivity of metal films. After validated, it was incorporated into a two-temperature model (TTM) to study transient optical and thermal response for a copper film irradiated by an ultrashort laser pulse. The dynamic changes of reflectivity (R) and absorptivity coefficient (α) during laser irradiation, electron and lattice temperature, and phase change were investigated. It was shown that for an ultrashort laser pulse with relatively high laser fluence, both R and α could drastically decrease, leading to significantly different thermal response than that described by using constant R and α at room temperature (RT).

Topics: Metals , Lasers
Commentary by Dr. Valentin Fuster
2012;():899-905. doi:10.1115/HT2012-58074.

Hot forming die quenching (HFDQ) is an emerging process in which sheet metal is successively stamped at high temperature and quenched by the cold tool. The microstructure and mechanical properties of ultra high strength steel parts formed by the HFDQ process are a function of the cooling rate achieved during the quenching step. Optimal energy absorption for crash performance can be obtained by a local reduction of the cooling rate in order to form softer, more ductile phases such as bainite. Accurate characterization of the heat transfer coefficient (HTC) between the sheet metal and the cold tool is required to evaluate the cooling rate and the resulting mechanical properties. Experiments were conducted in which Usibor 1500P® boron steel blanks were austenitized at 900°C then quenched by stamping between flat tool steel dies. The HTC was calculated using an inverse heat conduction analysis for the die and a lumped capacitance approach for the blank. The heat transfer coefficients measured during the approach phase were found to be significantly lower than the values predicted by semi-empirical models based on thermal conduction through the air gap.

Commentary by Dr. Valentin Fuster
2012;():907-912. doi:10.1115/HT2012-58123.

A dynamic model of the Tube Digester System in Alumina (TDSA) production was developed by using Matlab/Simulink package. A novel workflow simulation method based on Virtual Operating Environment (VOE) was proposed, and a prototype system based on a commercial control-build software was presented. In order to increase the reliability and visibility of simulations, animations demonstrating the simulation process were studied. Based on the workflow virtual operating environment, the simulation engine can impel the workflow running and deal with semi-automatic or manual activities automatically. The workflow simulation model of TDSA can be used to assess the performances of operating processes, diagnose the existing process and provide qualitative and quantitative analysis for the optional retrofit methods.

Commentary by Dr. Valentin Fuster
2012;():913-922. doi:10.1115/HT2012-58139.

Thermally induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Existing research has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Hence, for a more accurate estimation of the thermally induced residual stresses it is desired to incorporate the weld pool dynamics into the analysis. Various welding parameters (like, welding speed, current, arc length, surfactant activity, plasma drag etc.) influence the weld pool dynamics, which in turn affect the thermal history of the workpiece. Such integration would help in better quantification of thermal stress evolution and residual stress distribution in the welded joint. In this study, a three-dimensional numerical model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of a butt joint of thin stainless steel plates has been developed. The effects of welding parameters on the residual stress distribution are documented.

Commentary by Dr. Valentin Fuster
2012;():923-935. doi:10.1115/HT2012-58162.

This article presents a fixed-mesh approach to model convective-diffusive particle deposition onto surfaces. The deposition occurring at the depositing front is modeled as a first order reaction. The evolving depositing front is captured implicitly using the level-set method. Within the level-set formulation, the particle consumed during the deposition process is accounted for via a volumetric sink term in the species conservation equation for the particles. Fluid flow is modeled using the incompressible Navier-Stokes equations. The presented approach is implemented within the framework of a finite volume method. Validations are made against solutions of the total concentration approach for one- and two-dimensional depositions with and without convective effect. The presented approach is then employed to investigate deposition on single- and multi-tube arrays in a cross-flow configuration.

Commentary by Dr. Valentin Fuster
2012;():937-942. doi:10.1115/HT2012-58256.

Welding defects such as undercuts, porosity, irregular beads are frequently observed in laser welds due to the fast cooling rate and no filler metal addition in the process. In addition, increasing penetration depth is a challenging issue in laser welding. Some preliminary experimental studies indicated that applying electromagnetic force in laser welding could be an effective solution to some of these problems. However, the underlying physics behind this electro-magnetically assisted laser welding is not clear and needs further investigation. In this paper, mathematical models are used to study the transport phenomena, such as heat transfer and melt flow, in both spot and 3-D electro-magnetically assisted laser welding. Studies are focused on understanding the effects of electromagnetic forces on heat generation and transfer, weld pool dynamics, cooling and solidification, porosity prevention, weld shape control, and penetration depth.

