Combustion, Fire and Reacting Flow

2009;():1-5. doi:10.1115/HT2009-88017.

Tests were conducted with ISO 9705 room to investigate the combustion behavior of medium size wood cribs. Cribs were burnt at the center and corner inside ISO room and also under the hood of the ISO room. Effective heat of combustion and increase rate of heat release rate in growth phase is compared for cribs with different nominal heat release rate and in different positions. The relationship between scaled steady mass loss rate and porosity factor of wood crib is quite different from those in literatures. The average effect heat of combustion is 12.18 MJ kg−1 , which is close to commonly accepted value 12 MJ kg−1 for wood sample burning with diffusion flame.

Topics: Wood products , Fire
Commentary by Dr. Valentin Fuster
2009;():7-13. doi:10.1115/HT2009-88018.

A mathematical model for predicting sprinkler cooling effect on smoke layer was developed. Results of calculation showed that the temperature difference between droplets and smoke layer and the thickness of smoke layer are major factors of the cooling effect. The cooling effect might lose its efficiency when the sprinkler pressure became relatively high. Experiments were carried out to validate the model and compared with Morgan model. The experimental results show that temperatures predicted by the current model agree well with the experimental results when temperature difference is high. However Morgan model is more suitable for low temperature difference.

Commentary by Dr. Valentin Fuster
2009;():15-20. doi:10.1115/HT2009-88058.

Biomass reburn is a low NOx alternative to cofiring that effectively uses the high volatility and high char reactivity of biomass for NOx reduction. In this paper, computational fluid dynamics (CFD) and thermal modeling, and a NOx prediction model were used to evaluate the impacts of sawdust/coal reburn on the performance of a 250 MW opposed-fired boiler burning bituminous coal as the primary fuel. The results showed that the reburn system maintained overall boiler performance with a 50 – 70 °F reduction in the furnace exit gas temperature. Predicted losses in thermal efficiency were caused by the lower biomass fuel heating value (similar to biomass cofiring) and increase in unburned carbon. The higher unburned carbon emissions were attributed to an order of magnitude larger biomass mean particle size relative to bituminous coal. Thus, LOI emissions can be improved significantly by reducing the biomass mean particle size. The NOx predictions showed that for reburn rates above about 19%, adding dry sawdust biomass to a coal reburn system can improve NOx reduction; i.e., using pure dry sawdust as reburn fuel at 30% of the total heat input can lead to NOx levels about 30% less than those for pure coal reburn under for similar firing conditions.

Topics: Fuels , Biomass
Commentary by Dr. Valentin Fuster
2009;():21-25. doi:10.1115/HT2009-88083.

Computer thermal fire models are used in hazard assessment for performance-based fire design. Fire field model using Computational Fluid Dynamics (CFD) is now a popular design tool. The thermal fire environment can be predicted in a ‘microscopic’ picture with air flow pattern, pressure and temperature contours. However, most of the field models are only validated by some experiments not specially designed for such purpose. Whether those models are suitable for use is queried, leading to challenges. In this paper, prediction on smoke filling in a big atrium by the CFD tool Fire Dynamics Simulator developed at the National Institute of Standards and Technology in USA was justified by field tests. Smoke layer interface height and air temperatures inside the atrium were taken as the parameters. CFD results predicted were compared with the field measurement results.

Topics: Fire , Smoke
Commentary by Dr. Valentin Fuster
2009;():27-30. doi:10.1115/HT2009-88084.

Flame stretching in a room with a ceiling vent will be discussed in this short note. A real-scale model was constructed with a gasoline pool fire placed inside. Another pool fire of the same size and amount of fuel was burnt outside the model. Different pool sizes of diameters 0.07 m, 0.08 m, 0.11 m, 0.16 m and 0.2 m were set up. Volume of gasoline varied from 30 ml to 500 ml to give different burning durations. The flame lengths of the two fires were measured and compared. It is observed that the flame length of the pool fire inside the room was over 20% higher than that outside at the later stage of the fire.

Topics: Flames , Vents
Commentary by Dr. Valentin Fuster
2009;():31-40. doi:10.1115/HT2009-88156.

A study was conducted to examine the energy transfer from a reacted thermite placed on a steel target substrate. A high speed infrared camera captured a temporally evolving thermal distribution on the substrate, while the thermite, which was placed in a v-notch, self propagated. The thermite investigated for this experiment was Aluminum with Iron (III) Oxide (Al-Fe2 O3 ). An energy balance model was developed to predict temperatures near the v-notch in order to quantify the amount of energy transferred into the steel. Results quantified the percent of energy available from the reaction that was conducted through the substrate and energy losses were estimated. The thermite reaction transferred 10% of the heat of reaction to the steel. The Al-Fe2O3 exhibited greater heat losses to convection and radiation upon propagation through the powder mixture. The Al-Fe2 O3 reaction produced more gas by chemistry, 10% by mass, which contributed to transporting energy away from the v-notch. Much work had been performed that examined the combustion behaviors from a reacting thermite, but there are very few studies that quantify the energy transfer from a reacting thermite to a target. This diagnostic approach and numerical analysis were the first steps toward understanding energy transferred from a thermite into a target, and lost to the environment.

Commentary by Dr. Valentin Fuster
2009;():41-61. doi:10.1115/HT2009-88193.

Snapshot proper orthogonal decomposition (POD) was performed on the analysis of dominant structures of fire-induced flows. The data for POD analysis was obtained from large eddy simulation (LES). Identification and analysis of dominant flow patterns have been carried out for two important types of fire-induced flows, including vertical plumes induced by pool fire and fire-induced horizontal channel flows. The essential features or energetic motions of these fire-induced flows were identified by combination of desired orders of POD modes. For the fire plumes, the counter-rotating vortex tubes were identified as the most dominant flow patterns. It is revealed that the oscillation dynamics of fire plume were related to the vortices near these vortex tubes. A larger number of small-scale structures and more structure scales were found in the fire plumes with higher Reynolds number (or higher heat release rate). For fire-induced horizontal channel flows, both the energy fractions and the structure patterns associated with POD modes depend more strongly on Reynolds number than those for fire plumes. The energy fractions contained within the most energetic modes significantly decrease with the increase of Reynolds number (or extraction flow rate) for fire-induced channel flows. It is found that the locations of strong vortices areas identified by POD mode are higher than the interface heights estimated by Janssens’ method, especially at the positions where counter flow mixing is strong.

Topics: Flow (Dynamics) , Fire
Commentary by Dr. Valentin Fuster
2009;():63-71. doi:10.1115/HT2009-88195.

A combined experimental and numerical study has been conducted to investigate the impinging flame structure. Inflame temperature profiles were obtained and compared with corresponding simulated profiles. For detailed understanding of flame structure numerical simulations were carried out using commercial CFD code FLUENT. Simulated temperature, heat flux and species profiles were analyzed. Further investigations were done by plotting streamlines, velocity magnitude profiles and species profiles. It has been seen that bulk of the combustion products were burnt rapidly in the narrow reaction zone at the tip of the flame. This was because of exponential relationship between the chemical reaction rate and temperature. Simulation results show high temperature in the region between the inner premixed and the outer non-premixed (diffusion) reaction zones. The burnt gas along the inner zone expands and molecules change their directions from initially parallel to diverging lines. Flow accelerated from stagnation point and attained maximum velocity at the start of wall-jet region.

Topics: Flames , Methane
Commentary by Dr. Valentin Fuster
2009;():73-80. doi:10.1115/HT2009-88219.

Characterizing the combustion behaviors of energetic materials requires diagnostic tools that are often not readily or commercially available. For example, a jet of thermite spray provides a high temperature and pressure reaction that can also be highly corrosive and promote undesirable conditions for the survivability of any sensor. Developing a diagnostic to quantify heat flux from a thermite spray is the objective of this study. Quick response sensors such as thin film heat flux sensors can not survive the harsh conditions of the spray, but more rugged sensors lack the response time for the resolution desired. A sensor that will allow for adequate response time while surviving the entire test duration was constructed. The sensor outputs interior temperatures of the probes at known locations and utilizes an inverse heat conduction code to calculate heat flux values. The details of this device are discussed and illustrated. Temperature and heat flux measurements of various thermite spray conditions are reported. Results indicate that this newly developed energetic material heat flux sensor provides quantitative data with good repeatability.

Topics: Sprays , Heat flux
Commentary by Dr. Valentin Fuster
2009;():81-87. doi:10.1115/HT2009-88253.

This work used a pseudo three-dimensional discrete element method (DEM) to study the way gas supply patterns affect the thermodynamics characteristics in fluidized beds. During the simulations, gas-to-particle and particle-to-particle heat transfers were considered. Results indicate that there is a lateral temperature gradient of particles in fluidized bed using horizontal air distributor incorporated with even gas supply; nevertheless, in the case of inclined air distributor together with uneven gas supply, it takes a short time for particle phase to achieve a homogenous temperature field. Hydrodynamics analysis and comparison of solid fluxes between the two cases reveal that the bubbles’ lateral movement are reduced due to even gas supply, and the particle-to-gas heat transfer is localized; however, there is an global circulating solids stream in the bed with uneven gas supply, which is thought to expand the particles’ movement range and enhance the thermal transport performance of the fluidizing system.

Topics: Fluidized beds
Commentary by Dr. Valentin Fuster
2009;():89-95. doi:10.1115/HT2009-88259.

The structure and soot formation characteristics of a coflow laminar methane/air diffusion flame under conditions of constant p2 g and mass flow rates of the air and fuel streams were numerically investigated in order to examine the validity of the p2 g scaling relationship. The p2 g scaling relationship has been used to experimentally investigate soot formation in weakly-buoyant laminar diffusion flames by conducting experiments at reduced pressures. Detailed numerical calculations were conducted by solving the elliptic conservation equations of mass, momentum, species, and energy in axisymmetric cylindrical coordinates using a standard control volume method. Detailed multi-component thermal and transport properties and detail combustion chemistry were employed in the modelling. Soot formation was modeled using a semi-empirical acetylene based model in which two transport equations for the soot mass fraction and soot number density per unit mass were solved. Thermal radiation was calculated using the discrete-ordinates method and a 9-band non-grey model for the radiative properties of the CO-CO2 -H2 O-soot mixture. The flame structure and soot formation characteristics exhibit strong dependence on the ambient pressure even though p2 g and the mass flow rates are kept constant. Significantly more soot is produced with increasing the pressure and decreasing the gravity level. Numerical results clearly demonstrate that the p2 g scaling relationship is invalid as far as soot formation is concerned.

Commentary by Dr. Valentin Fuster
2009;():97-103. doi:10.1115/HT2009-88268.

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane-air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocities were measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean CH4 -air mixtures when the turbulent level in the unburned mixture is not changed. The increase rate of turbulent burning velocity is higher than that of the corresponding laminar burning velocity, suggesting that the increase in turbulent velocity due to hydrogen addition is caused by not only chemical kinetics effect, but also the variation in flame structure due to turbulence. The further analysis of flame surface characteristics and brush thickness indicate that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. As a result, turbulent burning velocity is intensified by the increase in total flame surface area and the increased laminar burning velocity, when hydrogen is added.

Commentary by Dr. Valentin Fuster
2009;():105-114. doi:10.1115/HT2009-88392.

The reduction of greenhouse gas emissions is essential to mitigate the impact of energy production from fossil fuels on the environment. Oxyfuel technology is a process developed to reduce emissions from power stations by removing nitrogen from air and burning the fossil fuels in a stream of pure oxygen. The remaining oxidiser is composed of recycled flue gas from the furnace to reduce temperatures. The product of this system is a flue gas with very high carbon dioxide concentration enabling more efficient capture and storage. Accurate modelling of oxyfuel is essential to gain better understanding of the combustion fundamentals and obtain accurate predictions of properties within the furnace that cannot be measured. Heat transfer to the furnace walls will be affected due to the different composition of the gases in the furnace. Carbon dioxide has higher heat capacity than nitrogen. Water vapour and carbon dioxide also exhibit absorption spectra of radiation in the infra-red region of the spectrum relating to wavelengths observed in combustion. Accurate CFD modelling of radiative heat transfer in oxyfuel combustion will require improvements to the radiative properties model to account for the spectral nature of radiation. In addition the impact of the solid fuel particles, soot and ash are considered. Several different radiative properties models have been tested to assess the impact on the predicted radiation and temperatures under air and oxy firing conditions. The results for radiation transferred to the walls are highly dependent upon the model chosen and the need for an accurate radiative properties model for oxyfuel firing, such as the full-spectrum k-distribution method is demonstrated.

Commentary by Dr. Valentin Fuster
2009;():115-123. doi:10.1115/HT2009-88415.

In this paper, a one-dimensional numerical approach is used to study the effect of various parameters such as micro combustor diameter, mass flow rate and external convection heat transfer coefficient on the temperature and species mass fraction profiles. A premixed mixture of H2-Air with a multi-step chemistry (9 species and 19 reactions) is used and thermal conductivity of the mixture is considered as a function of species thermal conductivity and temperature by using a set of new relations. The transient gas phase energy and species conservation equations result in an Advection-Diffusion-Reaction system (A-D-R) that leads to two stiff systems of PDEs, which can not be solved by conventional Computational Fluid Dynamics (CFD) methods. In the present work, Strang splitting method, which is suitable for nonlinear stiff system of PDEs, is used. The results show that both convection heat transfer coefficient and micro combustor diameter have a significant effect on the combustion and heat transfer rates in the micro scales. Also, increasing the convective heat transfer coefficient and decreasing the diameter and inlet mixture velocity, decreases the temperature and active radicals along the micro combustor.

Commentary by Dr. Valentin Fuster
2009;():125-129. doi:10.1115/HT2009-88425.

