bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrasonic Bonding of Membrane Electrode Assemblies for Low Temperature PEM Fuel Cells
Authors: Beck Joseph R. , Walczyk Daniel F. , Hoffman Casey J. , Buelte Steve J.
Published: July 2013

Ultrasonic bonding of low temperature PEM membrane electrode assemblies has shown to cut the cycle time and energy input of manufacturing by over an order of magnitude each as compared to the industry standard of thermal pressing. Ultrasonic bonding uses high-frequency mechanical oscillations to convert electrical energy into heat energy which bonds the membrane electrode assembly components. This reduction in manufacturing resource requirement and time helps make fuel cell energy more economical as an alternative electrical power source. This paper will discuss ultrasonic and thermal bonding for low temperature Nafion fuel cells with 10 cm2 active area including process optimization and the effects of electrode type and membrane conditioning on ultrasonically bonded MEA performance.

A design set of experiments was created for both ultrasonic bonding and thermal pressing process optimization using commercially available electrodes and conditioned Nafion 115 membrane. Analysis of Variance suggests that neither energy nor pressure have a statistically significant effect on the performance on ultrasonically bonded MEAs. For thermally pressed MEAs, temperature was found to have a significant effect on performance while pressure was not. Neither manufacturing technique found interaction effects to be statistically significant.

Three different electrode compositions were tested on both ultrasonic and thermal MEA bonding methods. Electrodes investigated include two that were custom made in-house with catalyst loadings of 0.16 and 0.33 mg Pt/cm2, and one commercial electrode with 0.5 mg Pt/cm2. The lower loaded custom electrode had greater performance than the commercial electrode, which had higher platinum loading, indicating electrode architecture is an important factor in the performance of ultrasonically bonded MEAs.

Membrane electrode assemblies made using Nafion membranes that were pretreated with a conditioning process showed decreased performance compared to MEAs ultrasonically bonded from dry, unconditioned membrane in short-term testing. MEAs thermally pressed with the custom made electrodes performed better with conditioned membranes while the commercial electrodes showed decreased performance with conditioning.

Current electrodes have been optimized for thermal pressing as demonstrated by the two commercial electrodes having the largest performance decreases between thermally and ultrasonically manufactured MEAs. Future work includes intelligently designing an electrode for optimizing the ultrasonic bonding process for low temperature fuel cells to increase the performance of this manufacturing technique.
Modeling of Proton Exchange Membrane Fuel Cell (PEMFC) Performance by Using Genetic Algorithm-Polynomial Neural Network (GA-PNN) Hybrid System
Authors: Mehrabi Mehdi , Rezazadeh Sajad , Sharifpur Mohsen , Meyer Josua P.
Published: July 2013

In the present study, a genetic algorithm-polynomial neural network (GA-PNN) was used for modeling proton exchange membrane fuel cell (PEMFC) performance, based on some numerical results which were correlated with experimental data. Thus, the current density was modeled in respect of input (design) variables, i.e., the variation of pressure at the cathode side, voltage, membrane thickness, anode transfer coefficient, relative humidity of inlet fuel and relative humidity of inlet air. The numerical data set for the modeling was divided into train and test sections. The GA-PNN model was introduced with 80% of the numerically-validated data and the remaining data was used for testing the appropriateness of the GA-PNN model by means of two statistical criteria.
Investigation of Liquid Water Content of a Compressed PEMFC GDL Using Micro-Computed Tomography
Authors: Yip Ronnie , Bazylak Aimy
Published: July 2013

With the current lack of understanding of the water transport phenomenon in the porous gas diffusion layers (GDLs) of the polymer electrolyte membrane fuel cells (PEMFCs), GDL designs are primarily implemented on a costly trial-and-error basis. In this work, an ex-situ device, suitable for micro-computed tomography (micro-CT) imaging, was designed to facilitate liquid water invasion of a GDL sample under flow field compression. The millimeter-scale apparatus allows for water injection from a point source with an opening diameter of 0.8 mm. A sample of felt-based Freudenberg GDL (H2315) was examined for the current study. Using micro-CT, the sample was scanned, before and after water invasion, to obtain high resolution, three-dimensional reconstructions of the dry GDL microstructures, as well as the liquid water patterns after breakthrough. These results were used to find the effect of liquid water content on the effective through-plane porosity for the felt-based Freudenberg GDL.
Influence of Wall Thermal Conductivity During Pool Boiling on Large Diamond and SiC Heaters
Authors: Kapitz Marek , Becker Adam , aus der Wiesche Stefan
Published: July 2013

The results of pool boiling experiments with synthetic (CVD) diamond and silicon carbide (SiC) heaters are presented for water as boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, also in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15mm × 15mm) for pure water under atmospheric pressure (1bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In a prior work, it was found that the influence of the thermophysical wall properties might be substantial in case of special heater geometries leading to trapped bubbles, but no significant differences between both materials were observed in case of conventional heater configurations.
Flow Pattern and Pipe Orientation Independent Semi-Empirical Void Fraction Correlation for a Gas-Liquid Two Phase Flow Based on the Concept of Drift Flux Model
Authors: Bhagwat Swanand M. , Ghajar Afshin J.
Published: July 2013