Commentary by Dr. Valentin Fuster
2012;():943-946. doi:10.1115/HT2012-58315.

Nanoscale-synthesized materials hold great promise for the realization of future generation devices. In order to fulfill the exceptional promise, new techniques must be developed that will enable the precise layout and assembly of the heterogeneous components into functional ‘superblocks’. As one promising route to this end, rapid and spatially confined heating capability of laser irradiation has enabled precisely controlled nucleation and subsequent direct growth of nanowires at an arbitrary local region based on vapor-liquid-solid (VLS) mechanism. Spatial confinement of the nanowire growth region via focused laser beam illumination provides a convenient way to examine multiple growth parameters (temperature, time, illumination direction, and composition), thereby elucidating fundamental nanowires growth mechanisms. Furthermore, the work demonstrates an advanced method for direct synthesis of nanostructures for the purpose of practical rapid patterning including on demand multi-bandgap materials based nanowires.

Commentary by Dr. Valentin Fuster
2012;():947-953. doi:10.1115/HT2012-58367.

Nanoimprint Lithography (NIL) is becoming a powerful tool for nanolithography, nanofabrication and nanomanufacturing for nanotechnology applications. However, there is still a lack of systematic study of key processing parameters, which determine the imprinted pattern quality in terms of uniformity and replication fidelity.

This research focuses on identifying the most important parameters in a nanoimprint process, in which microscale patterns were imprinted into polymethyl-methacrylate (PMMA) polymer with polydimethylsiloxane (PDMS) mold. The effects of several parameters such as pre-imprint temperature, pre-imprint pressure, imprint temperature, imprint pressure, imprint time, venting temperature and venting time, were varied in a certain range during the imprinting process. The imprinting results were analyzed with a three-level design of experiments (DOE) analysis. It was found that the pre-imprint pressure and imprint temperature are the key parameters. In addition, the DOE analysis is a powerful tool for NIL process optimization.

As a practice, a vacuum assisted and selective coating (VASC) method based on a commerical nanoimprinting tool was developed to fabricate micro-hole arrays on a PbSe nanocrystal film to study its spectral response to IR radiation for applications such as IR detection and photovoltaic. The process optimization significantly improves imprinting quality.

Commentary by Dr. Valentin Fuster
2012;():955-958. doi:10.1115/HT2012-58375.

Sulfur poisoning can deactivate nickel catalysts in solid oxide fuel cells (SOFCs), resulting in a significant drop in fuel cell performance. In this paper, a Ni/YSZ SOFC anode exposed to 100 ppm H2S was examined using x-ray absorption contrast imaging for its microstructure and x-ray fluorescence (XRF) spectroscopy to probe and map Ni, S, and YSZ phases. It was observed that S was frequently found to be collocated with Ni, with higher concentrations being located on or near the surface of the Ni particles exposed to gas, while little S was found near the YSZ phase.

Commentary by Dr. Valentin Fuster
2012;():959-962. doi:10.1115/HT2012-58415.

