ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V001T00A001. doi:10.1115/HT2016-NS1.

This online compilation of papers from the ASME 2016 Heat Transfer Summer Conference (HT2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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: Applications

2016;():V001T01A001. doi:10.1115/HT2016-7140.

In this work, the heating process for apple, eggplant, zucchini and potato by means of evaluation of their thermal properties and the Biot number determined in experimental form is presented. The heating process is carried out using a solar cooker box-type as heating device. The thermal experimental properties determined are conductivity (k), density (D), specific heat (C), diffusivity (Dif) and the Biot number (Bi) for each product evaluated. In the experimentation, temperatures for center and surface in each product and water were measured in controlled conditions. For those measures, a device Compact Fieldpoint and thermocouples placed in the points studied were used. By using correlations with temperature as function, k, D and C were calculated, while by using equations in transitory state for the products modeled as sphere and cylinder was possible to estimate the Biot number after calculation of the heat transfer coefficient for each case. Results indicate the higher value for k, C and Dif correspond to zucchini (0.65 W/m °C, 4084.5 J/kg °C, 1.5 × 10−7 m2), while higher value for D correspond to potato (1197.5 kg/m3). The lowest values for k and C were obtained for potato (0.59 W/m °C, 3658.3 J/kg °C) while lowest values for D and Dif, correspond to zucchini (998.2 kg/m3) and potato (1.45 × 10−7 m2/s) respectively. The maximum and minimum values for Bi corresponded to potato (21.4) and zucchini (0.41) in respective way. The results obtained are very useful in applications for solar energy devices, where estimates for properties are very important to generate new results, for example, numerical simulations. Also, results could be used to evaluate the cooking power in solar cookers when the study object is oriented in that direction.

Commentary by Dr. Valentin Fuster
2016;():V001T01A002. doi:10.1115/HT2016-7142.

A evaluation of the conduction heat loss over their cover for four different solar cookers box-type (1. Square solar cooker with inner reflectors placed in right angles, 2. Square solar cooker with inner reflector placed in different angles, 3. Rectangular solar cooker with inner reflectors placed in different angles and 4. Octagonal solar cooker with inner reflectors placed in right angles) is presented. In the heating process in a solar cooker box-type, the conduction heat loss in their cover is the most important in comparative with convection and radiation losses. The cover in solar cookers is made with clear glasses, which allows the inlet solar radiation inside of it. When the heating process happen, the temperature in the cover glasses is important and is important for this part. To evaluate the magnitude for the heat loss, controlled tests were planned, where a solar radiation simulator was used as energy source over the solar cookers considered. In the experiments, thermocouples to determine the gradient temperature for thickness among glasses were placed. In this activity, a Compact Field and LabView software were used. Also, in the experimental tests, thermographic imagines for some instants during the heating process were taken. According results, the conduction heat losses are bigger than 25 % of the inlet energy Flux in the cookers. The biggest values for temperature on the glasses correspond to the solar cooker 3, while minimum values are obtained for the solar cooker 1. The solar cooker 1 present the biggest conduction heat losses and the cooker 4, has the minimum values for the losses. Results of this work can be useful and important for design proposes which could impacts on save of money and cooking time.

Commentary by Dr. Valentin Fuster
2016;():V001T01A003. doi:10.1115/HT2016-7255.

In this paper, a numerical investigation on Latent Heat Thermal Energy Storage System (LHTESS) based on a phase change material (PCM) in a metal foam is accomplished. A vertical shell and tube LHTESS made with two concentric aluminum tubes is investigated. The internal surface of the hollow cylinder is at a constant temperature above the PCM melting temperature to simulate the heat transfer from a hot fluid. The other external surfaces are assumed adiabatic. The phase change of the PCM is modeled with the enthalpy porosity theory while the metal foam is considered as a porous media that obeys to the Darcy-Forchheimer law. Local thermal non-equilibrium (LTNE) model is assumed to analyze the metal foam and some comparison are accomplished with the local thermal equilibrium model assumption. The governing equations are solved employing the Ansys-Fluent 15 code. Numerical simulations for PCM, PCM in the porous medium in LTE and in LTNE assumptions are obtained. Results as a function of time for the charging phase are carried out for different porosities and assigned pore per inch (PPI).

The results show that at high porosity the LTE and LTNE models have the same melting time while at low porosity the LTNE has a larger melting time. Moreover, the presence of metal foam improves significantly the heat transfer in the LHTESS giving a very faster phase change process with respect to pure PCM, reducing the melting time more than one order of magnitude.

Commentary by Dr. Valentin Fuster
2016;():V001T01A004. doi:10.1115/HT2016-7291.

Heat and water recovery using Transport Membrane Condenser (TMC) based heat exchangers is a promising technology in power generation industry. In this type of innovative heat exchangers the tube walls are made of a nano-porous material and have a high membrane selectivity which is able to extract condensate water from the flue gas in the presence of the other non-condensable gases such as CO2, O2 and N2. Considering the fact that for industrial applications, a matrix of TMC heat exchangers with several TMC modulus in the cross section or along the flow direction is necessary. Numerical simulation of multi-stage TMC heat exchanger units is of a great importance in terms of design, performance evaluation and optimization. In this work, performance of a two-stage TMC heat exchanger unit has been studied numerically using a multi-species transport model. In order to investigate the performance of the two-stage TMC heat exchanger unit, parametric study on the effect of transversal and longitudinal pitches in terms of heat transfer, pressure drop and condensation rate inside the heat exchangers have been carried out. The results indicate that the heat transfer and condensation rates both increase by reducing TMC tube pitches in the second stage and increasing the number of TMC tube pitches in the first stage of the units.

Commentary by Dr. Valentin Fuster
2016;():V001T01A005. doi:10.1115/HT2016-7300.

Heat transfer enhancement (HTE) of supercritical CO2 flowing in heated vertical tube with an inner diameter of 6.32 mm was investigated numerically in present paper. The studies were performed for HTE cases with the pressure being 8.12 MPa, the mass flux being 400 and 1000 kg/m2s, and the heat flux being 30 and 50 kW/m2. Four turbulence models, including the RNG k-ε, the SST k-ω and two low-Reynolds number models (LB and LS), were evaluated with the experimental data collected from literatures. The SST k-ω model was shown to be the best among the four models, and then was used for the simulation in this study. The effect of five factors, including buoyancy, thermal acceleration, thermal conductivity, specific heat and viscosity of the fluid, on HTE were respectively analyzed according to the numerical results. It was shown that the buoyancy had a little negative influence on HTE and was negligible for the heat transfer. The thermal acceleration effect was detrimental to the HTE by accelerating the fluid near the wall and at the same time reducing the turbulence kinetic energy in the core of the flow. The rapid decrease of thermal conductivity of the fluid at the pseudo-critical region was also bad for HTE. Variation of the specific heat of the fluid had strong positive effect on the HTE. When most of the buffer layer was occupied by fluid with the large specific heat, the heat absorbing capability of the fluid was increased and more heat was carried away efficiently. Moreover, decrease in viscosity of the fluid with increasing temperature also significantly promoted the HTE because of the increase in flow turbulence and the reduction in thermal-conduction resistance. After that, the weight of above five factors effect on HTE was compared quantitatively.

Commentary by Dr. Valentin Fuster
2016;():V001T01A006. doi:10.1115/HT2016-7304.

Supercritical pressure water has been widely used in many industrial fields, such as fossil-fired power plants and nuclear reactors because mainly of its high thermal efficiencies. Although many empirical correlations for heat transfer coefficients of supercritical pressure water have been proposed by different authors based on different experimental data base, there exist remarkable discrepancies between the predicted heat transfer coefficients of different correlations under even the same condition. Heat transfer correlations with good prediction performance are of considerable significance for developing supercritical (ultra-supercritical) pressure boilers and SCWRs. In this paper, the experimental data (about 7389 experimental data points) and 30 existing empirical correlations for heat transfer of supercritical pressure water (SCW) flowing in vertical upward tubes are collected from the open literatures. Evaluations of the prediction performance of the existing correlations are conducted based on the collected experimental data, and a detailed multi-collinearity analysis has been made on different correction factors involved in the existing correlations, and then based on the collected experimental data, a new heat transfer correlation is developed for the supercritical pressure water flowing in vertical upward tubes under normal and enhanced heat transfer mode. Compared with the existing correlations, the new correlation exhibits good prediction accuracy, with a mean absolute deviation (MAD) of 9.63%.

Commentary by Dr. Valentin Fuster
2016;():V001T01A007. doi:10.1115/HT2016-7394.

This research is an experimental investigation of a double-pipe heat storage unit. The inner pipe of the unit, through which a heat-transfer fluid (HTF) is supplied, is made of aluminum and has an outer helix-like fin. The annular space between the pipes is filled with a phase change material (PCM). Actually, this research presents a novel design of the heat storage unit, which, unlike traditional designs with e.g. radial (circumferential) or longitudinal fins, has a single fin which does not divide the shell volume into separated cells. Moreover, this research focuses on close-contact melting (CCM), a process which is characterized by detachment of the solid bulk from the unit envelope and its sinking towards the hot fin surface. In previous investigations, performed in our laboratory, this effect has been achieved in units with above-mentioned traditional fin configurations. It was demonstrated that CCM reduces the overall melting time, i.e. the rate of unit charging, significantly as compared with commonly encountered melting in which the fins serve just to enlarge the heat transfer area. The experimental system employed in this study includes a vertically-oriented double-pipe heat storage unit and thermostatic baths capable of providing hot or cold HTF. The unit has a transparent Perspex shell which makes visualization possible. The entire unit may be placed in a heated water tank with transparent walls. In the latter case, close-contact melting is achieved by detaching the solid phase from the envelope and thus allowing its gravity-induced motion. Regular melting is compared to CCM and advantages of the latter are demonstrated. Also demonstrated are the advantages of the novel fin, including in solidification. Possible mathematical and numerical modeling of the melting processes is discussed.

Topics: Heat storage , Pipes
Commentary by Dr. Valentin Fuster

Heat Transfer in Energy Systems: Design and Performance Analysis

2016;():V001T01A008. doi:10.1115/HT2016-7056.

