bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Asymmetric Disk-Shaped and Flat-Plate Heat Pipes
Authors: Vafal K. , Zhu N. , Wang W.
Published: February 2008

An analytical investigation and conceptual design of a disk-shaped asymmetric heat pipe is presented in this work. Using the conservative formulations for the steady incompressible vapor and liquid flow for a disk-shaped heat pipe, an in-depth integral analysis is applied. Analytical results for the asymmetric vapor velocity profile, the vapor and liquid pressure distributions, and the vapor temperature distribution in the heat pipe are obtained and compared to those of rectangular flat-plate heat pipe. The analysis establishes the physics of the process and the intrawick interactions for the disk-shaped heat pipe. The effects of variations in the thicknesses of the vapor channel and the wick as well as the heat pipe on the performance of both disk-shaped and rectangular flat-plate heat pipes are analyzed in detail and compared with each other. The factors limiting heat pipe performance are discussed and the results show that the disk-shaped heat pipe, while utilizing a smaller surface area and being more adaptable to several application areas, significantly increases the heat transfer capability per unit surface area compared to rectangular flat-plate heat pipe.
Heat Generation and Transport in Submicron Semiconductor Devices
Authors: Fushinobu K. , Majumdar A. , Hijikata K.
Published: February 2008

The reduction of semiconductor device size to the submicrometer range leads to unique electrical and thermal phenomena. The presence of high electric fields (order of 107 V/m) energizes the electrons and throws them far from equilibrium with the lattice. This makes heat generation a nonequilibrium process. For gallium arsenide (GaAs), energy is first transferred from the energized electrons to optical phonons due to strong polar coupling. Since optical phonons do not conduct heat, they must transfer their energy to acoustic phonons for lattice heat conduction. Based on the two-step mechanism with corresponding time scales, a new model is developed to study the process of nonequilibrium heat generation and transport in a GaAs metal semiconductor field effect transistor (MESFET) with a gate length of 0.2 μm. When 3 V is applied to the device, the electron temperature rise is predicted to be more than 1000 K. The effect of lattice heating on electrical characteristics of the device shows that the current is reduced due to decrease in electron mobility. The package thermal conductance is observed to have strong effects on the transient response of the device.
Local Heat Transfer in Axially Feeding Radial Flow Between Parallel Disks
Authors: Prata A. T. , Pilichi C. D. M. , Ferreira R. T. S.
Published: February 2008

Experiments and computations were performed to determine the local heat transfer in radial flows between parallel concentric disks. The flow is supplied axially by a feeding orifice placed in one of the disks and becomes radial after being deflected by the frontal disk. The frontal disk is kept isothermal and the other solid surfaces washed by the flow are kept adiabatic. Local heat transfer coefficients were determined using the naphthalene sublimation technique and the analogy between heat and mass transfer. Computations were performed using a finite volume methodology. The local Nusselt number distribution showed a valley at the stagnant region in front of the feeding orifice and a peak at the diffuser entrance where the flow impinges on the frontal disk prior to becoming radial. Depending on the Reynolds number and on the gap between the disks a secondary peak was observed in the diffuser region. The secondary peak is believed to be caused by nonparallelism or unsteadiness of the flow field and was not captured by the numerical model.
Heat Transfer and Pressure Drop Characteristics of Spirally Fluted Annuli: Part I—Hydrodynamics
Authors: Garimella S. , Christensen R. N.
Published: February 2008

This paper is the first of two papers that present the results of a comprehensive study of heat transfer and pressure drop in annuli with spirally fluted inner tubes for the laminar, transition, and turbulent flow regimes. To understand the underlying physical phenomena responsible for heat transfer enhancement, flow mechanisms documented elsewhere are combined with pressure drop measurements to propose friction factor correlations for the low Re range (Re < 800) and the high Re range (800 < Re < 43,500). Friction factors were found to be functions of the flute depth, pitch and angle, and the annulus radius ratio. These correlations are used in the second paper to develop Nusselt number correlations in terms of the fluted annulus friction factor. Transition in these annuli occurred in the 310 < Re < 1000 range. In Part II of this study, this early transition is shown to be responsible, in part, for significant heat transfer enhancement. Friction factor increases were typically between 1.1 and 2.0 in the laminar regime, and up to 10 in the turbulent regime. These enhancement values can be used in conjunction with heat transfer enhancement values reported in Part II to determine appropriate ranges of applicability for spirally enhanced annuli.
Heat Transfer Coefficients in an Orthogonally Rotating Duct With Turbulators
Authors: Hsieh Shou-Shing , Hong Ying-Jong
Published: February 2008