A significant fraction of energy is lost during power generation for transportation as a form of waste heat. Approximately over three quarters of waste heat have the temperature range from 200 to 700 ° C. Solid solution materials made of magnesium silicide and magnesium tin have the potential of utilization in thermoelectric (TE) devices in such temperature ranges due to their availability, relatively low density (3.02 g/cm3), non-toxicity compared to classical Te-Pb TE materials and high stability from room temperature (RT) to 600 ° C. The environmentally friendly n-type Mg2 (Si, Sn) thermoelectric solid solution bulk materials were prepared from powder elements via direct melting and current-assisted hot-press sintering. For the direct melting method, raw element magnesium, tin, and silicon powders were homogeneously mixed and put into a stainless steel container. Then, the sample was heated under a vacuum condition at 500–600 ° C for several hours to obtain crystalline Mg2Si, Mg2Sn, and Mg2 Si0.4Sn0.6 ingots respectively. These ingots were ground by using a high energy ball miller to obtain their particles whose sizes are distributed from tens of nanometers to micro-meters. Several doping material with different doping ratio were added to compare doping effect on Mg2 (Si, Sn) solid solutions. The samples were hot-pressed with electrical current (400–600A) in order to create phonon scattering centers such as nano-size particles and lamellar structures for improving the dimensionless figure of merit, ZT value. Samples were cut into slender pieces flash method. Then, ZT value was calculated from ZT = α 2 Tσ /κ and plotted as a function of temperature. The results were also compared with the results obtained from to test electrical conductivity, by using a four-probe method as well as the Seebeck coefficient,. Thermal conductivity measured as a function of temperature from RT to 600 °C by using a the Harman technique. The nano-structure size, size distribution as well as crystallinity were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray diffraction (XRD). The chemical composition was examined by energy-dispersive x-ray spectroscopy (EDS).

Commentary by Dr. Valentin Fuster
2012;():963-966. doi:10.1115/HT2012-58453.

The paper presents the thermoelectric properties of Mg2Si coatings by Atmospheric Plasma Spray (APS), High Velocity Oxy-Fuel Spray (HVOF) and Vacuum Plasma Spray (VPS). Thermal spraying, a flexible, industry-scalable and cost-effective manufacturing process, was first applied to prepare Mg2Si as thermoelectric materials. The characterization of Mg2Si coatings including thermal conductivity, electric conductivity, and Seebeck coefficient are reported. XRD and SEM analysis of the coatings are presented as well.

Commentary by Dr. Valentin Fuster
2012;():967-972. doi:10.1115/HT2012-58454.

Thermoelectric Generators (TEG) can be used on automobiles to harvest energy from exhaust waste heat. Besides the improvement on material side, the optimization of geometry of the module is also important to maximize the output power density and will be addressed in this paper. A thermal resistance network is established based on heat conduction and radiation from heat source to heat sink. Although the heat transfer model is based on cylindrical exhaust pipe geometry, the thermo-element is approximated as plane geometry because the ceramic layer and thermoelectric layer are much smaller compared with the exhaust pipe diameter. The TE material we proposed to recover waste heat energy is magnesium silicide (Mg2Si), which has a reasonable figure of merit in the automobile exhaust temperature range, and the process is thermal spray compatible, which is a mass productive method currently under investigation. Another material that used for comparison is titanium oxide. Based on the Seebeck coefficient, thermal and electrical conductivity of our thermal sprayed samples, the thermoelectric leg length and the area ratio between thermoelectric element and total module area are optimized for maximum power density output. The optimal leg length is around 0.85mm, and the air gap is as small as possible. A parameter sensitivity analysis is conducted to investigate the influence of ceramic layer thickness, exhaust pipe radius, electrical contact resistance, hot and cold side temperature, Seebeck and electrical conductivity on the optimal leg length.

Commentary by Dr. Valentin Fuster
2012;():973-975. doi:10.1115/HT2012-58551.

We introduce the modulation-doping strategy in bulk SiGe nanostructures to improve the thermoelectric power factor. By separating charge carriers from their parent atoms via embedding heavily doped nanoparticles inside an intrinsic host matrix, the ionized impurity scattering rate could be largely reduced, resulting in enhanced mobility. By band engineering, the carriers can spill over from nanoparticles into the host matrix, resulting in similar carrier concentrations, Fermi levels and consequently Seebeck coefficients as those of the uniform nanocomposites. In addition, nanoparticles with low thermal conductivities can further reduce the overall thermal conductivity of the sample. Combining the enhanced electrical conductivity, the reduced thermal conductivity and the unaffected Seebeck coefficient, we were able to enhance the thermoelectric properties of Si-rich Si95Ge5. And therefore were able to fabricate a low-cost sample with a competitive performance as those of the state of the art Si80Ge20.

Commentary by Dr. Valentin Fuster
2012;():977-982. doi:10.1115/HT2012-58594.

