bluuebunny/crossref_metadata_2025 huggingface dataset:
CFD Analysis of a Possible Leakage of Coolant in the MEGAPIE Spallation Source Target
Authors: Smith Brian L. , Shepel Sergey V.
Published: April 2009

The paper reports first steps in an analysis, using Computational Fluid Dynamics (CFD), of a leak of hot (>250°C) lead bismuth eutectic (LBE) from the MEGAPIE spallation source target into the helium-filled gap between the target hull and a close-fitting, double-walled safety container. Issues addressed are: (1) determining the initial impact pressure of the LBE on the inner shell of the safety container, which could result in the gap between the shells being closed, and its cooling capacity compromised; (2) LBE freezing on the cold shell surface could delay action of the leak detection system, and hence the beam trip control; and (3) heat transfer from the hot LBE to the D2O circulating between the safety container walls could induce boiling and subsequent pressurization of the circuit. Steps in the model development needed for the simulation are described, which include, in particular, the implementation of a liquid/gas interface-tracking model based on the Level Set approach. Results confirm that the safety container will be adequately cooled by the D2O during normal target operation, but first indications are that boiling might occur within the D2O circuit if substantial amounts of LBE leak from the target vessel unless its outer surface is coated with an insulating layer. Pressures generated by the impact of a slug of LBE on the safety window do not threaten its structural integrity, and no delay of the leak detection equipment as a consequence of LBE freezing is anticipated.
Water Transportation in Porous Media at Pore Scale
Authors: Yu Mingzhi , Peng Xiaofeng , Fang Zhaohong
Published: April 2009

Heat and mass transfer, particularly water transport was investigated at pore scale in unconsolidated porous media with a thermal probe as a heater. A CCD camera combined with a microscope was employed to observe and record water morphology variation during heating. The water near the heater firstly evaporated and then diffused to the region relatively far away where it condensed. As local temperature increased, the water would evaporate again and diffuse to even farther region. Even though the temperature and ambient conditions were the same, it could be induced that the liquid might have different chemical potentials with different interface curvatures. The chemical potential relationship between liquid and its surrounding vapor determined the phase change direction. At certain conditions, liquid evaporation and vapor condensation can coexist at the same region.
Numerical Investigation of Cooling Strategy for Reducing Blast Furnace Hearth Erosions
Authors: Yan Fang , Zhou Chenn Q. , Huang D. , Chaubal Pinakin
Published: April 2009

Hearth wearing is the key limit of a blast furnace campaign life. Hot metal flow pattern and temperature distributions are the two key variables to determine the rate and style of the hearth wearing. There are several strategies to control and reduce the hearth erosion, such as changing cooling water temperature and changing the heat transfer coefficient. In this paper, both cooling strategies are investigated using a comprehensive computational fluid dynamics (CFD) code, which was developed specifically for the simulation of blast furnace hearth. That program can predict the liquid flow patterns and temperature distributions of the hot metal as well as temperature profiles in the hearth refractory materials under different conditions. The results predicted by the CFD code were compared with actual industrial operation data. The cooling strategies are evaluated based on the energy analysis and effect on the hearth erosion.
Use of Computational Fluid Dynamics in the Design and Optimization of Microchannel Heat Exchangers for Microelectronics Cooling
Authors: Patterson Michael K. , Wei Xiaojin , Joshi Yogendra
Published: April 2009

Increasing circuit density and adherence to Moore’s law is driving advanced cooling systems for the next generation microprocessors. One method receiving considerable study is that of microchannel heat exchangers in silicon substrate. These very fine channels in the heat exchanger provide a greatly enhanced convective heat transfer rate and have been shown to be able to meet the demands of the cooling challenge for microprocessors for many generations to come. While the thermal performance has been demonstrated, the design methodology and analysis for fluid structures at this size scale remains difficult. This paper reviews the use of CFD analysis for the design and optimization of microchannel heat exchangers. These results are compared with classical approaches to the same design and demonstrate the need for CFD analysis. Errors using the standard correlations and methods are demonstrated through the results of an optimization study on microchannels. The effects of entrance lengths, spatial variation of the Nusselt number, and temperature dependencies are considered. The literature has widely varying reports on comparisons between experimental results and corresponding theoretical results in microchannel heat exchangers. Experimental validation of the CFD analysis has also been performed and demonstrates that current CFD techniques are actually well suited to heat exchanger designs of this size.
Convective Vaporization of Fuel Droplet: Effects of Thermal Radiation Absorption and Variable Liquid Properties
Authors: Abramzon B. , Sazhin S.
Published: April 2009