Experiments were conducted to determine the influence of rotation on local heat transfer coefficient for the turbulent flow in a short square duct (L/DH = 15) with a pair of opposite rib-roughened walls. The ribs are configured in an in-line arrangement with an attack angle of 90 deg to the main flow. The coolant used was air with the flow direction in the radially outward direction. The Reynolds numbers ranged from 5000 to 25,000; the rib pitch-to-height ratio was 5; and the rib height-to-hydraulic diameter ratio was kept at a value of 0.20. The rotation number range was 0 to 0.5. Local Nusselt number variations along the duct were determined over the trailing and leading surfaces. In addition, local heat transfer measurements on all sides of a typical rib as well as on a typical exposed base surface between two consecutive ribs in a fully developed region were conducted at various rotational speeds. It is shown that the Coriolis acceleration tends to improve the heat transfer due to the presence of strong secondary flow. Centripetal buoyancy is shown to influence the heat transfer response with heat transfer being suppressed on both leading and trailing surfaces as the wall-to-coolant temperature difference is increased with other controlling parameters hold constant. Results are also compared with previous investigations. It was found that the results agree very well with those reported by other works in this field.
Heat Transfer From an Asymmetrically Heated Channel Partially Enclosing a Rotating Disk
Authors: Schmidt R. R. , Patel P.
Published: February 2008

Experiments have been performed to determine the heat transfer from an asymmetrically heated channel partially enclosing a vertically oriented rotating disk. Parallel rectangular surfaces enclose the rear portion of a disk (slightly less than 1/2 of the disk is enclosed) allowing air to enter and exit the perimeter of the channel, except the rear vertical portion bridging the two parallel surfaces. The experiments encompassed data runs where one of the parallel walls was isothermal and the other was adiabatic. The experiments encompassed a range of spacings between the rotating disk and the adjacent parallel surfaces and a range of rotational speeds varying by a factor of 30. The experiments were performed using the naphthalene sublimation technique. From the experimental results a dimensionless correlation equation suitable for predicting average heat and mass transfer coefficients of the enclosing surfaces for various rotational Reynolds numbers and disk-to-wall spacings was deduced. Finally, to gain some insight into the air flow pattern along the enclosing walls, a visual flow technique was employed, the results of which will be described herein.
Turbulence Dissipation in a Free-Surface Jet of Water and Its Effect on Local Impingement Heat Transfer From a Heated Surface: Part 1—Flow Structure
Authors: Wolf D. H. , Viskanta R. , Incropera F. P.
Published: February 2008

This study investigates the relationship between jet turbulence and local impingement heat transfer for a free-surface, planar jet of water. Employing a thermal anemometer system, measurements of the mean velocity and turbulence intensity are reported at different streamwise and spanwise locations throughout the jet. The flow conditions at the nozzle discharge were controlled by using different nozzle designs (parallel-plate and converging) and flow manipulators (wire grid and screens). Measurements of the velocity gradient along the impingement surface, known to influence heat transfer from analytical considerations of a laminar impinging jet, were also made for the same sets of nozzle conditions. The test matrix also included variations in the Reynolds number (23,000 and 46,000) and distance from the nozzle discharge to the surface (0 to 30 nozzle widths). The local heat transfer results corresponding to the flow structure measurements are reported in Part 2 of this paper.
A Unified Field Approach for Heat Conduction From Macro- to Micro-Scales
Authors: Tzou D. Y.
Published: February 2008

A universal constitutive equation between the heat flux vector and the temperature gradient is proposed to cover the fundamental behaviors of diffusion (macroscopic in both space and time), wave (macroscopic in space but microscopic in time), phonon–electron interactions (microscopic in both space and time), and pure phonon scattering. The model is generalized from the dual-phase-lag concept accounting for the lagging behavior in the high-rate response. While the phase lag of the heat flux captures the small-scale response in time, the phase lag of the temperature gradient captures the small-scale response in space. The universal form of the energy equation facilitates identifications of the physical parameters governing the transition from one mechanism (such as diffusion or wave) to another (the phonon–electron interaction).
Turbulence Dissipation in a Free-Surface Jet of Water and Its Effect on Local Impingement Heat Transfer From a Heated Surface: Part 2—Local Heat Transfer
Authors: Wolf D. H. , Viskanta R. , Incropera F. P.
Published: February 2008

