bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Investigation of a Phase Change Material (PCM) Module With Internal Fins
Authors: Gharebaghi Maryam , Sezai Ibrahim
Published: April 2009

The main objective of this study is to investigate the enhancement of energy storage rate of a thermal energy storage unit filled with a PCM by inserting a fin array system. Heat is transferred to the unit through its base plates, to which aluminum fins are attached. The PCM, a commercial paraffin wax, is stored between the fins. A mathematical model, based on finite volume method, for a 2-Dimensional domain is developed for solving the melting problem. Transient simulation has been performed for different fin and PCM layer thicknesses, while the ratio of PCM layer to fin thickness is held constant, for different base plate temperatures. Computational results show that the phase change time and consequently heat absorption and release rate depends on thermal and geometrical parameters such as fin spacing, thickness and height. Results are presented in terms of Nusselt number and melt fraction versus time.
Construction of a Broad-Based Experimental and Computational Test Capability for High Power Wide Bandgap Semiconductor Devices
Authors: Carter Jason A. , Roth Matthew D. , Horgan Michael W. , Shellenberger Lisa , Hoffmann Daniel P. , Stitt Mark D.
Published: April 2009

In this paper, the authors will discuss the development and implementation of a test stand to assess the impact of temperature on the performance of commercial X-band gallium nitride (GaN) on silicon carbide (SiC) high electron mobility transistors (HEMTs) designed for radio frequency (RF) communications platforms. The devices are tested under a range of operating temperatures and under a range of electrical operating conditions of variable gate and source-drain voltages to assess the impact of temperature on core operational parameters of the device such as channel resistance and transconductance. This test capability includes infrared thermography and transient thermal impedance measurements of the device. In addition to the experimental effort, the initial construction of a finite-volume numerical analysis model of the device will be discussed. The focus of these models will be the accurate assessment of device thermal impedance based on assumed thermal loads and eventually the assessment of accumulated thermal stresses at the material interfaces within the device and package structure.
An Algorithm for Variable Density Flows
Authors: Rahul S. , Eswaran Vinayak , Shyam Sundar P.
Published: April 2009

An algorithm for solving variable density incompressible flows is presented. This algorithm is implemented on a finite volume based fully implicit solver employing a non-staggered hexahedral structured grid arrangement. Momentum interpolation is used for pressure-velocity coupling and the resulting Poisson equation for pressure is modified to a Helmholtz equation for better convergence. This algorithm is used for solving problems involving the variation of density due to temperature, fluid mixing and turbulent combustion flows.
Factor Analysis for Forced and Mixed Convection Laminar Heat Transfer in a Horizontal Tube Using Artificial Neural Network
Authors: Tam L. M. , Ghajar A. J. , Tam H. K. , Tam S. C.
Published: April 2009

Artificial neural network (ANN) has shown its superior predictive power compared to the conventional approaches in many studies. However, it has always been treated as a “black box” because it provides little explanation on the relative influence of the independent variables in the prediction process. In our previous work (Tam et al., 2006), an index of contribution extracted from the ANN correlation was primarily introduced to analyze the relative importance of the associated independent variables on our forced convective turbulent heat transfer data in a horizontal tube (Ghajar and Tam, 1994). The most and the least important variables were determined quantitatively and found to be thoroughly conforming to the empirical correlation and physical phenomena. In this study, we have extended the method to a more complicated data set, forced and mixed convection developing laminar flow in a horizontal tube with uniform wall heat flux. The parameters influencing the Nusselt number for this data set were Reynolds number, Grashof number, Prandtl number, the length-to-diameter ratio, and the bulk-to-wall viscosity ratio. Due to the complexity of the problem it is difficult to determine the influence of the individual independent variables. According to literature, for laminar heat transfer involving entrance and mixed convection effects, Rayleigh number and Graetz number are both important. Through the re-arrangement of those variables, the factor analysis clearly showed that the Rayleigh number has a significant influence on the mixed convection heat transfer data and the forced convection heat transfer data is more influenced by the Graetz number. The results clearly indicate that the factor analysis method can be used to provide an insight into the influence of different variables or a combination of them on complicated heat transfer problems.
Robust Thermal Design for Power Device Package Using Response Surface Method and Monte Carlo Simulation
Authors: Yokono Yasuyuki , Hisano Katsumi , Hirohata Kenji
Published: April 2009