A flow pattern and pipe orientation independent void fraction correlation is proposed in the present study. The correlation is based on the concept of drift flux model and proposes two separate expressions to model distribution parameter and drift velocity. The distribution parameter is expressed as a function of pipe orientation, phase superficial velocities and the void fraction in implicit form, while the drift velocity parameter is modeled as a function of fluid thermo physical properties, pipe orientation and void fraction. The drift velocity equation proposed by Zukoski [1] is extended for downward inclined pipe orientations. The performance of the proposed void fraction correlation is verified against void fraction data set of 5928 data points including the data for fifteen pipe diameters and eight different fluid combinations. The superiority of the proposed correlation is also illustrated by comparing it against the top performing correlations in horizontal, vertical upward and vertical downward pipe orientations and the predictions of the Woldesemayat and Ghajar [2] and Chexal et al. [3] correlations for incline pipe orientations.
Experimental Investigation and Performance Evaluation of Isothermal Frictional Two Phase Pressure Drop Correlations in Vertical Downward Gas-Liquid Two Phase Flow
Authors: Bhagwat Swanand M. , Mollamahmutoglu Mehmet , Ghajar Afshin J.
Published: July 2013

The correct prediction of gas-liquid two phase pressure drop is of immense significance for proper sizing of industrial equipment and safety operations involved in chemical, energy and petrochemical applications. The hydrostatic component of the two phase pressure drop is predicted based on the accurate estimation of void fraction. However, there exists a complexity in correct estimation of the frictional component of two phase pressure drop owing to interfacial friction at dynamic gas-liquid interface. The present study is focused on the experimental measurements of gas-liquid two phase frictional pressure drop and the performance evaluation of eleven correlations for its prediction in vertical downward orientation. The experimental determination of two phase frictional pressure drop is carried out for a 0.01252 m I.D. pipe with surface roughness of 0.0000152 m using air-water as the fluid combination. Unlike most of the other studies centered towards annular flow, this experimental study is spanned over different flow patterns and the entire range of the void fraction. In addition to the experimental measurements, the scope of this study also includes the performance analysis of eleven frictional pressure drop correlations available in the literature. These correlations are those based on the separated flow model initially proposed by Lockhart and Martinelli [1].The available frictional pressure drop correlations are compared against the data measured in the present study. Based on the experimental data available in the literature, the influence of the pipe diameter and fluid viscosity on the frictional pressure drop is also analyzed.
Numerical Simulation of the Thermodynamic Process of the Molten Salt Furnace
Authors: Ning Lian , Zhou Chenn Q. , Zhou Jiemin
Published: July 2013

In this paper, a numerical model of the thermodynamic process was developed, by using CFD (software) technique and considering the gas flow, the diffused combustion and the radiative heat transfer in the molten salt furnace. This model aims to optimize the operating parameters. Simulation results demonstrate that the performances of the salt furnace can be improved by optimization. The temperatures along the fire wall circumference are quite even, and the deviant combustion phenomenon is not observed. A back-flow formed in the upper part of the furnace chamber enhances the circulation and the mixing of the gas, helping to effectively stabilize the combustion in the furnace. The behaviors of CO, CO2, NOx and H2O are presented in terms of the gas flow, temperature distribution and volumetric concentration distribution. The furnace with the constant air flow rate of 15500Nm3/h and the angle of guide vane at 48∼50 ° can increase the combustion effectiveness.
Three Dimensional Transient Heat Transfer Model for Steel Billet Heating in Reheat Furnace
Authors: Dubey Satish Kumar , Agarwal Neelesh , Srinivasan P.
Published: July 2013

In steel rolling mills reheat furnaces are used to heat the billets prior to rolling processes. Reheating is one of the most energy intensive processes in the steel industries. Inadequate temperature measuring techniques and extremely complex analytical solution for temperature filed calculations demands suitable numerical model. In the present work a three dimensional transient heat transfer model is developed for billet heating in reheat furnaces. Conduction heat transfer within the billets is modeled using Finite Difference Method (FDM). Fully implicit spatial discretization approximation was used for three dimensional heat diffusion equation of billet. The three dimensional model takes into account the temperature dependent thermo physical properties, reaction heat effect and growing oxide layer. Algorithm is implemented in MATLAB® to solve three dimensional discretization equations. Model is capable of predicting the temperature field for billet and oxide scale thickness for any residence time. The predicted results are in reasonable concurrence with available data. The main objective of this work is to predict billet temperature field and oxide scale thickness for the various residence times, which may be vital for development of energy efficient optimization strategy for reheating process.
Effect of Heater Size on Confined Subcooled Jet Impingement Boiling
Authors: Abishek S. , Narayanaswamy R. , Narayanan V.
Published: July 2013