This paper describes an improved, simplified model of convective vaporization of a fuel droplet injected into a hot gas environment, based on the previous publications of the authors (Abramzon and Sazhin, 2005a,b). The model represents a generalization of the so-called “effective conductivity” model developed by Abramzon and Sirignano (1989). In addition to the effects of the Stefan flow (vapor blowing) and internal liquid circulation taken into account in the previous model, the proposed new model includes the effects of thermal radiation absorption and variable physical properties in the liquid phase. The simplified model for thermal radiation absorption suggested by Dombrovsky and Sazhin (2003) is employed for calculation of the heat sources distribution within the droplet. The computations were performed for n-decane and diesel fuel droplets whose spectral absorption data are available in the literature. It was found that, the effect of thermal radiation on the vaporization rate of the diesel fuel is considerably greater than for n-decane, especially in the regions of semi-transparency (λ not close to 3.4 μm). The effect of variable physical properties in the liquid phase is exhibited at the initial stage of the droplet heating when evaporation rate is relatively low and the droplet radius may increase due to the thermal expansion of the liquid. The results obtained using the “effective-conductivity” model with the uniformly distributed internal radiation heat source match very closely the predictions of the “extended vaporization” model with the non-uniform distribution of radiation absorption. The agreement is exceptionally good even for a very coarse finite-difference mesh within the droplet. Therefore, combining the relatively low computational cost and sufficient accuracy, the above “effective-conductivity” model with uniformly-distributed radiation absorption can be employed in sophisticated CFD codes for spray combustion analysis.
Simplified Model for the Thermal Boundary Condition in Polymer Injection
Authors: Barros I. , Teixeira J. C. F. , Cunha A. M. , Teixeira S. F. C. F.
Published: April 2009

The present paper discusses an alternative method for the definition of the thermal boundary condition in polymer injection molding. The model assumes that the cooling channels behave as a continuous heat sink located at the channels plane. Therefore, the mold thermal resistance can be easily calculated and added to those resulting from the interface polymer/mold and from the convective boundary at the channels. In addition, the heat dissipated into the environment is introduced as an additional heat sink. The thermal profiles in the polymer as well as the cooling times are very similar to those obtained with a full finite volume model for the mold/polymer combination. The simulated results were compared with experimental data taken on a fully instrumented mold. Good agreement was observed with a comprehensive reduction in the computational cost.
Investigation of Natural Convection in Nanofluids by Lattice Boltzmann Method
Authors: Wu Xuan , Kumar Ranganathan
Published: April 2009

Introduction of suspended nanoparticles into a base liquid will remarkably enhance energy transport process of the original liquid, which has been proved by a few experiments carried out by many authors. Irregular displacement and random distribution of the suspended nanoparticles as well as the interaction between nanoparticles and the adjacent liquid molecules make the modeling of flow and heat transfer in nanofluids very difficult. In this paper, a Lattice Boltzmann (LB) model for nanofluids has been developed. The external and internal forces, such as buoyancy, gravity, drag and Brownian force, and the mechanical and thermal interactions among the nanoparticles and their impact on the equilibrium velocity have been introduced. Along with a Gauss white noise model for Brownian motion, the double-distribution-function (DDF) approach, which treats temperature as a passive diffusing scalar and simulates it by a density-independent distribution function, was used to simulate free convection in nanofluids. By this model, the possible sedimentation and fluctuation of nanoparticles, and their impacts on the free convection in nanofluids have been observed and studied. A correlation formula for Nusselt number which characterized by properties of nanofluids has been obtained. The comparison of characteristics of natural convections between the nanofluids and its corresponding pure liquid has been done, and the possible mechanisms which enhance the heat transfer of natural convection in nanofluids have been discussed and revealed.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Ab Initio Investigation of Ionic Hydration With the Polarizable Continuum Model
Authors: Karampinos Dimitrios C. , Georgiadis John G. , Martinez Todd J.
Published: April 2009

The formulation of an ab initio method for the quantification of the energetics of ionic hydration is reviewed from the viewpoint of thermodynamics and statistical mechanics. The numerical approach, termed as the Polarizable Continuum Model, solves the exact quantum mechanical problem for the solute coupled with the electrostatic problem of the solvent, the latter being described as an effective continuous medium. The results show that the method can reproduce the experimental values of solvation energy for 3 cations and 3 anions by using only one adjustable parameter (scaled ionic radius) and can therefore be used in the furthter study of energetics and structure of hydrated ions.
Analysis and Optimized Design of a Packaged Liquid Chiller: Part I — Analytical Modelling
Authors: Heydari Ali , Kargar Saeed
Published: April 2009