This paper deals with one of the most interesting methods of lowering heat losses: thermochemical recuperation (TCR) of waste heat present in high-temperature flue gases via methane steam reforming. An earlier proposed process flow diagram for TCR in a glass production facility was revised and a mathematical model was developed to predict the heat and mass transfer phenomena that take place in the catalytic chemical reactor included in that process. Comparison of data obtained by the new proposed model and published data confirmed the feasibility of using it in the investigation and synthesis of TCR processes. Obtained results also permit to find optimal temperature profile in order to increase the hydrogen rate formation.

Commentary by Dr. Valentin Fuster
2009;():131-138. doi:10.1115/HT2009-88429.

This research aims at developing a turbulent diffusion combustion model based on the chemical equilibrium method and chemical kinetics for simplifying complex chemical mechanisms. This paper presents a combustion model based on the chemical equilibrium method and the eddy dissipation concept (CE-EDC model); the CE-EDC model is validated by simulating a H2 -air turbulent diffusion flame. In this model, the reaction rate of fuels and intermediate species is estimated by using the equations of the EDC model. Further, the reacted fuels and intermediate species are assumed to be in chemical equilibrium; the amount of the other species is determined from the amount of the reacted fuels, intermediate species, and air as reactants by using the Gibbs free energy minimization method. An advantage of the CE-EDC model is that the amount of the combustion products can be determined without using detailed chemical mechanisms. The results obtained by using this model were in good agreement with the experimental and computational data obtained by using the EDC model. Using this model, the amount of combustion products can be calculated without using detailed chemical mechanisms. Further, the accuracy of this model is same as that of the EDC model.

Commentary by Dr. Valentin Fuster
2009;():139-144. doi:10.1115/HT2009-88430.

Nowadays, atrium building is very popular because it can provide extensity and attraction to the users even if they are inside the enclosed environment. Similar to other buildings, fire safety is one of the major concerns especially the atrium are linked to shopping arcades. The major challenge is to control the smoke movement in the case of fire and maintain a stable smoke layer clear height to allow sufficient time for the occupants to evacuate from the building. Therefore, an efficient smoke management system (SMS) is necessary. For the SMS to function properly, the smoke behaviour inside the atrium must be studied. One of the phenomena affecting the operation of the SMS is smoke stratification. That is, due to the vertical temperature gradient inside the atrium, a thermally stratified environment is formed and smoke will not be able to reach the smoke detectors/smoke outlets in the ceiling. In the past decade, various studies were conducted to study the smoke filling process in the atrium. Only a few studies were carried out to study smoke stratification in atrium. This paper attempted to study the factors leading to the development of thermally stratified environment in an atrium and the formation of smoke stratification under the ceiling space of an atrium building using scale model. These factors included the temperature of the smoke plume, the air temperature under the ceiling, the configuration of roof ceiling and the ambient air temperature. Two types of ceiling configurations such as a cuboid and a triangular prism are used. Data concerning the ceiling air temperature, smoke plume temperature, effect of different ceiling configuration and maximum smoke layer height in a thermally stratified environment are collected. Comparisons are conducted with the calculated results from National Fire Protection Association (NFPA) 92B equations. With all these information, better design criteria of smoke detection system, SMS in an atrium building can be developed. Finally, the experimental results can be used to investigate the discrepancies between the experimental measurement and the calculated results from NFPA 92B equations. Put abstract text here.

Topics: Smoke
Commentary by Dr. Valentin Fuster
2009;():145-150. doi:10.1115/HT2009-88439.

In this paper, piston wind effect on smoke diffusion characteristic in subway tunnel is studied by using three-dimensional transient computational fluid dynamics (CFD) method. In the first simulation case, fire disaster is simulated with homogeneous resting initial field condition. In the second simulation case, the train’s decelerating process till stopping in the tunnel is simulated for getting three-dimensional tunnel air velocity field distribution. Then the final heterogeneous air velocity field when the train stops in the tunnel is taken as initial field condition and the same fire scenario as the first case is simulated again. The data obtained under both initial conditions are compared by detecting people evacuation safety and the influence of initial air velocity field is analyzed. The results show that the inertial air velocity field caused by train’s movement has significant influence on smoke diffusion at the first few minutes of fire disaster, which is the key time for people’s evacuation. The adopted method in this paper and the simulation result could be used in establishing more effective subway fire evacuation plan.

Commentary by Dr. Valentin Fuster
2009;():151-159. doi:10.1115/HT2009-88470.

Coal combustion with oxygen is considered one of the most effective methods to improve thermal efficiency, reduce pollutant emissions such as NOX , and facilitate capture of CO2 pollutant from flue gas. This paper presents calculations of oxygen coal combustion with flue gas recirculation. The coal is burned in oxygen/CO2/H20 mixture. In addition to solving transport equations for the continuous phase (gas), a discrete second phase (spherical particles) is simulated in the Lagrangian frame of reference. Reaction is modeled by a mixture fractions/PDF approach. Discrete phase modeling is used for the prediction of discrete phase trajectory and heat and mass transfer to/from particles. Drayton coal with a lower heating value of 27.8 MJ/Kg is used in this study. Coal is burned in oxygen/CO2/H20 mixture with a composition of VC02+H20 /VO2 = 0 to 4. The results obtained in this study show clearly the benefit of burning coal with oxygen/CO2/H20 mixture compared to coal combustion with air. The CO2 emissions increases which will help to reduce the cost of CO2 capture, NOX emissions will also decrease because of the replacement of nitrogen in air by CO2/H20 mixture, and better devolatization and burnout of coal particles for coal combustion with oxygen/CO2/H20 mixture. In addition to that, with a CO2/H20 to oxygen volume ratio of 0.67, the gas temperature is the same as the gas temperature for coal combustion with air. No modifications of the combustor materials is required during the retrofitting of power plant with oxygen coal combustion systems.

Commentary by Dr. Valentin Fuster
2009;():161-171. doi:10.1115/HT2009-88493.

Transportation accidents and the subsequent fire present a concern. Particularly energetic accidents like an aircraft impact or a high speed highway accident can be quite violent. We would like to develop and maintain a capability at Sandia National Laboratories to model these very challenging events. We have identified Smoothed Particle Hydrodynamics (SPH) as a good method to employ for the impact dynamics of the fluid for severe impacts. SPH is capable of modeling viscous and inertial effects for these impacts for short times. We have also identified our fire code Lagrangian/Eulerian (L/E) particle capability as an adequate method for fuel transport and spray modeling. A fire code can also model the subsequent fire for a fuel impact. Surface deposition of the liquid may also be acceptably predicted with the same code. These two methods (SPH and L/E) typically employ complimentary length and timescales for the calculation, and are potentially suited for coupling given adequate attention to relevant details. Length and timescale interactions are important considerations when joining the two capabilities. Additionally, there are physical model inadequacy considerations that contribute to the accuracy of the methodology. These models and methods are presented and evaluated. Some of these concerns are detailed for a verification type scenario used to show the work in progress of this coupling capability. The importance of validation methods and their appropriate application to the genesis of this class of predictive tool are also discussed.

Topics: Fire , Modeling , Sprays
Commentary by Dr. Valentin Fuster
2009;():173-181. doi:10.1115/HT2009-88520.

A series of fire tests was conducted involving a 2.44-m-(8-ft)-diameter, 4.57-m-(15-ft)-long, 2.54-cm-(1-inch)-wall thickness pipe calorimeter suspended 1-m above a 7.93-m-diameter pool that contained 7.57 m3 (2000 gallons) of jet fuel. The wind conditions, calorimeter temperature, participating media temperature and speed, and radiant heat flux, were measured at several locations as functions of time in three fire tests. The first two had relatively light winds and lasted roughly 40 minutes, while the third had much stronger winds and consumed the fuel in 25 minutes. The purpose of this paper is to describe the experimental facilities and certain fire characteristics. The large amount of data acquired cannot be fully presented in this paper. A website is available by contacting the first author so that the full data set may be used to quantitatively benchmark large-fire simulations and models.

Commentary by Dr. Valentin Fuster
2009;():183-191. doi:10.1115/HT2009-88620.

The uniform flow rate is a fundamental requirement in the design of air distributors for the hydrogen reformer furnace. Constraints of flow rate primarily demands on configuration of air distributors. Particularly for the air with different temperature, velocity and pressure, an even distribution of air distributors is especially important. Air distributors containing one inlet and eleven outlets are connected with burners so that uniform flow rate of each outlet is required. Based on CFD (Computational Fluid Dynamics) method, temperature, velocity and pressure distribution in the air distributors are simulated. The results show that flow rate is sensitive to the rate of pressure and velocity change but not for temperature change. The maldistribution of each outlet cannot accord with engineering standard. So, it is necessary to take some methods to decrease the maldistribution of each outlet. The dampers exist at each outlet are controlled individually. Hence, the flow rate can be constrained by adjust pressure according to the proportion of maldistribution.

Topics: Furnaces , Hydrogen
Commentary by Dr. Valentin Fuster
2009;():193-199. doi:10.1115/HT2009-88624.

The air curtain can be adopted to slow down or stop smoke spread, in case of channel fires, with no influence of the normal usage of the channels and on the human evacuation. The flow field of air curtain jet from ceiling to floor was divided into 3 sections, i.e. the section of impinging mixing, non-isothermal jet and impinging floor and the theoretical analysis of fire-induced smoke control by air curtain in long channels was carried out. The formulas of the critical conditions for absolutely preventing smoke from intruding the protected side were obtained. With the Realizable k–ε model and the numerical solutions of the formulas as the initial and boundary conditions, an application example was simulated. Results showed that the three-section analysis method was reasonable and the simulated results verified the rationality of the theoretical prediction model.

Commentary by Dr. Valentin Fuster
2009;():201-209. doi:10.1115/HT2009-88634.

In a potential accident scenario with a solid-propellant fire, aluminum present in the propellant and in surrounding structures is exposed to high-temperature environments. The enthalpy present in the aluminum particles is a substantial component of the heat release, both in terms of the particle sensible energy and its chemical energy. This paper examines the consequences of the deposition of aluminum particles present in the propellant in terms of heat transfer to surfaces. Also examined is the possibility that deposited aluminum will ignite in the high-temperature oxidizing environment. The examination is made using a computational fluid dynamics approach with some new models to describe the aluminum oxidation. In addition, these models provide a means to predict the aluminum ignition criteria that will be discussed.

Commentary by Dr. Valentin Fuster

Heat Transfer in Multiphase Systems

2009;():211-219. doi:10.1115/HT2009-88032.

Industrial processes use mechanical draft cooling towers (MDCT’s) to dissipate waste heat by transferring heat from water to air via evaporative cooling, which causes air humidification. The Savannah River Site (SRS) has cross-flow and counter-current MDCT’s consisting of four independent compartments called cells. Each cell has its own fan to help maximize heat transfer between ambient air and circulated water. The primary objective of the work is to simulate the cooling tower performance for the counter-current cooling tower and to conduct a parametric study under different fan speeds and ambient air conditions. The Savannah River National Laboratory (SRNL) developed a computational fluid dynamics (CFD) model and performed the benchmarking analysis against the integral measurement results to accomplish the objective. The model uses three-dimensional steady-state momentum, continuity equations, air-vapor species balance equation, and two-equation turbulence as the basic governing equations. It was assumed that vapor phase is always transported by the continuous air phase with no slip velocity. In this case, water droplet component was considered as discrete phase for the interfacial heat and mass transfer via Lagrangian approach. Thus, the air-vapor mixture model with discrete water droplet phase is used for the analysis. A series of parametric calculations was performed to investigate the impact of wind speeds and ambient conditions on the thermal performance of the cooling tower when fans were operating and when they were turned off. The model was also benchmarked against the literature data and the SRS integral test results for key parameters such as air temperature and humidity at the tower exit and water temperature for given ambient conditions. Detailed results will be published here.

Topics: Cooling towers
Commentary by Dr. Valentin Fuster
2009;():221-229. doi:10.1115/HT2009-88047.

This study investigates heat flux performance for a LHP that includes a fractal based evaporator design. The prototype Fractal Loop Heat Pipe (FLHP) was designed and manufactured by Mikros Manufacturing Inc. and validation tested at NASA Goddard Space Flight Center’s Thermal Engineering Branch laboratory. Heat input to the FLHP was supplied via cartridge heaters mounted in a copper block. The copper heater block was placed in intimate contact with the evaporator. The evaporator had a circular cross-sectional area of 0.877 cm2 . Twice distilled, deionized water was used as the working fluid. Thermal performance data was obtained for three different Condenser/Subcooler temperature combinations under degassed conditions (Psat = 25.3 kPa at 22°C). The FLHP demonstrated successful start-ups in each of the test cases performed. Test results show that the highest heat flux demonstrated was 75 W/cm2 .

Commentary by Dr. Valentin Fuster
2009;():231-235. doi:10.1115/HT2009-88112.

This work investigates the subcooled flow boiling of aqueous based nanofluids in a 510 μm single microchannel with a focus on the effect of nanoparticles on the critical heat flux (CHF). The surface temperature distribution along the pipe, the inlet and outlet pressures and temperatures are measured simultaneously for different concentrations of alumina nanofluids and dionized water. The experiment shows a remarkable increase ∼ 31% in the CHF under very low nanoparticle concentrations (∼0.1v%) and a nonlinear influence of nanoparticles on the subcooled boiling heat transfer.

Commentary by Dr. Valentin Fuster
2009;():237-243. doi:10.1115/HT2009-88125.

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1×1.1 cm2 . At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–160 W/cm2 . Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dry-out appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance rose in response to the appearance and expansion of partial dry-out. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.

Commentary by Dr. Valentin Fuster
2009;():245-252. doi:10.1115/HT2009-88126.