This paper presents local heat transfer data for a planar, free-surface jet of water impinging normal on a uniformly heated surface. The hydrodynamic conditions of the jet were altered through the use of different nozzle types (parallel-plate and converging) and flow manipulators (wire grid and screens) to investigate the relationship between jet turbulence and local impingement heat transfer. The flow structures for each of the various nozzle conditions are reported in a companion paper (Wolf et al., 1995), and results are used in this paper to interpret their effect on local heat transfer. In addition to qualitative interpretations, correlations are developed for both the onset of transition to turbulence and the dimensionless convection coefficient at the stagnation point. Higher levels of jet turbulence are shown to induce transition to a turbulent boundary layer at smaller streamwise distances from the stagnation point. The effect of stream-wise turbulence intensity on the convection coefficient is shown to scale approximately as the one-quarter power.
On Hyperbolic Heat Conduction and the Second Law of Thermodynamics
Authors: Bai C. , Lavine A. S.
Published: February 2008

For situations in which the speed of thermal propagation cannot be considered infinite, a hyperbolic heat conduction equation is typically used to analyze the heat transfer. The conventional hyperbolic heat conduction equation is not consistent with the second law of thermodynamics, in the context of nonequilibrium rational thermodynamics. A modified hyperbolic type heat conduction equation, which is consistent with the second law of thermodynamics, is investigated in this paper. To solve this equation, we introduce a numerical scheme from the field of computational compressible flow. This scheme uses the characteristic properties of a hyperbolic equation and has no oscillation. By solving a model problem, we show that the conventional hyperbolic heat conduction equation can give physically wrong solutions (temperature less than absolute zero) under some conditions. The modified equation does not display these erroneous results. However, the difference between results of these two models is negligible except under extreme conditions.
The Thermal Contact Conductance of Hard and Soft Coat Anodized Aluminum
Authors: Lambert M. A. , Marotta E. E. , Fletcher L. S.
Published: February 2008

An experimental investigation of the thermal contact conductance of anodized coatings, synthesized at different bath temperatures and in different electrolyte solutions, was performed, and the results were compared with previously published information. Electrolyte solutions of sulfuric acid at bath temperature of 7°C (Type III) and 25°C (Type II) and chromic acid at a bath temperature of 54°C (Type I) were used to grow coating thicknesses ranging from 3.2 to 61 μm (0.11 to 2.4 mil). Experimental thermal contact conductance data were obtained for a junction between anodized aluminum 6101-T6 and uncoated aluminum A356-T61 as a function of apparent contact pressure and anodized coating thickness. Apparent contact pressure ranged from 172 to 2760 kPa (25 to 400 psi) and the mean interface temperature was maintained at 40°C (104°F). The thermal contact conductance for the low-temperature sulfuric acid anodized (Type III) coatings varied from 300 to 13,000 W/m2 K, while the conductance of the room temperature sulfuric anodized (Type II) coatings varied between 100 to 3000 W/m2 K. The chromic acid (Type I) coatings yielded conductance values of 60 to 3000 W/m2 K. In general, the use of elevated temperatures for the anodizing bath will lead to lower surface microhardness and lower thermal contact conductance. The greatest conductance measurements were obtained for coatings grown in low-temperature sulfuric acid.
Prandtl Number Influence on Heat Transfer Enhancement in Turbulent Flow of Water at Low Temperatures
Authors: Ravigururajan T. S. , Bergles A. E.
Published: February 2008

The paper presents the results of an experimental study that was carried out to determine the Prandtl number influence on the thermal-hydraulic performance of enhanced tubes. The enhanced tubes were identical in all aspects, except the roughness height. Emphasis was placed on turbulent flow, with water as the working fluid. Experiments were performed for Prandtl numbers from 10.2 to 5.8. Data for friction factor were obtained under both isothermal and heating conditions. The study confirmed the strong influence of rib roughness on heat transfer and pressure drop increase. In addition, the results of the investigation strongly suggest that the optimum roughness is application specific, because it is dependent on Reynolds and Prandtl numbers. The results show that the presence of entrance effects and the influence of temperature on the friction factor of enhanced tubes may be significant. The experiments show that a rib height-to-diameter ratio of 0.02 provides the best overall performance for water at Pr = 10; while at Pr = 6.0, the optimum e/d depends on the Reynolds number.
Combined Effect of Free-Stream Turbulence and Unsteady Wake on Heat Transfer Coefficients From a Gas Turbine Blade
Authors: Zhang L. , Han J.-C.
Published: February 2008