In the present study, the robust thermal design of a power device package was accomplished using thermal conduction calculation, design of experiment, response surface method and Monte Carlo simulation. Initially, the effects of the design parameters on the solder strain were examined in terms of the thermal expansion difference as a result of unsteady thermal conduction simulation. From the factorial effects of design parameters, the design proposals were screened. Then, robustness of the thermal resistance was evaluated for the three design proposals obtained. The thermal resistances were calculated by solving the steady thermal conduction equation under the design of experiment conditions. The solder thickness, the substrate thickness, and the cooling fin performance were considered as the fluctuation factors, assuming the error associated with manufacturing process. Using a response surface method, the values of thermal resistance were expressed as a function of the design variables. The variances of the thermal resistance were examined based on Monte Carlo simulations. Related to the cooling fin design, the Pareto line showing the trade-off relation between the fin dimension and the fan velocity was obtained. By repeating the Monte Carlo simulations, the Pareto solution was calculated so that the thermal resistances satisfy the criteria in the position of 95 percrntile of the thermal resistance variation. Under the same flow velocity conditions, the fin dimensions become about 10% higher compared to the case where the manufacturing error was not taken into account. By carrying out the thermal design following this Pareto line, even if the manufacturing error was taken into consideration, the thermal resistance could satisfy the desired value.
Sensitivity of the Human Comfort Equation: An Example of the Use of Global Sensitivity to Evaluate Sensitivity Interactions
Authors: Emery A. F. , Wu MingHang H.
Published: April 2009

Many models of engineering and scientific systems involve interactions between and among the parameters, stimuli, and physical properties. The determination of the adequacy of models for predictions and for designing experiments generally involves sensitivity studies. This is particularly true for experiments by which properties are to be estimated. Good designs mandate that the experiments be sensitive to the parameters sought with little interaction between them because such interaction generally confuse the estimation and reduce the precision of the estimates. For design purposes, analysts frequently want to evaluate the sensitivities of the predicted responses to specific variables but it the variables interact it is often difficult to separate the effects. Global sensitivity is a technique by which one can evaluate the magnitude of the interactions between multiple variables. In this paper the global sensitivity approach is applied to the human comfort equation. The aim is twofold: 1) to demonstrate the usefulness of the global sensitivity approach, 2) to increase our understanding of how human comfort is affected by activity, clothing, and environmental conditions. It is found that when occupants are uncomfortable there is little interaction and that one can predict the effects of changing several environmental conditions at once by adding the separate effects. But when occupants are comfortable there is a large interaction and the effects cannot be treated separately.
Simulation of Heat Transfer and Effects on Reaction-Coupled Transport Processes in SOFCs
Authors: Yuan Jinliang , Lv XinRong , Sunde´n Bengt , Yue Dantin
Published: April 2009

For anode-supported plate solid oxide fuel cells (SOFCs), the size of the porous anode is bigger than that of the fuel gas flow duct in terms of thickness and cross-sectional area. The heat transfer rates are controlled by various operating and design parameters, and have significant effects on chemical reactions and coupled transport processes. In this study, the considered composite duct is relevant for an intermediate temperature (600–800°C) plate design and consists of a porous anode layer for the internal reforming reactions of methane, the fuel gas flow duct and solid plate. A fully three-dimensional calculation method is developed to simulate and analyze heat transfer and combined effects on internal reforming/electrochemical reactions and the coupled transport processes, with purpose to reveal the importance of various parameters. The results show that the operating temperatures have significant effects on the chemical reactions, fuel gas distribution and overall performance.
Transient Heat Transfer for Parallel Flow of Helium Gas Over a Horizontal Plate
Authors: Liu Qiusheng , Fukuda Katsuya , Shibahara Makoto , Kikumoto Shingo
Published: April 2009

Forced convection transient heat transfer for helium gas at various periods of exponentially increase of heat input to a horizontal plate (ribbon) was experimentally and theoretically studied. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over a horizontal plate under wide experimental conditions. The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, τ, ranged from 46 ms to 17 s. The pressures were from 400 to 800 kPa. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. It was obtained that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities.
Numerical Investigation of Unsteady Mixed Convection Flow
Authors: Singh Param Jeet , Roy S.
Published: April 2009

Unsteady mixed convection flow over a vertical slender cylinder under the combined effects of buoyancy force and thermal diffusion with injection/suction has been studied where the slender cylinder is inline with the flow. The unsteadiness is introduced by time dependent free stream velocity of the fluid. The effect of surface curvature is also taken into account, especially for the applications such as wire and fiber drawing, where accurate predictions are desired. The governing boundary layer equations along with the boundary conditions are first converted into dimensionless form by a non similar transformation, and then resulting system of coupled nonlinear partial differential equations is solved by an implicit finite difference scheme in combination with the quasilinearization technique. The effects of various parameters on velocity, temperature profiles, on skin friction coefficient and heat transfer rate at the wall are reported in the present study.
Development of a Transparent Heater to Measure Surface Temperature Fluctuations for Pool Boiling and Spray Cooling
Authors: Griffin Alison R. , Vijayakumar Arun , Chen Ruey-Hung , Sundaram Kalpathy B. , Chow Louis C.
Published: April 2009