The purpose of this work is to develop a physical simulation model for the purpose of studying the effect of various design parameters on the performance of a packaged liquid chiller. A computer model which simulates the steady-state cyclic performance of a vapor compression chiller is developed for the purpose of performing detailed physical design analysis of actual industrial chillers. The model can be used for optimizing design and for detailed energy efficiency analysis of packaged liquid chillers. The simulation model takes into account presence of all chiller components such as compressor, shell-and-tube condenser and evaporator heat exchangers, thermostatic expansion valve and connection pipes and tubing’s by thermo-hydraulic modeling of heat transfer, fluids flow and thermodynamics processes in each one of the mentioned components. To verify the validity of the developed model, a 7.5 USRT packaged-liquid chiller is used and a laboratory test stand for brining the chiller to its standard steady-state performance condition is build. Experimental results obtained from testing the chiller in various load and temperature conditions are shown to be in good agreement with those obtained from simulating the performance of the chiller using the computer prediction model. In part II of this work, an entropy-minimization-optimization analysis is performed based on the developed analytical performance model of the chiller. The variation of design parameters in construction of shell-and-tube condenser and evaporator heat exchangers are studied using the developed performance and optimization analysis and simulation model and a best-match condition between the physical design and construction of chiller heat exchangers and its compressor is found to exist. It is expected that manufacturers of chillers and research organizations interested in developing energy-efficient design and analysis of chillers can take advantage of the presented study and its results.
Calculation of Occluded Diffuse and Nondiffuse Radiative Exchange Shape Factors Between Polygons
Authors: DiLaura David L.
Published: April 2009

A procedure is described to calculate the radiative exchange shape factor between polygons. The spatial distribution of the emitting surface can be arbitrary and occluding surfaces are taken into account. Emitted radiant power is discretized into collimated, homogeneous beams leaving in a 3-dimensional, angular spray. Since a beam is homogeneous, the flux transported to a receiving surface is proportional to the intersection area of beam and surface. Occluding surfaces change the shape of the areas of intersection by casting collimated shadows. The total radiant power transported is proportional to the sum of the areas of intersection of all beams. Test cases are presented that demonstrate the utility and accuracy of the procedure and compare the computational expense with ray casting.
A Variable Effectiveness Model for Indirect Thermal Storage Devices
Authors: Boies A. M. , Homan K. O. , Davidson J. H. , Liu Wei
Published: April 2009

The performance of indirect thermal storage systems is critically dependent on the degree of thermal contact between the energy storage medium and the energy transfer medium. For liquid-liquid systems, the energy transfer occurs across a heat exchanger for which the overall effectiveness is determined by both tube-side and storage-side convection coefficients. While the tube-side convection is essentially constant throughout a draw at a constant flow rate, the storage-side convection process depends intimately on the natural convection flow driven by the temperature difference between the two fluids. This temperature difference is inherently transient during the discharge process. In the present work, analytical models are developed which predict system behavior for constant and variable heat exchanger effectiveness. The accuracy of each model is quantified in relation to empirical data obtained by Liu et al. [1, 2] in a physical system motivated by application to integral collector storage (ICS) solar water heating devices. From analysis of the empirical data, discharge-averaged values in the constant effectiveness model and in the variable effectiveness model are determined for a range of empirical conditions. The results show that the initial flow transients generated by the start of the discharge process are flow rate dependent and have a significant impact on the observed overall heat transfer coefficients.
Thermal Performance of a Loop Heat Pipe Having Polypropylene Wick in a Flat Evaporator
Authors: Boo Joon Hong , Chung Won Bok
Published: April 2009

A small-scale loop heat pipe (LHP) with polypropylene (PP) wick was fabricated and tested for its thermal performance. The container and tubing of the system were made of stainless steel and several working fluids were used including methanol, ethanol, acetone, and ammonia. The heater and the evaporator were sized so that the system can be applied to a local thermal control including electronics cooling. The heating area was 35 mm × 35 mm and there were nine axial grooves in the flat evaporator (40 by 50 mm) to provide a vapor passage. The pore size of the polypropylene wick inside the evaporator was varied from 0.5 μm to 25 μm. The size of condenser was 40 mm (W) × 50 mm (L) in which ten coolant paths were provided. The inner diameters of liquid and vapor transport lines were 2.0 mm and 4.0 mm, respectively and the length of the two lines was 0.5 m each. The start-up transient as well as steady-state operation was investigated with maximum system operating temperature of 90°C, which was imposed to protect the PP from permanent deformation. The minimum thermal load of 10 W (0.8 W/cm2) and maximum thermal load of 80 W (6.5 W/cm2) were achieved using methanol as working fluid with the condenser temperature of 20°C at horizontal position. For a LHP with ammonia as working fluid, the minimum thermal load of 1 W and maximum thermal load of 87 W (7.1 W/cm2) were achieved for condenser temperature of 0°C at horizontal position. The minimum system thermal resistance was 0.65 K/W.
Characteristics of Nucleation and Bubble Growth During Microscale Boiling
Authors: Tian Yong , Liu Jiang-Tao , Peng Xiao-Feng
Published: April 2009