The combined effect of free-stream turbulence and unsteady wakes on turbine blade surface heat transfer was studied. The experiments used a five-blade linear cascade in a low-speed wind tunnel facility. A turbulence grid and spoked-wheel type wake generator produced the free-stream turbulence and unsteady wakes. The mainstream Reynolds numbers based on the cascade inlet mean velocity and blade chord length were 100,000, 200,000, and 300,000. Results show that the blade time-averaged heat transfer coefficient depends on the mean turbulence intensity, regardless of whether this mean turbulence intensity is from unsteady wake only, turbulence grid only, or a wake and grid combination. The higher mean turbulence promotes earlier boundary layer transition and causes much higher heat transfer coefficients on the suction surface. It also significantly enhances the heat transfer coefficients on the pressure surface. The unsteady wake greatly affects blade heat transfer for low oncoming free-stream turbulence; however, the wake effect diminishes for high oncoming turbulence. The free-stream turbulence also strongly affects blade heat transfer for a low wake passing frequency, but the oncoming turbulence effect diminishes for a high unsteady wake condition.
Experimental Investigation of Corona Wind Heat Transfer Enhancement With a Heated Horizontal Flat Plate
Authors: Owsenek B. L. , Seyed-Yagoobi J. , Page R. H.
Published: February 2008

Corona wind enhancement of free convection was investigated with the needle-plate geometry in air. High voltage was applied to a needle suspended above a heated plate, and heat transfer coefficients were computed by measuring the plate surface temperature distribution with an infrared camera. Local heat transfer coefficients greater than 65 W/m2 K were measured, an enhancement of more than 25:1 over natural convection. The enhancement extended over a significant area, often reaching beyond the 30 cm measurement radius. At high power levels, Joule heating significantly reduced the effective impingement point heat transfer coefficient. The corona wind was found to be more efficient with positive potential than with negative. The heat transfer efficiency was optimized with respect to electrode height and applied voltage. The needle-plate heat transfer effectiveness improved rapidly with increasing height, and became relatively insensitive to height above a threshold value of about 5 cm.
Effect of Ferric Chloride on Silica Fouling
Authors: Chan S. H. , Chen Z. J. , He P.
Published: February 2008

This study presents the results of an experimental investigation of the effects of Fe3+ ion in aqueous solution on silica fouling of heat exchanger and assesses the relative effect of Fe3+, Na+, and K+ ions on silica fouling. Experiments were conducted at 165–125°C, 1.58 MPa, and under turbulent flow conditions. It was shown that adding a few ppm of Fe3+ into silica solution was sufficient to formulate chemical coagulation and deposition of silica colloidal particles, which in turn caused a significant increase in the fouling rate. It was also observed that when Fe3+ was added to the silica solutions containing either Na+ or K+ ions, the fouling behaviors were found to be similar. Thus Fe3+ was found to be dominant over Na+ and K+ ions in its effects on silica fouling.
Direct Simulation of Double-Diffusive Layered Convection
Authors: Hyun M. T. , Bergman T. L.
Published: February 2008

A two-layer, salt-stratified system destabilized and mixed by lateral heating and cooling is simulated numerically using a spectral methodology. As noted in previous studies, the mixing time is delayed as the stabilizing buoyancy ratio increases, and as the Rayleigh number decreases. Depending on the regime of operation, however, distinct physical mechanisms are responsible for interface erosion and system mixing. At low Ra, erosion is gradual with overlying (underlying) fluid sheared from the interface and mixed into the adjacent thermally convecting layer. At high Ra, an intermittent mechanism is predicted to occur with solutally distinct plumes bombarding the interface and ejecting fluid into the convecting layers. Predictions obtained with the spectral method are in relatively good agreement with experimental data.
Characteristics of Radiation Absorption in Metallic Particles
Authors: Qiu T. Q. , Longtin J. P. , Tien C. L.
Published: February 2008