A heater designed to monitor surface temperature fluctuations during pool boiling and spray cooling experiments while the bubbles are simultaneously being observed has been fabricated and tested. The heat source was a transparent indium tin oxide (ITO) layer commercially deposited on a fused quartz substrate. Four copper-nickel thin film thermocouples (TFTCs) on the heater surface measured the surface temperature, while a thin layer of sapphire or synthetic fused silica provided electrical insulation between the TFTCs and the ITO. The TFTCs were micro-fabricated using the liftoff process to deposit the nickel and copper metal films. The TFTC elements were 50 μm wide and overlapped to form a 25 μm by 25 μm junction. TFTC voltages were recorded by a DAQ at a sampling rate of 50 kHz. A high-speed CCD camera recorded bubble images from below the heater at 2000 frames/second. From these images, the bubble outer diameter and contact ring diameter were clearly visible. A trigger sent to the camera by the DAQ program synchronized the bubble images and the surface temperature data. As the bubbles and their contact rings grew over the TFTC junction, correlations between bubble behavior and surface temperature changes were made. The 1–2°C temperature drops occurred as the contact ring moved over the TFTC junction during bubble growth and as the contact ring moved back over the TFTC junction during bubble departure.
Validation of a General Heat Transfer Correlation for Non-Boiling Two-Phase Flow With Different Flow Patterns and Pipe Inclination Angles
Authors: Tang Clement C. , Ghajar Afshin J.
Published: April 2009

A general heat transfer correlation for non-boiling gas-liquid two-phase flow with different flow patterns and inclination angles was developed. To verify the correlation, local heat transfer coefficients and flow parameters were measured for air-water flow in a pipe for the horizontal and slightly upward inclined (2°, 5°, and 7°) positions, and all the flow patterns in the entire flow map. The test section was a 27.9 mm stainless steel pipe with a length to diameter ratio of 95. A total of 763 data points were collected for horizontal and slightly upward inclined positions by carefully coordinating the liquid and gas superficial Reynolds number combinations. The heat transfer data were collected under a uniform wall heat flux boundary condition ranging from about 1800 to 10900 W/m2. The superficial Reynolds numbers ranged from about 740 to 26000 for water and from about 560 to 48000 for air. The general heat transfer correlation was validated with the 763 data points that were experimentally collected. The validation confirmed the robustness of the general two-phase heat transfer correlation to adequately predict heat transfer data for various flow patterns and inclination angles. The accuracy of the correlation to correlate the experimental data was further explored by applying various available void fraction correlations. The performance of the correlation when applied with the different void fraction correlations were compared and appropriate recommendations are made.
Characterization of Cryogen Spray Cooling With Thin-Film Thermocouple
Authors: Jia Wangcun , Nguyen Thang , Gill Jaskaran , Nelson J. Stuart
Published: April 2009

Cryogen spray cooling (CSC) has been used effectively to protect the epidermis during laser dermatologic surgery. However, the temperature reduction in human skin induced by CSC has not been reliably determined due to the short cooling period and significant temperature gradient in the skin. Therefore, CSC has not been optimized for different laser dermatologic surgery procedures. Although it would be desirable to measure in situ human skin temperature, embedding a sensor within 100 μm beneath the skin surface is not feasible. In addition, infrared skin temperature measurement is also not workable because the skin is covered with a cryogen layer of unknown thickness. In this study, we selected an epoxy which has similar thermal properties to that of human skin as our cooling target. Thin-film thermocouples (TFTC) were deposited directly onto the epoxy substrate using micro-fabrication techniques to minimize the thermal contact resistance between TFTC and the substrate. The negligible mass of TFTC also creates a minimal disturbance to heat flow across the surface. Due to the difference in thermoelectric property between thin film and leading wire, special sensor design and calibration procedures were developed. TFTC were calibrated from −46 to 50°C. The thermoelectric sensitivity is around 50–60% of that of bulk material. Skin phantom temperature reductions produced by a commercial medical laser nozzle at different spray durations were measured using the TFTC. The results not only help to elucidate the mechanisms involved in interaction between cryogen spray and human skin but also provide a thermal boundary condition for numerical modeling of laser dermatologic surgery.
Design Considerations for High Temperature, Ceramic Heat Exchangers
Authors: Wilson Merrill A. , Lewinsohn Charles , Cutts James
Published: April 2009