In this paper the Navier-stokes equations for a single liquid slug have been solved in order to predict the circulation patterns within the slug. Surface tension effects on the air-water interface have been investigated by solving the Young–Laplace equation. The calculated interface shape has been utilized to define the liquid slug geometry at the front and tail interfaces of the slug. Then the effects of the surface tension on the hydrodynamics of the two-phase slug flow have been compared to those where no surface tension forces exist. The importance of the complex flow field features in the vicinity of the two interfaces has been investigated by defining a non-dimensional form of the wall shear stress. The latter quantity has been formulated based on non-dimensional parameters in order to define a general Moody friction factor for typical two-phase slug flows in microchannels. Moreover, the hydrodynamics of slug flow formation has been examined using computational fluid dynamics (CFD). The volume-of-fluid (VOF) method has been applied to monitor the growth of the instability at the air-water interface. The lengths of the slugs have been correlated to the pressure fluctuations in the mixing region of the air and water streams at an axisymmetric T-junction. The main frequencies of the pressure fluctuations have been investigated using the Fast Fourier Transform (FFT) method.

Commentary by Dr. Valentin Fuster
2009;():253-257. doi:10.1115/HT2009-88144.

From the viewpoint of protecting the ozone layer and preventing global warming, there is now strong demand for science and technology based on ecologically safe ‘natural’ working fluids. A CO2 refrigeration method has been proposed and developed several years ago, using CO2 solid-gas two phase fluid as refrigerant. Heat transfer of the CO2 solid-gas two phase flow in a horizontal tube is important to design of such a refrigeration system. In the present paper, an experiment work is conducted to measure its heat transfer characteristics in the horizontal tube. The results show an average value 310 W/(m2 -K) of heat convective coefficient is experimentally obtained, which is much higher than that of gas flow. In the sublimation area, the Nusselt number is observed to increase slowly along the tube length, the phenomena may be physically explained that the sublimation process changes the thermal boundary layer thickness; makes the flow turbulence stronger; or changes the flow and the pressure fields.

Commentary by Dr. Valentin Fuster
2009;():259-267. doi:10.1115/HT2009-88153.

A closed-loop two-phase microchannels cooling system using a micro-gear pump was built in this paper. The microchannels heat sink was made of oxygen-free copper, and 14 parallel microchannels with the dimension of 0.8mm(W)×1.5mm(D)×20mm(L) were formed by electric spark drilling followed by linear cutting which separated the channels from each other. The heat transfer performance was evaluated by the fluid temperature, the pressure drop across the micro-channels and the volumetric flow rate. Experiments were performed with refrigerant FC-72 which spanned the following conditions: initial pressure of Pin = 73 kPa, mass velocity of G = 94 – 333 kg/m2 s, outlet quality of xe,out = 0 – superheat and heat flux of q″ = 25–140 W/cm2 . The result showed that, the maximum heat flux achieved 96 W/cm2 , as the heating surface temperature was kept below 85 °C and critical heat flux occurred in the condition of low flow rate. Average two-phase heat transfer coefficients increased with the heat flux at low mass flux (G = 94 and 180 kg/m2 s) and all heat fluxes, high mass flux (G = 333 kg/m2 s) and all heat fluxes, and moderate mass fluxes (G = 224kg/m2 s) under low and moderate heat fluxes (q″ <110 W/cm2 for G = 224 kg/m2 s), which was a feature of nucleate boiling mechanism. Pressure drop through microchannels heat sink was found to be below 4kPa.

Commentary by Dr. Valentin Fuster
2009;():269-277. doi:10.1115/HT2009-88155.

Most energy conversion systems and cooling devices employ nucleate pool boiling because of its high efficiency of heat exchange. It is a liquid-vapor phase change process associated with ebullition, characterized by cyclic growth and departure of vapor bubbles from heated wall and greatly influenced by the bubble growth mechanism. Bubble dynamics is difficult to simulate due to the difficulty of tracking the liquid vapor interface without smearing it, the discontinuity in material properties due to high density ratio and the need to take surface tension into account that introduces a jump in the pressure field. This paper focuses on the accurate representation of surface tension effects on bubble dynamics in nucleate pool boiling. The complete Navier-Stokes equations are solved and liquid-vapor interface is captured using a conservative level-set technique, curvature of interface is computed using the level set function and surface tension forces are evaluated as a body force according to the continuum surface force method. This enables us to simulate flows with large density and viscosity differences, to capture the shape of the deforming interface of the bubble while maintaining good mass conservation. The ability of the model is demonstrated with the numerical example of a growing bubble.

Commentary by Dr. Valentin Fuster
2009;():279-290. doi:10.1115/HT2009-88165.

Measurements of space and time resolved subcooled pool boiling of pentane in earth gravity environments were made using a microscale heater array. Data from individual heater elements in the array were synchronized with bottom and side view images from two highspeed cameras. The bubble growth was primarily due to energy transfer from the superheated liquid layer. Transient conduction and/or microconvection was found to be the dominant heat transfer mechanism. A composite model consisting of microlayer evaporation and transient conduction was developed and compared with the experimental data.

Commentary by Dr. Valentin Fuster
2009;():291-300. doi:10.1115/HT2009-88180.

An experimental investigation on the effects of condenser temperatures, heating modes and heat inputs on a miniature, three dimensional flat-plate oscillating heat pipe (3D FP-OHP) was conducted visually and thermally. The 3D FP-OHP was charged with acetone at a filling ratio of 0.80, had dimensions of 101.60 × 63.50 × 2.54 mm3 , possessed 30 total turns, and had square channels on both sides of the device with a hydraulic diameter of 0.762 mm. Unlike traditional flat-plate designs, this new three-dimensional, compact design allows for multiple heating arrangements and higher heat fluxes. Transient and steady-state temperature measurements were collected at various heat inputs and the activation/start-up was clearly observed for both bottom and side heating modes during reception of its excitation power for this miniature 3D FP-OHP. The neutron imaging technology was simultaneously employed to observe the internal working fluid flow for all tests directly through the heat pipe’s copper wall. The activation was accompanied with a pronounced temperature field relaxation and the onset of chaotic thermal oscillations — all occurring with the same general oscillatory pattern at locations all around the 3D FP-OHP. Qualitative and quantitative analysis of these thermal oscillations, along with the presentation of the average temperature difference and thermal resistance, for all experimental conditions are provided. The novelty of the three-dimensional OHP design is its ability to still produce the oscillating motions of liquid plugs and vapor bubbles and, more importantly, its ability to remove higher heat fluxes.

Commentary by Dr. Valentin Fuster
2009;():301-310. doi:10.1115/HT2009-88217.

During the operation of parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow-rate pumping, with a pressure regulating tank, and a constant heating pre-heater, the system may experience severe alternations between these two states of boiling and non-boiling with a period of minutes. This cyclical system instability has been modeled. In the model, the existence of the liquid phase flow happens at conditions of inlet subcooling and low surface heat flux that the boiling inception is hard to occur. Accordingly, the system instability criteria are established in terms of a System binary states parameter S and a non-dimensional surface heat flux, which is normalized with the boiling incipient heat flux. This model has been validated experimentally.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2009;():311-315. doi:10.1115/HT2009-88227.

Microencapsulated phase change material (MPCM) suspensions have large specific heat due to the latent heat of the phase change material and enhance the convective heat transfer consequently. However low thermal conductivity of the phase change material diminishes the heat transfer performance of the MPCM suspensions. To improve the thermal conductivity of the MPCM suspensions, TiO2 nanoparticles were added into the MPCM suspensions to formulate a novel thermal fluid—nanoparticle compound microencapsulated phase change material suspensions. In this paper, the rheological characteristics and shear viscosities of such slurries using a Bolin CVO rheometer (Malvern Instruments) over a range of shear rate (5–500s−1 ), MPCM concentration (0–20wt%) and TiO2 nanoparticle concentration 0.5wt% at temperature (20°C–40°C). The result shows that the viscosities of NCMPCM suspensions are almost independent of the shear rate, indicating Newtonian fluid under the conditions of this work and the viscosities depend strongly on temperature which fits well with the VTF function. Based on the effective volume fraction method and Vand equation, two methods that predict the viscosity of nanoparticle compound microencapsulated phase change material suspensions was analyzed and the result shows that the prediction data the effective volume fraction method fit the measurements well.

Commentary by Dr. Valentin Fuster
2009;():317-326. doi:10.1115/HT2009-88240.

Evaporating fronts propagate through porous media during drying processes, underground coal gasification, geothermal energy production from hot dry rock, and around nuclear waste repositories. Present work will focus on the one-dimensional heat transfer at the interface between vapor saturated porous matrix and water saturated porous region and evaluate the conditions for which various approximations yield an accurate representation of front velocity. An implicit finite difference scheme is utilized to simulate the propagation of an evaporating front in a porous medium saturated with water and undergoing the phase change process. The assumption of local thermal equilibrium (LTE) which results in a one-equation model and a simple two-equation model that does not assume LTE are examined by comparison with a quasi-analytic numerical model. We consider the case for low Reynolds number, hence Nusselt number is assumed constant. Results illustrate that the one-equation model does not yield accurate results even if the length scale for diffusion in the solid phase is relatively small. The one-equation model predicts faster front propagation than the two-equation model. It is illustrated that the one-equation model yields satisfactory results only when thermophysical properties characterized by the volume weighted ratio of thermal diffusivities is reduced to an order of magnitude less than those for the applications of interest. In addition, consistent with the established “rule of thumb”, for Biot < 0.1, the traditional two-equation model which makes the lumped capacitance assumption for the solid phase compares well with a two-equation model that more accurately predicts the time dependent diffusion in the solid phase using Duhamel’s theorem.

Commentary by Dr. Valentin Fuster
2009;():327-333. doi:10.1115/HT2009-88257.

Biporous evaporator wicks, generated by sintering copper particles into semi-uniform clusters, were demonstrated to achieve high flux, heat transfer performance for use in heat pipes by Semenic (2007). The effective thermal conductivity of thick biporous wicks at high heat fluxes was found to be reduced because the region next to the wall dried out prematurely allowing the wall interface temperature to rise well above the saturation temperature. The region above the dried out portion of the wick continued to work with the large pores between the clusters being primarily occupied with vapor and the small pores between the particles being occupied with the liquid. In this work, we report our efforts to reduce the size of the wall-wick interface dry-out region by sintering a thin layer of uniform size particles on the wall as originally suggested in a thesis by Seminic (2007). The boiling curve for this “double layer” wick diverges from a standard “single layer” biporous wick at the point of nucleation by reducing the wall temperature, and concurrently the overall temperature drop across the wick needed to drive a given heat flux. The temperature drop across the wick is reduced because the thin layer of particles between the biporous wick and the wall reduces the wall-wick interface resistance and also provides additional capillary channels underneath the biporous wick. Experimental data supports this hypothesis by showing a clear divergence between measured wall temperatures for the double layer wick from its single layer counterpart. The presumed point of nucleation in both wicks is similar, with the heat flux increasing much more rapidly than the liquid superheat and it is clear that this slope is much steeper for the double layer wick. This finding has great potential to expand the performance capabilities of heat pipes and vapor chambers because the new double layered wick can transfer more heat with less superheat thereby increasing the effective thermal conductivity of the wick and decreasing the wall-wick interface temperature for a given heat flux.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2009;():335-343. doi:10.1115/HT2009-88314.

Jet/spray cooling is a way of efficiently removing heat from a hot surface like in the back plate for high power electronic devices. In the present work, a close loop liquid jet/spray setup with a novel effluent/spent liquid removal system is established for large area cooling. A 4×4 nozzle array is used to generate the spray/jet targeting the heat source area of 4 (2×2) cm2 . Flourinert FC-72™ is used as coolant liquid. The thermal performance data for multi-jet/spray cooling in a closed and confined system are obtained at various liquid temperatures and flow rates. The experimental work illustrates the multi-jet cooling system can reach critical heat fluxes up to 101 W/cm2 . The results showed that the heat flux is increased with liquid sub-cooled temperature and flow rates in single phase as well as in phase change zone. It is further observed that the critical heat flux (CHF) increases with liquid sub-cooled temperature and flow rates.

Commentary by Dr. Valentin Fuster
2009;():345-357. doi:10.1115/HT2009-88315.

This paper presents an experimental investigation on the co-current downward condensation of R134a inside a tube-in-tube heat exchanger. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The condensing temperatures are between 40 and 50°C, heat fluxes are between 9.78 and 50.69 kW m−2 . The temperature difference between the saturation temperature of refrigerant and inlet wall varies between 1.66–8.94°C. Condensation experiments are done at mass fluxes varying between 340 and 456 kg m−2 s−1 while the average qualities are between 0.76–0.96. The quality of the refrigerant in the test section is calculated considering the temperature and pressure measured from the test section. The pressure drop across the test section is directly measured by a differential pressure transducer. The average experimental heat transfer coefficient of the refrigerant is calculated by applying an energy balance based on the energy transferred from the test section. Experimental data of annular flow are examined such as the alteration of condensation heat transfer coefficient with the vapor average quality and temperature difference respectively according to different mass fluxes and condensing temperatures. The relation between the heat flux and temperature difference, besides this, the relation between the condensation heat transfer coefficient and condensing pressure are shown comparatively and the effects of mass flux and condensation temperature on the pressure drop are also discussed. The efficiency of the condenser is considered comparing with various experimental data according to tested condensing temperatures and mass fluxes of refrigerant. Some well known correlations and models of heat transfer coefficient were compared to show that annular flow models were independent of tube orientation provided that annular flow regime exists along the tube length and capable of predicting condensation heat transfer coefficient in the test tube.

Commentary by Dr. Valentin Fuster
2009;():359-365. doi:10.1115/HT2009-88340.

New experimental results have been obtained on the enhancement of heat transport by a pulsating heat pipe (PHP) using a self-rewetting fluid. Self-rewetting fluids have a property that the surface tension increases with temperature unlike other common liquids. The increasing surface tension at a higher temperature could cause the liquid to be drawn towards a heated surface if a dry spot appears, and improving boiling heat transfer. In the present experiments, 1-butanol was added to water at a concentration of less than 1 wt% to make the self-rewetting fluid. A pulsating heat pipe made from an extruded multi-port tube was partially filled with the butanol-water mixture and tested for its heat transport capability at different input power levels. The experiments showed that the maximum heat transport capability was enhanced by a factor of four when the maximum heater temperature was limited to 120 °C. Thus, the use of a self-rewetting fluid in a PHP has been shown to be highly effective in improving the heat transport capability of pulsating heat pipes.