Thermal radiation absorption in metallic particles is an important phenomenon in many contemporary laser-processing techniques, including laser cladding of coating materials and laser cleaning of particulate contaminations. In this work, the Drude free-electron theory and electromagnetic wave theory are utilized to characterize the internal absorption of CO2 laser radiation in aluminum, chromium, and nickel particles. The results show that metallic particles have unique radiation properties. Radiation absorption in large particles occurs only in a very narrow region of the front particle surface, which results in inefficient radiation absorption. On the other hand, micron and submicron particles can absorb radiation very efficiently, due to the strong diffraction effect at the particle surface. For extremely small particles (e.g., nanometer particles), radiation absorption becomes less efficient. The particle absorption efficiency is found to increase with temperature, and this temperature dependence can be determined from those of flat metal surfaces at the normal incidence.
The Spectral Line-Based Weighted-Sum-of-Gray-Gases Model in Nonisothermal Nonhomogeneous Media
Authors: Denison M. K. , Webb B. W.
Published: February 2008

An approach is developed to extend the previously developed spectral-line weighted-sum-of-gray-gases (SLW) model to nonisothermal, nonhomogeneous media. The distinguishing feature of the SLW gas property model is that it has been developed for use in arbitrary solution methods of the radiative transfer equation (RTE). A spatial dependence of the gray gas absorption cross sections on local temperature, pressure, and mole fraction is introduced through the absorption-line blackbody distribution function. Incorporating this spatial dependence results in significant improvement over the use of spatially uniform gray gas absorption cross sections in comparisons with line-byline benchmarks.
Bolometric Response of High-Tc Superconducting Detectors to Optical Pulses and Continuous Waves
Authors: Chen Rong-Chang , Wu Jhy-Ping , Chu Hsin-Sen
Published: February 2008

This work presents a thermal analysis on predicting the temperature increase and the voltage response of high-Tc superconducting bolometers. Two heat transfer models, that is, the surface heating model and the heat generation model considering the skin depth, are introduced and compared. The surface heating model is found to be valid only for situations where the skin depth is much smaller than the film thickness. To consider the thermal boundary resistance between film and substrate, a radiation-boundary-condition model based on acoustic mismatch model (AMM) and an interfacial-layer model (ILM) are employed. The thermal boundary resistance significantly influences the voltage response. Additionally, several common substrates are examined. SrTiO3 (100) or LaAlO3 (100) is a better substrate for high-Tc superconducting bolometers. One interesting finding was that when compared with experimental data, all the theoretical values from the present study as well as the other previously theoretical treatment overestimate the voltage response near the transition temperature.
Two Approaches to Optimal Sensor Locations
Authors: Fadale T. D. , Nenarokomov A. V. , Emery A. F.
Published: February 2008

Accurate modeling of thermal systems depends upon the determination of the material properties and the surface heat transfer coefficients. These parameters are frequently estimated from temperatures measured within the system or on the surface or from measured surface heat fluxes. Because of sensor errors or lack of sensitivity, the measurements may lead to erroneous estimates of the parameters. These errors can be ameliorated if the sensors are placed at points of maximum sensitivity. This paper describes two methods to optimize sensor locations: one to account for signal error, the other to consider interacting parameters. The methods are based upon variants of the normalized Fisher information matrix and are shown to be equivalent in some cases, but to predict differing sensor locations under other conditions, usually transient.
Low-Velocity Subcooled Nucleate Flow Boiling at Various Orientations
Authors: Kirk K. M. , Merte H. , Keller R.
Published: February 2008

Subcooled forced convection nucleate boiling experiments with R-113 were conducted at low velocities using both thin film semitransparent gold-on-quartz and gold-coated copper substrate flat heaters at various orientations. The experiments demonstrate that if buoyancy is significant relative to bulk liquid momentum, then a decrease in the buoyant force normal to and away from the heater surface enhances the heat transfer, with the effect being greatest at low values of heat flux. Furthermore, the effect of the bulk liquid velocity is shown to be dependent on the surface orientation.
Pool Boiling of n-Pentane, CFC-113, and Water Under Reduced Gravity: Parabolic Flight Experiments With a Transparent Heater
Authors: Oka T. , Abe Y. , Mori Y. H. , Nagashima A.
Published: February 2008