The recent developments in the energy industry have kindled renewed interest in producing energy (alternative fuels and electricity) more efficiently. This has motivated the development of higher temperature cycles and their associated equipment. In this paper we will discuss several design configurations coupled with the inherent properties of preferred ceramic materials to assess the viability and design reliability of ceramic heat exchangers for next generation high temperature heat exchangers. These analyses have been extended to conceptually compare the traditional shell and tube heat exchanger with shell and plate heat exchangers. These analyses include hydrodynamic, heat transfer, mechanical stress and reliability models. It was found that ceramic micro-channel heat exchanger designs proved to have the greatest reliability due to their inherent mechanical properties, minimal thermo-mechanical stresses while improving the performance efficiency in a compact footprint.
Buoyancy-Driven Circulation Flow of an Electrical Conductive Liquid in a Rectangular Annulus
Authors: Li Peiwen , Chan Cho Lik , Chen C. F.
Published: April 2009

A study on the characteristics of a buoyancy-driven flow in a rectangular circulation channel in a solar-energy-harvesting device is presented in this paper. The solar-energy-harvesting device is projected to convert solar radiation into electrical energy. As a first step of the energy conversion in the device, a flow is generated by an imbalance of buoyancy forces in the heating and cooling sections for a liquid in the circulation channel. Whereas solar energy is collected to provide the heat, free convection of ambient air provides the cooling in the device. The fluid used in the circulation channel is electrically conductive and has high thermal expansion coefficient. The present investigation focuses on the effects of channel dimensions on the buoyancy-driven flow field and uniformities of velocities. Both analytical and numerical approaches are applied in the study. Analytical closed-form solution is obtained by assuming uni-direction flow. Steady-state two-dimensional laminar solutions are obtained by numerical computation using QUICK scheme and SIMPLE algorithm.
Indirect Measurements of Coherent Thermal Emission From a Truncated Photonic Crystal Structure
Authors: Lee B. J. , Chen Y. -B. , Zhang Z. M.
Published: April 2009

Recently, coherent thermal emission characteristics have been demonstrated for both polarizations from a multilayer structure consisting of a one-dimensional (1D) photonic crystal (PC) coated on a silver layer, i.e., PC-on-Ag structure. The key to enabling coherent emission is to excite a surface wave at the PC-Ag interface in the stop band of the PC. A detailed experimental investigation is conducted to demonstrate coherent thermal emission in the near-infrared region from the fabricated PC-on-Ag structure. A Fourier-transform infrared spectrometer, together with a specular reflectance accessory and a polarizer, is used to measure the reflectance at incidence angles of 30° and 45°. In addition, an angle-resolved scatterometer is used to measure the reflectance at the wavelength of 891 nm. Measured reflectance shows very good temporal and spatial coherence. Furthermore, the magnitude of the evanescent waves at the PC-Ag interface is plotted in terms of the wavelength and the incidence angle. It is found that dip locations of the measured reflectance precisely match with the condition when the field intensity is largely enhanced.
Equilibrium Molecular Dynamics Simulation Study on the Effect of Nanoparticle Loading and Size on Thermal Conductivity of Nanofluids
Authors: Selvam R. Panneer , Sarkar Suranjan
Published: April 2009

Nanofluids have been proposed as a route for surpassing the performance of currently available heat transfer liquids for better thermal management needed in many diverse industries and research laboratories. Recent experiments on nanofluids have indicated a significant increase in thermal conductivity with 0.5 to 2% of nanoparticle loading in comparison to that of the base fluid. But the extent of thermal conductivity enhancement sometimes greatly exceeds the predictions of well established classical theories like Maxwell and Hamilton Crosser theory. In addition to that, these classical theories can not explain the temperature and nanoparticle size dependency of nanofluid thermal conductivity. Atomistic simulation like molecular dynamics simulation can be a very helpful tool to model the enhanced nanoscale thermal conduction and predict thermal conductivities in different situations. In this study a model nanofluid system of copper nanoparticles in argon base fluid is successfully modeled by equilibrium molecular dynamics simulation in NVT ensemble and thermal conductivities of base fluid and nanofluids are computed using Green Kubo method. The interatomic interactions between solid copper nanoparticles, base liquid argon atoms and between solid copper and liquid argon are modeled by Lennard Jones potential with appropriate parameters. For different volume fractions of nanoparticle loading, the thermal conductivities are calculated. The nanoparticle size effects on thermal conductivities of nanofluids are also systematically studied. This study indicates the usefulness of MD simulation to calculate thermal conductivity of nanofluid and explore the higher thermal conduction in molecular level.
Effect of Blockage Ratio on Heat Transfer Enhancement in a Channel by Flow Pulsation
Authors: Ji Tae Ho , Kim Seo Young , Hyun Jae Min , Rhee Gwang Hoon
Published: April 2009