Numerical study is performed on the effect of thermal conductivity of porous media (k) on the Nusselt number (Nu) and performance evaluation criteria (PEC) of a tube. Two-dimensional axisymmetric forced laminar and fully developed flow is assumed. Porous medium partially inserted in the core of a tube is investigated under varied Darcy number (Da), i.e., 10−6Da ≤ 10−2. The range of Re number used is 100 to 2000 and the conductivity of porous medium is 1.4 to 202.4 W/(m.K) with air as the working fluid. The momentum equations are used to describe the fluid flow in the clear region. The Darcy-Forchheimer-Brinkman model is adopted for the fluid transport in the porous region. The mathematical model for energy transport is based on the one equation model which assumes a local thermal equilibrium between the fluid and the solid phases. Results are different from the conventional thoughts that porous media of higher thermal conductivity can enhance the performance (PEC) of a tube. Due to partial porous media insertion, the upstream parabolic velocity profile is destroyed and the flow is redistributed to create a new fully develop velocity profile downstream. The length of this flow redistribution to a new developed laminar flow depends on the Da number and the hydrodynamic developing length increases with increasing Da number. Moreover, the temperature profile is also readjusted within the tube. The Nu and PEC numbers have a nonlinear trend with varying k. At very low Da number and at a lower k, the Nu number decreases with increasing Re number while at higher k, the Nu number first increases to reach its peak value and then decreases. At higher Re number, the results are independent of k. However, at a higher Da number, the Nu and PEC numbers significantly increases at low Re number while slightly increases at higher Re number. Hence, the change in Nu and PEC numbers neither increases monotonically with k, nor with Re number. Investigation of PEC number shows that at very low Da number (Da = 10−6), inserting porous media of a low k is effective at low Re number (Re ≤ 500) while at high Re number, using porous material is not effective for the overall performance of a tube. However, at a relatively higher Da number (Da = 10−2), high k can be effective at higher Re number. Moreover, it is found that the results are not very sensitive to the inertia term at lower Da number.

Commentary by Dr. Valentin Fuster
2016;():V001T01A009. doi:10.1115/HT2016-7189.

It is known that particle accumulation is beneficial for dust removal in industry. In order to understand better particle accumulation mechanism, experiments were carried out to analyze the influence of flue gas temperature and humidity on ash accumulation. It is found that the Engineering Acid Dew Temperature (EADT) of flue gas is an important parameter that determines the efficiency of particle accumulation. When the gas temperature is lower than the EADT, the sulfuric acid in the flue gas and ash humidity rise dramatically, which leads to particle accumulation. In order to improve the collection efficiency, the flue gas temperature can be controlled to trigger particle accumulation.

Commentary by Dr. Valentin Fuster

Heat Transfer in Energy Systems: Energy Conversion

2016;():V001T01A010. doi:10.1115/HT2016-7437.

Recent high energy density thin film material development has led to an increased interest in pyroelectric energy conversion. Using state-of-the-art lead-zirconate-titanate piezoelectric films capable of withstanding high electric fields we previously demonstrated single cycle energy conversion densities of 4.28 J/cm3. While material improvement is ongoing, an equally challenging task involves developing the thermal and thermodynamic process though which we can harness this thermal-to-electric energy conversion capability. By coupling high speed thermal transients from pulsed heating with rapid charge and discharge cycles, there is potential for achieving high energy conversion efficiency. We briefly present thermodynamic equivalent models for pyroelectric power generation based on the traditional Brayton and Ericsson cycles, where temperature-pressure states in a working fluid are replaced by temperature-field states in a solid pyroelectric material. Net electrical work is then determined by integrating the path taken along the temperature dependent polarization curves for the material. From the thermodynamic cycles we identify the necessary cyclical thermal conditions to realize net power generation, including a figure of merit, rEC, or the electrocaloric ratio, to aid in guiding generator design. Additionally, lumped transient analytical heat transfer models of the pyroelectric system with pulsed thermal input have been developed to evaluate the impact of reservoir temperatures, cycle frequency, and heating power on cycle output. These models are used to compare the two thermodynamic cycles. This comparison shows that as with traditional thermal cycles the Ericsson cycle provides the potential for higher cycle work while the Brayton cycle can produce a higher output power at higher thermal efficiency. Additionally, limitations to implementation of a high-speed Ericsson cycle were identified, primarily tied to conflicts between the available temperature margin and the requirement for isothermal electrical charging and discharging.

Commentary by Dr. Valentin Fuster

Heat Transfer in Energy Systems: Fundamentals

2016;():V001T01A011. doi:10.1115/HT2016-7062.

Heat transfer coefficients were experimentally determined for a free rotating disk in still air and water. These were obtained with an electrically heated disk placed in a cylindrical pool. The accuracy of the employed experimental apparatus was assessed by heat transfer measurements in air. For this fluid, an excellent agreement with reliable literature data was found. Essentially new experimental data were obtained for water as fluid. Based upon the experimental data, the validity of theoretical correlations and the effect of the Prandtl number on the convective heat transfer from a rotating disk were discussed. It was found that in laminar water flow, the value of the correlation exponent for the Prandtl number is practically identical to 1/2 as theoretically predicted in 1948 by Dorfman. In turbulent flow, its value is better given by 1/3 as in case of the classical turbulent boundary layer theory.

Commentary by Dr. Valentin Fuster
2016;():V001T01A012. doi:10.1115/HT2016-7122.

Bubble breakers have been shown to be effective at reducing bubble size and delaying transition from bubbly to slug and churn flow regimes in the two-phase vertical pipe flow. If used in bubble column reactors, bubble breakers can increase the surface area-volume ratio of the gas-liquid interface allowing for an enhanced mass transfer or chemical reaction rate. Studies have been done showing the effect of bubble breakers on bubbles size and flow regime but none exist to show the effect of a bubble breaker on heat transfer for a two-phase pipe flow. A new method of measuring the heat transfer for a two-phase vertical pipe flow is proposed in the current study. The method uses thermocouples inserted directly into the flow for bulk fluid temperature measurements and a thermal camera for surface temperature measurements of a thin walled stainless steel pipe. Heat transfer measurements, expressed as a Nusselt number, for a single phase laminar liquid flow are compared to accepted values to show the validity of the experimental method. Preliminary results of two-phase gas-liquid heat transfer rates with and without a bubble breaker present in the vertical pipe are compared. The liquid flowrates used in the experiment represented superficial Reynolds numbers of ReI<2000 and the gas flowrates used in the experiment represented superficial Reynolds numbers of Reg<100. Without a bubble breaker, the convective heat transfer coefficient, represented as Nusselt number, was found to decrease with increasing gas flowrate. When a bubble breaker was added, the effect on the heat transfer was dependent on the flow regime. For most cases, the bubble breaker had very little effect on the measured heat transfer rate. In a case where the bubble breaker was able to generate slug flow rather than churn flow that was generated when no bubble breaker was present, the measured Nusselt number was increased.

Commentary by Dr. Valentin Fuster
2016;():V001T01A013. doi:10.1115/HT2016-7210.

The present study deals with a new numerical approach for solid-liquid phase-change modeling. The new model is based on the enthalpy method and takes into account natural convection in the melt which can be coupled to the solid bulk sinking motion by the force balance on the solid bulk. A basic configuration is investigated, namely a two-dimensional rectangular cavity with a constant wall temperature. The effect of rather low values of the Archimedes and Rayleigh numbers on the sinking motion of the solid bulk, the melting rate and melting patterns, is explored. It is found that the density difference, between the solid and liquid phases, for the studied case does not affect considerably the results. However, it is shown that by natural convection alone the solid sinking motion is established, the melting rate is enhanced and the melting patterns are completely different in comparison with the first studied case.

Topics: Melting , Cavities
Commentary by Dr. Valentin Fuster
2016;():V001T01A014. doi:10.1115/HT2016-7310.

“T-history method” is widely used for characterization of thermal properties of Phase Change Material (PCM). In this study improvements are proposed to the experimental protocol used in the conventional T-History method. Experimental validation of numerical predictions for various samples of PCM were performed using the proposed measurement technique. This enabled the evaluation of the improvements in the proposed approach as well as for analyzing the experimental results. This involved measurement of temperature at the surface and in the center of the PCM samples (as well as that of the reference sample materials). The proposed modifications enable enhanced accuracy for estimation of the material properties (when compared to the conventional approaches). The estimates from the proposed approach were observed to be within 10% of the measured values obtained using Differential Scanning Calorimetry (DSC). The proposed approach is amenable to testing large sample sizes, is simpler to implement, provides more rapid data collection and is more cost-effective than that obtained using standard DSC protocols.

Commentary by Dr. Valentin Fuster

Thermophysical Properties: Measurements and Computations of Thermophysical Properties

2016;():V001T02A001. doi:10.1115/HT2016-7043.

The performance characteristics of thermal interface materials (TIMs) are quickly outpacing our ability to measure them using steady-state techniques. In fact, scientists have turned to photothermal techniques like Time-domain Thermoreflectance (TDTR) to measure the impedance to heat flow across TIMs, namely due to their relatively low measurement uncertainties. However, such techniques are costly, require significant sample preparation, only measure local thermal impedances and are not yet equipped to measure thermal resistance as a function of pressure. Instead, it is desirable to maximize the resolution of traditional steady-state equipment for these types of measurements. In this work, we develop a more robust and accurate methodology to determine the temperature difference across the junction of a traditional steady-state apparatus using high accuracy measurements of in-situ TIM thickness in tandem with infrared thermography. This methodology eliminates a significant fraction of the uncertainty associated with the measurement of thermal interface resistance. Importantly, the use of this method improves the accuracy of the measurement device by an order of magnitude at interfacial thermal resistance values on the order of 1·10−6m2·K/W when compared to state-of-the-art, thermal probe-based measurement systems.

Commentary by Dr. Valentin Fuster
2016;():V001T02A002. doi:10.1115/HT2016-7172.

In order to obtain a fluid with both drag reducing and heat transfer enhanced abilities, we use viscoelastic fluid for drag reducing and stimulate the elastic turbulence for the sake of heat transfer enhancement. To further enhance the heat transfer, carbon powder particles were added to the viscoelastic fluid where Polyacrylamide Ethylene Glycol solution was served as the base fluid. Fluid properties were measured at different solution concentration, shear rate and temperature by the rotary rheometer and thermal conductivity meter including viscosity and heat conductivity coefficients. The results showed that the addition of the carbon powder could significantly change the viscoelastic fluid properties. As for the viscosity, the shear-thinning behavior still exists after adding the carbon powder. Moreover, the heat conductivity coefficients increase about 7.5% after adding the carbon powder and nearly linearly increase with the temperature and volume concentration of the carbon particle.

Topics: Fluids , Carbon
Commentary by Dr. Valentin Fuster
2016;():V001T02A003. doi:10.1115/HT2016-7177.