Oxygen bottom blowing smelting for copper is a totally new metallurgical technology, which was set up in China and put into operation in 2008. This new technology is different from top blowing and side blowing which were popular in 70’s last century. Production practice demonstrate that the new process is characterized by having a good condition and high thermal efficiency for heat transfer, and its heat transfer rate is about 1.7 times that of Noranda process. In the furnace, therefore, raw feed molten is fast, smelting intensity is high. Under very strong continuously stirring in the bath, gas-liquid-solid there phases formed are of higher heat transfer index by convection.

Commentary by Dr. Valentin Fuster
2012;():983-989. doi:10.1115/HT2012-58600.

The successful development of thermoelectric generators (TEGs) for automotive applications requires advances on several fronts: (1) abundant, low cost, and efficient thermoelectric (TE) materials, (2) industry-scalable material synthesis and device fabrication process, and (3) cost-efficient and reliable integration into existing vehicle systems. Most vehicle TEG approaches to date use flat, prefabricated modules that are placed in contact with a custom-built exhaust component/heat exchanger to capture waste heat. We are developing a single-step, integrated approach involving non-equilibrium synthesis, 3D conformal deposition and rapid formation of patterned structures, which is well-suited for large-scale waste-heat TEG applications.

Commentary by Dr. Valentin Fuster

Heat and Mass Transfer in Biotechnology

2012;():991-999. doi:10.1115/HT2012-58025.

This paper presents a model of targeted drug delivery. The model is based on recent experimental results that reported synthesis and pharmacological efficiency tests of a tri-partite complex designed for axonal transport drug delivery. The developed model accounts for two populations of pharmaceutical agent complexes (PACs). The first population of PACs includes those that are transported retrogradely by dynein motors and the second population of PACs includes those that are accumulated in the axon at the Nodes of Ranvier. The transitions between these two populations of PACs are described by first-order reactions. Laplace transform is utilized to obtain an analytical solution of the coupled system of transient equations describing conservations of these two populations of PACs. Results for various combinations of parameter values are presented and their physical significance is discussed.

Commentary by Dr. Valentin Fuster
2012;():1001-1006. doi:10.1115/HT2012-58422.

This paper reports a simple technique based on multispectral image processing that can be used to recover the biomass concentration of an algal biofilm photobioreactor. Monitoring the biomass concentration of a culture is critical in achieving successful algae cultivation for biofuel or food supplement production. In particular, non-invasive and rapid detection techniques that can provide estimates of the biomass concentration can significantly aid in providing delay-free process control feedback in large scale cultivation platforms. In this technique, the digital images of the biofilms of the cyanobacteria Anabaena variabilis were obtained under consistent lighting conditions and analyzed as a function of their biomass content. The image analysis was carried out using a custom Matlab code where the red, green and blue content of the images were correlated with the biomass concentration. The obtained correlation was consistent across biofilms generated from different stock cultures of varying culture age. Challenges facing application of the image processing technique for scaled up outdoor photobioreactors under various lighting conditions and color backgrounds were discussed.

Commentary by Dr. Valentin Fuster
2012;():1007-1012. doi:10.1115/HT2012-58437.

One way for permanent correction of vision errors such as myopia is using surgically implanted lenses which are also called Phakic IOLs (intraocular lenses). These lenses can be placed between the iris and cornea. According to currently available statistics, these lenses could cause difficulties for the patients. The reasons for the discomfort and potential damages to the eye, however, are not fully understood. In this paper, the effect of intraocular lenses on the hydrodynamics of the eye anterior chamber was studied using the computational fluid dynamics (CFD) technique. The computational model was developed based on the eye geometry of a volunteer patient. The lens was selected to be compatible with mean anterior chamber depth of typical patients. The model provided estimation of aqueous humor flow streams in the anterior chamber of the eye under the effect of natural convection. The flow fields in the anterior chamber, as well as, the shear stress acting on the cornea for the cases before and after the surgery were obtained and compared.

Commentary by Dr. Valentin Fuster

Environmental Heat Transfer

2012;():1013-1020. doi:10.1115/HT2012-58118.