A series of pool boiling experiments have been conducted under reduced gravity condition (the order of 10−2 times the terrestrial gravity) available in an aircraft taking parabolic flight. A transparent resistant heater, a transparent indium oxide film plated on a glass plate, was employed so that the vapor/liquid behavior interacting with the heater surface could be observed from the rear side of the heater simultaneously with the side view of vapor bubbles above the heater surface. The experiments were performed for three different fluids—n-pentane, CFC-113, and water—under subcooled conditions. The critical heat fluxes for both n-pentane and CFC-113 under the reduced gravity were lowered to about 40 percent of the corresponding terrestrial values. Although the heat transfer characteristics in a low heat flux nucleate boiling regime for both n-pentane and CFC-113 showed no more than a slight change with the reduction in gravity, a significant heat transfer deterioration was noted with water in the reduced gravity boiling. The observation from the rear side of the heater suggested that this particular difference in the gravity dependency of heat transfer was ascribed to a considerable difference, between the organic fluids and water, in the behavior of attachment to the heater surface of the bubbles grown up, while the behavior of attachment must depend on the surface tension of each fluid and the wettability of the heater surface with the fluid.
Stochastic Modeling of Molecular Transport to an Evaporating Microdroplet in a Superheated Gas
Authors: Carey V. P. , Hawks N. E.
Published: February 2008

The investigation summarized in this paper explored the use of a stochastic, direct-simulation Monte-Carlo scheme to model heat and mass transfer associated with the vaporization of liquid nitrogen microdroplets in superheated nitrogen gas. Two different stochastic models of the molecule–surface interactions were used in the particle simulation scheme. The first, which imposes conservation of mass at the droplet surface, is appropriate for a heated or cooled solid sphere. The second models the net generation of vapor at the surface of an evaporating droplet by allowing the molecular flux to adjust itself dynamically to balance the energy exchange. Predictions of the particle simulation model with the mass conservation surface treatment are found to agree favorably with heat transfer data for a solid sphere in a rarefied gas. The effects of noncontinuum behavior and interface vapor generation on transport during microdroplet evaporation are explored and a closed-form relation for the heat transfer coefficient is developed, which closely matches all our simulation heat transfer predictions for evaporating microdroplets. This relation apparently is the first to account for the simultaneous effects of interface vapor generation and noncontinuum behavior on heat transfer controlled microdroplet evaporation.
Flow and Cold Heat-Storage Characteristics of Phase-Change Emulsion in a Coiled Double-Tube Heat Exchanger
Authors: Inaba H. , Morita S.
Published: February 2008

This paper dealt with the flow and cold heat-storage characteristics of the oil (tetradecane, C14H30, freezing point 278.9 K)/water emulsion as a latent heat-storage material having a low melting point. A coiled double-tube heat exchanger was used for the cold heat-storage experiment. The pressure drop, the heat transfer coefficient, and the finishing time of cold heat storage in the coiled tube were measured as experimental parameters. It was understood that the flow behavior of the emulsion as a non-New-tonian fluid had an important role in the present cold heat storage. The useful nondi-mensional correlation equations for the additional pressure loss coefficient, the heat transfer coefficient, and the cold heat storage time were derived in terms of modified Dean number and heat capacity ratio.
Direct Contact Melting of a Packed Bed
Authors: Kearns D. A. , Plumb O. A.
Published: February 2008

Direct contact melting of a packed bed is studied experimentally. The effect of several parameters, including material properties, particle size, bed loading (applied external force), and thermal boundary conditions, is examined. Two types of melting are observed. If the load on the bed is small and the top of the bed is maintained at a temperature below the melting temperature, then crusting can occur. During this type of melting liquid flows away from the heated surface due to capillary action, and resolidifies forming a crust. The crust prevents downward movement of the packed bed, and melting of the solids below the crust results from convection and conduction. Eventually the crust is melted and the process repeats itself, resulting in a highly unsteady melting rate. If the bed load and the temperature at the top of the bed are sufficiently high, then direct contact melting persists. In this case the melting rate is steady and the parametric behavior can be predicted using a modification of an available theory for direct contact melting of monolithic solids. Although this theory accurately predicts the parametric behavior, the prediction of the actual melting rate is more than an order of magnitude higher than the experimental results.
Finite Difference Modeling of Anisotropic Flows
Authors: Keyhani M. , Polehn R. A.
Published: February 2008