Jet impingement boiling heat transfer is a potential technique for the removal of very high heat fluxes concentrated at discrete locations, such as in power electronic components. In the present research, the effect of heater-nozzle size ratio (in the range 0.5 ≤ wH/wN ≤ 11) on jet impingement boiling is studied numerically. A steady-state submerged and confined subcooled jet impingement boiling of de-ionized and degassed water (at atmospheric pressure) on a polished isothermal heater surface is considered for a jet Reynolds number of Rew = 2500 and 20°C subcooling. The RPI wall boiling closure is used for the partition of heat flux on the surface into liquid phase, evaporation and quenching. Turbulence is modeled using the RNG-k-ε mixture model. The flow and heat transfer is simulated by considering the liquid and vapor phase to be an Euler-Euler interpenetrating continua; the interfacial momentum transfer is modelled using appropriate correlations for interphase momentum, heat and mass transfers. Validation of the numerical approach was performed by comparison of the present results with experimental data from literature for axisymmetric as well as slot jets. It was found that for any prescribed wall superheat, the heat flux was consistently larger for relatively smaller heaters (or smaller wH/wN). However, for any given wall superheat, the heat flux stagnated at an apparent asymptotic limit with increase in heater size, and this asymptotic limit was larger for larger wall superheats. It was also found that the quenching heat flux was the largest contributor to the total heat flux at relatively large degrees of superheat irrespective of heater-nozzle size ratio. A correlation is also developed for the heat flux as a function of the heater size and degree of superheat, for a given set of other controlling parameters.
Molecular Dynamics Simulation of Heat Conduction in Si Thin Films Induced by Ultrafast Laser Heating
Authors: Zou Yu , Huai Xiulan , Xin Fang , Guo Zhixiong
Published: July 2013

Molecular dynamics simulations are carried out to study the thermal and mechanical phenomena of ultra-high heat flux conduction induced by ultrafast laser heating in thin Si films. Nanoscale Si films with various depths in heat flux direction are treated as a semi-infinite model for the study of ultrafast heat conduction. A distribution of internal heat source is applied to simulate the absorption of the laser energy in films and the induced temperature distribution. Stress distribution and the evolution of the displacement are calculated. Thermal waves are observed from the development of temperature distribution in the heat flux direction, though the average temperature of the simulated Si films increases monotonically. The average stress shows periodic oscillations. The time development of strain has the same trend as the average stress, and the net heat flux shows the same trend as the stress at different depths of the Si films in the direction of heat flux. This reveals a close relationship between stress and net heat flux in the Si films in the process of ultrafast laser heating.
Modeling the Initial Stage of Crosslinked Aromatic Hydrocarbon Polymer Pyrolysis
Authors: Desai Tapan , Lawson John , Keblinski Pawel
Published: July 2013

Ablative materials are employed as thermal insulators in hypersonic space vehicles and for fire proofing in commercial applications. However, the chemical reactions involved in the transformation of ablative materials to char during pyrolysis are not well understood. Reactive molecular dynamics simulations were performed to study the initial stage of the pyrolysis of crosslinked aromatic hydrocarbon polymers. The products formed were characterized and acetylene was found to be the primary product. The value of the activation energy for acetylene formation was estimated. The acetylene formation mechanisms were analyzed at different degrees of crosslinking and the graphitic precursor formation mechanism was studied.
Experimental Study of Cylindrical Latent Heat Energy Storage Systems Using Lauric Acid as the Phase Change Material
Authors: Liu Chang , Murray Robynne E. , Groulx Dominic
Published: July 2013

Phase change materials (PCMs) inside latent heat energy storage systems (LHESS) can be used to store large amounts of thermal energy in relatively small volumes. However, such systems are complicated to design from a heat transfer point of view since the low thermal conductivity of PCMs makes charging and discharging those systems challenging on a usable time scale.

Results of experiments performed on both a vertical and a horizontal cylindrical LHESS, during charging, discharging and simultaneous charging/discharging, are presented in this paper. Both LHESS are made of acrylic plastic, the horizontal LHESS has one 1/2″ copper pipe passing through its center. The vertical LHESS has two 1/2″ copper pipes, one through which hot water flows, and the other through which cold water flows. Each of the pipes has four longitudinal fins to enhance the overall rate of heat transfer to and from the PCM, therefore reducing the time required for charging and discharging.

The objective of this work is to determine the phase change behavior of the PCM during the operation of the LHESS, as well as the heat transfer processes within the LHESS. Natural convection was found to play a crucial role during charging (melting) and during simultaneous charging/discharging (in the vertical LHESS). However, during discharging, the effect of natural convection was reduced in both systems.
Comparison of Heat Transfer Enhancement by Actuated Plates in Heat-Sink Channels
Authors: Yu Youmin , Simon Terrence , North Mark , Cui Tianhong
Published: July 2013