Experiments are performed to investigate heat transfer enhancement from a heated square cylinder in a channel by pulsating flow. For all the experiments, the amplitude of the pulsating flow is fixed at A = 0.05. The effects of the pulsating frequency (0 Hz < fp < 60 Hz) and the blockage ratio of the square cylinder (β = 1/10, 1/8, and 1/6) on convective heat transfer are examined. The “lock-on” phenomenon is clearly observed for a square cylinder in the present flow pulsation. The influence of blockage ratio on heat transfer enhancement is examined. The experimental results show that as the blockage ratio decreases, the heat transfer enhancement is more pronounced and lock-on regime broadens.
Effect of an Isothermal Baffle on Pseudosteady-State Natural Convection Inside Spherical Containers
Authors: Hosseinizadeh S. F. , Duan Yuping , Khodadadi J. M.
Published: April 2009

The influence of an isothermal thin baffle on pseudosteady-state natural convection within spherical containers is studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent, two-dimensional axisymmetric form of the governing continuity, momentum and energy equations are solved. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106 and 107, baffles with 3 lengths positioned at 5 different locations were investigated. In effect, a parametric study involving 60 cases were performed. The computational results were benchmarked against previous data available in the literature by comparing the heat transfer correlations, temperature distribution and streamline patterns for cases with no baffle. In general, regardless of the presence of an isothermal baffle, fluid that is heated adjacent to the surface of the sphere rises replacing the colder fluid which sinks downward. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid. This behavior can lead to onset of oscillations in the temperature and flow fields. Partly due to the blockage effect of an isothermal thin baffle and also the extra heating afforded by the baffle, multi-cell recirculating vortex structures are observed. The number and strength of these vortices depend on the position and length of the baffle. The additional heat that is brought into the baffle through the isothermal baffle is directly linked to creation of a counter clockwise rotating vortex next to the baffle. This baffle, in turn, directs hot fluid into the center of the sphere and disrupts thermal stratified layers. For the majority of the length and location combinations investigated, the Nusselt number is lower than the case with no baffle, however the time rate of rise of the bulk temperature can be greater for some combinations. The extent of heat transfer modifications depends on the Rayleigh number, length and location of the baffle.
Experimental Investigation of Solid-Liquid Phase Change in Cylindrical Geometry
Authors: Katsman L. , Dubovsky V. , Ziskind G. , Letan R.
Published: April 2009

The present study explores experimentally the process of melting of a phase change material (PCM) in cylindrical geometry. The study is performed with a commercially available paraffin-type material with the melting point of about 28 degrees Celsius. The experiments are conducted using vertical tubes of four different diameters, filled with the PCM and immersed in a water bath. In each tube the experiments are performed at the water bath temperatures of 10, 20 and 30°C above the melting point of the paraffin. The tubes are transparent, and the melting process is monitored and recorded by a digital camera. Each tube is thermally insulated at the bottom, and at its top open to atmosphere, to allow free expansion of the melt liquid. The digital pictures of the melting process were analyzed, and the results were graphically presented as melt fraction vs. time, showing for the plain tubes the effects of tube diameter and temperature difference. Numerical simulations are performed in order to provide an insight into the mechanisms governing the process. Generalization of the results is attempted based on the dimensionless groups, including the Fourier, Stefan, and Rayleigh numbers. A correlation connecting the melt fraction with these dimensionless groups is suggested.
Scale Modelling Studies on Smoke Movement in Vertical Shafts of Tall Buildings
Authors: Zhu J. , Huo R. , Chow W. K.
Published: April 2009

There is growing interest in understanding fire spreading in a vertical shaft for very tall buildings in the Far East. Experimental study on smoke movement induced by buoyancy in a model will be reported in this paper. A series of experiments had been performed in a 1/10 scale model of typical vertical shafts in China. Six scenarios were set up by varying the ventilation conditions by opening different side wall and top vents at the shaft. The smoke temperature and concentrations of carbon monoxide in the shaft model were measured. Those are useful to justify the reduction of buoyancy in the vertical shaft. Measured results are compared with numerical simulation with Computational Fluid Dynamics. Relations of buoyancy reduction and the toxic gases concentrations are studied.
A Quantitative Free Convection DNA Amplifier
Authors: Daly John , Davies Mark
Published: April 2009

The Polymerase Chain Reaction (PCR) has been used extensively to amplify targeted nucleic acids for many applications in molecular biology and, increasingly, in medical diagnostics. Outlined in this paper is a PCR device which takes account of the advantages offered by free convection. The design is, in it fundamental format a time-wise isothermal well-based thermocycler. A temperature gradient induced across the well causes convection forces to circulate the sample through the required temperatures necessary for amplification. Quantitative amplification is demonstrated with real time measurements of SYBR Green I fluorescence within the free convective DNA amplifier. Amplification of an 86-bp fragment of the pGEM®-T vector (Promega) is performed in a 25μl volume in eight minutes. A 10-fold dilution series and methods for calculating effective cycle times are presented. Also detailed within this paper are PIV and thermal imaging results of the free convection cavity. This device presents an opportunity for the development of a practical and inexpensive gene-expression measurement system.
The Effect of Nozzle Geometry on Pressure Drop and Heat Transfer to Free Surface Liquid Jet Arrays
Authors: Whelan Brian P. , Robinson Anthony J.
Published: April 2009