Topics: Fluids , Heat pipes
Commentary by Dr. Valentin Fuster
2009;():367-373. doi:10.1115/HT2009-88353.

A numerical study was conducted on the spreading behavior of liquid drops on flat solid surfaces. The model predicts the shape of liquid-vapor interface under static equilibrium using an unstructured surface grid composed of triangular elements. Incremental movement of base contour, i.e. solid-liquid-vapor contact line, is also captured such that the constrained boundary conditions, i.e. advancing and receding contact angles, can be satisfied. The numerical model is applied to a common experiment that studies the behavior of liquid drops on inclined surfaces, where the shape of the drops change in response to an alteration of total volume or gravitational direction. On a heterogeneous surface that has contact angle hysteresis, the shape of the base contour on the solid surface is not determined uniquely but rather dependent upon history. This study demonstrates such dependence by comparing the spreading of a liquid drop on a solid surface with different quasi-equilibrium paths.

Commentary by Dr. Valentin Fuster
2009;():375-382. doi:10.1115/HT2009-88410.

Micro heat exchangers are emerging as one of the most effective cooling technologies for high power-density applications. The design of micro heat exchangers is complicated by the presence of alternating flow regimes, which give way to flow boiling instability. Bubble formation inside microchannels can be correlated directly to flow boiling instability and can regulate flow characteristics and wall heat transfer when the bubbles grow to reach the microchannel hydraulic diameter. In this study, the growth of vapor bubbles in a single microchannel was examined using an experimental setup capable of measuring coolant flow rate, inlet and outlet liquid temperatures, and channel wall surface temperature. Liquid flow rate and wall heat flux were systematically varied while a high-speed camera was used to capture images of vapor bubbles forming in the channel. These images were used to compare bubble growth rates for a constant flow rate. The results provide fundamental understanding of the bubble growth process.

Commentary by Dr. Valentin Fuster
2009;():383-388. doi:10.1115/HT2009-88418.

In the melting process of a packed bed of ice particles in an ice thermal storage tank, water channel is formed by non homogeneity melting. Water channel causes some problems such as the decrease of heat exchange rate and the increase of residual of ice. In this study, the generation of water channel during melting process of an ice packed bed was investigated. The experiment and numerical analysis of melting of an ice packed bed were performed in a two-dimensional model. In this model, water at 0°C flows through the ice packed bed continuously. In order to realize the non homogeneous melting, the ice packed bed was intentionally melted by a line heat source. In the experiment, the shape of melting front in the ice packed bed was observed on some typical conditions. The effects of the permeability and porosity distribution in the ice packed bed and the flow velocity of water on the shape of melting front were investigated in the numerical analysis. As a result, it was found that the growth rate of water channel is affected by the permeability in the ice packed bed and the flow velocity of water. It was also found that the results of the numerical analysis considering the porosity distribution of the packed bed correspond to the experimental results.

Commentary by Dr. Valentin Fuster
2009;():389-397. doi:10.1115/HT2009-88428.

The problem of elevated heat flux in modern electronics has led to the development of numerous liquid cooling devices which yield superior heat transfer coefficients over their air based counterparts. This study investigates the use of liquid/gas slug flows where a liquid coolant is segregated into discrete slugs, resulting in a segmented flow, and heat transfer rates are enhanced by an internal circulation within slugs. This circulation directs cooler fluid from the center of the slug towards the heated surface and elevates the temperature difference at the wall. An experimental facility is built to examine this problem in circular tube flow with a constant wall heat flux boundary condition. This was attained by Joule heating a thin walled stainless steel tube. Water was used as the coolant and air as the segregating phase. The flow rates of each were controlled using high precision syringe pumps and a slug producing mechanism was introduced for segmenting the flow into slugs of various lengths at any particular flow rate. Tube flows with Reynolds numbers in the range 10 to 1500 were examined ensuring a well ordered segmented flow throughout. Heat transfer performance was calculated by measuring the exterior temperature of the thin tube wall at various locations using an Infrared camera. Nusselt number results are presented for inverse Graetz numbers over four decades, which spans both the thermally developing and developed regions. The results show that Nu in the early thermally developing region are slightly inferior to single phase flows for heat transfer performance but become far superior at higher values of inverse Gr. Additionally, the slug length plays an important role in maximizing Nusselt number in the fully developed region as Nu plateaus at different levels for slugs of differing lengths. Overall, this paper provides a new body of experimental findings relating to segmented flow heat transfer in constant heat flux tubes without boiling. Put abstract text here.

Commentary by Dr. Valentin Fuster
2009;():399-407. doi:10.1115/HT2009-88446.

Addition of surfactants to liquids helps to eliminate intermittent two-phase flow patterns and alleviate flow instability. These features are very desirable for two-phase microfluidic applications. However, very little information is available on two-phase flow patterns of surfactant solution in the microchannels. The present paper reports a study of adiabatic two-phase flow with surfactants in a circular microchannel of a 180-μm diameter. Air-water mixtures with trace quantities of sodium dodecyl sulfate (SDS) were used in the experiments. The maximum superficial velocities measured were 4 m/s for the liquid and 65 m/s for the gas. High-speed photographic technique was employed to visualize various two-phase flow patterns and to identify the transition boundaries between different flow regimes. The results were compared to data obtained from air-water flow without surfactants. It was found that addition of surfactants brings in significant modification to the two-phase flow regimes as well as their transition characteristics in microchannels; in particular, slug flow is effectively suppressed.

Commentary by Dr. Valentin Fuster
2009;():409-416. doi:10.1115/HT2009-88454.

Numerical analysis was conducted for a heat pipe application in a metal hydride (MH) reactor for hydrogen gas storage. The hydriding and dehydriding characteristics of MH strongly depend on temperature and pressure. Due to its extremely low thermal conductivity however, it is very difficult to control the temperature of MH, especially when it is of vast bulk as in an MH reactor. This study deals with heat pipes embedded into the MH to increase the effective thermal conductivity of the system and thus to enhance the thermal control characteristics. The existing model was a brine-tube type MH reactor having cylindrical container with outer diameter of 76 mm and length of 1 m, which was partially filled with 8 to 10 kg of MH material. The hydriding and dehydriding processes occur at 10°C and 80°C, respectively. The heat-pipe type reactor model replaced the brine tubes and channels with copper-water heat pipes of the same dimensions. Three-dimensional numerical analysis predicted that the heat-pipe type MH reactor model enhanced thermal performance with faster response to the change of boundary conditions and higher degree of isothermal characteristics. Discussion is presented based on the numerical results of the two models compared with experimental results.

Commentary by Dr. Valentin Fuster
2009;():417-424. doi:10.1115/HT2009-88480.

The heat and mass transfer in the condenser region of a variable conductance thermosyphon, consisting of two components (R11 + R113) has been studied and special attention has been devoted to pressure drop during reflux condensation. The mass, energy, and species conservation equations in conjunction with the overall mass conservation and continuity of momentum at liquid-vapor interface constraints and the thermodynamic equilibrium condition have been solved numerically by use of the integral method. In contrast to the flat-front model which assumes a sharp interface between the active and shut-off portions in a variable conductance thermosyphon, in this paper a continuous model has been used. In this model a continuous variation of physical properties with condensation of both components along condenser is assumed. The results of the present study have been compared with available numerical and experimental results of other investigators and pressure gradient profiles have been achieved. A calculation of the frictional, accelerational and gravitational components of the pressure drop shows that the gravitational component has the greatest magnitude due to the relatively high density of the vapor.

Commentary by Dr. Valentin Fuster
2009;():425-434. doi:10.1115/HT2009-88487.

A new heat transfer model for stratified flow boiling in a horizontal tube is proposed in this present study. In recent years, the subject of nonlinear dynamics has progressed and various tools of analysis have been proposed for complex systems. Coupled Map Lattice (CML) method is one such tool which makes it possible to simulate complex systems and to capture the qualitative nature of the phenomenon. In this work, steady stratified flow boiling of water is simulated qualitatively by using the CML model for laminar, hydrodynamically and thermally developing flow and heat transfer in a horizontal tube. The liquid enters in a constant wall temperature tube (Tw*>Tsat* at Pentrance*) in a subcooled or saturated condition. The present modeling by CML is based on the assumption that the flow boiling is governed by nucleation from cavities on the heated surface, migration of vapor into the core, forced convection and phase change in the bulk. The macroscopic variable chosen is temperature. The influences of mass flow rate, inlet subcooling and wall temperature have been studied. The results of the computations provide information on the effect of aforementioned parameters on the heat transfer coefficient and void fraction. The results show that the CML has been able to model flow boiling in a realistic manner.

Commentary by Dr. Valentin Fuster
2009;():435-444. doi:10.1115/HT2009-88495.

Passive condenser systems are used in a number of industrial heat transfer systems. Passive containment cooling system (PCCS) which is composed of a number of vertical heat exchanger serves as an engineered safety system in an advanced boiling water reactor. The PCCS condenser must be able to remove sufficient energy from the reactor containment to prevent containment from exceeding its design pressure. Experiments were designed to simulate the PCCS condensation with a tube bundle. Scaling analysis was performed to scale down the prototype PCCS with a tube bundle consisting of four tubes. The tubes in the bundle were of prototype height (1.8 m) and diameter (52.5 mm) and the operating conditions and boundary conditions such as the operating pressure, secondary cooling system were designed to represent prototype conditions. Steam condensation tests were carried out in complete condensation mode where all the steam entering the condenser bundle is completely condensed. Condensation heat transfer coefficients (HTC) were obtained for various steam flow rate. The condensation pressure depended on the inlet steam flow rate which happens to be the maximum condensation rate for the given test pressure. Data on condensation heat transfer were obtained for primary pressure raging from 110–270 kPa. The tube bundle condensation heat transfer rates were compared with single tube heat transfer rates from previous work. The results showed that the condensation heat transfer coefficient for the tube in bundle was comparable with single tube, however the secondary side heat transfer coefficients for the tubes in bundle was higher than for the single tube. Condensation heat transfer for tube in bundle ranged from 7500 W/ m2K to 20,000 W/ m2K for the range of pressure studied. A heat and mass analogy model was developed and the condensation heat transfer prediction from the model was compared with experimental data.

Topics: Condensation
Commentary by Dr. Valentin Fuster
2009;():445-453. doi:10.1115/HT2009-88496.

As the power and heat output of modern CPUs climb ever higher and the interest in compact, passively cooled devices grows, there is an urgent need for thinner and more effective vapor chamber technologies. Nanostructured wick technologies based on oxide and organic nanowires have been proposed as a method of improving heat pipe performance in such applications. This work performs finite difference simulations of a 2D heat pipe accounting for variable porosity in the wick. For heat fluxes of 10 and 100 W/cm2 , we find that temperature difference between the evaporator and condenser regions decreases by 10%, which is promising for spreading thermal energy. We find that spatially varying porosity yields improvements in spreading heat throughout the entire wick region. Finally, we observe that boiling is depressed in the evaporator region. These results verify the benefits of nanostructured wicks. This simulation tool provides the groundwork for future studies of 3D flat package heat pipes.

Commentary by Dr. Valentin Fuster
2009;():455-464. doi:10.1115/HT2009-88497.

One of the engineered safety systems in the advanced boiling water reactor is a passive containment cooling system (PCCS) which is composed of a number of vertical heat exchanger. After a loss of coolant accident, the pressurized steam discharged from the reactor and the noncondensable (NC) gases mixture flows into the PCCS condenser tube. The PCCS condenser must be able to remove sufficient energy from the reactor containment to prevent containment from exceeding its design pressure. The efficient performance of the PCCS condenser is thus vital to the safety of the reactor. In PCCS condenser tube three flow conditions are expected namely the forced flow, cyclic venting and complete condensation modes. The PCCS test facility consists of steam generator (SG), instrumented condenser with secondary pool boiling section, condensation tank, suppression pool, storage tank, air supply line, and associated piping and instrumentation. The specific design of condensing tube is based on scaling analysis from the PCCS design of ESBWR. The scaled PCCS is made of four tubes of diameter 52.5mm and height 1.8 m arranged in square pitch. Steam air mixture condensation tests were carried out in a through flow mode of operation where the mixture flows through the condenser tube with some steam condensation. Data on condensation heat transfer were obtained for two nominal pressures, 225 kPa and 275 kPa and for air concentration fraction from 0 to 13%. Test results showed that with increase in pressure the condensation heat transfer increased. The presence of the air in the steam decreased the condensation heat transfer coefficient from 10 to 45% depending on air fraction in the steam.

Topics: Condensation
Commentary by Dr. Valentin Fuster
2009;():465-469. doi:10.1115/HT2009-88509.

NASA plans human exploration near the South Pole of the Moon, and other locations where the environment is extremely cold. This paper reports on the heat transfer performance of a loop heat pipe exposed to extreme cold under the simulated reduced gravitational environment of the Moon. A common method of spacecraft thermal control is to use a loop heat pipe with ammonia working fluid. Typically, a small amount of heat is provided either by electrical heaters or by environmental design, such that the loop heat pipe condenser temperature never drops below the freezing point of ammonia. The concern is that a liquid-filled, frozen condenser would not re-start, or that a thawing condenser would damage the tubing due to the expansion of ammonia upon thawing. This paper reports the results of an experimental investigation of a novel approach to avoid these problems. The loop heat pipe compensation chamber is conditioned such that all the ammonia liquid is removed from the condenser and the loop heat pipe is non-operating. The condenser temperature is then reduced to below that of the ammonia freezing point. The loop heat pipe is then successfully re-started.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster
2009;():471-480. doi:10.1115/HT2009-88514.