This paper investigates heat transfer enhancement of an air-cooled plate-fin heat sink by introducing actively-driven agitating plates within its channels. The investigation was computationally conducted with a single actuated plate in a single channel constructed as two fin wall surfaces and one fin base surface. As air flows through the channel, the plate is vibrated transversely to agitate the channel flow and thereby enhance heat transfer. The channel flow and the actuated plate are considered to be driven by a fan and a piezoelectric stack, respectively. A Coefficient of Performance (COP), ratio of total heat dissipated from the fin channel to total electric power to drive the fan and the agitator plate, is employed to evaluate overall heat transfer enhancement. A short plate, i.e. a plate is only placed at the entrance of the channel, has been shown to possess higher COP than a longer plate, i.e. a plate that is extended to be over most of the channel. For the short plate, COP is higher when it is actuated than when it is stationary. Detailed turbulence-kinetic-energy contours indicate that the higher COPs are due to turbulence generated along the plate edges and streamwise acceleration and deceleration of the bulk channel flow; both are induced by the vibration of the plate. Within regions where the plate is present, the generated turbulence and the acceleration and deceleration augment heat transfer. For a short plate, the turbulence and unsteadiness are transported downstream of the actuated plate to increase heat transfer in that region. However, such turbulence and unsteadiness are drawn out of the channel without full benefit of agitation and heat transfer enhancement when the plate is long, as the plate’s trailing edge is already close to the channel exit. This leads to a conclusion that the short plate is a better choice for active heat transfer enhancement.
Numerical Investigation of Melting With Internal Heat Generation in a Vertical Cylindrical Geometry
Authors: Shrivastava Amber , Williams Brian , Siahpush Ali S. , Crepeau John
Published: July 2013

There have been significant efforts by the heat transfer community to investigate the melting phenomenon of materials. These efforts have included the analytical development of equations to represent melting, the numerical development of computer codes to assist in the modeling, and the collection of experimental data. The understanding of the melting phenomenon has application in several areas of interest, for example, the melting of a phase change material used as a thermal storage medium as well as the melting of the fuel bundle in a nuclear power plant during an accident scenario. The objective of this paper is to present a numerical investigation, using computational fluid dynamics (CFD), of melting with internal heat generation for a vertical cylindrical geometry. As a precursor to the development of this numerical model, two classical configurations were also modeled. The first configuration consists of pure convection (no phase change) of a liquid with an external heat source and the second is melting with an externally applied heat source. For both of these two configurations, the numerical results were compared with experimental data from previous work.
Critical Heat Flux in Microchannels With an Adjustable Inlet Orifice
Authors: Odom Brent A. , Ortiz Carlos A. , Phelan Patrick E.
Published: July 2013

The benefits of eliminating instabilities in two-phase microchannel flow with inlet orifices come with costs. This study describes the tradeoffs between microchannels with and without inlet orifices, focusing on results from critical heat flux data obtained for various orifice sizes and mass fluxes. An adjustable inlet orifice controlled with a micrometer was placed in front of an array of 31 parallel microchannels each with a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For mass fluxes ranging from 186 kg m−2 s−1 to 847 kg m−2 s−1, critical heat flux (CHF) data were obtained for 7 different orifice sizes. For low flow rates that provided a low quality saturated inlet condition, the difference in CHF values was found to be minimal between open and almost closed orifice conditions. The smallest orifice achieved a CHF value of 5 W cm−2 less than the largest orifice size for a mass flux of 186 kg m−2 s−1, and 7 W cm−2 less for a mass flux of 433 kg m−2 s−1. For mass fluxes higher than 433 kg m−2 s−1, subcooled conditions were present at the orifice inlet, and the highest CHF values occurred with an orifice hydraulic diameter of 35 percent of fully open. For the higher mass flux cases, orifice sizes in the range of 1.8 percent to 28 percent of fully open caused CHF to occur at lower values than less restrictive orifice sizes. This was due to loss of cooling capacity from rapid pressure drop through the orifice. Slightly higher average channel pressures also decrease the refrigerant’s latent heat of vaporization. For the orifice sizes from 35 to 70 percent of unrestricted flow, a very minimal increase in pressure drop over fully open inlet conditions occurred and the general trend was higher CHF values. Very small inlet orifices are beneficial for steady state conditions that do not approach CHF; however, overly restricting the flow at the inlet to microchannels reduces cooling capacity significantly and will cause early onset of CHF. A slightly restrictive inlet orifice will increase CHF.
Thin Film Evaporation Using Nanoporous Membranes for Enhanced Heat Transfer
Authors: Xiao Rong , Maroo Shalabh C. , Wang Evelyn N.
Published: July 2013