In this paper, both nucleus formation and bubble growth during boiling in microchannels were investigated. A series of visualized experiments were conducted to observe the boiling nucleation and bubble dynamics restricted within parallel microchannels on a silicon wafer. The channels were rectangular and had selected length scale ranging from 50 to 100 microns. A high-speed CCD camera was employed together with a microscope to dynamically record the boiling images. The rates of bubble growth were measured in the channels. The phase change nucleus formation theory was used to determine the initial position of the bubble. The bubble growth rate was described by two ordinary differential equations deduced from the microlayer evaporation theory. The calculation and experimental results were reasonably coincided.
Medium Resolution Transmission Measurements of Water Vapor at High Temperature
Authors: Bharadwaj Sudarshan P. , Modest Michael F. , Riazzi Robert J.
Published: April 2009

Medium resolution transmissivities of water vapor were measured at temperatures between 600 K and 1550 K for all important vibration-rotation bands as well as part of the purely rotational band. Measurements were made with an improved drop tube design, which guarantees a truly isothermal high-temperature gas column. Data were collected with an FTIR-spectrometer, allowing for much better spectral resolution than most previous high-temperature measurements. The measured data were compared with the HITEMP database, as well as with the data of Phillips for the 2.7 μm band of H2O. The data show minor discrepancies with the high-resolution database, particularly at higher temperatures, but in general agreement is acceptable.
Numerical Solutions of Transient Radiative Transfer Using Time and Frequency-Domain Approaches
Authors: Francoeur Mathieu , Rousse Daniel R.
Published: April 2009

The one-dimensional transient radiative transfer problem in the Cartesian coordinate system — an absorbing and scattering medium illuminated by a short laser pulse — is solved by the use of time and frequency-domain approaches. In both cases, a Discrete Ordinates–Finite Volume method is adopted. Results for transmittance show that even if high order spatial schemes coupled with flux limiters can minimize the non-physical results associated with the temporal approach (transmitted flux emerging earlier than the minimal time required by the radiation to leave the medium), early transmitted radiation are always present. Transmittances obtained from the space-frequency method are more accurate, without unrealistic behaviors at early time periods. However, the frequency-dependent approach is computationally expensive with respect to its temporal counterpart, and the implementation of a Fast Fourier Transform algorithm is therefore considered. Finally, promising applications regarding optical diagnosis of absorbing and scattering media, using the radiation transport equation in the space-frequency domain, are briefly discussed.
Spreadsheet-Based Design Tool for the Analysis of Thermal Shock
Authors: Bull Jeffrey D. , Yee Raymond K.
Published: April 2009

Elastic and inelastic thermal shock analysis for simple shapes subject to convective heat transfer and the use of spreadsheets to automate this analysis is described. The completed spreadsheet application serves as a design tool to quickly calculate the elastic thermal stress distribution at peak loading or the residual stress distribution for inelastic regime. Spreadsheets were developed for the heating and cooling of a flat plate or wall, a beam, solid cylinder, and disc. The design tool results for elastic behavior are in good agreement with numerical methods over a wide range of Biot number. Inelastic analysis assumes elastic perfectly plastic behavior, and an approximate method is used to model it. Design tool results for inelastic analysis are also in good agreement with the literature for moderate values of Biot number with constant yield strength. The inclusion of temperature dependent yield stress and comparison with numerical methods is shown to give fair agreement.
Variable Conductance Two-Phase Closed Flat Plate Thermosyphon With Binary Mixture
Authors: Shokouhmand H. , Sharifi N.
Published: April 2009

In this paper the variable conductance heat transfer in a flat plate thermosyphon has been considered. For this purpose the governing equations including mass and momentum conservation laws are solved numerically. It should be noted that liquid-film momentum advection, axial normal stress and interfacial shear stress were typically included and shown to be important to the thermosyphon performance. It is found that total system pressure and mean vapor temperature at different concentrations of mixture are nearly constant for a special range of absorbed power in evaporator. The obtained numerical results are in good agreement with available experimental data.
Heat Transfer Enhancement in Axial Taylor-Couette Flow
Authors: Gilchrist S. , Ching C. Y. , Ewing D.
Published: April 2009