Due to the current trend of miniaturization of electronic components, higher heat fluxes are encountered. Current fan cooling methods are approaching their maximum cooling capabilities leading to the investigation of liquid cooling techniques. An experimental study of the cooling capabilities of liquid water impinging jet arrays, with a view for use in the cooling of electronic chips, is presented. The 3.0 mm thick jet nozzle plate contained 45 individual jets of 1mm diameter with 5 mm spacing between each jet. The arrays impinged onto a copper surface of 31.5 mm diameter, which dissipated approximately 200 W of heat. The inlet and outlet geometries of the jets were varied with a view of determining an optimal configuration with regards to maximum heat transfer to the impinging jets for the minimal pressure drop across the nozzle plate. Each array was tested under free-surface flow conditions, with a constant jet-to-target spacing of H/d = 20 for a Reynold’s number range of approximately 300–10,000. The results indicate that chamfering of the inlet decreased the pressure drop for a given average heat transfer coefficient, whilst chamfering of the outlet provided a greater heat transfer for a given Reynold’s number.
Generic Correlation of Spray Distance With In-Flight Particle Behavior for Different Kinds of Materials in Air Plasma Spraying
Authors: Zhang Wei , Zheng Lili , Zhang Hui , Sampath Sanjay
Published: April 2009

This paper investigates the influence of spray distance on inflight particle status for Yttria Stabilized Zirconia (YSZ), Mo and NiCrAlY thermal spray powders in Ar-H2 plasma through modeling and experiments. Distributions of individual particle characteristics such as temperature, velocity and size at the point of the maximum particle flux and points of the squared grids were measured for a few plume cross-sections. Numerical simulations were performed using the experimental conditions, and parametric studies were also performed. In-flight particle characteristics and their distributions were presented at different standoff/spray distances to reveal the heating and melting behaviors of different kinds of materials such as metal, alloy and ceramics powders. It can be found that for ceramic, metal and alloy powders, the particle velocity and temperature all increase with spray distance and then decrease after reaching maximum. However the positions of maximum temperature and velocity depend strongly on the material and size of the particles. The statistical distributions of temperature and velocity at different spray distances were also presented. It can be observed that the temperature distribution becomes narrower as spray distance increases. The occurrence of such behavior can be explained by analyzing particle kinetic energy and molten status. The correlations between powder properties and spray conditions have been established and they can serve as guidance for spray distance optimization of air plasma spraying for metal, alloys and ceramics particles.
Flow Boiling Visualization of R-134a in a Vertical Channel of Small Diameter
Authors: Marti´n-Callizo Claudi , Palm Bjo¨rn , Owhaib Wahib , Ali Rashid
Published: April 2009

The present work reports on flow boiling visualization of refrigerant R-134a in a vertical circular channel with internal diameter of 1.33 mm and 235 mm in heated length. Quartz tube with a homogeneous ITO-coating is used allowing heating and simultaneous visualization. Flow patterns have been observed along the heated length with the aid of a digital camera with close-up lenses. From the flow boiling visualization, seven distinct two-phase flow patterns have been observed: Isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Finally, the experimental flow pattern map is compared to models developed for conventional sizes as well as to a microscale map for air-water mixtures available in the literature, showing a large discrepancy.
Single-Phase Heat Transfer in Micro-Tubes: A Critical Review
Authors: Krishnamoorthy Chandramoulee , Rao Rahul P. , Ghajar Afshin J.
Published: April 2009

This review paper specifically concentrates on heat transfer in micro-tubes and eleven experiments (on liquid flow) and two experiments (on gaseous flow) from 1991 to 2007 are reviewed critically with respect to measurement techniques, instrumentation; and factors like surface roughness and diameter that may play an important role at these small scales. Moreover, a comprehensive list of numerical and analytical results (for both liquid and gaseous flows) is presented in this paper. Interestingly, the effect of surface roughness on heat transfer does not seem to have been investigated thoroughly, as it has been observed to play a key role in influencing heat transfer at small diameters. The state-of-art review thus provides the contemporary experimenters in the field of mini-micro channel heat transfer, this tabulated data that can be used to understand how the different parameters affect the heat transfer in these small scales and a data-bank to validate future numerical and experimental work. The present study identifies the various factors that have contributed in the disparity of results found in the literature and finds that there is a need to investigate certain issues like the effects of roughness, diameter, and secondary flow due to buoyancy on heat transfer and transition. Moreover, it was observed that the start and end of the transition region at these small diameters are not validated by the any of the existing macro-scale correlations.
Visualization of Water Transport and Freezing Process in a Gas Diffusion Layer of PEFC During Low Temperature Operation
Authors: Nishida Kosuke , Murakami Takeshi , Kobayashi Kazutaka , Tsushima Shohji , Hirai Shuichiro
Published: April 2009