In nucleate boiling as the heat flux from the wall to the fluid is increased the heat transfer coefficient initially increases. At a sufficiently high heat flux called the critical heat flux (CHF) the heat transfer mechanism suddenly becomes less effective resulting in a rapid jump in wall temperature. In bubbly subcooled (or near-subcooled) conditions the CHF mechanism is referred to as departure from nucleate boiling. Departure from nucleate boiling (DNB) refers to the transition from nucleate boiling where liquid contacts the wall to film boiling in which a vapor layer contacts the wall. Various hypotheses have been used in modeling and predicting CHF. High speed video images of boiling water flows taken at Bettis Laboratory at the critical heat flux visually captured sufficient evidence of the DNB mechanism that improved insight into DNB modeling may be possible. This paper summarizes high speed video image analysis and the development of a new DNB critical heat flux model based on the image analysis findings. Using short window averages of image data, a significant increase in transmitted light intensity is seen near the wall just prior to CHF. The increase suggests that at CHF there is a transient reduction in the interfacial area concentration, ai , or bubble number density near the wall. This is believed to be the result of a sudden increase in bubble coalescence rates near the wall. The increase in coalescence rates results in a reduction in the interfacial area concentration causing it to reach a maximum at CHF. This near-wall maximum in ai at CHF under flow boiling conditions is consistent with recent pool boiling data in the literature. The image based observations motivated development of an interfacial area based CHF model to predict the maximum in the interfacial area concentration at CHF. The model predicts that a critical nucleation site density or a near-wall critical void fraction can be used as a DNB CHF criterion. This is a valuable simplification that can be directly implemented in three-dimensional thermal hydraulic codes. The critical nucleation site density result was used as an input to a simple wall heat transfer partition model to predict the critical heat flux. The model relies on correlation based estimates for the superheat temperature, bubble departure diameter, and bubble departure frequency. Model predictions are compared to CHF values taken from Groeneveld’s 2006 CHF look-up table.

Commentary by Dr. Valentin Fuster
2009;():481-490. doi:10.1115/HT2009-88515.

A fundamental departure from nucleate boiling (DNB) flow visualization experiment was designed to obtain a better understanding of flow boiling by visually capturing the mechanisms leading up to and including DNB for subcooled vertical flow boiling. At the critical heat flux (CHF) the heat transfer coefficient between the wall and fluid is greatly reduced, entering an inefficient heat transfer region that can cause a rapid increase in wall temperature. Most of the visual data on DNB in the open literature comes from experiments conducted with refrigerants or with water at relatively low pressure. One goal of this test was to capture high-quality photographs leading up to DNB for water at higher pressures, higher mass fluxes, and larger inlet subcooling than most of the data in the open literature. The fundamental DNB experiment consisted of three different run stages: incipient boiling, subcooled boiling, and CHF runs, which were intended to capture the behavior leading up to and including a departure from nucleate boiling crisis. At high heat flux conditions, the steep temperature and refractive-index gradients in the water near the wall act like lenses and bend the light away from the wall, which is the region of interest for discerning the DNB mechanism. By frosting the inner surface of the window on the light source side, the nearly collimated light was diffused as it entered the test section and enabled better visualization near the wall region. A high speed camera was used in testing. A typical run consisted of a 2 second image data set, with a resolution of 512 by 160 pixels, at 10,000 frames per second. Three excursive CHF runs were achieved, the last of which melted the test section. The trigger function on the camera captured images from before and after the power trip for the last CHF run. A trend can be seen of an increasing two-phase friction factor with power that begins to increase more rapidly at test section powers greater than 64.5kW. The 1995 Groenevel, et al. (1996) look-up table proved to be a good estimate of the heat flux at DNB.

Commentary by Dr. Valentin Fuster
2009;():491-495. doi:10.1115/HT2009-88516.

Dropwise condensation has shown the ability to increase condensation heat transfer coefficients by an order of magnitude over filmwise condensation. In standard dropwise condensation, liquid droplets forming on a sub-cooled nonwetting surface are removed from the surface by gravitational forces when the droplets reach a critical mass. The dependence on gravity for liquid removal limits the utilization of dropwise condensation in low gravity aerospace applications and horizontal surfaces. Presented in this study is a novel passive mechanism to remove droplets from a condensing surface using a surface energy gradient (wettability gradient) on the condensing surface. The wettability gradient creates a difference in contact angle across droplets condensing on the surface. The difference in contact angle across the droplets causes motion of the droplets to regions of increased wettability, without relying on additional forces. The movement of droplets away from the surface prevents flooding and allows for the condensation of new droplets on the surface. This paper presents an overall description of the wettability gradient mechanism and experimental condensation data acquired on surfaces with wettability gradients. A mechanism for creating the wettability gradients is also described, which involves varying the surface concentration of hydrophobic molecules through a self-assembled monolayer process.

Commentary by Dr. Valentin Fuster
2009;():497-503. doi:10.1115/HT2009-88525.

Loop Heat Pipes (LHPs) are two-phase devices that can passively transport heat over long distances relative to other passive two phase systems such as heat pipes. Most of the art of LHP fabrication is in the primary and secondary wick. The manufacturing steps for an LHP are described, including the tests to validate the LHP during manufacture. The tests include wick property testing (pore size, permeability, and thermal conductivity), secondary wick testing, and parallel flow balance design and testing. The required tests after the LHP is fabricated include low power starts, shutdown through compensation chamber heating, unbalanced condenser temperature tests, transient testing — both power cycling and condenser temperature changes, and maximum power tests.

Commentary by Dr. Valentin Fuster
2009;():505-510. doi:10.1115/HT2009-88536.

A new criterion has been developed to predict the onset of liquid (heavier fluid) entrainment from a stratified two-phase region through single and dual branches mounted on a vertical wall. This criterion was based on the local instability of the interface between two fluids due to the suction effect associated with the discharging of the lighter fluid. To validate the criterion, a three-dimensional model has been developed to predict the critical height at the onset of liquid entrainment. Comparisons between the theoretical critical heights with the available experimental data demonstrated a very good concurrence between the predicted and the measured values for both single and dual branches. This indicated that the onset of liquid entrainment mechanism occurs due to local flow instability of the interface analogous to Taylor instability.

Topics: Bifurcation
Commentary by Dr. Valentin Fuster
2009;():511-516. doi:10.1115/HT2009-88552.

Numerous methods, such as sintered or machined surface, have been proposed to enhance the boiling performance. On the other hand, nanofluids are also reported to increase the convective heat transfer and become attractive in many engineering applications. In this study, anodizing was tried to produce porous structure on the heater surface to enhance the boiling performance. Two electrolyes, phosphoric acid and oxalic acid, were used. It was found the anodized pores prepared by oxalic acid were smaller. However, the phosphoric acid anodized surface exhibited a CHF increment by 40% with almost no superheat increment. A nanofluid pre-boiling process was also proposed. An aluminum wire was used as the heater to boil nanofluid for a certain time at a certain heat flux. 0.1 wt% TiO2 nanofluid was used as the working fluid during the pre-boiling process. The wire then went on being the heater for pure water. It is found that aluminum wires exhibit boiling enhancement after preboiling in nanofluid. The most effective process heat flux and duration were found experimentally. The experimental results showed an 11° decrease of the surface superheat. According to SEM photos, two layers were deposited on the aluminum surface. The top layer is more like clusters of deposited nanoparticles, and the layer could be dissolved into water when perform subsequent pure water boiling. The other layer located beneath is more condensed and reliable. It is believed that the second layer could be the main mechanism to exhibit the boiling enhancement.

Commentary by Dr. Valentin Fuster
2009;():517-526. doi:10.1115/HT2009-88598.

In this paper the interfacial characteristics of a liquid flowing over a 1cm2 array of hydrophobic cylindrical micropillars located within a microchannel are investigated. The microchannel was 12mm wide and 32mm long with an average channel height of approximately 83μm. Hydrophobic coating of the channel was achieved via a controlled flow of a trichlorosilane and ethanol solution. A method to remove lodged gas bubbles from a microchannel was successfully demonstrated, while maintaining the favorable Cassie-Baxter wetting state (gas/vapor layer present) of the micropillar structures. This was achieved using degassed water to dissolve low-curvature gas bubbles, while ohmically heating the silicon substrate to reassert and maintain the Cassie-Baxter wetting state of the hydrophobic micropillars. During this experimentation it was discovered that the part wetting and dewetting of a superhydrophobic (SH) surface within a microchannel could be achieved using similar methods. The onset of surface wetting (Wenzel wetting state) was achieved by pumping degassed water through the microchannel. Surface dewetting was then accomplished through substrate heating by the increase in the trapped gas layer pressure, the water vapor pressure and outgassing from the lightly degassed fluid. These reactions force the gas/vapor layer to expand laterally throughout the micropillar array, thus restoring the Cassie-Baxter wetting state. The reported results demonstrate a low-power method for effectively reversing the Wenzel wetting state of a SH surface under microchannel flow conditions and may prove to be a useful technique for manipulating fluid flow within microfluidic devices.

Topics: Wetting
Commentary by Dr. Valentin Fuster
2009;():527-535. doi:10.1115/HT2009-88626.

In the current work, the mixing of a diffusive passive-scalar, e.g., thermal energy or species concentration, driven by electro-osmotic fluid motion being induced by an applied potential across a micro-channel is studied numerically. Secondary time-dependent periodic or random electric fields, orthogonal to the main stream, are applied to generate cross-sectional mixing. This investigation focuses on the mixing dynamics and its dependence on the frequency (period) of the driving mechanism. For periodic flows, the probability density function (PDF) of the scaled passive scalar (i.e., concentration), settles into a self-similar curve showing spatially repeating patterns. In contrast, for random flows there is a lack of self-similarity in the PDF for the interval of time considered in this investigation. The present study confirms an exponential decay of the variance of the concentration for the periodic and random flows.

Topics: Scalars
Commentary by Dr. Valentin Fuster
2009;():537-546. doi:10.1115/HT2009-88637.

Ice storage is currently the dominant cooling energy storage method. To more effectively utilize natural, renewable cooling sources, such as evaporative cooling and sky-radiative cooling, diurnal storage media operated on daily basis at the temperate range between 10 and 20 °C are the most desirable. In this paper, we presented our experimental investigation of micro-encapsulated paraffin slurry as cooling storage media for building cooling applications. The water slurry of micro-encapsulated N-hexadecane with a melting temperature of 18 °C were cooled to 5 °C and heated to 25 °C cyclically in a storage tank of 230 litre, and it was observed that full latent heat storage can only be realized at 5 °C due to supercooling, and the effective cooling storage capacity at the cooling temperature between 5 and 18 °C are obtained, which can be used to for cooling storage system design with various passive cooling possibilities.

Commentary by Dr. Valentin Fuster
2009;():547-560. doi:10.1115/HT2009-88644.

Basic results are considered of aerohydrodynamic and thermophysical experiments, in which secondary tornado-like jets (TLJ ) are revealed and investigated. These jets are self-organized under conditions of flow past surfaces with three-dimensional recesses (dimples) with a second-order curvilinear surface (TLJS – tornado-like jet surface). Exact solutions are given of unsteady-state Navier–Stokes and continuity equations, which describe the TLJ. The impact is considered, which is made on the flow in dimple by forces forming a flow of new type with built-in secondary tornado-like jets. These forces are absent in the case of flow past initially smooth surfaces. The problems are discussed of reducing the aerohydrodynamic drag on the TLJS , of enhancing the heat and mass transfer with the level of hydraulic loss lagging behind the degree of enhancement, of increasing the critical heat loads under conditions of boiling and supercritical flows of continuous medium past the TLJS , of preventing cavitation damage to the TLJS in hydraulic apparatuses, of reducing the adsorption of foreign matter on these surfaces, of reducing the friction between TLJS rubbing against one another, and of raising the efficiency of facilities for tornadolike conversion of energy of renewable low-potential sources. It is demonstrated that the obtained exact solutions of Navier–Stokes and continuity equations provide an adequate model of generation and evolution of swirling flow of blood in human blood circulation system, which enables one to proceed to development of safe and effective devices for substitution of organs in cardiac surgery. An inference is made about the universality of the flow of new type for raising the efficiency of technologies involving flows of various media.

Commentary by Dr. Valentin Fuster

Heat Transfer in Transport Phenomena in Manufacturing and Materials Processing

2009;():561-577. doi:10.1115/HT2009-88041.

Spherical glass and copper beads have been used to create bead packed porous structures for an investigation of two-phase heat transfer bubble dynamics under geometric constraints. The results demonstrated a variety of bubble dynamics characteristics under a range of heating conditions. At low heat flux of 18.9 kW/m2 , a single spherical bubble formed at nucleation sites of a heating surface and departed to the interstitial spaces of porous structure. When heat flux increased to 47 kW/m2 , a single bubble grew into a Y shape between beads layers and connected with others to generate a horizontal vapor column. As heat flux reached 76.3 kW/m2 , vertical vapor columns obtained strong momentum to form several major vapor escaping arteries, and glass beads were pushed upward by the vapor in the escaping arteries. According to Zuber’s hydrodynamics theory, choking will take place when the size of vapor columns reaches a certain value that is comparable to the critical hydrodynamic wavelength of the vapor column in plain surface pool boiling. The experimental and simulation results of this investigation illustrated that, under the geometric constrains of bead packed porous structures, similar characteristics had been induced to trigger the earlier occurrence of vapor column chocking inside porous structures. The bubble generation, growth, and detachment during the nucleate pool boiling heat transfer have been filmed, the heating surface temperatures and heat flux were recorded, and theoretical models have been employed to study bubble dynamic characteristics. Computer simulation results were combined with experimental observations to clarify the details of the vapor bubble growth process and the liquid water replenishing the inside of the porous structures. This investigation has clearly shown, with both experimental and computer simulation evidence, that the millimeter scale bead packed porous structures could greatly influence pool boiling heat transfer by forcing a single bubble to depart at a smaller size as compared to that in a plain surface situation at low heat flux situations, and could trigger the earlier occurrence of critical heat flux (CHF) by trapping the vapor into interstitial space and forming a vapor column net. The results also proved data for further development of theoretical models of pool boiling heat transfer in bead packed porous structures.