The current study uses phonon wave-packet simulations and calculates the phonon transmission rate to explore the contributions of the mass and the bond energy differences on the thermal boundary conductance at the interface between two dissimilar materials. The impact of interdiffusion and interface bond strength on the thermal boundary conductance are also studied. Results show that the difference in mass and bond energy of materials results in a difference in phonon dispersion relations. Thus the frequency dependence of phonon transmission rate is observed at the interface. The interdiffusion allows high frequency phonons to contribute to phonon energy transport by inelastically scattering into multiple lower frequency phonons. Therefore the different energy distribution at the interface is observed for different wavevectors when there is interdiffusion between two materials which results in increased strain at the interface. It is also found that applying different bond strengths has little effect on thermal boundary conductance at the interface unless this interface bond strength deviates significantly from the commonly used mixing rules.

Commentary by Dr. Valentin Fuster
2016;():V001T02A004. doi:10.1115/HT2016-7192.

Nanofluids are a class of fluids with nanoparticles suspended in a base fluid. The aim for using nanofluids is often to improve the thermophysical properties of the base fluid so as to enhance the energy transfer efficiency. As the technology develops; the size of devices and systems needs to get smaller to fulfill the engineering requirements and/or to be leading among competitors. The use of nanofluids in heat transfer applications seems to be a viable solution to current heat transfer problems, albeit with certain limitations. As an enhancing factor for the thermal conductivity of the base fluid, nanofluids are considered to be use in cooling system applications. For these applications, the base fluid, the refrigerant, exists as a two-phase liquid-vapor mixture in parts of the refrigeration cycle. To analyze, design and optimize the cycle in such applications, the thermophysical properties of the refrigerant based nanofluids for two-phase flow of refrigerant are needed. There are different models present in the literature derived for the thermophysical properties of nanofluids. However, a majority of the existing models for nanofluid thermophysical properties have been proposed for water- and other liquids-based nanofluids, through theoretical, numerical and experimental research. Therefore, the existing models for determination of the nanofluid thermophysical properties are not applicable for refrigerant based nanofluid applications when the results are compared. Thus, in this work, a new model is derived for the thermal conductivity and viscosity of refrigerant based nanofluids, using existing data from both heat transfer and thermophysical property measurement experiments. The effect of the nanoparticles on heat transfer in two phase flow of the refrigerant is considered by applying the two phase heat transfer correlations in the literature to experimental data. As a result, the thermophysical properties of the known states are determined through known heat transfer performance. Even though the model is developed from the analysis of flow in an evaporator and flow in a single tube with evaporating refrigerant, it is aimed to cover the flows in both evaporator and condenser sections in a vapor compression refrigeration cycle to provide the necessary models for thermophysical properties in heat transfer devices which will allow the design of both cycle and evaporator or condenser in terms of sizing and rating problems by performing heat transfer analysis and/or optimization. The model can also be improved by considering the effects of slip mechanisms that lead to slip velocity between the nanoparticle and base fluid.

Commentary by Dr. Valentin Fuster
2016;():V001T02A005. doi:10.1115/HT2016-7362.

The negative influence of substrate on in-plane phonon transport in graphene has been revealed by intensive research, whereas the interaction between phonons couplings across graphene/substrate interface and within graphene is still needed to figure out. In this work, we put forward a two-step Raman method to accomplish interface thermal resistance characterization of graphene/SiO2 and in-plane thermal conductivity measurement of supported graphene by SiO2. In order to calculate the interfacial thermal resistance, the temperature difference between graphene and its substrate was probed using Raman thermometry after the graphene film was uniformly electrically heated. Combing the ITR and the temperature response of graphene to laser heating, the thermal conductivity was computed using the fin heat transfer model. Our results shows that the thermal resistance of free graphene/SiO2 is enormous and the thermal conductivity of the supported graphene is significantly suppressed. The phonons scattering and leakage at the interface are mainly responsible for the reduction of thermal conductivity of graphene on substrate. The morphology change of graphene caused by heating mainly determines the huge interfacial thermal resistance and partly contributes to the suppression of thermal conductivity of graphene. This thermal characterization approach simultaneously realizes the non-contact and non-destructive measurement of interfacial thermal resistance and thermal conductivity of graphene interface materials.

Commentary by Dr. Valentin Fuster
2016;():V001T02A006. doi:10.1115/HT2016-7386.

In this paper, a numerical model employing 3D foam structure represented by Weaire-Phelan foam cell is developed to study the steady heat conduction of high porosity open-cell metal foam/paraffin composite at the pore-scale level. Two conduction problems are considered in the cubic representative computation unit of the composite material: one with constant temperature difference between opposite sides of the cubic unit (that can be used to determine the effective thermal conductivity (ETC)) and the second with constant heat flux at the interface between metal foam and paraffin (that can be used to determine the interstitial conduction heat transfer coefficient (ICHTC)). The effects of foam pore structure parameters (pore size and porosity) on heat conduction are investigated for the above two problems. Results show that for the first conduction problem, the effect of foam structure on heat conduction (i.e. the ETC) is related to porosity rather than pore size. The essential reason is due to the thermal equilibrium state between metal foam and paraffin indicated by the negligible interstitial heat transfer. While for the second conduction problem with inherent thermal non-equilibrium effect, it shows that both porosity and pore size significantly influence the interstitial heat conduction (i.e. the ICHTC). Furthermore, the present ETC and ICHTC data are compared to the results in the published literature. It shows that our ETC data agree well with the reported experimental results, and are more accurate than the numerical predications based on body-centered-cubic foam cell in literature. And our ICHTC data are in qualitative agreement with the published numerical results, but the present results are based on a more realistic foam structure.

Commentary by Dr. Valentin Fuster

Theory and Fundamentals in Heat Transfer: Fundamentals of Boiling and Condensation

2016;():V001T03A001. doi:10.1115/HT2016-7296.

Heat transfer characteristics of high pressure boiling water flowing in vertical internally-ribbed tubes are numerically investigated with the RPI wall boiling model. Specifically, Tolubinsky’s bubble departure diameter model is applied in the present study. The numerical results indicate that the value of dref in Tolubinsky’s bubble departure diameter model needs to be modified at high pressures so that reasonable wall temperature values can be obtained. A recommended value of dref is achieved for the cases at 14.2 MPa in the present paper, and the simulated value of wall temperature is in good agreement with the corresponding experimental data collected from the published literature. Based on the present modified RPI wall boiling model, the effects of rib geometries on heat transfer characteristics of high pressure boiling water in internally-ribbed tubes are investigated. The numerical results show that the heat transfer capability of high pressure boiling water in internally-ribbed tubes increases with the increase in rib heights or the number of ribs, but decreases with the increase in helix angle of the rib. Meanwhile, the increase in rib width has little influence on the heat transfer of boiling flow in the internally-ribbed tube. The results obtained in this study will be helpful for optimizing the rib geometries of internally ribbed tubes.

Commentary by Dr. Valentin Fuster

Theory and Fundamentals in Heat Transfer: Fundamentals of Convection in Porous Media

2016;():V001T03A002. doi:10.1115/HT2016-7256.

Combined natural convection and forced convection gets a great attention for its importance in practical applications in various modern systems such as electronic cooling, chemical vapor deposition and solar energy systems. Buoyancy force due to the heating of the lower cavity wall induces secondary flows hence the local heat transfer increases. The onset point of the secondary flows is important because it delineates the region after which the two-dimensional laminar flow becomes three-dimensional and a transition motion from laminar to turbulent flow is observed.

In this paper, mixed convection in a horizontal channel partially filled with a porous medium and the lower wall heated at uniform heat flux is studied experimentally and numerically. The heated wall temperature profiles as a function of the Gr/Re2 values are presented. Pictures of flow visualization along longitudinal, transversal and horizontal section are given. Average Nusselt numbers are evaluated.

Results in terms of wall temperature profiles, local and average Nusselt numbers are presented for different Reynolds and Rayleigh number values. Some comparison between experimental and numerical results are accomplished.

Commentary by Dr. Valentin Fuster
2016;():V001T03A003. doi:10.1115/HT2016-7257.

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.

Commentary by Dr. Valentin Fuster
2016;():V001T03A004. doi:10.1115/HT2016-7294.

Results of a numerical study considering the periodic natural convection inside a fluid saturated porous medium are presented. The porous medium is obtained by placing four, large and uniformly distributed solid obstacles of regular (square) shape inside the enclosure, a structure that hinders the option of seeking a porous-continuum modeling approach. The periodic heating is achieved by imposing a time-periodic and spatially uniform high temperature condition at one of the walls of the enclosure, while the other wall is maintained at a constant, uniform and low temperature; the horizontal surfaces are set as adiabatic. Heat transfer results are obtained then by following a continuum modeling approach, and reported on a parametric form with the Prandtl number fixed equal to 7, and the Rayleigh number inside the enclosure varying from 103 to 107. The boundary layer interference phenomenon, observed for the case of constant horizontal heating, is also observed in the case of periodic heating. The visualization of the natural convection process via isotherms and streamlines, together with the periodic (time-varying) Nusselt number, allows the identification of a singular dynamic behavior, including the storage of thermal energy inside the enclosure.

Commentary by Dr. Valentin Fuster
2016;():V001T03A005. doi:10.1115/HT2016-7377.

The utilization of porous media can enhance the heat transfer process due to its large heat transfer area within limited space. The natural convection in porous media widely exists in various heat transfer equipment and the related flow and heat transfer in porous spaces is one complicated transport phenomenon, for which the accurate prediction is challenging. Pore-scale models can predict transport phenomena in porous media in pore space, which can be used in the modeling of flow and heat transfer in porous media under local thermal non-equilibrium condition. The pore-scale study includes the reconstruction of porous structure and the direct numerical simulation of transport phenomena in the pore spaces. In this paper, the geometrical reconstruction approach was developed to generate the porous region using the tomographic reconstruction, which is one nondestructive imaging technique. The porous sample was scanned on a micro-CT scanner with micrometer resolution. 2D sliced scan images were obtained and then stacked to reconstruct the 3D porous geometry. A double-population thermal lattice Boltzmann model was established to predict the natural convection in reconstructed porous media at pore scale.

Commentary by Dr. Valentin Fuster
2016;():V001T03A006. doi:10.1115/HT2016-7395.