During low temperature operation of polymer electrolyte fuel cell (PEFC), water flooding in a cathode electrode is a critical barrier to achieve high power density, because liquid water condensed in the porous gas diffusion layer (GDL) blocks oxygen transport to the active reaction sites. Furthermore, during startup below the freezing point, liquid water in the cathode GDL or on the catalyst layer (CL) surface is frozen and the cell voltage is suddenly decreased. In this study, we experimentally investigate liquid water transport and freezing process in a cathode GDL of PEFC at low temperature operations using an optical diagnostic. The results showed that liquid water behavior and ice formation at the cathode electrode are largely affected by operating parameters such as current density and ambient temperature. Furthermore, in order to prevent water flooding, we propose the structure of cathode GDL that has several slits for water removal. With this cathode GDL structure, it was found that liquid water accumulated inside the cathode electrode is rapidly removed and cell voltage is improved.
Subcritical and Supercritical Nanodroplet Evaporation: A Molecular Dynamics Investigation
Authors: Mikkilineni S. , Landry E. S. , McGaughey A. J. H.
Published: April 2009

Molecular dynamics simulations are used to investigate the subcritical and supercritical evaporation of a Lennard-Jones (LJ) argon nanodroplet in its own vapor. Using a new technique to control both the ambient temperature and pressure, a range of conditions are considered to define a transition line between subcritical and supercritical evaporation. The evaporation is considered to be supercritical if the surface temperature of the droplet reaches the LJ argon critical temperature during its lifetime. Between ambient temperatures of 300 K and 800 K, the transition from subcritical to supercritical evaporation is observed to occur at an ambient pressure 1.4 times greater than the LJ argon critical pressure. For subcritical conditions, the droplet lifetimes obtained from the simulations are compared to independently predicted lifetimes from the D2 law.
Numerical Simulation of Laminar Mixed Convection of Air Flow Through Uniformly Heated Vertical Tubes
Authors: Raisee M. , Shahraeeni M.
Published: April 2009

This paper discusses laminar mixed convection of air flow through vertical tubes. Calculations were performed by solving the Navier-Stokes and energy equations for a number of heating lengths. The Reynolds number based on the fluid bulk velocity and diameter of the tube is Re = 500 and Grashof number based on wall heat flux is Gr = 106. The numerical results have been obtained using a finite-volume code which solves the governing equations in axi-symmetric coordinate system. The pressure field is obtained with the SIMPLE algorithm. The HYBRID scheme is used for the convective terms. The computer code was validated by comparing its predictions with the reported analytical, numerical and experimental results. For various heating lengths the values of Nusselt number and friction coefficient are presented and the effects of heating length on these parameters are studied. It was found that for the buoyancy-aided convection, the velocity in the vicinity of the wall increases while decreases in the core region. These result in an enhancement of wall heat transfer coefficient.
High Power Density Dissipations by Spray Cooling
Authors: Escobar Sergio , Sharma Ratnesh , Fabris Drazen , Gonzalez Jorge E. , Bash Cullen
Published: April 2009

An experimental set up is developed to deliver monodisperse microdroplets at different volumetric flows over a hot surface. In-house built circuitry activates a Thermal Ink Jet (TIJ) atomizer to spray controlled amounts of deionized water to cool a small area of 5 mm by 3 mm on a copper block. The temperature history is monitored through K-type thermocouples and the dissipated power is calculated using Fourier’s law. Digital cameras are used to track the wetted area unveiling considerable changes in the wetted area present close to the CHF. The controlled flow delivery and small droplets (50 μm diameter) result in small wetted areas near the CHF. The wetted area is measured and used to compute the dissipated heat. Experimental results show dissipation of heat fluxes up to 4 kW/cm2 based on the best estimate of the wetted area.
Optical Resonance in Dielectric Micro-Sphere for Temperature Measurement
Authors: Rahman Anisur , Kumar Sunil
Published: April 2009