Commentary by Dr. Valentin Fuster
2009;():579-587. doi:10.1115/HT2009-88068.

Low temperature sintering of metal nanoparticle (NP) inks is a promising technique in realizing large area and flexible electronics. Previous research efforts on laser patterning of nanoparticle-laden materials in micro-fabrication as well as potential applications in nano-fabrication have sparked great interest in understanding the fundamental phenomena involved in nanoscale phase change and coalescence transformation. Molecular dynamics simulation has been applied to study nanoparticle sintering behavior. Optical methods including in-situ pump and probe technique and spectroscopic ellipsometry have been employed to study the characteristics of nanoparticle sintering and coalescence including coalescence time, temperature history and percolation transition, etc.

Commentary by Dr. Valentin Fuster
2009;():589-594. doi:10.1115/HT2009-88095.

A modified 3-ω method applied to a suspended platinum microwire was employed to measure the thermal conductivity and convective heat transfer coefficient of water-based single-walled carbon nanotubes (CNT) solution, and an expression for calculating the convective heat transfer coefficient in a free convective fluid was introduced. The measurement technique was validated for three model systems including vacuum, air, and deionized water. It is found that there is excellent agreement of these three model systems with theoretical predictions. In addition, the frequency dependence on the third harmonic response measured in deionized water reveals existence of a very low working frequency below 60 mHz. The thermal conductivity and convective heat transfer coefficient of a nanofluid (water-based single wall CNTs colloidal suspension) were determined to be 0.73±0.013 W/m·K and 14900±260 W/m2 ·K respectively, which corresponds to enhancement of 19.4% in thermal conductivity and 18.9% in convective heat transfer as compared to water.

Commentary by Dr. Valentin Fuster
2009;():595-599. doi:10.1115/HT2009-88124.

Even though a large number of applications for multiwalled carbon nanotubes have been proposed, there is relatively limited knowledge about the optimal conditions in which to create multiwalled carbon nanotubes (MWNTs). Computational models have been shown to be a promising tool to determine the best carbon nanotube growth conditions. In this paper the growth of MWNTs in a tube flow CVD reactor was studied through the use of the commercial software package COMSOL, where details steps have been described to reformulate an existing single walled carbon nanotube (SWNT) growth model to accommodate MWNTs followed by validation and growth rate prediction. Higher growth rates were predicted for MWNTs than SWNTs which is a result of the increase in pathways for carbon to form carbon nanotubes based on the additional walls. Results indicate that selecting the correct number of walls can be important to the results of the model.

Commentary by Dr. Valentin Fuster
2009;():601-606. doi:10.1115/HT2009-88130.

Nanosecond laser ablation is studied using a theoretical model combined with experimental data from laser ablation of metal films. The purpose of the research is to obtain the recoil pressure boundary condition resulting from explosive phase change. The ablation experiments are performed using a Nd:YAG laser of 1064 nm wavelength and 7 ns pulse width at full width half maximum. Three samples, 200 and 1000 nm aluminum films and 1000 nm nickel films, are used in the experiments. The transient shock wave positions are obtained by a time-resolved shadowgraph technique. A N2 -laser pumped dye laser with 3 ns pulse width is used as an illumination source and is synchronized with the ablation laser to obtain the transient shock wave position with nanosecond resolution. The transient shock position is used in a model for finding the shock wave speed as well as the pressure, temperature, and velocity just behind the shock wave. A power law is used for fitting curves on the experimentally obtained shock wave position. Knowing the shock wave position, the normal shock equations are used to calculate the thermo-fluid properties behind the shock wave. The solutions are compared with the Taylor-Sedov solution for spherical shocks and the reason for the deviation is described. The thermo-fluid property results show similar trends for all tested samples. The results show that the Taylor-Sedov solution under-estimates the pressure behind the shock wave when compared to the normal shock results.

Commentary by Dr. Valentin Fuster
2009;():607-616. doi:10.1115/HT2009-88142.

The present work presents how scaling analysis can be applied into multiphysics and multicoupled problems related to welding processes. The formation of the weld pool surface depression in high current and velocity Gas Tungsten Arc Welding (GTAW) is dominated by the gas shear acting on the weld pool. Considering this dominant force the weld penetration was estimated and compared to experimental results. Plastic deformation and heat flow are coupled phenomena in Friction Stir Welding (FSW), the maximum temperature was estimated using scaling analysis and compared with experimental and numerical results reported in the literature. Although the simplicity of the scaling models, they are capable of capturing correct trends and order of magnitudes of the unknown estimations in a problem. Moreover, they are capable of determining the dominant forces that act on the process studied.

Commentary by Dr. Valentin Fuster
2009;():617-625. doi:10.1115/HT2009-88220.

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The microduct is of diameter of around 200μm and the flow takes place at turbulent Reynolds number. The outer wall temperature of microduct and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and varying liquid mass flux. The experimental results shows that the average Nusselt numbers for the short micro-duct are higher than those predicted by conventional correlations for large diameter ducts. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.

Commentary by Dr. Valentin Fuster
2009;():627-630. doi:10.1115/HT2009-88223.

Multi-wall carbon nanotube (MWCNT)-reinforced nickel composites have been manufactured in a bulk form by using a laser deposition technique, commercially known as the laser engineered net shaping (LENS™) process. These nanocomposites have been characterized in detail by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and high-resolution TEM study has also been conducted on these nanocomposites to characterize the nanotube/metal matrix interface. In addition, the thermal conductivities of Ni/CNT composites deposited by the LENS™ process have been measured by using Fourier Law of conduction in vacuum. The measurement did not show enhancement of thermal properties, which is caused by the inherent formation of voids and carbide formed during the LENS™ process.

Commentary by Dr. Valentin Fuster
2009;():631-638. doi:10.1115/HT2009-88225.

Nano-patterns are generated on semiconducting and metallic surfaces through coupling an apertured near field scanning optical microscope (NSOM) with a pulsed laser source in this study. To understand the dominant mechanisms for the generation of the nano-patterns, a series of experimental measurement of the size and shape of nano-patterns generated on targets under different experimental conditions with different targets is conducted. The characteristic dimensions of nano-patterns show dependence on optical properties of the target material. The qualitative trend of the variation of nano-patterns as a function of laser and material conditions indicates that the dominant mechanisms for the generation of nano-patterns through a combination of nanosecond laser and an apertured NSOM under different conditions studied is near field laser-material interaction.

Commentary by Dr. Valentin Fuster
2009;():639-644. doi:10.1115/HT2009-88265.

In this study, the thermal transport issues for a nanocomposite material used in the blow molding process are addressed in the fabrication of the composite mold. For low production cycles, there is a significant interest in using lower cost composite molds to replace the expensive traditional metal molds used for making polymer parts by the blow molding process. A critical issue in using a polymer matrix composite as an alternative to a metal mold is the large difference in the thermal transport property. The composite mold design must integrate enhanced cooling so that the product can cool sufficiently within each cycle time. Nanocomposites that use carbon nanofiber offer improvements in thermal and mechanical properties; therefore they are potential candidates for making molds for polymer products. This paper describes the design of the cooling system for a nanocomposite blow mold using numerical simulations; and the processing steps by which the design is incorporated in the fabrication of the mold.

Commentary by Dr. Valentin Fuster
2009;():645-654. doi:10.1115/HT2009-88321.

A combined experimental and analytical investigation of single proton exchange membrane (PEM) fuel cells, during cold start has been conducted. The temperature influence on the performance of a single PEM fuel cell and the cold start failure of the PEM fuel cell was evaluated experimentally to determine the failure mechanisms and performance. The voltage, current and power characteristics were investigated as a function of the load, the hydrogen fuel flow rate, and the cell temperature. The characteristics of cold start for a single PEM fuel cell were analyzed, and the various failure mechanisms explored and characterized. In an effort to better understand the operational behavior and failure modes, a numerical simulation was also developed. The results of this analysis were then compared with the previously obtained experimental results and confirmed the accuracy of the failure mechanisms identified.

Commentary by Dr. Valentin Fuster
2009;():655-660. doi:10.1115/HT2009-88384.

Automotive industry frequently needs to test new products, according to different production parameters, in order to determine the actual thermal behavior of bodies before mass production is implemented. Numerical simulation of these processes can reduce the very expensive and time consuming experimental procedures. For the drying and hardening process of the top paint applied in the coating process, the body temperature must be raised according to the paint manufacturer regulations. Consequently, prediction of temperature distribution of the car body during various zones of ovens is very vital in the design and performance analysis of the paint dryers. In this research, a novel semi-analytical approach has been used to predict the body temperature variation during the curing process. Considering the energy balance for the body, a set of differential equation has been extracted, depending on the oven zone. These equations can be solved numerically to find the transient temperature profile of the car body. Some parameters in these equations have been achieved by experimental procedure. The results show that the present model predictions are in a good agreement with the experimental data. Therefore, the developed model has a reasonable accuracy and can be used as an efficient robust approach to distinguish overall thermal behavior of the body. These techniques can be used to optimize the design of curing paint oven.

Commentary by Dr. Valentin Fuster
2009;():661-666. doi:10.1115/HT2009-88388.

The extraction of functional components from natural plant is one of important processing in the development and further practical application of plant product. Microwave assisted extraction (MAE) has been widely used in the extraction of many samples for its unique heating mechanism, short extraction time and high yield of extract. Astragalus slice is a special and typical plant porous media. We describe an approach by scanning electronic microscope (SEM) to characterize the trachea and aperture of Astragalus slices irradiated 20 min by microwave at 600 W and 900 W, with the aim to analyze the effect of the microwave power on the extraction yield by SEM and discuss further the relationship between the microstructure characteristics of sample and mechanism on mass transfer in micro-scale. It is found that extract with the 20 min irradiation of microwave at 600 W is higher than that at 900 W because the apertures on the trachea wall remain open at 600 W, but shrink at 900 W. Moreover, we confirm the important role of the aperture in the extraction of plant materials. Therefore, this study has significant meanings to develop the optimized extraction technology of plant porous media and maintain or improve the quality of extract.

Commentary by Dr. Valentin Fuster
2009;():667-674. doi:10.1115/HT2009-88389.

Astragalus slice is one species of stem and root medicinal herb with the widely curative effects, also a special and typical plant porous material, and the drying operation is one of important processing technologies in its storage and further practical application. This paper characterizes the microstructure of Astragalus slices dried by microwave technique at 200 W by using scanning electronic microscope (SEM). The study also compares Astragalus slices dried by microwave with those untreated and discusses the drying mechanism. Result shows that as compared to the untreated sample, the microwave dried sample behaves much shorter drying time with more and larger pore and open structure on the surface layer of matrix, but without significant change about the distribution status of cytoplasm inside parenchyma cells. Further analysis suggests that the vapor diffusion is the dominant mode of moisture transfer inside matrix during the microwave drying process of sample, resulting in the well-preserved structures of sample, including parenchyma cell and trachea. This is also helpful for maintaining the distribution status of cytoplasm, particularly avoiding the agglomeration of biological macro-molecular, which is benefit to improving the permeability of moisture transfer path, leading to the rapidly dehydration of moisture. This work seems to be helpful for developing the optimized drying technology of plant porous material focused on micro-mechanism and the quality of dried products.

Commentary by Dr. Valentin Fuster
2009;():675-684. doi:10.1115/HT2009-88407.

This paper studied the influences of shielding gas compositions on the transport phenomena in the metal domain during gas metal arc welding (GMAW). A comprehensive model was developed to simulate the time-dependent processes of the electrode melting; the droplet formation, detachment, transfer and impingement onto the workpiece; the weld pool dynamics and bead formation and their transient coupling with the arc plasma. The transient melt-flow velocity and temperature distributions in the metal shielded by pure argon and argon-helium mixtures with various mixing ratios are presented. It is predicted that the increase of helium content and the resulting arc contraction induce an upward electromagnetic force at the bottom of the droplet to sustain the droplet at the electrode tip. As a result, the more oblate droplet and the longer droplet formation time are produced. The behaviors of the predicted droplet shape and detachment frequency are in agreement with the published results. It is also found that, under the identical energy input, the weld bead has a shallower penetration depth and broader bead width when helium content increases.

Commentary by Dr. Valentin Fuster
2009;():685-694. doi:10.1115/HT2009-88562.

The time dependent temperature distribution due to a moving plane heat source of hyperelliptical geometry is analytically studied in this work. The effect of the heat source shape is investigated starting from the general solution of a moving heat source on a half space. Selecting the square root of the heat source area as a length scale, it is observed that the temperature distribution becomes a weak function of the heat source shape. Variation of temperature field with respect to the source aspect ratio, velocity and depth is studied. The analysis presented in this work is valid for both transient and steady-state conditions. In addition, the hyperellipse formulation provided here covers a wide range of shapes including star, rhombic, ellipse, circle, square, rectangle and rectangle with rounded corners.

Commentary by Dr. Valentin Fuster
2009;():695-698. doi:10.1115/HT2009-88587.

Preliminary evidence of density and mechanical properties enhancement of binary alloys by solidification subject to vibrations is presented. The frequency of vibrations was increased from 0 to 100 Hz by using sound waves as the vibration source. The latter shows that the solidified microstructure, the ultimate tensile strength, and the hardness improve as the frequency increases. The chosen alloy for this study was Pb-Sb 4.4% (lead antimony 4.4%) and was selected because of its low melting temperature. The cast chosen was of a rod shape having a diameter of 10mm and a length 500mm. This choice is consistent with assuming an infinite length and therefore ignoring boundary effects in a planned theoretical follow-up analysis. Also due to the geometry of the mould it can be assumed that the cast was cooled due to conduction alone.