The porous composite system is consists of porous medium and free fluid layer, which has extensive industrial applications. The study method for the flow field in the porous composite system includes the microscopic, mesoscopic and macroscopic approaches. When the two-domain approach is adopted, which is one of the macroscopic methods, the momentum transport boundary conditions at the interface between porous medium and free fluid layer is essential to analyze the flow field in the system. When Darcy equation is adopted to describe the flow in porous region, the Beavers-Joseph (BJ) interface condition can be used. When Darcy-Brinkman equation is adopted to describe the flow in porous region, the stress-jump (Ochoa-Tapia & Whitaker: OTW) interface condition can be used. To utilize these interface conditions, the velocity slip coefficient used in the BJ interface condition and the stress-jump coefficient used in the OTW interface condition should be specified. In this paper, a brush configuration is approximately treated as the equivalent porous media in the composite system. A numerical simulation method is used to obtain the microscopic solution for the flow in the system based on the Navier-Stokes equation applied in whole system, and an analytical method is used to obtain the corresponding macroscopic solution based on the two-domain approach. By comparing the microscopic and macroscopic solutions, the velocity slip coefficient and the stress-jump coefficient are determined since they can be treated as adjustable parameters. The influence of different flow types, including Poiseuille flow, Couette flow, and free boundary flow, are investigated. Also the impact of free fluid layer thickness and porous structure on the velocity slip coefficient and the stress-jump coefficient are discussed. The results indicate that, the velocity slip coefficient and the stress-jump coefficient are not only the parameters which depend on the porous structure, but also depend on the thickness of free fluid layer and flow type. When the thickness of free fluid layer is lower than a certain value, the impact of free fluid layer thickness on the velocity slip coefficient and the stress-jump coefficient is much obvious. In addition, when the thickness of free fluid layer is small, these coefficients are found to be dependent on the flow type. However, when the thickness of free fluid layer is large, the stress jump coefficient is independent of the thickness of free fluid layer and the flow type. Thus the stress jump coefficient obtained for a specific case can be used to predict velocity for different flow types and different thickness of free fluid layers.

Commentary by Dr. Valentin Fuster
2016;():V001T03A007. doi:10.1115/HT2016-7405.

The lid-driven flow inside a porous square cavity is numerically simulated. The porous media is modelled on the microscopic scale (heterogeneous porous medium) with a square heat conductive single block representing the solid constituent. Conversely, the fluid relies between the block and the cavity surfaces. A vertical positive thermal gradient, obtained by keeping the sliding-lid temperature TH higher than the base one TC, aligned with the gravity force enables a gravitational stable condition where the buoyant-induced flow does not occurs spontaneously. Instead, the flow comes about as the cavity top surface slides with constant velocity. Conservation equations are applied separately for each constituent and are coupled by boundary conditions at the fluid to solid interface (block surface). The Boussinesq-Oberbeck approximation accounts for the buoyant effects. The equations are solved via the finite volume method with the use of the SIMPLE algorithm for the pressure-velocity coupling and QUICK interpolation scheme for the treatment of the advection terms. The aim of the present work is to investigate how variations on the flow parameters and the block size affect the thermal process throughout the cavity. A top lid velocity based Reynolds number evaluates the intensity of the forced convection process while the Grashof number is associated with the intensity of buoyancy. The flow parameters cover only the laminar regime, such as 102Re≤103 and 103Gr≤107. The Re and the Gr numbers are also analyzed by the means of the Richardson number, Ri, which accounts the relative predominance of buoyancy over the inertia effects. Moreover, a clear fluid cavity and enclosure configurations with three different block dimensions, namely B = 0.3, 0.6 and 0.9, are simulated. The heat transfer across the cavity can be characterized as a competitive effect, since the flow is hindered as the buoyancy effect rises. Results show that an increase in Re, or decrease in Gr, enhances the heat transfer, revealing a convection dominant regime. Alternatively, an increase in Gr, or a decrease in Re, leads the fluid to a stagnant-prone condition where a conduction dominant regime is verified. Thus, the surface-average Nusselt number, Nuav, tends to unity as the flow is confined to the adjacency of the sliding-lid. The placement of the single block in the cavity can enhance or hinder the heat transferred, depending on the flow regime. For instance, if a B = 0.6 block is inserted in the presence of a convection dominant regime, the Nuav is increased. Conversely, if the fluid is quiescent, a B = 0.6 block alters the flow path and the Nuav decreases. Intense blockage effects are observed for larger values of B since the block interferes on the flow more significantly. For a convection dominant regime, for instance, a B = 0.9 block causes the Nuav to drop. However, in the presence of stagnant fluid, the same obstacle forces the flow to circumvent it. Thus, the Nuav number increases, indicating that heat transfer mode returns to a convective pattern.

Commentary by Dr. Valentin Fuster
2016;():V001T03A008. doi:10.1115/HT2016-7492.

Cost effective solutions involving thermal-fluid transport need to be developed for various energy applications. The miniaturization of the chip architecture in the electronic devices have caused the challenge of increased heat dissipation and higher power consumption. Hence there is an immediate need for developing efficient and cost-effective solutions for next generation cooling systems. This thermal management challenge can be addressed with maximizing the surface area of the heat exchanging surfaces that can allow dissipation of high heat fluxes. Fractal structures are known to maximize the surface area for compact volumes and are explored as a potential technique to addressing the issue of maximizing heat fluxes in compact volumes.

Fractal structures are effective in maximizing the surface area in compact volumes. Fractals possess the property of self-similarity (the pattern is similar to itself at different levels of magnifications) and infinite recursion (created by repeating a simple process infinitely). Enhanced heat transfer in microelectronic devices can be achieved by increasing the available surface area for heat exchanging fluids within a compact volume. Fractal structures can provide the appropriate technology for the enhancement in heat transfer for these devices.

The proposed model is used to predict the thermal-fluid flow characteristics in microchannel geometries with fractal hierarchies. The chosen fractal architectures in this study are observed to enhance the heat transfer due to the augmentation of surface area in the fractal branching networks of varying length-scales.

Commentary by Dr. Valentin Fuster

Theory and Fundamentals in Heat Transfer: Fundamentals of Nanomaterials and Nanostructures for Energy Applications

2016;():V001T03A009. doi:10.1115/HT2016-7123.

We theoretically demonstrate a novel, efficient and cost effective thermal emitter using a Mie-resonance metamaterial for thermophotovoltaic (TPV) applications. We propose for the first time the design of a thermal emitter which is based on nanoparticle-embedded thin film. The emitter consists of a thin film of SiO2 on the top of tungsten layer deposited on a substrate. The thin film is embedded with tungsten nanoparticles which alter the refractive index of the film. This gives rise to desired emissive properties in the wavelength range of 0.4 μm to 2 μm suitable for GaSb and InGaAs based photovoltaics. Effective dielectric properties are calculated using Maxwell-Garnett-Mie theory. Our calculations indicate this would significantly improve the efficiency of TPV cells. We introduce a new parameter to gauge the efficacy of thermal emitters and use it to compare different designs.

Topics: Metamaterials
Commentary by Dr. Valentin Fuster
2016;():V001T03A010. doi:10.1115/HT2016-7493.

Microscopic thin films are known to have narrow band selective thermal radiation whch have potential applications in selective emitter and absorbers. We propose a methodology to shift the wavelength selectivity in the desired location. Calculation of emissivity and near field radiation for different materials and structures is presented. Emissivity calculations are performed using the standard expressions for Fresnel reflection coefficients. For the near field thermal radiation, we use the analytical expression for transitivity between two half spaces obtained using the dyadic Greens function formalism. We use Maxwell-Garnett-Mie theory to calculate effective dielectric function of media doped with nanoparticles. We observe that spectrum of thermal radiation in far-field and in near-field can be altered using nanoparticles. Moreover spectral properties of mixtures can be characterized using refractive indices. Influence of nanoparticle size and concentration is also studied.

Commentary by Dr. Valentin Fuster

Theory and Fundamentals in Heat Transfer: Fundamentals of Nanoscale Transport in Flows

2016;():V001T03A011. doi:10.1115/HT2016-7316.

Literature review of molten salt nanofluids is performed in this study with focus on the thermo-fluidic properties and performance in thermal management applications. The colloidal mixture of nanoparticles in a base liquid phase is called nanofluid. Molten salts such as alkali nitrate eutectics, alkali carbonate eutectics and alkali chloride eutectics have high melting temperatures. These materials are suitable for various high temperature applications, including as Heat Transfer Fluid (HTF), Thermal Energy Storage (TES), Concentrated Solar Power (CSP) plants, nuclear power, etc. The major drawback of molten salt materials is their low thermal conductivity and specific heat capacity. Enhancing the thermo-physical properties of molten salt materials can lower the cost of power production involving these materials (e.g., as HTF and/ or TES in CSP or nuclear power plants. Mixing molten alt eutectics with nanoparticles (e.g., molten salt nanofluids) can provide a cost-effective technique for enhancing the specific heat capacity and thermal conductivity which in turn can enable the reduction in the cost of power production. In this review - the following topics involving molten salt nanofluids were explored: thermo-physical property measurements, numerical modeling (e.g., Molecular Dynamics/ MD simulations), materials characterization (e.g., using electron microscopy techniques — such as SEM and TEM).

For example, SEM studies in conjunction with MD simulation results confirm the formation of a dense layer of fluid molecules on the surface of nanoparticles that can enhance the specific heat capacity of these molten salt nanomaterials. Subsequently the concepts of nanofins was explored (which involves the study of interfacial thermal impedance, such as resistance, capacitance and diodicity). The contribution of these interfacial thermal impedances to the enhancement of specific heat capacity and thermal conductivity are also explored. Specific heat enhancement as high as 100% has been observed for various molten salt eutectics when doped with 1.5% (weight) silica nanoparticles. Various synthesis protocols such as one-step, two-step and three-step methods as well as conventional experimental methods used for specific heat capacity measurement are compared and examined. A review of the effects of concentration, nanoparticle size, temperature, base fluid, and nanofluid chemical properties is also performed. Other topics of interest are the anomalous enhancement of thermal conductivity in molten salt nanofluids which contradict typical predictions obtained from using the effective medium theory.

The available data in literature shows enhancement in thermal conductivity by as much as 35–45% for carbonate eutectics doped with silica nanoparticles at 1% mass fraction. The possible mechanisms suggested for this improvement are briefly discussed and compared with experimental observations (e.g., using SEM). In addition, nanofluids often display non-Newtonian rheological behavior. This necessitates a rigorous study, since the applications of nanofluids will impact the required pumping power. Studies show that the rheological properties of molten salt nanofluids are a function of base salt composition, shape of nanoparticles selected, chemical formula of nanoparticles, concentration of nanoparticles, size of nanoparticles, temperature, shear rate and synthesis protocol of the nanofluid. Several models are introduced to predict the viscosity variation along with their advantageous and disadvantages. SEM results show agglomeration of nanoparticles can be reduced by doping the nanofluids with very small values of mass fractions of additives such as Gum Arabic.