This study investigates the thermal performance of typical residential attic insulations under wildfire ash deposition. Adding a layer of wood ash to the surface of other materials, such as insulation batts, may theoretically increase the overall thermal resistance of the integrated layers; however, in case of penetrating the wood ash into the fibers of the porous insulation materials, the direction and magnitude of change in overall thermal resistance remains uncertain. There have been many questions in public whether the layers of ash can affect the thermal performance of insulation batts, and if it does, how much this effectiveness will be, although the studies reported in open literature on this topic are limited. This study attempts to address these questions through analytical and experimental approaches.

Topics: Insulation
Commentary by Dr. Valentin Fuster
2012;():1021-1029. doi:10.1115/HT2012-58443.

Sintered copper porous media has found many uses in the electronics cooling industry as it effectively transfers energy while maintaining low heater side temperatures. Evaporator wicks of this type transfer heat through sensible and latent heat as the liquid evaporates. A biporous wick is particularly effective for this application as there are two distinct size distributions of pores; small pores to provide ample capillary pressure in order to drive flow through the wick and large pores to provide high permeability for escaping vapor. The modeling proposed in this work was inspired by the work by Kovalev (1987), which used a pore size distribution in order to determine the most probably pore size at a given position. The model distinguishes phases by choosing a “cutoff” pore size, above which all pores were assumed to be filled with vapor and below which filled with liquid. For a given thickness and thermophysical properties of the liquid, this 1-D model predicts a temperature difference across the wick for a given input heat flux. The modeling proposed in this work was compared to experimental data collected on biporous evaporators at UCLA for validation. It is hoped that this modeling is the starting point for more extensive modeling and optimization of biporous evaporators for phase change heat transfer devices.

Commentary by Dr. Valentin Fuster
2012;():1031-1038. doi:10.1115/HT2012-58456.

This paper presents energy performance of hybrid ground source heat pump system used in a hotel building under a hot and humid climate condition. The performance data of a heat pump used in the building have been measured for several weeks. Based on the measured performance data of the heat pump, the building using a hybrid ground source heat pump system is first simulated using an energy simulation program. The building simulation model is then calibrated using monthly utility data. Finally, the calibrated building model using a hybrid ground source heat pump system is used to determine an economical effect of climatic and other factors to this building. The results are compared to those of the building using a conventional air source heat pump system and are used to find the advantage and disadvantage of a hybrid ground source heat pump system against a conventional system under a hot and humid climate condition.

Topics: Climate , Heat pumps
Commentary by Dr. Valentin Fuster
2012;():1039-1048. doi:10.1115/HT2012-58463.

This study is focused to the heat transfer analysis in air cooled rooms, which is important to achieve pleasant indoor comfort conditions, fundamentally in terms of air velocity, temperature and air quality. Three dimensional numerical results of a rectangular ventilated room with three different inlet configurations are presented. The study was carried out considering a turbulent flow and the radiative exchange between the walls. The assumed heat flux on a vertical wall was 300 W/m2 (Rayleigh number of 1.07×1012). The inlet velocity was 0.5 m/s (Reynolds number of 3145) and the emissivity of the walls was fixed as 0.8. The mathematical model was solved numerically with Computational Fluid Dynamics software. The temperature fields, flow patterns, heat transfer coefficients and temperature distribution effectiveness are presented and discussed. It was found that the heat transfer due the radiative effect is around 50% for the three studied cases.

Commentary by Dr. Valentin Fuster
2012;():1049-1055. doi:10.1115/HT2012-58498.

Ventilation is air circulation inside a building. Two main approaches can be applied for building ventilation: (1) a fan is used to drive the airflow, and (2) natural convection due to temperature difference is used to drive the airflow. The cost of electricity on ventilation is significant, especially when it is considered together with space cooling. The second approach can take advantage of the renewable resource such as solar energy to lower energy cost. This paper presents a numerical model to investigate the solar chimney performance. Several configurations of solar chimney are examined in this study to predict the ventilation of the building. The commercial software package, Fluent, is adopted. The effect of solar chimney height, air gap width as well as the brick width is investigated. It is found that the flow rate increases by 100% as the chimney height increases from 1.5 to 3 m, while the air gap width has much smaller influence. It also shows that the solar chimney works for different seasons although the solar radiation changes significantly. In addition, using an unsteady state model, it can be observed that the chimney with 0.3 m thick brick wall can work the whole day even in the night. It is expected that this research can help design the solar chimney in a better way.