We describe the effect of Transverse Electric (TE) wave propagation in dielectric micro-spheres for optical resonances based on Whispering Gallery Mode (WGM). TE waves are characterized both theoretically and experimentally by considering the fact that the size parameter (pi times diameter divided by wavelength of light) of the micro-spheres is very large at optical wavelengths. Asymptotic expressions have been developed based on electromagnetic theory derivations at the large size parameter limits. The developed expressions for optical resonance condition of TE wave are very simple and can accurately characterize resonances in dielectric microspheres. The theoretical development is mathematically robust and significantly less complicated than existing approaches based on quantum physics presented in the literature. The theoretical result of size parameter for consecutive morphology dependent resonance (MDR) peaks are validated by experimental data obtained via this study using a tunable semiconductor laser and from the literature. The comparisons are shown to be very accurate for large size parameters. The quality factor of experimental resonance spectra observed in the laboratory is calculated approximately in the order of 104 which is sensitive enough to detect micro or nano level temperature changes in the surrounding medium. The sensitivity of the MDR temperature sensor is wavelength change of 10−9 meter for one degree centigrade change in temperature. This sensor could potentially be used for nano technology, Micro-Electro-Mechanical Systems (MEMS) devices, and biomedical applications.
Energy Conversion From Fossil Fuel Into Spectral-Controlled Radiation for TPV by Super-Adiabatic Combustion in Porous Quartz Glass
Authors: Kumano Tomoyuki , Hanamura Katsunori
Published: April 2009

Characteristics of energy conversion from fossil fuel into radiant energy in the energy-recirculated thermophotovoltaic (TPV) generation system with piled porous quartz glass plates have been investigated through numerical simulation. When the total thickness of the quartz glass plates is fixed, it is revealed that the conversion efficiency of the system does not almost depend on a combination of the number of the quartz glass plates and the individual thickness. However, the spectral efficiency with respect to the specific TPV cell may be improved as both the number of the quartz plates is larger and the individual thickness is smaller. As a result, it is suggested that the achievable total efficiency of the TPV system is expected to be over 15% under the condition that the emitter of the system is regarded as gray body and the total thickness of the quartz media is 30mm.
An Experimental and Theoretical Investigation of the Pumping Performance of Geometrically Similar Flow Fields Within Miniature-Scale Centrifugal Pumps
Authors: Kearney Daniel , Punch Jeff , Grimes Ronan
Published: April 2009

Thermal management has become a key point in the development of contemporary electronics systems. It is evident that heat fluxes are currently approaching the limits of conventional forced air cooling, and that liquid cooling technologies are now under consideration. Most recent research in liquid cooling has focused on primary heat removal, with relatively little attention paid to liquid pumping. As the space available to incorporate a pump is often limited, miniature-scale pumps are required. Because such pumps operate at low Reynolds numbers, their operation may deviate from that predicted from the conventional pump affinity laws, and their efficiencies reduced. This paper investigates such deviations, and reduced efficiency, through experimental measurements of the performance of three pumps of varying scale. The pumps were fabricated using transparent material to allow optical access for PIV measurements. A characterization facility is described, which allows measurement of bulk pressure-flow performance characteristics. The measurements demonstrate that variations in diameter, without changed pump height, cause deviation from the affinity laws, whilst the characteristics of pumps whose diameter and height are varied proportionately obey the affinity laws. Efficiency was seen to reduce significantly with Reynolds number. PIV measurements of flow in the pump blade passages are also presented, which show a clear increase in the absolute velocity vectors within the blade passage and conversely a decrease in fluid velocity directly after blade tip in the expanding volute of the pump. The paper concludes that pump scaling laws predict accurately the pump performance in downsizing a geometrically identical pump but fail to predict when all aspects of the pump are scaled except height. It is also concluded that there is a significant decrease in efficiency in pump performance at low Reynolds numbers.
Hydrogen Transport in Single-Walled Carbon Nanotube by Molecular Dynamics Simulation
Authors: Oh Kyung Su , Park Seungho , Kwon Ohmyoung , Choi Young Ki , Lee Joon Sik
Published: April 2009

Carbon nanotubes have been strongly considered as promising hydrogen storage materials. In this paper, hydrogen transport mechanisms in single-walled carbon nano-tubes (SWNTs) for various temperatures and chiral indices were studied using molecular dynamics simulation. The SWNT models of zigzag (10,0), chiral (10,5) and armchair (10,10) with hydrogen molecules inside were simulated at temperatures ranging from 253K to 373K. Movements of hydrogen molecules (H2) inside a SWNT were analyzed using mean-square displacements and velocity autocorrelation functions.
Characteristics of Heat Transfer for Hydrogen and Wall During Filling Hydrogen Into Actual Tank at High Pressure
Authors: Woodfield Peter L. , Takano Toshio , Monde Masanori
Published: April 2009

An experiment has been made to measure the rise in temperature of hydrogen and tank wall during filling of actual tanks to 35 and 70 MPa. Three different tank configurations are used, having volumes of 205, 130 and 39 liters. The filling time is 5 to 20 minutes. A governing equation for the filling process is proposed, which includes unknown values for heat transfer coefficients between the hydrogen and the wall and the wall and surrounding air. The values are tentatively assumed to be 500 W/(m2K) during filling and 250 W/(m2K) after filling for the inside tank wall and 4.5 W/(m2K) for the outside tank wall. The measured temperatures of the hydrogen gas and the wall are in good agreement with the calculated ones.