A modification to the finite difference equations is proposed in modeling multidimensional flows in an anisotropic material. The method is compared to the control volume version of the Taylor expansion and the finite element formulation derived from the Galerkin weak statement. For the same number of nodes, the proposed finite difference formulation approaches the accuracy of the finite element method. For the two-dimensional case, the effect on accuracy and solution stability is approximately the same as quadrupling the number of nodes for the Taylor expansion with only a proportionately small increase in the number of computations. Excellent comparisons are made with a new limiting case exact solution modeling anisotropic heat conduction and a transient, anisotropic conduction experiment from the literature.
A Variable Heat Flux Model of Heat Transfer in Grinding: Model Development
Authors: Jen T.-C. , Lavine A. S.
Published: February 2008

In any grinding process, thermal damage is one of the main limitations to accelerating the completion of the product while maintaining high quality. Therefore, the objective of the present study is to understand the thermal behavior in the grinding process and possibly achieve the ultimate goal of avoiding thermal damage in the grinding process. A model previously developed is improved to analyze the heat transfer mechanisms in the grinding process. Heat generated at the interface between the abrasive grains and workpiece (i.e., the wear flat area) is considered. A conjugate heat transfer problem is then solved to predict the temperature in the grinding zone. In the previous model, all the heat fluxes were assumed to be uniformly distributed along the grinding zone. This led to a contradiction in the temperature matching condition. This reveals that the heat fluxes into each of the various materials are not uniform along the grinding zone. An improved model, accounting for the variation of the heat fluxes along the grinding zone, is presented. The temperature and heat flux distributions along the grinding zone are presented, along with comparisons to previous theoretical results.
Modeling of Nitrogen and Oxygen Recombination on Partial Catalytic Surfaces
Authors: Deutschmann O. , Riedel U. , Warnatz J.
Published: February 2008

In connection with recombination coefficients derived from experimental data described in the literature, a reaction scheme including detailed rate expressions for O and N atom recombination on the surface of re-entry vehicles is established, consisting of elementary reaction steps. To validate the reaction mechanism derived, surface chemistry and fluid mechanical processes are coupled assuming a one-dimensional stagnation flow field. A quantitative agreement is achieved between recombination coefficients resulting from the numerical computations and those calculated from experiments. The temperature dependence of the recombination coefficient is explained by elementary reaction steps. Furthermore, the reaction scheme established is implemented in a two-dimensional Navier–Stokes code computing the re-entry flow around a simple geometry to show the importance of a detailed modeling of surface reactions.
A Kelvin–Clapeyron Adsorption Model for Spreading on a Heated Plate
Authors: Reyes R. , Wayner P. C.
Published: February 2008

A new adsorption model for the spreading dynamics of completely wetting fluids on a heated solid substrate that emphasizes interfacial phenomena is developed and evaluated. The model is based on the premise that both interfacial intermolecular forces and temperature affect the vapor pressure in change-of-phase heat transfer and (therefore) the spreading velocity. Classical change-of-phase kinetics, and interfacial concepts like the Clapeyron, Kelvin, and the augmented Young–Laplace equations are used to evaluate the effects of stress (change in apparent dynamic contact angle), temperature, and superheat on the rewetting velocity. Explicit equations are obtained for the velocity, heat flux, and superheat in the contact line region as a function of the initial plate temperature. Comparisons with experimental data for substrate superheats below a critical value demonstrate that the resulting interfacial model of evaporation/condensation in the contact line region can describe the effect of the saturation temperature and superheat on the rewetting velocity.
The Effect of the Thermal Boundary Condition on Transient Method Heat Transfer Measurements on a Flat Plate With a Laminar Boundary Layer
Authors: Butler R. J. , Baughn J. W.
Published: February 2008

The heat transfer coefficient distribution on a flat plate with a laminar boundary layer is investigated for the case of a transient thermal boundary condition (such as that produced with the transient measurement method). The conjugate problem of boundary layer convection with simultaneous wall conduction is solved numerically, and the predicted transient local heat transfer coefficients at several locations are determined. The numerical solutions for the surface temperature are used to determine the Nusselt number that would be measured in a transient method experiment for a range of (nondimensionalized) surface measurement temperatures (liquid crystal temperatures when they are used as the surface sensor). These predicted transient method results are compared to the well-known results for uniform temperature and uniform heat flux thermal boundary conditions. Measurements are made and compared to the numerical predictions using a shroud (transient) experimental technique for a range of nondimensional surface temperatures. The numerical predictions and measurements compare well and both demonstrate the strong effect of the (nondimensional) surface temperature on transient method measurements. Transient method measurements will give heat transfer coefficients that range from as low as that of the uniform temperature case to higher than that of the uniform heat flux case (a 36 percent difference). These results demonstrate the importance of the temperatures used with the transient method.
Structure of Turbulent Hydrogen Jet Diffusion Flames With or Without Swirl
Authors: Takahashi F. , Vangsness M. D. , Durbin M. D. , Schmoll W. J.
Published: February 2008