An experimental investigation was performed to determine the effect that surface roughness has on the heat transfer in an axial Taylor-Couette flow. The experiments were performed using an inner rotating cylinder in a stationary water jacket for Taylor numbers of 106 to 5×107 and axial Reynolds numbers of 900 to 2100. Experiments were performed for a smooth inner cylinder, a cylinder with two-dimensional rib roughness and a cylinder with three-dimensional cubic protrusions. The heat transfer results for the smooth cylinder were in good agreement with existing experimental data. The change in the Nusselt number was relatively independent of the axial Reynolds number for the cylinder with rib roughness. This result was similar to the smooth wall case but the heat transfer was enhanced by 5% to 40% over the Taylor number range. The Nusselt number for the cylinder with cubic protrusions exhibited an axial Reynolds number dependence. For a low axial Reynolds number of 980, the Nusselt number increased with the Taylor number in a similar way to the other test cylinders. At higher axial Reynolds numbers, the heat transfer was initially independent of the Taylor number before increasing with Taylor number similar to the lower Reynolds number case. In this higher axial Reynolds number case the heat transfer was enhanced by up to 100% at the lowest Taylor number of 1×106 and by approximately 35% at the highest Taylor number of 5×107.
A PDF/Photon Monte Carlo Method for Radiative Heat Transfer in Turbulent Flames
Authors: Wang Liangyu , Haworth Daniel C. , Modest Michael F.
Published: April 2009

Thermal radiation plays a dominant role in heat transfer for most combustion systems. Accurate predictions of radiative heat transfer are essential for the correct determination of flame temperature, flame structure, and pollutant emissions in combustion simulations. In turbulent flames, transported probability density function (PDF) methods provide a reliable treatment of nonlinear processes such as chemical reactions and radiative emission. Here a second statistical approach, a photon Monte Carlo (PMC) method, is employed to solve the radiative transfer equation (RTE). And a state-of-the-art model for spectral radiative properties, the full-spectrum k-distribution (FSK) method, is employed. The FSK method provides an efficient and accurate approach for spectral integration in radiation calculations. The resulting model is applied to simulate radiation and turbulence/radiation interactions in nonluminous turbulent non-premixed jet flames. The initial results reported here emphasize sensitivities of computed results to variations in the physical and numerical models. Results with versus without radiation, results obtained using two different RTE solvers, and results with a gray-gas approximation versus a spectral FSK method are compared.
Effect of Swirl Intensity on the Heat Performance of a Premixed Circular Impinging Flame Jet With Swirl Induced
Authors: Huang X. Q. , Leung C. W. , Chan C. K.
Published: April 2009

Swirl has been successfully introduced to a small scale premixed circular impinging flame jet under low Reynolds number. The study on the flow and heat transfer characteristics of such a swirling impinging flame found that it could provide a more uniform heat flux and flame temperature distribution around the stagnation point. The swirl ratio, which is indicated by swirl angle and axial velocity at the exit of the nozzle is changed to investigate the effect of swirling intensity on the heat performance of the flame jet.
Flow Visualization of Bicycle Helmets for Optimal Ventilation Design
Authors: Sznitman Josue´ , Kritter Fabien , Ro¨sgen Thomas , Bru¨hwiler Paul A.
Published: April 2009

Bicycle helmets present complex geometries intended for protection and ventilation. Models often seem driven by esthetic considerations rather than by designs for optimal ventilation. To quantify the amount of flow entering a helmet, four bicycle helmets were investigated using flow visualization in a water tunnel, at representative riding speed for two helmet inclinations. Design features such as location of vents, visors and wall thickness of openings were examined as potential factors influencing ventilation efficiency. Cross-sections of streamtubes available for ventilation ahead of a helmet were evaluated. Results revealed that several helmet openings were seldom ventilated making them largely ineffective for forced convection under the given conditions. Velocities of the flow entering helmet vents were estimated to decrease by a factor > 3 relative to the freestream and varied from one vent to another. The general trend observed for velocities was described by a simple analytical model based on channel flow.
Effect of Nano-Scale Surface Conditions for Boiling Heat Transfer and Its Enhancement
Authors: Chiba S. , Yuki K. , Hashizume H. , Toda S.
Published: April 2009

In this paper, the Leidenfrost phenomena and water mist cooling are described from the viewpoint of surface conditions of heat transfer interfaces. The effect of nano-scale structures on boiling heat transfer phenomena is researched. It is clarified that the Leidenfrost phenomena on a substrate with adhered nanoscale carbons (nano carbons) are different from the one in case of a normal heat transfer interface. The photographs taken by a high-speed camera show that the boiling on a substrate with nano carbons takes the different form in comparison with the one on a normal interface. In case that the surface temperature of a copper substrate is about 140 degree C, a water droplet has a neck of water between itself and the substrate with nano carbons. On the other hand, the nucleate boiling is observed on a normal copper substrate. From the relation between evaporation time and initial surface temperature, heat transfer enhancement can be achieved under the nucleate boiling conditions. Also, the critical heat flux of water mist cooling could be enhanced by adhering nano carbons on heat-transfer interfaces. It is supposed that the wettability between water and copper is improved by the nano carbons.
Comparative Analysis of Vapor Compression and Hybrid Liquid Desiccant Dehumidification Systems
Authors: Al-Jaafari A. M. , Sherif S. A.
Published: April 2009