Recent advancements in integrated circuits demand the development of novel thermal management schemes that can dissipate ultra-high heat fluxes with high heat transfer coefficients. Previous study demonstrated the potential of thin film evaporation on micro/nanostructured surfaces [1–11]. Theoretical calculations indicate that heat transfer coefficients on the order of 106 W/m2K and heat fluxes of 105 W/cm2 can be achievable with water [1, 5–6]. However, in previous experimental setup, the coolant has to propagate across the surface which limits the increase in heat flux and the heat transfer coefficient, while adding complexity to the system design. This work aims to decouple the propagation of the coolant from the evaporation process through a novel experimental configuration. Thin nanoporous membranes of 13 mm diameter were used where a metal layer was deposited on the top surface to serve as a resistance heater. Liquid was supplied from the bottom of the membrane, driven through the nanopores by capillary force, and evaporated from the top surface. Heat transfer coefficient over 104 W/m2K was obtained with isopropyl alcohol (IPA) as the coolant, which is only two orders of magnitude smaller than the theoretical limit. This work offers insights into optimal experimental designs towards achieving kinetic limits of heat transfer for thin film evaporation based thermal management solutions.
Effects of Geometrical Parameters on Natural Convective Heat Transfer From Vertically-Mounted Rectangular Interrupted Fins
Authors: Mostafavi Golnoosh , Ahmadi Mehran , Bahrami Majid
Published: July 2013

Steady-state external natural convection heat transfer from vertically-mounted rectangular interrupted fins is investigated numerically and experimentally. To perform an experimental study, a custom-designed testbed was developed to verify the analytical and numerical results. FLUENT software was used in order to develop a 2-D numerical model for investigation of interruption effects. After regenerating, and validating the existing analytical results for fin spacing, a systematic numerical and experimental study was conducted on effect of fin interruption. Results show that adding interruptions to vertical rectangular fins enhances the thermal performance of fins. In a parametric study optimum interruption length for maximum fin performance was found and correlated.
Rice Husk Gasification Using N2 and Air-Steam for Partial Oxidation
Authors: Gordillo Gerardo , Mendez Maria C.
Published: July 2013

The concern for the reduction of the environmental impact caused by greenhouse emissions (CO2) from fossil fuel combustion processes is growing around the world. This has increased research on new energy technologies to produce clean fuels. One of them is the use of biomass as feedstock in gasification processes. The rice agriculture industry around the world produces a great amount of rice husk wastes (RHW) which show the potential for water, soil, and air pollution (including global warming by way of potent greenhouse emissions such as CH4) since waste handling system and structures for storage and treatment frequently are not appropriate. However, the concentration of the rice husk in industrial units makes this low Btu feedstock a viable source for locally based thermal gasification. The current paper presents results on both HRW adiabatic gasification modeling using air-steam blends for partial oxidation and pyrolysis kinetic model to determine, by thermogravimetric analysis (TGA), the RHW activation energy (E). The Chemical Equilibrium with Applications program (CEA), developed by NASA, was used to estimate the effect of both the equivalence ratio (ER) and the steam to fuel ratio (S:F) on adiabatic temperature, gas quality (gas composition and energy density), and energy recovery of an unlimited number of species (∼150). The thermogravimetric analysis (TGA) was carried out using N2 as carrier gas and under different heating rates (β: 10, 20, 40, and 50 °C/min). Furthermore, the activation energy (E) was estimated based in the results from TGA and using the isoconversional method (i.e., free-model). In general, for the range of parameters studied (0.2 < S:F < 0.8 and 1.5 < Φ < 6), the results from equilibrium adiabatic modeling (CEA) showed that increased ER and (S:F) ratios increase the production of H2 and CO2 but decrease the production of CO. Equilibrium temperature decreases with increased ER until ER = 3.0 whereas at ER > 3.0, the effect of ER on equilibrium temperature is negligible. Also, the activation energy average value, estimated from the kinetics model, results to be 233 kJ/kmol.
Failure Resistant Data Center Cooling Control Through Model-Based Thermal Zone Mapping
Authors: Zhou Rongliang , Wang Zhikui , Bash Cullen E. , Cader Tahir , McReynolds Alan
Published: July 2013

Due to the tremendous cooling costs, data center cooling efficiency improvement has been actively pursued for years. In addition to cooling efficiency, the reliability of the cooling system is also essential for guaranteed uptime. In traditional data center cooling system design with N+1 or higher redundancy, all the computer room air conditioning (CRAC) units are either constantly online or cycled according to a predefined schedule. Both cooling system configurations, however, have their respective drawbacks. Data centers are usually over provisioned when all CRAC units are online all the time, and hence the cooling efficiency is low. On the other hand, although cooling efficiency can be improved by cycling CRAC units and turning off the backups, it is difficult to schedule the cycling such that sufficient cooling provisioning is guaranteed and gross over provisioning is avoided. In this paper, we aim to maintain the data center cooling redundancy while achieving high cooling efficiency. Using model-based thermal zone mapping, we first partition data centers to achieve the desired level of cooling influence redundancy. We then design a distributed controller for each of the CRAC units to regulate the thermal status within its zone of influence. The distributed controllers coordinate with each other to achieve the desired data center thermal status using the least cooling power. When CRAC units or their associated controllers fail, racks in the affected thermal zones are still within the control “radius” of other decentralized cooling controllers through predefined thermal zone overlap, and hence their thermal status is properly managed by the active CRAC units and controllers. Using this failure resistant data center cooling control approach, both cooling efficiency and robustness are achieved simultaneously. A higher flexibility in cooling system maintenance is also expected, since the distributed control system can automatically adapt to the new cooling facility configuration incurred by maintenance.
Effect of Voids on Solidification of Phase Change Materials Infiltrated in Graphite Foams
Authors: Sedeh Mahmoud Moeini , Khodadadi J. M.
Published: July 2013