Commentary by Dr. Valentin Fuster
2009;():699-707. doi:10.1115/HT2009-88621.

A multiscale model is developed to simulate filtration process for the fabrication of composite material with nanoparticle additives. The model consists of two components. One is a particle trajectory tracking model (PTTM) which can predict the deposition rate of nanoparticle on the fiber matrix in a single pore structure, and the other one is a macroscale transport model of fluid flow in porous fiber structures. The flow of the fluid in the porous media with a free moving surface is solved by using the meshless SPH method. The integrated model is used to predict the local deposition rate coefficient and the distribution of the nanoparticle concentration in the carrier fluid and on the fiber surface. We envision this as the first step of a systematic study towards to an advanced understanding of the process as well as the optimization of the operational parameters for achieving homogeneous material properties of the materials.

Commentary by Dr. Valentin Fuster

Heat and Mass Transfer in Biotechnology

2009;():709-714. doi:10.1115/HT2009-88138.

Human eye is one of the most sensitive parts of the body when exposed to radiation effects. Since there is no barrier (such as skin) to protect the eye against the absorption of the external thermal waves, radiation can readily interact with cornea. On the other hand, lack of blood flow in the interior part of the eye makes it more vulnerable compared to other organs even in the case of weak heat interaction. Further, blood flow circulation alone cannot establish thermal equilibrium between the eye and body organs effectively. There are limitations in measuring human eye temperature profile experimentally due to the required invasive procedures in monitoring the inner layers. Therefore, there is a need to develop an accurate model to represent the eye structure and energy transport through it. Thermal modeling of the eye is important to investigate the effect of external heat sources as well as in predicting the abnormalities within the eye. Modeling of heat transport through the human eye has been the subject of interest for years, but the application of porous media models in this field is new and will be one of the themes of this study. In this work, iris/sclear is considered as a porous medium and energy transport is modeled using the tissue local thermal equilibrium equations. The eye is assumed to include six different parts: cornea, anterior chamber, posterior chamber, iris/sclera, lens and vitreous. A two-dimensional finite element simulation will be performed. Results are shown in terms of transient corneal surface temperature, isothermal lines in different regions and local temperature of pupillary axis. Effects of external radiation sources, convection coefficient of the surrounding air, blood temperature, blood convection coefficient and ambient temperature on different regions of the eye are also investigated.

Topics: Modeling , Human eye
Commentary by Dr. Valentin Fuster
2009;():715-721. doi:10.1115/HT2009-88261.

Thermal analysis of a cylindrical tissue subject to a train of ultrashort pulse irradiations was made by developing a combined time-dependent radiation and conduction bio-heat transfer model. Ultrashort pulsed radiation transport in the cylindrical tissue is simulated using the transient discrete ordinate method. Treatment of focused beam is introduced. The model skin tissue is stratified as three layers with different optical, thermal and physiological properties. Comparisons between the collimated irradiation and focused beam are conducted. The effect of pulse train is investigated.

Commentary by Dr. Valentin Fuster
2009;():723-729. doi:10.1115/HT2009-88345.

Many neurodegenerative diseases, such as Alzheimer’s disease, are linked to swellings occurring in long arms of neurons. Many scientists believe that these swellings result from traffic jams caused by the failure of intracellular machinery responsible for fast axonal transport; such traffic jam can plug an axon and prevent the sufficient amount of organelles to be delivered toward the synapse of the axon. Mechanistic explanation of the formation of traffic jams in axons induced by overexpression of tau protein is based on the hypothesis that the traffic jam is caused not by the failure of molecular motors to transport organelles along individual microtubules but rather by the disruption of the microtubule system in an axon, by the formation of a swirl of disoriented microtubules at a certain location in the axon. This paper investigates whether a microtubule swirl itself, without introducing into the model microtubule discontinuities in the traffic jam region, is capable of capturing the traffic jam formation. The answer to this question can provide important insight into the mechanics of the formation of traffic jams in axons.

Topics: Simulation , Traffic
Commentary by Dr. Valentin Fuster
2009;():731-739. doi:10.1115/HT2009-88462.

The implementation of in vivo imaging technologies, such as digital photography, dermoscopy and confocal scanning laser reflectance microscopy (CSLM) in dermatology has led to recent improvements in recognizing skin lesions. Specifically, in the case of skin cancer, a key issue is that the rate of cancerous tissue growth and changes in its spatial extent with time are linked to the energy released locally by these uncontrolled metabolic processes. We believe that with a properly designed infrared (IR) imaging and measurement system combined with thermal analysis, one can characterize healthy and diseased tissue. This paper augments our previous work, in which we introduced a computational model to estimate the location and size of lesions using IR imaging data. In this paper, we focus on calibrating the IR camera and correcting its inherent artifacts. Calibration and corrections are first performed on a blackbody object and then on human skin images in order to acquire accurate surface temperature distributions. As future work, in addition to these correction steps, several other steps, such as accounting for emissivity variations will be developed for clinical studies. In addition to IR imaging, images acquired by in vivo confocal scanning laser microscopy are used to examine the structure of the human skin for different skin types. Our aim is to generate additional data necessary for the IR imaging model by further analyzing the 3D structure of healthy tissue and the lesion. Specifically, in clinical studies, confocal images will be used to describe thermal associations with skin lesion and its blood supply in order to refine our transient thermal model of skin tissue.

Commentary by Dr. Valentin Fuster
2009;():741-748. doi:10.1115/HT2009-88575.

Cryosurgery is also called as cryoablation or cryoleision. The third generation of cryo-machine use argon gas for cooling and helium for rewarming to destroy cancer cells. The probes may be put into the tumor during surgery or through the skin (percutaneously). After cryosurgery, the frozen tissue thaws and is either naturally absorbed by the body (for internal tumors), or it dissolves and forms a scab (for external tumors). The main purpose of this paper is to establish a preliminary computer assisted simulation in prostate tumor cryosurgery. A radiologist and an urologist in a medical center in addition to the engineering specialist from the university participated in this interdisciplinary research program. The first step of this simulation protocol is to trim hundreds of two-dimensional medical imaging photos from a patient through the imaging reconstructive software into building a three-dimensional solid modeling. The image data for each patient can be obtained from the x-ray computed tomography (CT), or magnetic resonance imaging (MRI) in the imaging department of hospital. It has successfully built up the related knowledge to overcome the complicacy between the medical imaging modalities and engineering graphic solid modeling with high resolution. It is worthy to mention here that the present solid modeling of prostate can demonstrate the variable diameters and courses of the prostate urethra in vivo. The second step focuses on thermal calculation. So far, there has been no existing commercial software for the specific purpose of the bioheat transfer problem. Hence, user subroutines must be added to the existing commercial software to simulate the clinical situation of cryosurgery. For example, the occurrence of phase change during some specified temperature range and the latent heat of fusion are also incorporated into bio-heat transfer model. It has successfully incorporated bioheat transfer model into the software program to fit the reality in thermal medicine. The third step supplies the data and knowledge concerned with the position of a tumor and the related mechanism of metabolism of living tissue and vessels. The number of probes, the position of each probe, and the operating time of each probe will be explored to ensure a complete killing of the tumor tissue while saving as much healthy surrounding tissue as possible. In this study, the three-dimensional transient temperature distributions based on cryosurgery for prostate tumors have been performed for several cases to find the optimal operating conditions. Different cryoprobes with different freezing time are considered to find the temperature distribution. The simulation results for cryosurgery of prostate tumors will be supplied for practicing physicians as reference to greatly improve the effectiveness of cryosurgery.

Commentary by Dr. Valentin Fuster

Low Temperature Heat Transfer

2009;():749-756. doi:10.1115/HT2009-88087.

A one-dimensional mathematical model is derived for a three-stage pulse-tube refrigerator (PTR) that is based on the conservation laws and the ideal gas law. The three-stage PTR is regarded as three separate single-stage PTRs that are coupled via proper junction conditions. At the junctions there are six fluid flow possibilities each defining its own boundary conditions for the adjacent domains. Each single stage cools down the gas in the regenerator to a lower temperature such that the system reaches its lowest temperature at the cold end of the third stage. The velocity and pressure amplitudes are decreasing towards the higher stages and there is an essential phase difference between them at different positions. The system of coupled PTRs is solved simultaneously first for the temperatures and then for the velocities and the regenerator pressures. The final result is a robust and accurate simulation tool for the analysis of multi-stage PTR performance.

Topics: Simulation
Commentary by Dr. Valentin Fuster
2009;():757-766. doi:10.1115/HT2009-88169.

Cyrocoolers are notorious for being difficult to design and optimize. Reasons for this include subsystem complexity, large unknowns associated with material and transport parameters, and high sensitivity to manufacturing tolerances. The purpose of this paper is to address this topic by incorporating design uncertainty itself as a constraint during the optimization of a Joule-Thomson sorption cryocooler. In our method a Markov Chain Monte Carlo sampler is used as the means to develop a suitable ensemble from a practical set of computational results which circumscribe the power/efficiency characteristics of a cryocooler as a function of several dimensionless stochastic optimization parameters. The ensemble is used to estimate the covariance structure of the design uncertainty, which is then projected into the best low rank subspace where tests of hypothesis under the dominant generalized parameters can be formulated; growth in fluctuations of the generalized parameters along optimization trajectories becomes clearly evident and quantifiable. The method results in a classical power/efficiency diagram, with the addition of quantified design uncertainty. The utility of these diagrams is that they enable rapid-prototyping efforts to target the best cooler design that is most likely to function as expected either for model validation or production. This paper will present the methodogy and a comprehensive computation model of a JT metal hydride cryocooler and demonstrate its application.

Topics: Design
Commentary by Dr. Valentin Fuster
2009;():767-771. doi:10.1115/HT2009-88181.

The authors have been proposed and developed snow-melting system using geothermal and solar energy. In summer, solar heat is stored into underground from road surface to underground piles. In winter, the underground heat is utilized to melt snow on the road surface. This system was applied to parking lots and bridges of relatively small scale (less than 1000 m2 ). Numerical simulation program was also developed to predict temperature field of the system and to evaluate system performance. This program was verified by experimental data only for relatively small scale test area. In addition, appropriate design conditions, such as pile diameter, length and number, can not be easily estimated when road surface area and ability (average heat flux) of snow-melting are given. This paper aims to demonstrate the system for relatively large scale (larger than 1000 m2 ), and to obtain optimal design conditions of the system at given road surface area and ability. The snow-melting system using geothermal and solar energy was applied to a parking lot and a bridge of large scale. Both sites were under practical use which means cars were sometimes parked and run over the bridge. Obtained experimental data of temperature field of the system and snow melting situation show that numerical simulation program predicted system performance and temperature field adequately even though the program contains several simplifications. To discuss the optimal design conditions, numerical simulation was conducted by changing the following parameters: diameter, length, number and pitch of piles, pitch and diameter of heat dissipation pipe, flow rate of circulating water, road surface area. All these parameters are considered to affect system performance. The simulation results revealed that pile surface area determined by diameter, length and number of piles is the dominant parameter for deciding snow-melting ability. Namely, when road surface and snow-melting ability are given, necessary pile surface area can be obtained from the simulation results, and system design of piles becomes possible with considering cost for embedding piles.

Commentary by Dr. Valentin Fuster

Environmental Heat Transfer

2009;():773-778. doi:10.1115/HT2009-88024.

Sustainability of life form on the earth is a major concern of every nation, which stems from the continued global warming trend, which has become a major policy, political, and economic issue. Global warming is the most important challenge thrown by the human activities largely due to rapid pace of industrialization in the twenty first century. The impact is likely to extend to next few centuries and unless controlled there would be irrevocable damage to the life form on this planet. Human made halocarbons have a high global warming potential, and some still have the potential to cause damage to the ozone layer as well if released to the atmosphere. The implications of global warming have far-reaching effects beyond the imagination of common person. Rise in global temperature, rise in sea level, food shortages, large scale spread of diseases & infections, catastrophic economic consequences and colossal loss of bio-diversity are some of the major implications of global warming trend. Although many methods are in vogue for comparison of impact of global warming of different compounds, yet the concept of Global warming potential with reference to Carbon dioxide is the simplest one and is widely used. An endeavor has been made in this paper to correlate and develop empirical relations of global warming potential and atmospheric lifetimes of Halocarbons. A new parameter Glife has been evolved for this purpose.

Topics: Climate change
Commentary by Dr. Valentin Fuster
2009;():779-787. doi:10.1115/HT2009-88105.

Continued efforts to reduce the environmental impact of products and services are leading to the increased prevalence of life-cycle assessment (LCA) during the design phase. A key challenge in traditional process-based LCA is the validation of vast life-cycle inventory data, which easily numbers in the tens of thousands of data points. In this paper, we utilize an ‘object-based’ approach from software engineering to manage the large amount of data and combine this approach with simple thermodynamic principles, thus significantly reducing the chance of error in LCA inputs. The approach is demonstrated for the test case of a typical industrial air-conditioning unit. We find that implementation of the object-based approach identifies numerous potential errors in the life-cycle inventory data. Thus, the proposed approach enables the designer to perform a more robust environmental analysis of a given system. This facilitates a better identification and prioritization of opportunities for reducing the environmental footprint across the system life-cycle.

Commentary by Dr. Valentin Fuster
2009;():789-799. doi:10.1115/HT2009-88158.

We analyze non-isothermal absorption of soluble atmospheric trace gases by the falling rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the concentration of soluble trace gases and temperature in the atmosphere varies in a vertical direction. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. It is shown that the non-uniform vertical distribution of absorbate and temperature in a gaseous phase strongly affects mass transfer during gas absorption by a falling droplet.