The objective of this study was to evaluate the energy savings of a commercially available hybrid liquid desiccant (HLD) cooling system relative to a conventional vapor compression system used for an existing school building where 100% outside air is used for ventilation. Psychrometric analysis and hour-by-hour simulations for three energy models were developed for three cities in the United States using available weather data assuming normal operation and typical building occupancy. Energy calculation software such as the Carrier Hourly Analysis Program (HAP 4.1) and Desicalc along with generated spreadsheets was used to compute the energy consumption for the models under study. Results of each model are summarized and comparisons are made. The annual energy savings employing the HLD system were found to reach 46% for Chicago, 37% for Gainesville and 32% for Miami. Simple cost analysis and associated payback periods were also performed.
Effect of Contact Angle on the Heat Transfer to an Evaporating Meniscus on a Moving Heated Surface
Authors: Mukherjee A. , Kandlikar S. G.
Published: April 2009

Numerical simulation is carried out to study a 2D evaporating meniscus formed on a moving wall. The complete Navier-Stokes equations along with continuity and energy equations are solved. The liquid vapor interface is captured using the level set technique. The meniscus is fed with saturated water from the top whereas the bottom wall is maintained at a higher temperature and is also imparted with a velocity. The meniscus attains a steady shape when all the incoming liquid gets evaporated due to heat transfer from the wall. The advancing and receding contact region of the meniscus are provided with different contact angles. Results indicate that the average heat flux at the meniscus base increases with increase in contact angle. The primary reason for heat transfer from the wall is attributed to the liquid circulation inside the meniscus and the corresponding transient conduction from the wall. As the meniscus contact angle increases the liquid circulation is found to disturb the thermal boundary layer more effectively thereby resulting in increased wall heat transfer. The effect of contact angle on wall heat transfer to the moving and evaporating meniscus is compared to partial nucleate pool boiling.
Anisotropic Nature of Thermal Transport in Nanoscale Materials
Authors: Wang Xinwei
Published: April 2009

In this work, an equilibrium technique is developed to study the thermal transport in nanomaterials. By directly tracking the relaxation behavior of energy carriers, the developed technique is able to determine the effect of boundary scattering on thermal transport. Since no temperature differential across the material is required to determine its thermal conductivity, the developed technique is applicable to nanomaterials of different shapes and capable of capturing the anisotropic nature of the thermal transport inside. Applying this technique, the thermal transport in several typical nanomaterials—nanofilms, square and round nanowires, and spherical and cubic nanoparticles are studied in detail. A strong anisotropic nature of thermal transport in nanomaterials is observed. For nanofilms and nanowires, the thermal conductivity in the restricted directions (thickness and radial) is smaller than that in the unrestricted direction. This anisotropic nature is more obvious and important when the characteristic size of nanomaterials becomes comparable to or smaller than the mean free path of energy carriers. Our results comparison shows that with the same characteristic size, the shape of the cross section of nanowires has appreciable effect on the thermal transport in the axial direction. For spherical and cubic nanoparticles, little difference is observed between their thermal conductivities.
A Model of Nanofluids Thermal Conductivity
Authors: Wang Jinbo , Chen Gang , Zhang Zongqin
Published: April 2009

Since the experimental observations were reported that nanofluids exhibit unusually high thermal conductivity, different candidate models have been proposed by several research groups in order to reveal the underlying physics. Despite the efforts, mechanisms of the thermal conductivity enhancement are still being hotly debated. In this paper, we report a new model that correlates nanofluids thermal conductivity to particle size, volume fraction, temperature, surface zeta potential, and suspension electrolyte ion concentration. Our model provides new insights on the mechanisms and guidelines for future experimental exploration.
Bubble Formation and Detachment Under the Influence of Electric Fields and the Role of Gravity
Authors: Herman C. , Liu Z. , Iacona E.
Published: April 2009

Boiling is an attractive solution to cooling problems in aerospace engineering because of the high heat transfer coefficients associated with phase change processes. Bubble detachment from an orifice shows some resemblance to bubble departure in boiling. The buoyancy force is responsible for bubble removal from the surface in terrestrial conditions. In space, with the gravity level being orders of magnitude smaller than on earth, bubbles formed during boiling can remain attached to the surface. As a result, the amount of heat removed from the heated surface can decrease, and it is difficult to predict reliably and accurately. The impact of electric fields is investigated with the aim to control bubble behavior and help bubble removal from the surface on which they form in reduced gravity. The behavior of single gas bubbles injected through an orifice into an electrically insulating liquid is studied in reduced gravity under the influence of static electric fields and the results of the experiments are compared with data obtained using a simplified model. The bubble life cycle was visualized in terrestrial conditions and reduced gravity. Bubble departure, volume and dimensions at detachment were measured and analyzed for different parameters such as gravity level, electric field magnitude and electric field uniformity. Results suggest that these parameters significantly affect bubble behavior, shape, volume and dimensions at detachment.
Thermal Performance of Ventilated Passenger Seats
Authors: Ratts Eric B.
Published: April 2009