Topics: Nanofluids
Commentary by Dr. Valentin Fuster
2016;():V001T03A012. doi:10.1115/HT2016-7487.

Using of oscillatory flow and phase change material (PCM) microcapsules to enhance heat transport efficiency in micro-/minichannels are among many new methodologies that have been proposed. In this paper, we propose a novel and simple heat spreader concept that integrates the technologies of oscillating flow streaming and PCM microcapsules. Phenomena of the flow streaming can be found in oscillating, zero-mean-velocity flows in many channel configurations. The pumpless flow can be generated by simple heating or by channel wall vibration. Discrepancy in velocity profiles between the forward and backward flows causes fluid and particles suspended in fluids near the walls to drift toward one end while particles near the centerline drift to the other end. Preliminary work of computer simulations on fluid and suspended particle streaming in multichannel mini-bifurcation networks flows has been conducted and verified by visualization experiments. Results show that flow streaming with PCM microcapsules entrainment has the potential to be used as a cost-effective technology in a heat spreader.

Commentary by Dr. Valentin Fuster
2016;():V001T03A013. doi:10.1115/HT2016-7494.

Near-field thermal radiation and van der Waal force between flat plates and curved surfaces have been probed in the past; however the peculiarities of radiative heat transfer and van der Waals stress due to fluctuations of electromagnetic fields for micro/nano-sized spherical objects have not been studied in great details. We demonstrate how fluctuational electrodynamics can be used to determine emissivity and van der Waals contribution to surface energy for various spherical shapes in a homogeneous and isotropic medium. The dyadic Green’s function formalism of radiative energy and fluctuation-induced van der Waals stress for different spherical configurations has been developed. We present the calculations for a single sphere, a bubble, a spherical shell and a coated sphere. We observe that emission spectrum ofmicro/nanoscale spheres displays several sharp peaks as the size of object reduces. Our calculations indicate that surface energy becomes size dependent (r-3) due to van der Waals phenomena for small radii.

Topics: Shapes , Emissions
Commentary by Dr. Valentin Fuster

Nanoscale Thermal Transport: Advances in Modeling and Simulation of Nanoscale Heat Conduction

2016;():V001T04A001. doi:10.1115/HT2016-7145.

In prior work an effective medium approach (EMA) has been developed to evaluate composite physical properties such as thermal conductivity, dielectric function or elastic modulus (C.-W. Nan, Prog. Mat. Sci. V. 37, 1993). This model combined with the Kapitza interface resistance can predict the effective thermal conductivity of randomly dispersed long fibers for a very low volume fraction (f < 0.01). The present study compares finite-element (FEA) computations and the EMA model for CNT-matrix compositions with low to moderate volume fractions, 0.001 to 0.02. The FEA results obtained show that the EMA model underestimates the effective thermal conductivity of the composite when the particles are very close to each other, even for small particle volume fractions. For aligned fibers the Kaptiza resistance cannot be neglected in the longitudinal direction. This paper proposes a general correction function for the dependence on particle to particle interaction based on the near neighbor distances and the number of near neighbors. This correction function reduces the EMA under prediction to within several percent (< 5%) in most cases.

Commentary by Dr. Valentin Fuster
2016;():V001T04A002. doi:10.1115/HT2016-7219.

Phonon scattering from media with embedded spherical nanoparticles has been studied extensively over the last decade due to its application to reducing the thermal conductivity of thermoelectric materials. However, similar studies of thermal transport in fiber-embedded media have received little attention. Calculating the thermal conductivity tensor from microscopic principles requires knowledge of the scattering cross section spanning all possible incident elastic wave orientations, polarizations and wavelengths including the transition from Rayleigh to geometric scattering regimes. In this paper, we use continuum mechanics to develop an analytic treatment of elastic wave scattering for an embedded cylinder and show that a classic treatise on the subject contains important errors for oblique angles of incidence, which we correct. We also develop missing equations for the scattering cross section at oblique angles and study the sensitivity of the scattering cross section as a function of elas-todynamic contrast mechanisms. In particular, we find that for oblique angles of incidence, both elastic and density contrast are important mechanisms by which scattering can be controlled, but that their effects can offset one another, similar to the theory of reflection at flat interfaces. The solution developed captures the scattering physics for all possible incident elastic wave orientations, polarizations and wavelengths including the transition from Rayleigh to geometric scattering regimes, so long as the continuum approximation holds. The method thus enables incorporation of coherent scattering models into calculations of the thermal conductivity tensor for media with nanofibers.

Commentary by Dr. Valentin Fuster
2016;():V001T04A003. doi:10.1115/HT2016-7367.

Field plated GaN high electron mobility transistors (HEMTs) are widely preferred amongst other GaN HEMT devices because of their ability to regulate electric field at high power densities. When operated at high power densities, GaN HEMTs suffer significantly from the concentrated heating effects in a small region called hotspot located closer to the drain edge of the gate. Although; the stabilizing effect of field plate on the electrical field distribution in HEMTs is known by researchers, its effect on temperature distribution and the hotspot temperature is still not studied to a greater extend. For this purpose, finite element thermal modelling of devices with different sizes of field plates are performed using the joule heating distribution data obtained from 2D electrical simulations. Results obtained from such combined model show that the existence of a field plate changes the electrical field, therefore the heat generation distribution within device. Moreover; increasing the size of the field plate has an effect on the maximum temperature at the hotspot region. The results are used to analyze these effects and improve usage of field plates for high electron mobility transistors to obtain better temperature profiles.

Commentary by Dr. Valentin Fuster

Nanoscale Thermal Transport: Hard-Soft Material Interfaces and Thermal Interface Materials

2016;():V001T04A004. doi:10.1115/HT2016-7374.

In this paper, a kind of highly conductive thermal paste is investigated, which consists of liquid metal alloy (LMA) and copper particles. The LMA used in the current research is a gallium-indium-tin eutectic alloy (Ga62.5In21.5Sn16). The copper particles dispersing into LMA have an average diameter of 9 μm. During the dispersing process, a degassing process was conducted in order to reduce air bubbles and increase the thermal conductivity of the investigated paste. A new method based on laser flash (LFA) was used to test the total thermal conductivities of the samples. Three types of thermal pastes were prepared and tested, i.e., LMA, oxidized liquid metal alloy (OLMA), and OLMA mixed with copper particles. Results show that when LMA, OLMA, and OLMA mixed with copper particles at a ratio of 5wt%, the resulting thermal conductivities of the investigated thermal pastes can achieve 44.48 W/mK, 13.55 W/mK, and 24.34 W/mK, which result in the corresponding thermal contact resistances of 4.044 mm2K/W, 5.638 mm2K/W, and 4.075 mm2K/W, respectively. In addition, the effect of the copper particle ratio on the thermal performance was investigated. Results show that when the ratio of copper particles increased from 5wt% to 10wt%, the thermal conductivity of investigated thermal paste increased from 24.34 W/mK to 29.07 W/mK, and the thermal contact resistance decreased from 4.075 mm2K/W to 3.37 mm2K/W.

Commentary by Dr. Valentin Fuster
2016;():V001T04A005. doi:10.1115/HT2016-7413.

Metal nanoparticle has been a promising option for fillers in thermal interface materials due to its low cost and ease of fabrication. However, nanoparticle aggregation effect is not well understood because of its complexity. Theoretical models, like effective medium approximation model, barely cover aggregation effect. In this work, we have fabricated nickel-epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Smaller particles are also found to show higher thermal conductivity, contrary to classical models indicate. A two-level EMA model is developed to account for aggregation effect and to explain the size-dependent enhancement of thermal conductivity by introducing local concentration in aggregation structures.

Commentary by Dr. Valentin Fuster
2016;():V001T04A006. doi:10.1115/HT2016-7414.

In this work, we have observed 60% reduction in total interfacial resistance by adding an intermediate metal layer nickel between gold and aluminum oxide. Two temperature model is applied to explain the change of interfacial resistance, including both lattice mismatch with diffuse mismatch model and electron-phonon coupling effect. Simulation result agrees reasonably well with experimental data. Even though interfacial resistance due to electron-phonon coupling effect for Au-aluminum oxide is much larger than that of Ni-aluminum oxide interface, lattice mismatch is still the dominant factor for interfacial resistance.

Commentary by Dr. Valentin Fuster

Nanoscale Thermal Transport: Micro/Nano-Structured Surfaces for Phase-Change Heat Transfer

2016;():V001T04A007. doi:10.1115/HT2016-7331.

Electrowetting has drawn significant interests due to the potential applications in electronic displays, lab-on-a-chip devices and electro-optical switches, etc. Current understanding of electrowetting-induced droplet dynamics is hindered by the inadequacy of available numerical and theoretical models in properly handling the dynamic contact angle at the moving contact line. A combined numerical and experimental approach was employed in this work to study the spatiotemporal responses of a droplet subject to EW with both direct current and alternating current actuating signals. The time evolution of the droplet shape was measured using high-speed photography. Computational fluid dynamics models were developed by using the Volume of Fluid-Continuous Surface Force method in conjunction with a selected dynamic contact angle model. It was found that the numerical models were able to accurately predict the key parameters of the electrowetting-induced droplet dynamics.

Commentary by Dr. Valentin Fuster

Nanoscale Thermal Transport: Nanobubbles, Nanodroplets, and Nanofluids

2016;():V001T04A008. doi:10.1115/HT2016-7343.

The oscillatory flows are often in order to augment heat transfer rates in various processes. It is also well known fact that nanofluids provide significant enhancement in heat transfer at certain conditions. In this research, heat transfer in an oscillatory pipe flow of both water and water-alumina nanofluid were studied experimentally under low frequency regime flow conditions. The aim of the conducted research is parametric experimental investigation of the convective heat transfer in the oscillatory pipe flow. Firstly, the nanofluids were prepared and thermophysical properties weare measured. The experimental apparatus consist of a capillary pipe bundle connecting two reservoirs which are placed at the top and bottom side of the capillary pipe bundle. Upper reservoir contains the hot fluid while lower reservoir and capillary pipe bundle filled with cold fluid. The oscillatory flow in the pipe bundle is driven by the periodic vibrations of a surface mounted on the bottom end of the cold reservoir. The effects of the maximum displacement amplitude of the vibrations and volumetric concentration of nanoparticles on heat transfer were evaluated based on the measured temperature and acceleration data. It is found that heat transfer rate increases with increasing vibration displacement in the fluid.