Commentary by Dr. Valentin Fuster
2012;():1057-1060. doi:10.1115/HT2012-58500.

Nowadays the Baking industry faces two main economic problems: low efficiency (significant waste heat losses) and negative environmental impact (air pollutions). Indeed, hot (about 400° F) stack gas flow, with content of vapor, carries about 2/3 of fuel that consumed by bakery oven to the ambient.

Moreover, stack gas contains of the ethanol vapor (produced by yeast in oven bakery process). Amount of ethanol in stack gas is equivalent up to 8% of the fuel consumed by bakery oven at nominal operation. In addition, U.S. EPA regulates the bakery exhaust, so that plants are forced to install expensive oxidizing equipment to combust the dangerous pollutant (including ethanol) emissions prior to releasing the stack gas into atmosphere. It results in additional fuel consumption and extra heat losses generated at the bakery site. Ethanol is easy solvable in water so the traditional methods of its extraction are associated with condensing/evaporating processes. However, at the low concentration (in the oven stack it is about 0.5%) the ethanol starts condensing at 10.6 ° F. The optimal conditions for ethanol diffusion into water droplets, which forms clouds in the cooled to ambient temperature oven stack, were found.

Commentary by Dr. Valentin Fuster
2012;():1061-1065. doi:10.1115/HT2012-58570.

Bacillus anthracis spores have shown extreme resistance to heat treatment methods. Various novel ideas have emerged including the use of thermite reactions for the de-activation of bacterial spores, focusing on the anthrax forming spore Bacillus anthracis. The basis of de-activation is dependent on the heat transfer to the spore and chemical interaction with the halogen gas. The objective of this work was to observe the mechanisms of de-activation as related to the thermal and halogen gas effect on the spore. Research focused on the specific roles of the heat transfer and the combination of heat and halogen gas. Results showed heat transfer in the spore greatly enhanced the effectiveness of the halogen gasses in the deactivation process. The observed results strengthen the hypothesis that the heat transfer affects the permeability of the bacterial spores, enabling the halogen gas to deactivate the spores. This novel observation leads to further studies in the combustion properties of thermites. Results from this study suggest that thermite formulations with increased heat of reaction will increase the thermal wave promoting spore neutralization.

Commentary by Dr. Valentin Fuster

Visualization of Heat Transfer

2012;():1067-1076. doi:10.1115/HT2012-58226.

The present study deals with natural convection heat transfer within water (Pr = 7.2) filled inclined square cavities for hot bottom wall (case 1: isothermal heating/case 2: non-isothermal heating) and cold side walls in the presence of adiabatic top wall. The Galerkin finite element method has been used to solve the nonlinear coupled partial differential equations governing the fluid flow and thermal fields. This method is further used to solve the Poisson equation for streamfunction and heatfunction. The streamlines (Ψ), isotherms (θ) and heatlines (Π) are obtained for various inclination angles (φ = 0°, 30° and 60°) in the range of Rayleigh numbers (103 ≤ Ra ≤ 105). The physical significance of heatlines have been demonstrated for a comprehensive understanding of heat energy distribution within the inclined square cavities. The flow pattern is symmetric for φ = 0° whereas asymmetric flow pattern is observed for the φ = 30° and 60° due to tangential and normal components of buoyancy forces. At Ra = 103, weak fluid circulation and orthogonal heat-lines on isothermal surface, indicate conduction dominant heat transfer for both cases. Strong closed loop heatlines are found due to strong fluid convective circulation cells at Ra = 105. Heat transfer rates are obtained in terms of local and average Nusselt numbers. In general, the overall amount of heat transfer along the right wall increases with inclination angle and that decreases along the left wall with increase in inclination angle. The non-isothermal heating case exhibits greater heat transfer rates at the center of the bottom wall than the isothermal heating whereas average Nusselt number shows that overall heat transfer rate is larger for the isothermal heating case as compared to that of non-isothermal heating case.

Commentary by Dr. Valentin Fuster
2012;():1077-1083. doi:10.1115/HT2012-58227.