As a fundamental process during production of composite thermal energy storage systems, infiltration of phase change materials (PCM) leads to formation of voids (air pockets) inside the pores of graphite foams. The presence of voids inside graphite cells (i.e. the presence of air pockets next to the conductive walls of the porous structure) markedly affects the thermal and phase change behavior of the composite. Therefore, it is vitally important to investigate the effect of voids on phase change behavior of latent heat energy storage composites. In complementing recent work devoted to modeling of the infiltration of PCM into graphite foams, a numerical approach was employed to study the solidification of PCM infiltrated into a graphite pore in the presence of a void. For this purpose, a two-dimensional model of the porous structure was developed based on the typical geometrical features of the pores. Grid independence study was performed on different unstructured grid systems. Since more than one fluid phase is present in this problem (PCM being the liquid phase and air pocket or void as the gas phase), the volume-of-fluid (VOF) method was utilized for investigation of solidification problem and tracking the interface. Considering various forces operating at the scale of the pore (i.e. 500 microns in diameter), this problem is under the influence of surface tension, gravity, and pressure gradient. The simulation was transient and continued until the entire liquid PCM inside the pore freezes. The volume of final void space will represent a combination of infiltration and shrinkage voids. Results of the simulation indicate the presence of 9.8% void (from the infiltration process) that can greatly alter the solidification rate of the PCM inside the pore. It is concluded that formation of shrinkage void during solidification can be predicted using this multi-phase model. For verification purposes, the volume of the predicted infiltration void was compared to reported experimental measurements and the volume of shrinkage void was compared to theoretical volume change. Good agreements were found in both cases.
Cooling of High-Performance Server Modules Using Direct Immersion
Authors: Sridhar Aravind , Styslinger Sarah , Duron Christopher , Bhavnani Sushil H. , Knight Roy W. , Harris Daniel , Johnson R. Wayne
Published: July 2013

An alternative to air-cooling of high performance computing equipment is presented. Heat removal via pool boiling in FC-72 was tested. Tests were conducted on a multichip module using 1.8 cm × 1.8 cm test die with multiple thermal test cells with temperature sensing capability. Measurements with the bare silicon die in direct contact with the fluid are reported. Additional testing included the test die directly indium-attached to copper heat spreaders having surface treatments. A screen-printed sintered boiling-enhanced surface (4 cm × 4 cm) was evaluated. Tests were conducted on an array of five die. Parameters tested include heat flux levels, dielectric liquid pool conditions (saturated or subcooled), and effect of neighboring die. Information was gathered on surface temperatures for a range of heat flux values up to 12 W/cm2. The highest heat dissipated from a circuit board with five bare die was 195 W (39 W per die). Addition of the heat spreader allowed heat dissipation of up to 740 W (from a five-die array). High-speed imaging was also acquired to help examine detailed information on the boiling process. Numerical modeling indicated that placing multiple boards in close proximity to each other did not degrade performance until board spacing was reduced to 3 mm.
Analysis of a Hybrid PCM-Air Heat Sink
Authors: Kozak Y. , Abramzon B. , Ziskind G.
Published: July 2013

Phase-change materials (PCMs) can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect may be utilized in thermal energy storage and passive thermal management. In order to enhance the rate of heat transfer into PCMs, various techniques have been suggested, like fins, metal and graphite-compound matrices, dispersed high-conductivity particles inside the PCM, and micro-encapsulation.

The present work deals with a hybrid PCM-air heat sink. The heat is dissipated on the heat sink base, and may be either absorbed by the PCM stored in compartments with conducting walls, or dissipated to the air using fins, or both. The heat sink is made of aluminum 6061. Eicosane (C20H42, 96% purity, nominal melting temperature 36.7°C) is used as the PCM. In order to exclude the effect of sensible heating below the melting temperature, a controllable environment is used. The latter is created in a programmable forced-circulation oven.

A simplified thermal model is developed for a conservative estimation of temperature growth of the heat sink base. The results of this model are compared to the experimental results. The relative contributions of heat accumulation, both by latent and sensible heat, and of heat removal by air are presented and discussed.
Analysis of the Equilibrium Approximation in Chemical Ablation of Thermal Protective Systems
Authors: Barr B. W. , Ezekoye O. A.
Published: July 2013

A quasi-steady-state ablation model is used to investigate the behavior of thermochemically ablating systems in equilibrium and nonequilibrium surface thermochemistry regimes. The model is simplified to allow extraction of relevant nondimensional parameters and comparison with existing experimental data on solid carbon combustion. Good agreement is found between model predictions and experimental data, and the data and model are collapsed in terms of the B number and surface Damkohler number. A new formulation for the surface Damkohler number is proposed, and a relationship between the B number and this Damkohler number is derived for the surface equilibrium and nonequilibrium regimes. The Damkohler formulation is applied to the reentry scenario, and the behavior of the B number in this context is explored. Nondimensional parameters governing behavior in the nonequilibrium regime are determined for graphite oxidation, and the results are extrapolated to more complex surface thermochemistry conditions.
Experimental Analysis of Dual-Evaporators Hybrid Two-Phase Cooling Loop
Authors: Park Chanwoo , Crepinsek Michael
Published: July 2013