Commentary by Dr. Valentin Fuster
2009;():801-808. doi:10.1115/HT2009-88170.

Ecologically Sustainable buildings are being designed for the University of Technology, Sydney (UTS) where the building façade and equipment may serve as a project-based environment for engineering students learning about energy efficiency. Building Integrated Photovoltaic (BIPV) panels with poly-crystalline Photovoltaic (PV) module were designed and experimentally tested. The power input, and power output, surface temperatures, and channel spacing(s) between the PV module and glazed layer(s) were measured at cooling conditions with both fan-on and fan-off conditions. The electric energy conversion efficiency and the heat transfer ratios of conduction, natural convection, forced convection, and radiation reflection were determined for BIPV panels and optimized against the surface temperature and the channel spacing(s). An optimum BIPV with compact spacing for the single glazed (double-skinned) facade was suggested to be retrofitted to existing buildings, while an optimum BIPV with compact spacing for the double glazed (triple-skinned) façade was suggested for the building development at UTS. The project is ongoing and serves as a collaborative educational platform for students and staff.

Commentary by Dr. Valentin Fuster
2009;():809-816. doi:10.1115/HT2009-88232.

This paper presents experimental data showing the response of a computer room air conditioning unit (CRAC) to chilled water (CHW) pump restart. The data are offered to improve and develop modeling of cooling equipment restart events following data center power failure. There are estimates that power failures will increase and limits on availability will affect data center operations at more than 90 percent of all companies over the next five years. Because providing backup power to cooling equipment increases data center first cost, it is important to have accurate models for cooling events and processes following power failure that help predict server inlet temperatures during the transient phase caused by a power failure. Since power density of computing equipment continues to rise, the temperature rise of air within the data center has been predicted to rise more quickly to an unacceptable level, increasing concern. Accurate models of CRAC response to pump restart can aid in data center cooling design, backup power infrastructure provisioning, and even compute equipment selection by predicting the air supply temperature after the generator provides power to the chilled water pump. Previous transient models include zonal models with large time scales and CFD/HT models with boundary conditions developed for steady state. These models can be improved by comparison with experimental data. The experiment consists of measuring the response of the CRAC heat exchanger to the step change in CHW flow rate upon pump restart. Inlet and outlet temperatures of both CHW and air were measured, as well CHW flow rate. A point measurement of air at the CRAC fan outlet was also taken to verify that airflow remained relatively constant. Outlet temperatures from the CRAC follow a first order response curve; it is found that the CRAC under consideration has fan outlet temperature time constant of 10 seconds. A delay of 20 seconds is observed between the fan outlet temperature response and the CHW return temperature response.

Commentary by Dr. Valentin Fuster
2009;():817-823. doi:10.1115/HT2009-88289.

A method to stabilize the draft through natural cooling towers is introduced. Natural draught dry cooling towers are widely used in arid regions of the world for the power industry especially those employing nuclear reactors. Their presence has become iconic of the process industry for their dominance of the landscape. These towers control the overall efficiency of power plants, and with the ongoing energy crisis it is desirable to raise efficiency by stabilising the draught through the tower. Energy comsumption is a substantial part of the overall cost of plant operation, and therefore even with a conservative 5 per cent improvement is feasible. It has been noted by some researchers like Baer, Ernst and Wurz (1980) that cooling towers do experience unstable flow with breezes. This phenomenon can be explained by Jörg and Scorer (1967) to occur even in a still ambience with cold air inflow down into the tower shell from exit. Jörg and Scorer (1967) developed a correlation to predict cold inflow to a glass tube for various fluids in a laboratory. By using their formula, it is found that under typical exit bulk velocities, of 3–5 m/s or below, cold air is liable to ‘sink’ into the shell, even in a quiescent surrounding. Indeed this phenomenon was demonstrated in the laboratory using a duct of size 457 × 457 mm2 of a heat exchanger by employing a smoke generator to detect that cold air did flow into the duct rather than the hot air filling the entire cross sectional area of the duct exit. A device was applied by Chu (1986) to prevent this cold air from sinking into the duct and enhance the stability and quantity of the updraft. In this paper, for the first time data obtained from a 700 × 700 mm2 cross-sectional flow area model air-cooled heat exchanger are presented that proves the air flow rate enhancement due to this device. It is hoped that more tests can be conducted to optimize the design for application in boiler chimneys and natural draught dry cooling towers.

Commentary by Dr. Valentin Fuster
2009;():825-831. doi:10.1115/HT2009-88344.

The simulated triple vacuum glazing consists of three, 4 mm thick glass panes with two vacuum gaps with each internal glass surface coated with a low emittance coating. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. Both solder glass and indium based sealants have been successfully applied in vacuum glazing previously. The thermal performance of the triple vacuum glazing was simulated using a finite volume model. The simulation results show that although the thermal conductivity of solder glass (1 W.m−1 .K−1 ) and indium (83.7 W.m−1 .K−1 ) are very different, the increase in heat transmission of triple vacuum glazing with a 10mm frame rebate resulting from the use of an indium edge seal compared to a solder glass edge seal was 0.48%. Increasing both edge seal widths from 3 mm to 10 mm led to a 24.7% increase in heat transmission of the triple vacuum glazing without a frame and an 18.3% increase for a glazing with a 10 mm frame rebate depth. Increasing the rebate depth in a solid wood frame from 0 to 15 mm decreased the heat transmission of the triple vacuum glazing by 32.9%. The heat transmission of a simulated 0.5 m by 0.5 m triple vacuum glazing was 32.2% greater than that of 1 m by 1 m triple vacuum glazing.

Topics: Vacuum
Commentary by Dr. Valentin Fuster
2009;():833-841. doi:10.1115/HT2009-88350.

The thermal behavior of Utah Lake, situated in northern Utah, is modeled over a spring-to-fall period using environmental forcing data from the year 2007. Results compare favorably with previously obtained data for temperature distributions around the lake during midsummer 2007. During the spring months, when experimental data is not available, the model predicts strong and rapid variations in the water temperature, which correlate well with significant storms on the lake. A heat balance shows that the largest components of heat fluxes into and out of the lake are due to short wave solar and evaporative cooling, respectively. Both numerical and experimental results also indicate that, due to the shallow nature of the lake and occurrence of significant wind events, thermal stratification is never achieved.

Topics: Lakes
Commentary by Dr. Valentin Fuster
2009;():843-850. doi:10.1115/HT2009-88596.

Numerical simulations have been performed to study the effects of size and slip coefficient of a porous manifold on the thermal stratification in a storage tank. The model is used to predict the development of flow and temperature fields during a charging process. Computations have covered a wide range of the Grashof number (1.8 × 105 < Gr < 1.8 × 108 ) and Reynolds number (10 ≤ Re ≤ 104 ), or in terms of the Richardson number, 10−2 < Ri < 105 . The results obtained compare favorably well with the experimental data. In addition, the present results have confirmed the effectiveness of porous manifold in the promotion of thermal stratification and provide useful information for the design of such system.

Commentary by Dr. Valentin Fuster

Heat Transfer Education

2009;():851-860. doi:10.1115/HT2009-88212.

In this paper we discuss the application of the certified reduced basis method and the associated software package rbMIT© to “worked problems” in steady and unsteady conduction. Each worked problem is characterized by an input parameter vector — material properties, boundary conditions and sources, and geometry — and desired outputs — selected fluxes and temperatures. The methodology and associated rbMIT© software, as well as the educational worked problem framework, consists of two distinct stages: an Offline (or “Instructor”) stage in which a new heat transfer worked problem is first created; and an Online (or “Lecturer”/“Student”) stage in which the worked problem is subsequently invoked in (say) various in-class, project, or homework settings. In the very inexpensive Online stage, given an input parameter value, the software returns both (i) an accurate reduced basis output prediction, and (ii) a rigorous bound for the error in the reduced basis prediction relative to an underlying expensive high-fidelity finite element discretization; as required in the educational context, the response is both rapid and reliable. We present illustrative results for two worked problems: a steady thermal fin, and unsteady thermal analysis of a delamination crack.

Commentary by Dr. Valentin Fuster
2009;():861-869. doi:10.1115/HT2009-88544.

At Saint Martin’s University, the Mechanical Engineering Department incorporates design throughout the curriculum, starting from the freshman course ‘Introduction to Engineering’ up through the senior capstone design. Undergraduate Mechanical Engineering students are required to take three Mechanical Engineering elective courses. Each of these elective courses is 3 credits; one of these credits must be design. For ME 436 Thermal Design of Heat Exchangers course, the students were assigned a project to meet the Accrediting Board of Engineering and Technology student outcome criteria a, b, c, d, e, g, and i. In addition to course work the students designed, market searched for parts, budgeted, manufactured and built, then tested a finned helical coil heat exchanger experiment. A limited budget was assigned for this project. Currently, the apparatus is being used as an additional experiment in the Thermal Engineering Laboratory. This article includes the preliminary design, final design, experimental results, and assessment by students and faculty.

Commentary by Dr. Valentin Fuster

Visualization of Heat Transfer

2009;():871-879. doi:10.1115/HT2009-88234.

Heat transfer in fluid flows traditionally is examined in terms of temperature field and heat-transfer coefficients. However, heat transfer may alternatively be considered as the transport of thermal energy by the total convective-conductive heat flux in a way analogous to the transport of fluid by the flow field. The paths followed by the total heat flux are the thermal counterpart to fluid trajectories and facilitate heat-transfer visualisation in a similar manner as flow visualisation. This has great potential for applications in which insight into the heat fluxes throughout the entire configuration is essential (e.g. cooling systems, heat exchangers). To date this concept has been restricted to 2D steady flows. The present study proposes its generalisation to 3D unsteady flows by representing heat transfer as the 3D unsteady motion of a virtual fluid subject to continuity. The heat-transfer visualisation is provided with a physical framework and demonstrated by way of representative examples. Furthermore, a fundamental analogy between fluid motion and heat transfer is addressed that may pave the way to future heat-transfer studies by well-established geometrical methods from laminar-mixing studies.

Commentary by Dr. Valentin Fuster
2009;():881-888. doi:10.1115/HT2009-88417.

When a heat exchanger in a Fast Breeder Reactor cracks, highly pressurized water or steam escapes into the surrounding liquid sodium. A sodium-water reaction then occurs, forming disodium oxide (and hydrogen gas). It can cause secondary damages to the heat exchangers by the reaction heat and erosion corrosion. The released flow of steam from the cracks of the heat exchanger is an underexpanded jet because the ambient pressure outside the tubes is lower than the critical pressure. When the pressure of a jet released at high pressure cannot be reduced to the low pressure of the ambient fluid, the flow is said to be underexpanded. Because this expansion causes a reduction of pressure and the pressure is lower than the critical pressure, the velocity of the flow can reach supersonic speed. Several studies have examined the underexpansion of the gas-gas phase. However, there have been few studies on the underexpansion of gas-liquid two-phase flows. The flow characteristics of the gas-liquid two-phase flow differ from the gas-gas flow because breakups of the bubbles appear in the gas-liquid two-phase flow. Therefore, in this study qualitative measurement was carried out for the purpose of revealing the flow with the underexpanded gas jet injected into water. The gas jet distance L and the expansion angle θ were then obtained from averaged image of a high-speed camera. L and θ increased approximately linearly with increasing pressure. The entrainment velocity and the velocity of entrained water droplets into the gas jet were obtained by PIV. Images of unstable expansion near the jet nozzle were captured for the first time.

Topics: Water
Commentary by Dr. Valentin Fuster
2009;():889-896. doi:10.1115/HT2009-88435.

It is thought that the pressure fluctuation can occur due to the interaction between flow through guide vanes and flow into runner blades, resulting in a vibration of turbine and a blade cracking, in a hydraulic turbine operated in a wide range for flexible power demand. High accurate velocity measurement with high time/spatial resolution can help to clarify the mechanism of the interaction and to provide good experimental data for the validation of numerical procedure. So the aim of present study is to estimate the unstable velocity field quantitatively in the area between guide vanes and runner blades, using high time-resolved particle image velocimetry (PIV). Two types of velocity measurements were carried out, i.e., phase-locked measurement and high time sequential velocity measurement, in a pump-turbine model with 20 guide vanes and 6 runner blades. The characteristic of the flow field varied corresponding to the operating conditions such as flow rate and rotational speed. Opening angles of guide vanes were kept uniform. A clockwise vortex was generated at inside of the runner blade under smaller rotational speed. A counterclockwise vortex was separated at the backside of the runner blade under higher rotational speed. At any operating conditions, the velocity between guide vanes and runner blades oscillated periodically at the blade passing frequency.

Commentary by Dr. Valentin Fuster
2009;():897-905. doi:10.1115/HT2009-88438.

The present study is an experimental and numerical analysis on the natural convection of air in square enclosures with partially active side walls. The experimental equipment is based on two different systems: an holographic interferometer and a 2D-PIV. The test cell is a square enclosure filled of air with vertical partially active side walls at different temperatures. The hot and cold regions on these sides are located in the middle of the cavity. The remaining vertical walls are made up of glass to allow an optical access to the cavity. The top and bottom surfaces of the enclosure are made up of plexiglas to reduce heat leakages. The experimental study is carried out both through the holographic interferometry, in order to obtain the average Nusselt numbers at different Rayleigh numbers, and through the 2D-PIV, in order to analyse the dynamic behaviour of the phenomenon at the same Rayleigh numbers. The average Nusselt numbers are obtained measuring the temperature distribution in the air layer trough the real-time and double-exposure holographic interferometry; the dynamic structures are the velocity vector distribution, the streamlines and the velocity maps. Finally these experimental data are compared to the results obtained through a numerical study carried out using the finite volume code, Fluent 6.2.3. The aim of this comparison is the validation of the numerical procedure. In this way it is possible to use the numerical code to enlarge the Rayleigh number range.

Commentary by Dr. Valentin Fuster

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