Automotive seats are now actively cooled as well as heated to provide thermal comfort. Some seats are cooled by thermoelectric devices and others through simple forced ventilation. To ultimately determine passenger comfort requires knowing the seats ability to remove water vapor and thermal energy from the passenger’s skin. This paper presents work on measuring the mass and heat transfer performance of a forced-air ventilated seat. Comparing the thermal behavior to a semi-infinite body, an effective thermal effusivity was measured as well as an effective mass diffusivity. Average thermal effusivities were 64 and 244 W s1/2/m2 K for the non-ventilated and ventilated seats, respectively. Average mass diffusivities were 0.0031 and 0.0142 m2/s for the non-ventilated and ventilated seats, respectively.
High Speed Friction Stir Welding: A Computational and Experimental Study
Authors: Raikoty Harsha , Ahmed Ikram , Talia George E.
Published: April 2009

A three-dimensional numerical analysis of friction stir welding at high speed (HS-FSW) is presented here. The temperature distribution in the workpiece has been calculated for a number of processing conditions. The analysis adopts a thermal model based on the simple laws of friction. This model translates to having a moving heat source along the weld-line on the top surface of the workpieces. Results have been validated experimentally using an infrared camera as well as thermocouple measurements. By comparing actual welds performed on Aluminum 6061-T6 and the numerical predictions, it is observed that the appropriate range for the (maximum) surface temperatures for obtaining a sound weld is between 570°C and 530°C, and that these temperatures are achieved between spindle translation velocities of 125 mm/min and 250 mm/min, respectively.
Coupled Conduction-Convective Heat Transfer in the Louvered Fin Heat Exchanger
Authors: Li Longjian , Cui Wenzhi , Chen Qinghua , Liao Quan , Jen Tien-Chien
Published: April 2009

A comprehensive numerical analysis on the heat transfer process in the louvered fin heat exchanger was conducted. In this paper, the physical and mathematical models on the coupled conduction-convective heat transfer for the louver fin geometry were developed. The control volume-based finite element method (CVFEM) was employed to solve the problem numerically. The computed results revealed the flow structure and heat transfer in the geometry in detail. Compared with the empirical correlation of heat transfer and friction coefficients previously proposed, the computed results showed better agreement with the experimental results.
Gaussian Roughness in Thermal Contact Conductance, Microtubes and Microfins
Authors: Yovanovich M. M. , Bahrami M. , Culham J. R.
Published: April 2009

The significant contributions of Professor B. B. Mikic and collaborators to fields of thermal contact conductance and contact mechanics are summarized in this technical note. Professor Mikic’s research is original and fundamental. The results have been cited and utilized in many practical applications.
Integral Transforms for Three-Dimensional Steady Turbulent Dispersion in Rivers and Channels
Authors: de Barros Felipe P. J. , Cotta Renato M.
Published: April 2009

A three-dimensional steady-state mathematical model is considered for predicting the fate of dissolved contaminants in rivers and channels under turbulent flows. The model allows for variable velocity fields and non-uniform turbulent diffusivities. Making use of the Generalized Integral Transform Technique (GITT), a hybrid numerical-analytical solution is then obtained. The solution convergence behavior is investigated and the criterion for reordering the terms in the infinite series is discussed, with the aim of reducing the computational effort associated with the double eigenfunction expansion. A test case is presented to illustrate the proposed approach.
Application of an Adaptive Wavelet Method to Natural-Convection Flow in a Differentially Heated Cavity
Authors: Wirasaet Damrongsak , Paolucci Samuel
Published: April 2009

We describe an adaptive wavelet method based on interpolating wavelets applied to the solution of a natural-convection flow. The adaptive wavelet method, owing to the approximation capabilities and spatial localization of wavelet functions, enables the solution of problems with local grid resolution consistent with the local demand of the physical problem. The adaptive method is applied to simulate the flow in a differentially heated square cavity at large Rayleigh numbers. Numerical results, whenever possible, are compared with those previously published.
Reduce Vibration Analysis Errors With Precise Weight Predictions
Authors: Wang Frank Fan
Published: April 2009