A mechanical pump-assisted and capillary-driven (hybrid) two-phase cooling loop, with dual-evaporators in the same loop placed in parallel and series, was constructed to experimentally investigate the performance of the multi-evaporators cooling loop. This paper discusses various heat input experiments using the dual-evaporators loop that were tested up to 1200 Watts, or 600 Watts (102 W/cm2) for each evaporator. Difficulties and limitations experienced with both parallel and series tests are discussed. It is found from the tests that the total heat inputs in the system determine the system temperatures and pressures and the individual heat input to each evaporator determines the evaporator temperatures. Setting up the evaporators in parallel allows for more cooling than series.
CFD Simulation of Confined Non-Premixed Flames
Authors: Dabade Tanaji M. , Li Xianchang , Chen Daniel , Lou Helen , Martin Christopher , Li Ku-Yen , Singh Kanwar Devesh
Published: July 2013

Material processing furnaces are the key component of the manufacturing industries. The burners used in these furnaces require precise control over the flame structure such as flame shape, height, and width. This study mainly focused on the simulation of the flame structure with Computational Fluid Dynamics (CFD) approach. ANSYS Fluent 13.0 was used to predict the flame characteristics in an enclosed cylinder. Non-premixed combustion model was applied to this combustion phenomenon. To control the flame structure, a micro jet at the centre of the burner is introduced. The effect on flame parameters with varying flow rates of micro jet, fuel jet and co-flow jet is examined. This study confirms the experimental study by Sinha et al. [1], which concluded that an air micro jet at the center of a non-premixed flame can control the flame height and luminosity. Moreover, this paper visualizes the thermo chemistry and transport phenomenon of non-premixed combustion process. Emissions from the combustion are monitored for different boundary conditions. This study shows that innovative strategies can be developed for the precise control over the different types of flames with the help of numerical modeling.
Experimental Determination of Temperature-Dependent Thermal Conductivity of Solid Eicosane-Based Nanostructure-Enhanced Phase Change Materials
Authors: Nabil Mahdi , Khodadadi J. M.
Published: July 2013

The effective thermal conductivity of composites of eicosane and copper oxide nanoparticles in the solid state was measured experimentally by using the transient plane source technique. Utilizing a controllable temperature bath, measurements were conducted at various temperatures between 10 and 35°C for the solid samples. In the course of preparation of the solid specimen, liquid samples (0, 1, 2, 5 and 10 wt%) were poured into small diameter molds and were degased within a vacuum oven. The molds were then subjected to either ambient solidification or ice-water bath freezing method. Measured thermal conductivity data of the composites were found to be nearly independent of the measurement temperature for a given loading of CuO nanoparticles regardless of the solidification procedure. Irrespective of the solidification method, as the melting temperature was approached, thermal conductivity data of the solid disks rose sharply for both sets of experiments. The composites prepared using the ice-water bath solidification scheme consistently exhibited lower values of thermal conductivity when compared to the samples which prepared under ambient solidification method. This behavior might be due to the greater void percentage of ice-water bath samples and/or crystal structure deviations due to phase transition method.
Experimental Study of Heat Transfer Characteristics for Horizontal-Tube Falling Film Evaporation
Authors: Mu Xingsen , Yang Yong , Shen Shengqiang , Liang Gangtao , Gong Luyuan
Published: July 2013

The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation (MED) desalination plant due to the higher heat transfer coefficient under quite small temperature differences. In the present study, an experimental platform for horizontal-tube falling film evaporation was set up to measure its heat transfer characteristics. Results indicate that heat transfer coefficient (h) for both fresh water and seawater are almost independent with heat flux. The h increases firstly and then decreases with growth of Re. Along the tube circumference, the h increases after decreasing. In addition, the distribution of h for fresh water and seawater at the different evaporation temperatures and Reynolds number (Re) are also provided.
Points to Note in Modeling Fire With Computational Fluid Dynamics
Authors: Qu L. , Chow W. K.
Published: July 2013

Computational Fluid Dynamics (CFD) is a popular design tool in many projects for ensuring fire safety through performance-based design. However, there are always challenges on the quality and uncertainties of the CFD simulated results. Two points raised are on the grid size and free boundary conditions. A simple corridor fire with a small design fire is taken as an example to address these two points in this paper. The CFD model Fire Dynamics Simulator (FDS) version 5 was taken as the simulation tool Two-dimensional and three-dimensional simulations are compared. The geometry is proposed to outside for better description on minimizing opening boundary.
Selection of Fuel in Different Occupancy Types by Computational Fluid Dynamics
Authors: Lau K. W. , Chow W. K. , Fong N. K.
Published: July 2013

Computational Fluid Dynamics (CFD) has been widely used to evaluate the effects of fire on people and buildings by predicting temperature, radiation heat flux, smoke visibility and combustion products. In this paper, CFD was used to study plume temperature, radiation heat flux, visibility and carbon monoxide concentration in a building fire. The CFD software Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology of USA is generally adopted to model the various fire scenarios in buildings.