Commentary by Dr. Valentin Fuster
2016;():V001T04A009. doi:10.1115/HT2016-7385.

Nanofluids — colloidal suspensions of nanoparticles in base fluids — are known to possess superior thermal properties compared to the base fluids. Various theoretical models have been suggested to explain the often anomalous enhancement of these properties. Liquid layering around the nanoparticle is one of such reasons. The effect of the particle size on the extent of liquid layering around the nanoparticle has been investigated in the present study. Classical molecular dynamics simulations have been performed in the investigation, considering the case of a copper nanoparticle suspended in liquid argon. The results show a strong dependence of thickness of the liquid layer on the particle size, below a particle diameter of 4nm. To establish the role of liquid layering in the enhancement of thermal conductivity, simulations have been performed at constant volume fraction for different particle sizes using Green Kubo formalism. The thermal conductivity results show 100% enhancement at 3.34% volume fraction for particle size of 2nm. The results establish the dominant role played by liquid layering in the enhanced thermal conductivity of nanofluids at the low particle sizes used. Contrary to the previous findings, the molecular dynamics simulations also predict a strong dependence of the liquid layer thickness on the particle size in the case of small particles.

Commentary by Dr. Valentin Fuster

Heat Transfer in Equipment: Advances in Enhanced Heat Transfer

2016;():V001T05A001. doi:10.1115/HT2016-7119.

Heat transfer enhancement in laminar regime by split and recombine (SAR) mechanism, based on the baker’s transformation, is investigated. Two different heat exchangers, called SAR1 and SAR2, are studied. Their geometries are inspired from the previous studies reported in the literature. The working fluid on both, shell and tube side, is water and the temperature on the shell side is kept constant. Experiments are carried out for the Reynolds number range 100<Re<3000 when the Prandtl number is between 4.5 and 7.5. The results show that the convective heat transfer coefficient in the first element of heat exchanger SAR1 is higher than that in the second one, i.e. SAR2. However, the variation in the convective heat transfer coefficient from the first to the third element along the heat exchanger SAR2 is less significant than that observed for SAR1. Moreover, SAR2 causes a higher pressure drop, especially when Re>1000, and provides a less uniform temperature field at the outlet.

Commentary by Dr. Valentin Fuster
2016;():V001T05A002. doi:10.1115/HT2016-7176.

Pillow-plate heat exchangers (PPHE) are manufactured as stacks of pillow plates, with a typical wavy surface. They represent a promising alternative to conventional equipment for the process industry; however, the lack of published design methods hinders their widespread application. Over the past years, our group has been intensively studying PPHE, using both theoretical and experimental methods. In the present contribution, the results of a CFD-based investigation of single-phase flow and heat transfer in both the inner and the outer pillow-plate channels are presented. In particular, the influence of using an advanced eddy viscosity turbulence model, namely, the elliptic blending k-ε model, on pressure drop and heat transfer in the outer channel is shown. Furthermore, a comparison between the thermo-hydraulic efficiencies of PPHE and pipes is undertaken, in order to identify the benefits of PPHE.

Commentary by Dr. Valentin Fuster
2016;():V001T05A003. doi:10.1115/HT2016-7215.

Experimental and simulation studies were performed to reveal local heat transfer coefficients under jet impinging in micro domain with Nitrogen gas. The experimental device was made of a 500 μm thick Pyrex and 400 μm thick silicon wafers. On the Pyrex wafer, four 100 nm thick resistance temperature detector (RTD) thermistors and a heater were fabricated from titanium. Jet orifices were etched by deep reactive ion etching (DRIE) on a silicon wafer, which was attached to the Pyrex wafer through a vinyl sticker (250 μm thick). A 1.9 mm × 14.8 mm × 250 μm micro channel was formed by laser drilling into the sticker.

Varying flow rates of Nitrogen gas and heat fluxes of the heater, temperatures of the four thermistors were collected and local heat transfer coefficients were inferred enabling to divulge the jet impinging cooling characteristics. Initial simulations were used to complement experiments and to obtain detailed flow patterns of the jet, temperature distribution on the heater area, and fluid temperature distribution.

Commentary by Dr. Valentin Fuster
2016;():V001T05A004. doi:10.1115/HT2016-7262.

We develop a semi-analytical solution for the Nusselt number for fully-developed flow of liquid between parallel plates, one of which is textured with isothermal parallel ridges. The opposite plate is smooth and adiabatic. The liquid is assumed to be in the Cassie state on the textured surface, on which a mixed boundary condition of no slip on the ridges and no shear along menisci applies. An existing solution for the velocity field is valid. The thermal energy equation is subjected to a mixed isothermal-ridge and adiabatic-meniscus boundary condition on the textured surface. Given the nature of the isothermal boundary condition, the analysis concerns a three-dimensional developing temperature profile, and the results are obtained for a streamwise location that tends to infinity. We assume that the temperature field is governed by an infinite sum of the product of a function of the streamwise coordinate and a second function of the spanwise co-ordinates. The latter functions are eigenfunctions satisfying a two-dimensional Sturm-Liouville problem from which the eigenvalues follow. The fully-developed Nusselt number follows from the first eigenvalue.

Commentary by Dr. Valentin Fuster
2016;():V001T05A005. doi:10.1115/HT2016-7284.

Cooling performances of perforated-finned heat sinks (PFHS) are investigated in the laminar forced convection heat transfer mode, through detailed experiments. Perforations like windows with square cross sections are placed on the lateral surfaces of the fins. Cooling performances are evaluated due to changes in both porosities and perforation sizes. Thermal characteristics are reported based on pumping power, in order to provide more practical insight about performances of PFHSs in real applications. It is found that at a constant perforation size, there is an optimum porosity that results in the largest heat transfer coefficient. For a fixed porosity, increasing the number of perforations (reducing the perforation size) results in an enhancement of heat transfer rate due to repeated interruption of the thermal boundary layer. The opposite trend is observed for PFHSs with larger perforation sizes. This indicates that there is an optimum perforation size and distance between perforations in order to achieve the maximum heat transfer coefficients at a constant porosity. Also, a PFHS results in a smaller temperature non-uniformity across the heat sink base, as well as a more rapid reduction in temperature non-uniformity on the heat sink base by increasing pumping power. In addition, the advantage of a PFHS to reduce the overall weight of the cooling system is incorporated into thermal characteristics of the heat sinks, and demonstrated by the mass specific heat transfer coefficient.

Topics: Cooling , Heat sinks
Commentary by Dr. Valentin Fuster
2016;():V001T05A006. doi:10.1115/HT2016-7327.

Turbulent natural convection in a two-dimensional horizontal composite square cavity is numerically analyzed using the finite volume method and the thermal non-equilibrium approach. Distinct energy equations for the working fluid and for the porous matrix are proposed reflecting different energy balances for each phase. The composite square cavity is formed by three distinct regions, namely, clear, porous and solid region. It was found that the fluid begins to permeate the porous medium for values of Ra greater than 10^6. Nusselt number values show that for the range of Ra analyzed there are no significant variation between the laminar and turbulent model solution. When comparing the effects of Ra and Da on Nu, results indicate that the solid phase properties have a greater influence in enhancing the overall heat transferred trough the cavity.

Commentary by Dr. Valentin Fuster
2016;():V001T05A007. doi:10.1115/HT2016-7329.

Extensive studies are being carried out by several researchers on the performance prediction of aluminum heat exchangers with different fin and tube geometrical configurations mostly for Reynolds number higher than 100. In the present study, the air-side heat transfer and pressure drop characteristics of the louvered fin micro-channeled, Aluminum heat exchangers are systematically analyzed by a 3D numerical simulation for very low Reynolds number from 25 to 200. Three different heat exchanger geometries obtained for the experimental investigation purposes with constant fin pitch (14 fins per inch) but varied fin geometrical parameters (fin height, fin thickness, louver pitch, louver angle, louver length and flow depth) are numerically investigated. The performance of the heat exchangers is predicted by calculating Colburn j factor and Fanning friction f factor. The effect of fin geometrical parameters on the heat exchanger performance at the Reynolds number range specified is evaluated. The air-side performance of the studied heat exchangers for the specified Reynolds number range is compared with experimental heat exchanger performance data available in the open literature and a good agreement is observed. The present results show that at the studied range of Reynolds number the flow through the heat exchanger is fin directed rather than the louver directed and therefore the heat exchanger shows poor performance. The effect of geometrical parameters on the average heat transfer coefficient is computed and design curves are obtained which can be used to predict the heat transfer performance for a given geometry.

Commentary by Dr. Valentin Fuster
2016;():V001T05A008. doi:10.1115/HT2016-7388.

Jet impingement flow is known to generate one of the highest single-phase heat transfer rates, with potential for micro-electronics cooling applications. Although free-surface jets have been studied extensively, existing models are either too complex for practical use or do not consider all relevant parameters, such as the impinging jet’s velocity profile. Recently the authors have shown that the stagnation zone heat transfer is dictated by the jet’s centerline velocity upon impingement, and that going between the limiting cases (uniform vs. parabolic profiles, laminar flow) corresponds to a two-fold increase in heat transfer.

In the present study, which is motivated by cooling at micro-scales (predominantly laminar flows), this simplified analysis is extended leading to a first-order analytical approximation, which is valid not only for the limiting cases but over the entire profile range. Thereby, the development of the jet flow both in the nozzle (pipe-type) and subsequently during its flight (before impingement) is incorporated in this model over a broad range of parameters.

For validation of the model, as well as for additional insight into the governing physics, direct numerical simulations were conducted. Through which it is shown that the jet’s velocity profile and its evolution during free “flight” are dependent on the level of the flow’s upstream development in the nozzle, both of which depend on a single self-similar scale: distance travelled normalized by the nozzle diameter and Reynolds number. This one-way coupling requires incorporation of both stages of development for an accurate description, as done in the present model. During pipe-flow, the first-order model employs a more-rapid development rate than during jet-flight (due to the additional pressure-driven flow) — converging to more complex, well-known models, within a few pipe diameters (for Re = 200 to 2300). During flight, the model describes velocity profile relaxation, which is dominated by viscous diffusion and assisted by jet contraction. Jet contraction is dependent on the emerging velocity profile and liquid-vapor surface tension. For most relevant conditions surface tension is negligible, under which the first-order model centerline velocity decay prediction agrees well with both present simulations and previous works.