Due to global demand of conservation and optimization of energy consumption it is very important to analyze the convective heat flow field and find out the detailed clarification of the convective flow parameters. While due to the complexity of convective flow fields both in forced and natural convection, it has remained very difficult to have three dimensional (3D) experimental analyses in detail. Image processing and measurement has got a very important role in finding solutions in this regard. For this purpose both steady and unsteady quantitative analysis of flow fields has to be analyzed separately, as they have got different density values throughout the flow field. This paper deals with quantitative image and CT analyses of natural convection using color-stripes background oriented schlieren (CSBOS) method from a heat generating source in natural atmospheric conditions.

Commentary by Dr. Valentin Fuster
2012;():1085-1091. doi:10.1115/HT2012-58426.

A power plant boiler is widely used as a heat source for generating steam through fuel combustion. Operations trainees at NIPSCO coal-fired power stations receive short-term training in boiler operation procedures and are given 2D, non-scaled representations of the plant’s steam, coal cycle, and limited key components. This study focused on pursuing a more efficient way of representation by using a three-dimensional (3D) numerical model of the power plant boiler and a Virtual Reality platform. Coal and air are injected into ten cyclones of the boiler to undergo violent combustion and release the heat to the tubes along both cyclone and furnace walls. In order to obtain a better understanding of the boiler operation process, which cannot be achieved in reality, Computational Fluid Dynamics (CFD) was employed to simulate the boiler components and the entire combustion procedure. Simulation results presented detailed transient flow characteristics and temperature gradients inside cyclones and the furnace to achieve a thorough understanding of the internal gas flow pattern. Also, the Virtual Reality (VR) platform of a power plant boiler was developed by combining the simulation data inside the boiler and visualization of model image outside the boiler to provide a vivid 3D representation for trainees.

Commentary by Dr. Valentin Fuster

Education and Future Directions in Heat Transfer

2012;():1093-1101. doi:10.1115/HT2012-58190.

The two-semester, capstone senior design courses of the Mechanical Engineering Department of Saint Martin’s University prepare students for the workforce by having them participate as teams to solve open-ended, real-world design problems. Students design, manufacture and assemble their project. The students use knowledge from all previous engineering courses as well as their own creative improvisations. Team work is emphasized. Problem recognition, constraints, alternative solutions and their evaluation, consideration of economic and environmental concerns, manufacturing, and scheduling are stressed. The design project further hones the oral and written communication skills of the team.

A 2011 senior team designed and constructed a unique, safe, miniature, Rankine vapor power cycle experimental apparatus. The apparatus is currently used to support instruction in four engineering courses: Heat Transfer, Thermodynamics II, Thermal Design of Heat Exchangers, and Energy Systems. A comprehensive literature and market search was performed to choose the working fluid. The cycle was tested up to a high pressure of 260 kPa and the highest temperature reached was 142 °C. The lowest pressure and temperature reached were approximately 101 kPa and 20°C, respectively. The apparatus is equipped with temperature, pressure, and flow rate instrumentation, connected to a real time data acquisition system. Technical details are given in the article.

Commentary by Dr. Valentin Fuster
2012;():1103-1111. doi:10.1115/HT2012-58544.

The present work describes development of an educational web portal for performing virtual experiments. Graduate students often conduct laser based optical measurements to study fundamentals of heat transfer and fluid flow. An attempt has been made to meet the same requirements with virtualization using computational tools. OpenFOAM, an open source fluid flow simulator serves as a backbone for this initiative. A new Foam solver is developed to incorporate an additional convective heat transfer equation, account for natural convection effects, implement specific boundary conditions of interest and represent all the required class of physics as required from a pedagogical perspective. The solver is tested for accuracy by comparing results with experiments and available data in the literature. Apart from access of the source code to students, open source methodology makes it free from any licensing restrictions allowing wider deployment across multiple schools simultaneously. Even if large number of students access the web portal at the same time, all simulations can be launched without any licensing restrictions, provided computing power is available. A suite of twelve examples covering steady and unsteady flow patterns is studied in the first phase. The graphical nature of results helps students to easily recall specialized phenomena such as transients, flow circulation patterns in natural convection and vortex shedding. A comparison with laser based experiments via interferograms and schlieren images proves useful to correlate the experimental results with physically realizable temperature fields.

Commentary by Dr. Valentin Fuster