For FDS modeling, the type of fuel is considered an important input parameter which has a significant effect on the simulation results of different types of combustibles, hence the fire hazard evaluation. In the combustion model of FDS, the fuel and oxygen are burnt to give fire products such as carbon dioxide, water, carbon monoxide and soot. Fuels including propane, wood, polyurethane and polyvinyl-chloride were used in the simulation. By using the same fire size and fuel bed for different combustibles under study, a peak fire size of 3 MW was adopted. Simulations were then carried out with different fuel and respective reaction parameters of these combustibles. Simulation results, the output quantities of plume temperature, visibility, carbon monoxide concentration and radiation heat flux for different scenarios obtained were analyzed and evaluated.

It was observed that there were reasonable differences in the results from the fuel packages of different combustibles. These differences were evaluated and their significance was discussed.
CFD Simulation and Performance Analysis of CJD Burner for Intensified Flash Smelting Process
Authors: Chen Zhuo , Long Peng , Sun Zhiqiang , Zhou Jun , Zhou Jiemin
Published: July 2013

The flash smelting process has been widely acknowledged as a successful modern pyro-metallurgical technology because of its good production flexibility. In past decades, great efforts have been put on the equipment improvement in order to achieve a highly intensive and efficient flash smelting process. However, along with the increasing of the productivity and the intensification of the process, technical problems such as the un-smelted materials accumulated in the settler and the dust generation ratio going higher are found occurring more frequently than before. All these problems however indicate degeneration in the performance of the central jet distributor (CJD) burner. A study was then made on the combustion and reaction processes in the flash furnace equipped with a CJD burner. A steady-state turbulent model was developed and a discrete phase model was included to investigate the velocity and temperature changes of both the gaseous and particle phases in the reaction shaft. The deviation of the numerical model is estimated to be less than 6%. The simulation results reveal a serious delay in the ignition of concentrate particles after they are fed into the furnace. Minor modification was also made by CFD computation, attempting to improve the particle ignition speed, but it was found not so effective. The main reason for the decreased smelting efficiency is found to be the poor mixing between the gaseous and particle phases under the intensified condition. These appeal for a great improvement in the performance of the CJD burner.
Deflagration Analysis of Aluminum Droplet Combustion
Authors: Dikici Birce , Pantoya M. L. , Shaw B. D.
Published: July 2013

The evaporation and combustion of nanometric aluminum particles with an oxidizer MoO3 is analyzed. The analysis was performed to correlate individual Al particle gasification rates to macroscopic flame propagation rates observed in flame tube experiments. Examination of various characteristic times relevant to propagation of a deflagration reveals that particles below about 1.7 nm in diameter evaporate before appreciable chemical reactions occur. Experimental studies use Al particles greater than 1.7 nm in diameter such that a diffusion flame model was developed to better understand the combustion dynamics of multiphase Al particles. The results showed that it is unlikely that droplets will fully evaporate before reacting in the gas phase. A droplet evaporation and combustion model was further applied to quantify single droplet reaction velocities in comparison to the bulk flame propagation measurements observed in the literature. The diffusion flame model predicted orders of magnitude slower propagation rates than experimentally observed. These results imply that another reaction mechanism is responsible for promoting reaction propagation or modes other than diffusion play a more dominant role in flame propagation.
Analysis of Thin-Film Evaporation Through Sintered Wick Microstructures
Authors: Bodla Karthik K. , Murthy Jayathi Y. , Garimella Suresh V.
Published: July 2013

Heat pipes are passive heat transport devices capable of transporting heat over long distances without a substantial drop in temperature. The topology and microstructure of the wick material play a crucial role in determining the thermal performance of such devices. Accurate modeling of pore-scale transport phenomena is thus important. In this study, pore scale analysis of thin-film evaporation through sintered copper wicks is performed. X-ray microtomography is employed to generate geometrically-faithful, feature-preserving meshes. Three commercially used sintered wicks of varying particle size ranges (45–60 μm, 106–150 μm and 250–355 μm) and with approximately 61% porosity are considered. The capillary pressure, effective pore radius, percentage thin film area and evaporative mass and heat fluxes are computed using the Volume of Fluid (VOF) model in FLUENT. Two different solution strategies are employed to stabilize the numerical solution and to improve convergence. After verifying that these strategies yield the correct solution, the VOF model is used to obtain static meniscus shapes in the pore space of the sintered wick samples. The meniscus shape is then held fixed and steady-state, thin-film evaporation analysis is performed. Liquid-vapor phase change heat transfer is modeled using a modified Schrage equation. Based on the present analysis, the best performing sample (particle size range) is identified along with the optimum contact angle.