Thereby, the present work lays the foundation for a simpler, more useable model for predicting heat transfer under an impinging free-surface jet, over a wide range of conditions (various liquids, pipe-type nozzles of different lengths, flow-rates and nozzle-to-plate distances), as part of an ongoing study into micro-jet array heat transfer.

Commentary by Dr. Valentin Fuster

Heat Transfer in Equipment: Heat Transfer in Energy-Water Conservation

2016;():V001T05A009. doi:10.1115/HT2016-7237.

This paper presents a design analysis framework for a transient cold storage unit that uses solid-liquid phase change for thermal storage. The analytical framework developed in this study establishes non-dimensional parameters that dictate the energy efficiency of the transient energy input and extraction processes, and specifies the links between physical parameters for the system and dimensionless parameters. The resulting governing equations in non-dimensional form are partial differential equations that can be solved numerically. Solutions of the equations predict the thermodynamic efficiency (effectiveness) of the energy storage and retrieval processes, and the time required to input or extract energy from storage for specified values of the dimensionless parameters. The paper illustrates how a high efficiency design target can be established for specified operating conditions using this framework. Application of this framework to a typical example application involving cold thermal storage is described, and the usefulness of this methodology is demonstrated. The use of this methodology for predicting the performance of cold thermal storage for a broad range of potential applications is also discussed.

Commentary by Dr. Valentin Fuster
2016;():V001T05A010. doi:10.1115/HT2016-7312.

Plate-fin heat exchangers are widely used in industries especially aerospace, cryogenics, food and chemical process industries where high heat flux surface area per unit volume is of prime importance. These heat exchangers consists of series of corrugated plates (herringbone or chevron), separated by gasket sealing. Chevron angled plates are one of the most commonly used type of geometry. The complex design of chevron plate heat exchanger, induces high turbulence and flow reversals causing high heat transfer through the plates. This paper discusses about the computational fluid dynamics simulations conducted over a simplified geometry of Chevron Plate Heat Exchanger to understand the formulation of vortices at different Reynold’s number for various aspect ratios. A single phase laminar flow with periodic boundary condition is used for analysis of the fluid behavior in a unit pattern of the corrugation geometry. Based on different flow and geometric conditions, varying amounts of swirl-flows are observed and different behavior of shear stress and heat transfer plot along the length of the plate is observed. At higher Reynolds numbers (Re), the re-circulations and mixing by the induced vortices causes significant rise of heat flux, with marginal increase in friction factor.

Commentary by Dr. Valentin Fuster
2016;():V001T05A011. doi:10.1115/HT2016-7328.

We present numerical results for turbulent heat transfer past a backward-facing-step channel with a porous insert. A non-linear eddy viscosity model was applied to handle turbulence. For a constant Darcy number, the thickness of the porous insert was varied in order to analyze its effects on the flow pattern, particularly the damping of the recirculating bubble past the insert. Further, the reduction of the Nusselt number along the bottom heated surface, when using porous materials inside the channel, was investigated. The numerical technique employed for discretizing the governing equations was the control-volume method. The SIMPLE algorithm was used to correct the pressure field and the classical wall function approach was utilized in order to handle flow calculations near the wall. Comparisons of results simulated with different porous materials were presented.

Commentary by Dr. Valentin Fuster

Heat Transfer in Fire and Combustion

2016;():V001T06A001. doi:10.1115/HT2016-7095.

In designing industrial cylindrical furnaces, it is important to predict the radiative heat flux on the wall with high accuracy. In this study, we consider CO2 and H2O which have strong absorption in the infrared range. The absorption coefficients of the gases are calculated by using the statistical narrow band (SNB) model. The spectrum is divided into 15 bands to cover all the absorption regions of the two non-gray gases. The radiative transfer equation is solved by the finite volume method (FVM) in cylindrical coordinates. To make the FVM more accurate, we discretize the solid angle into 80 directions with the S8 approximation which is found to be both efficient and less time consuming. Based on the existing species and temperature fields, which were modeled by the FLUENT commercial code, the radiative heat transfer in a cylinder combustor is simulated by an in-house code. The results show that the radiative heat flux plays a dominant part of the heat flux to the wall. Meanwhile, when the gas is considered as nongray, the computational time is very huge. Therefore, a parallel algorithm is also applied to speed up the computing process.

Commentary by Dr. Valentin Fuster
2016;():V001T06A002. doi:10.1115/HT2016-7277.

A Thermal Partial Oxidation (TPOX) based “Swiss-Roll” Reformer is being developed to convert hydrocarbon fuels into hydrogen rich syngas for subsequent use in a solid oxide fuel cell (SOFC) for electrical power generation. The “Swiss-roll” reformer is an efficient countercurrent heat exchanger that recuperates heat from the hot reformate stream generated internal to the reformer and preheats the cold reactants prior to entrance into a central region where the high temperature reforming reactions occur. Effective heat recuperation enables super-adiabatic temperatures to be achieved within the reformer which drives the reformate composition towards chemical equilibrium without the need for catalysts and/or external energy. The simplicity, compactness, lack of catalysts and fuel flexibility of the reformer are attractive. In this work, experimental data obtained using a 3D printed Swiss-roll reformer is reported that shows the conversion of representative fuel (propane) to syngas. The associated temperature and species concentrations taken at different locations within the reformer are also reported. These measurements provide information on the effectiveness of the heat exchange process as well as the reforming reactions inside the compact TPOX reformer.

Topics: Fuels , Syngas
Commentary by Dr. Valentin Fuster
2016;():V001T06A003. doi:10.1115/HT2016-7281.

Carbon monoxide (CO) boilers play an important role in the petroleum refining industry, completing the combustion of CO in the flue gas generated by the regeneration of fluidized cracking catalyst. The heat released by the CO combustion is used to generate steam for use in the refinery. The flue gas flow path can have a significant effect on the thermal efficiency and operation safety of the boiler. In this paper, a CO boiler which had been experiencing low thermal efficiency and high operation risks was studied. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed with detailed description on the combustion process, flow characteristics and heat transfer. The results obtained from the model have good agreement with the plant measurement data. The heat transfer between the tubes and the combustion flue gas was optimized by adding a checker wall.

Commentary by Dr. Valentin Fuster
2016;():V001T06A004. doi:10.1115/HT2016-7334.

In Hong Kong, the number of parallel traders has increased quickly within these years. Passengers bring not only handbags as planned for the subway system four decades ago. There are heavy luggage put in trolleys carried by parallel traders. Goods include milk powder, electronic devices, food and drinks. The increase in fire load will enhance the fire risk. Although the subway management limits the maximum allowable luggage to 23 kg, the combustible amount is still large in train cars.

Heat release rate (HRR) is the most important parameter in fire hazard assessment. A 1:15 scale modeling test was carried out to find the HRR in a train car under limited ventilation with parallel goods in this paper. The goods were simulated by a propanol pool fire. HRR for six scenarios with all closed door to all opened door were studied. Mass loss rate of fuel, oxygen consumption, air velocity and fire duration were measured. Ventilation was provided by opening different numbers of doors of the train car. The train car fire can then be ventilation-controlled or fuel-controlled. The burning phenomena of fire in the train under limited ventilation including steady burning, oscillating fire and ghosting flame were studied.

It was found that the fire size and duration depended mainly on ventilation when oxygen was very limited. When the amount of oxygen supply reached a critical value (the fire at the decay period with continuous supply of oxygen), the fire tended to be fuel-controlled. Burning characteristics of train fire under limited ventilation were also observed. They were different from fires burning in open area. Oscillating flame, ghosting flame and self-extinction were observed. Results are useful to assess fire hazards associated with parallel trading activities. Three repeated experiments on the captioned subject have been done with average results of all experiments presented.

Topics: Flames , Trains
Commentary by Dr. Valentin Fuster
2016;():V001T06A005. doi:10.1115/HT2016-7423.

For 3D observation of high speed flames, non-scanning 3D-CT technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “Schlieren 3D-CT” is based on (i) simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this technique, for simultaneous schlieren photography, the custom-made 20-directional schlieren camera has been constructed and used. This camera consists of 20 optical systems of single-directional quantitative schlieren system. Each system is composed of two convex achromatic lenses of 50 mm in diameter and 300 mm in focal length, a light source unit, a schlieren stop of a vertical knife edge and a digital camera. The light unit has a flash (9 micro-sec duration) light source of a uniform luminance rectangular area of 1 mm × 1 mm. Both of the uniformity of the luminosity and the definite shape are essential for a quantitative schlieren observation. Sensitivity of the digital cameras are calibrated with a stepped neutral density filter. Target flames are located at the center of the camera. The image set of 20 directional schlieren images are processed as follows. First the schlieren picture brightness is shifted by no-flame-schlieren picture brightness in order to obtain the real schlieren brightness images. Second, brightness of these images is scaled by Gladstone-Dale constant of air. Finally, the scaled brightness is horizontally integrated to form “density thickness images”, which can be used for CT reconstruction of density distribution. The density thickness images are used for CT reconstruction by MLEM (maximum likelihood-expectation maximization) CT-algorithm to obtain the 3D reconstruction of instantaneous density distribution. In this investigation, the “density thickness” projection images of 400(H) × 500(V) pixel (32.0 mm × 40.0 mm) are used for 3D-CT reconstruction to produce 3D data of 400(x) × 400(y) × 500(z) pixel (32.0 mm × 32.0 mm × 40.0 mm). The voxel size is 0.08 mm each direction. In this investigation, the target flame is spark-ignited flame kernels. The flame kernels are made by spark ignition for a fuel-rich propane-air premixed gas. First, laminar flow is selected as the premixed gas flow to establish the spherically expanding laminar flame. The CT reconstruction result show the spherical shape of flame kernel with a pair of deep wrinkles. The wrinkle is considered to be caused by spark electrodes. Next turbulent flows behind turbulence promoting grid is selected. The corrugated shape flame kernel is obtained. The schlieren 3D-CT measurements are made for the complicated kernels. CT results expresses the instantaneous 3D turbulent flame kernel shapes.

Topics: Density , Fuels , Flames
Commentary by Dr. Valentin Fuster
2016;():V001T06A006. doi:10.1115/HT2016-7448.