Airborne electronics face harsh environmental, high reliability and high design margin requirements. Mass and volume constraints dictate that the design be more compact than commercial electronics. Proper usage of Finite Element Analysis (FEA) modeling in designing high performance avionics will accomplish the optimized design within limited time and budget constraints. The idea of FEA analysis is to utilize computer calculation power to save time and money over iterative testing of real products. Building one design evaluation unit to calibrate the CAD and analysis model can reduce or eliminate the need for building several costly units in the traditional way. Engineers should learn to evaluate design options on computer models as much as possible instead of on real products. If similar product models have been calibrated before, some stages of mockup can be eliminated and more time and money savings will be achieved. To accurately calculate the vibration response of electronic equipment, precise weight estimation is the most important factor. Weight estimation is the determinative part of vibration analysis. This paper discusses the FEA modeling, design and testing strategies and their relations to weight estimation. This paper proposed two weight estimation methods and showed a few real life examples.
Enhancing Thermoelectric Energy Recovery Via Modulations of Source Temperature for Cyclical Heat Loadings
Authors: McCarty R. , Hallinan K. P. , Sanders B. , Somphone T.
Published: April 2009

Very recent thermoelectric (TE) device materials improvements have pushed this technology to the cusp of usefulness in converting waste heat to electricity in a variety of applications — from automotive to aerospace. For applications where the heat loading is cyclical or non-constant, the effect of active control to maintain the source temperature at or near the peak allowable temperature while maximizing the temperature difference across a TE temporally on the overall thermoelectric efficiency is investigated. Efficiencies for constant heat loading applications that are not at near peak allowable temperatures are also investigated. The modulation of the source temperature would be achieved through the use of a ‘thermal switch’ or ‘active thermal potentiometer’ between the heat source and the thermoelectric device. Two methods are used to model the thermoelectric energy recovery system. First, an RC equivalent model is used to define the controlling factors for efficiency on a first order basis. Second, a numerical model is created to investigate the system in more detail. Both models demonstrate that maximizing the exergy of the source by maximizing its temperature during off-peak heat loadings is capable of improving the time-averaged efficiency of a thermoelectric device. For some conditions, improved time averaged efficiencies of more than 4 times are realized. Criteria defining the operation space where efficiency improvements are realized are also developed.
Failure Site Transition During Drop Testing of Printed Wiring Assemblies
Authors: Varghese J. , Dasgupta A.
Published: April 2009

This paper is part of an ongoing effort to develop a test methodology to examine the durability of surface mount interconnects under impact loading conditions. Literature indicates that as the drop height is increased, there is a transition in the failure site from the ductile solder to the brittle intermetallic. Not much work has been done to understand this phenomenon. This study considers damage accumulated in the interconnects in terms of the local strain in the printed wiring assembly (PWA), local strain rate and component acceleration. The advantage is that the results are less dependant on structure and loading, because damage is quantified in terms of specimen response rather than the loading. A simple test specimen is fabricated to concentrate the study on interconnect failure mechanisms. An instrumented, repeatable test setup is developed to conduct high speed bend tests and drop tests on the specimen. All tests are replicated twice for proof of consistency of the test data. The paper presents the results of the high speed bend tests. As expected, the durability of the specimen decreases monotonically with PWA strain. On the other hand, the durability first increases and then decreases as the PWA strain rate increases. Failure analysis shows a transition in the failure site from solder to intermetallic. It is hypothesized that for a given package design, the rate dependent material properties determine the partitioning of the strain energy of deformation. Yield stress of the solder and fracture toughness of the intermetallic are identified as the key parameters. More tests are needed to understand this rate dependent strain energy partitioning. The end goal is to develop a consistent, accurate and generic methodology for ranking the impact durability of different surface mount interconnects technologies.
Development of Integrated Process-Ageing Modeling Methodology for Flip Chip on Flex Interconnections With Non-Conductive Adhesives
Authors: Zhang Xiaowu , Wong E. H. , Rajoo Ranjan , Iyer Mahadevan K. , Caers J. F. J. M. , Zhao X. J.
Published: April 2009

This paper presents a comprehensive methodology to model the static temperature-humidity (TH) ageing test (85°C/85%RH over 1000 hours) of flip chip on flex interconnections with non-conductive adhesives (NCAs). Nonconductive adhesives, being a special form of conductive adhesives, are chosen, as they allow bringing the pitch further down. The methodology combines experimental techniques for material characterization, finite element modeling (FEM) and model validation. A non-conductive adhesive (NCA) has been characterized using several techniques. The thermomechanical properties and the moisture properties were obtained for the NCA. A temperature dependent viscoelastic constitutive model was also obtained for the NCA. The viscoelastic model was defined by the Prony series expansion. The shift factor was approximated by the Williams-Landel-Ferry (WLF) equation. Finite element modeling has been performed to analyze the flip chip interconnects on flex with the NCA under process condition and reliability ageing conditions. The viscoelastic constitutive relation has been used to model the NCA in ageing modeling. An integrated process-ageing modeling methodology has been developed to combine the thermo-mechanical stress and hygro-mechanical stress, followed by stress relaxation analysis. To verify the finite element models, the static TH ageing test (85°C/85%RH) were also performed. The contact resistance was monitored with high measuring resolution during the accelerated test. The simulation results are good agreement with the experimental results. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters and good reliability performances.