Fire safety is critical for safety of airplane operation. During an emergency landing, airplane goes through dramatic external pressure change from cruise altitude to sea level, considering the impact caused by low pressure atmosphere. The objective of this work is to examine the effect of dynamic pressure on the behavior of a horizontally burning diffusion flame over a pool fuel surface based on experimental approach. The experiments were conducted in a large-scale altitude chamber of size 2 m × 3 m × 4.65 m. The pressure rise process was examined under different dynamic pressures from respectively 38 kPa, 64 kPa and 75 kPa to 90 kPa with various pressure rise rates of 100 Pa/s, 150 Pa/s, 200 Pa/s, 250 Pa/s and 300 Pa/s, which is to simulate the airplane landing process from different altitudes. The whole system of the altitude chamber is of unique capability that the pressure in the chamber can be exactly controlled by a powerful pressure controlling system, and the oxygen concentration can maintain at the level about 20%, which are achieved through controlling inlet air flow for oxygen level and outlet gas flow for pressure (static or dynamic) level. A round steel fuel pans of 34 cm in diameter and 15 cm in height were chosen for the pool fire tests. The fuel pan was filled with 99% pure liquid n-Heptane. Cold water is added beneath the fuel layer to cool the pan and minimize the temperature rise in the fuel. Parameters such as mass, mass burning rate, chamber pressure were measured. The results of those tests demonstrated the significant impact to fire behaviors caused by high altitude or low pressure atmosphere.

Commentary by Dr. Valentin Fuster

Transport Processes in Fuel Cells and Heat Pipes

2016;():V001T23A001. doi:10.1115/HT2016-1071.

The catalytic dehydrogenation reaction of isobutane to isobutylene is simulated in a commercial-scale heterogenous fixed bed reactor (FBR). The porous medium method in ANSYS Fluent combined with the reaction model capability was utilized to predict the flow behavior and species transport in a bed of spherical particles. Physical and material properties of a dehydrogenating catalyst of Chromium Oxide (Cr2O3) on Aluminum Oxide Support (Al2O3) were employed in the model. Several reaction models were implemented using a customized User-defined Function (UDF) subroutine. Simulation results were validated against literature data for a similar process. Good agreement was observed for the conversion of alkanes to alkenes within acceptable accuracy. It is concluded that the power-law model showed the least fit for the feed conversion and product selectivity compared to the other studied reaction models.

Commentary by Dr. Valentin Fuster

Boiling and Condensation in Macro, Micro and Nanosystems

2016;():V001T24A001. doi:10.1115/HT2016-1002.

Boiling heat transfer impacts the performance of various industrial processes like quenching, desalination and steam generation. At high temperatures, boiling heat transfer is limited by the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect), where the low thermal conductivity of the vapor layer inhibits heat transfer. Interfacial electrowetting (EW) fields can disrupt this vapor layer to promote liquid-surface wetting. This concept works for a variety of quenching media including water and organic solvents. We experimentally analyze EW-induced disruption of the vapor layer, and measure the resulting enhanced cooling during quenching. Imaging is employed to visualize the fluid-surface interactions and understand boiling patterns in the presence of an electrical voltage. It is seen that EW fundamentally changes the boiling pattern, wherein, a stable vapor layer is replaced by intermittent wetting of the surface. This switch in the heat transfer mode substantially reduces the cool down time. An order of magnitude increase in the cooling rate is observed. An analytical model is developed to extract instantaneous voltage dependent heat transfer rates from the cooling curve. The results show that electric fields can alter and tune the traditional cooling curve. Overall, this study presents a new concept to control the mechanical properties and metallurgy, by electrical control of the quench rate.

Commentary by Dr. Valentin Fuster
2016;():V001T24A002. doi:10.1115/HT2016-1003.

Two-phase flows involving phase change are ubiquitous in a diverse range of scientific and technological applications. There has been great recent interest in the enhancement of boiling heat transfer processes by means of additives such as surfactants. Surfactants can influence boiling through convection currents in the bulk fluids as a result of changes in the surface tension caused by local surfactant concentration due their adsorption/desorption from the bulk regions. This can result in changes in bubble release patterns and higher heat transfer rates if such changes lead to higher rate of vapor formation. We intend to study this effect in the context of film boiling. Our computational approach augments the CLSVOF method with bulk energy and diffusion equations along with a phase change model and an interface surfactant model. The challenge here is to accurately calculate the tangential gradients of the interfacial surfactant concentration in the presence of discontinuous bulk concentration gradients near the interface. We discuss a simplified model in which the interfacial surfactant concentration is always in equilibrium with the changing bulk concentrations and then present validation results to assess the accuracy of this approach. Finally, initial studies of surfactant enhanced film boiling will be presented and interpreted.

Commentary by Dr. Valentin Fuster
2016;():V001T24A003. doi:10.1115/HT2016-1004.

Flow boiling of R-134a refrigerant was experimentally conducted in a test section which is a stainless steel tube having internal diameter of 1 mm. The DC power supply was connected to the test section to provide constant surface heat flux conditions. Flow pattern and heat transfer data were obtained for a mass flux range of 252–820 kg/m2s, a heat flux range of 1–21 kW/m2 and a saturation pressure of 8 bar. The flow visualization results showed four different flow patterns including slug flow, throat-annular flow, churn flow, and annular flow. The flow boiling heat transfer behaviors were also compared with those based on non-boiling two-phase air-water flow in the same test section under constant surface heat flux conditions. For non-boiling two-phase flow experiment, an air-water T-shaped mixer was served to introduce fluids smoothly along the test section. The results indicated that based on the same gas and liquid Reynolds numbers, flow boiling tends to have Nusselt number higher than that for non-boiling gas-liquid flow.

Commentary by Dr. Valentin Fuster
2016;():V001T24A004. doi:10.1115/HT2016-1006.

It is well known that a high-power laser could breakdown liquid [1, 2]. Laser-induced breakdown of liquids is characterized by fast plasma formation after evaporation of liquid and subsequent vapor expansion accompanied by shock wave emission [2]. The bubble wall velocity after the shock departure has been found to be sufficiently high to produce emission of light at the collapse point [3]. Recently, bubble formation on the surface of gold nanoparticles irradiated by a high-power laser in water [4, 5] has been studied for medical applications such as cancer diagnosis and possible therapy [5]. However, it is very hard to perform these experiments and to obtain good data from the bubble formation on the surface of laser-irradiated nano-particles because the nanoparticles dispersed in liquid cannot be controlled properly. In this study, laser-induced bubble formation on a micro gold particle levitated at the center of a spherical flask under ultrasound was investigated experimentally. The obtained results are compared with the results for laser cavitation without the gold particle, i.e., typical laser-induced cavitation.

Figure 1 shows a schematic of the experimental setup used to investigate the laser-induced bubble formation on a micro gold particle. Two disk-type lead zirconate titanate (PZT) transducers (Channel Industries Inc.; 15 mm in diameter and 5.0 mm in thickness) attached to the side of the wall of the cell produced a velocity stagnation point near the center of the flask. The driving frequency of the PZT transducers was approximately 27.0 kHz which was close to the resonance frequency of the LRC circuit (Its capacitor unis is PZT.) and the acoustic resonance frequency of the water-filled flask. A drop of water containing gold particles with an average diameter of 10 μm are dispersed was injected into a 100-ml pyrex spherical flask filled with degassed water. When the body force of a gold particle in liquid is slightly lower than the Bjerknes force [6] induced by ultrasound, the particle will stay near the pressure antinode, i.e., the center of the flask.

A Q-switched Nd:Yag laser delivered a single pulse of 0.5 ns in width with an energy of 7.5 mJ at a wavelength of 1064 nm to the gold particle or liquid at the center of the cell. The laser light was focused at the center of the flask using a lens with an effective focal length of 36.3 mm. Bubble formation and subsequent growth and collapse were visuallized by a high-speed camera (V2511, Phantom, USA) with 0.45 Mfps (million frames per second). The time-dependent radius was also obtained by the light scattering method. The scattering angle chosen was 80 degree where one-to-one relationship exists between the scattered intensity and the bubble radius [7]. The scattered intensity from a bubble illuminated by a 5-mW He-Ne laser was received by a photomultiplier tube (PMT: Hamamatsu, R2027) and was recorded in an oscilloscope. The scattering data were calibrated using the maximum radius for different bubble, which was obtained by high-speed camera.

The shock strength during the expansion stage of bubbles was measured by a calibrated needle hydrophone (HPM1, Precision Acoustics, UK) at various distances from the center of the cell for different bubbles. The hydrophone can measure acoustic signals ranging from 1 kPa to 20 MPa. The hydrophone was attached to a three-dimensional micro stage so that fine control of the positioning of the hydrophone was possible.

Commentary by Dr. Valentin Fuster
2016;():V001T24A005. doi:10.1115/HT2016-1007.

In this paper, the IR laser photothermally induced phase change characteristics in the microchannels with different wettabilities were studied using visual experiments. The hydrophobic microchannel was obtained by the hydrophobic nature of the PDMS material while the hydrophilic microchannels were obtained by the inert gas plasma surface treatments. Effects of the contact angle, laser power and spot position were investigated. It is interesting to find the change of the wettability and laser power could alter the phase change behaviors. The hydrophobic microchannel showed the interface advancement at low laser power and the liquid slug formation accompanying with the interface receding at high laser power. For less hydrophilic microchannel, the liquid slug formation accompanying with the interface receding was observed at low laser power while the sole interface receding was observed at high laser power. For more hydrophilic microchannel, the sole interface receding was observed for both low and high laser powers. Besides, it was also found that increasing the distance between the initial interface location and laser spot led to the increased thermal resistance, lowering the evaporation rate and thereby the above-mentioned effect.

Topics: Lasers , Microchannels
Commentary by Dr. Valentin Fuster
2016;():V001T24A006. doi:10.1115/HT2016-1033.

This work reports a novel Quartz Crystal Microbalance (QCM) based method to analyze the droplet-micropillar surface interaction quantitatively during dropwise condensation. A combined nanoimprinting lithography and chemical surface treatment approach was utilized to directly fabricate the micropillar based superhydrophobic surface on the QCM substrate. The normalized frequency shift of the QCM device and the microscopic observation of the corresponding nucleation, drop growth, and drop coalescence processes clearly demonstrate the different characteristics of these condensation states. In addition, a synchrosqueezed wavelet spectrum based multi-resolution technique was utilized to analyze the resonant signal from the QCM sensor in both time and frequency domains simultaneously. An integrated discrete system modeling along with a hybrid signal and image processing approach was adopted to identify the response of the micropillars under different stages of dropwise condensation (DWC). The outcome of this signal processing research leads to a fundamental understanding of DWC spanning multiple time and length scales. The proposed study will also contribute to an in-depth understanding of different hydrophobic surfaces and DWC through this advanced signal processing and surface treatment. The developed QCM system provides a valuable tool for the dynamic characterization of different condensation processes.

Commentary by Dr. Valentin Fuster

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