The near-field turbulent structure of double-concentric hydrogen-air jet diffusion flames, with or without swirl, has been investigated using conditionally sampled, three-component laser-Doppler velocimetry and coherent anti-Stokes Raman spectroscopy. The turbulent flame zone became thinner and shifted inward as the mean jet velocity was increased, whereas swirl created a radial velocity even at the jet-exit plane, thereby broadening and shifting the flame zone outward. The probability-density functions of velocity components, their 21 moments (up to fourth order), mean temperature, and root-mean-square temperature fluctuation were determined in the near field. The data can be used to validate advanced turbulent combustion models.
The Absorptance of Infrared Radiation by Methane at Elevated Temperatures
Authors: Fuss S. P. , Ezekoye O. A. , Hall M. J.
Published: February 2008

In large-scale fires and flames, radiative transport can be an important factor determining the rate of fuel volatilization and flame spread in condensed fuels, and in general can affect the amount of soot that is produced by the flame. The radiant flux can be significantly attenuated by core hydrocarbon gases that have absorption features in the infrared. The spectral absorptance of the υ3 (centered at approximately 3020 cm−l) and υ4 (centered at approximately 1306 cm−l) fundamental bands of methane were measured at elevated temperatures. The measurements were made using a FTIR spectrometer coupled to a gas cell that was maintained at a constant temperature in a furnace. The partial pressure of the methane was varied between 5 and 95 percent, yielding pressure path lengths between 1.14 and 21.72 atm-cm. The total pressure was maintained at 1 atm. Measurements were made at temperatures between 296 and 900 K. The effect of spectral resolution on the measurements and derived parameters was examined. Spectral resolutions between 4 and 32 cm−1 were used. The spectral mean parameters of line strength and line shape were determined for the Elsasser narrow band radiation model using the data taken at a resolution of 4 cm−1. The band model parameters were incorporated into RADCAL, a narrow band model used to predict spectral intensity and transmittance. The results are compared with lower resolution predictions and experimental spectral transmittance data. Tabulated narrow band parameters are available on the Internet (WWW) at URL http://www.me.utexas.edu/~combust/students/paul/research.
Angle of Incidence and Size Effects on Dependent Scattering in Fibrous Media
Authors: Lee S.-C.
Published: February 2008

The influence of fiber size and angle of incidence on the radiative behavior of fibrous media containing closely spaced, aligned fibers is investigated by utilizing the dependent scattering theory. Numerical analyses are performed to calculate the coherent and incoherent scattering properties of dense fibrous media for several fiber sizes and angles of incidence. Results indicate that as the fiber concentration increases, the radiative properties deviate from their independent scattering values due to dependent scattering interactions. Dependent scattering becomes dominant at a lower volume fraction at normal than at oblique incidence. Increasing the fiber concentration causes the apparent refractive index of the fibrous medium to deviate from unity, thus giving rise to specular reflection due to Fresnel reflection.
Perspective: Issues in CHF Modeling—The Need for New Experiments
Authors: Sadasivan P. , Unal C. , Nelson R.
Published: February 2008

Nucleate boiling and critical-heat-flux (CHF) phenomena have been studied extensively for several decades. However, a satisfactory mechanistic description remains elusive. Although the influences of some system parameters such as heater geometry, body forces, etc., have been elucidated, the influences of several others remain in dispute. In this paper, we present our perspective on the current state of CHF modeling. We list possible parameters that are relevant in the process and discuss the interactions among these parameters. The consequences of such interactions are also discussed. We focus on the simplest configuration—saturated pool boiling on flat heaters. Additional complexities such as orientation effects, flow effects, enhanced surfaces, etc., are not addressed. We highlight specific areas on which we believe experimental efforts should focus to obtain improved mechanistic models of CHF. Experimental techniques used in previous studies are evaluated, and recommendations for new or modified techniques are discussed. We believe CHF must be looked at in the boiling plane (q and ΔT) rather than merely as a single heat-flux point. Mechanistically, this leads us to view CHF as the limiting point of the nucleate boiling region rather than as an independent entity. Experimentally, this means that issues related to the high-heat-flux region must be studied and their effects on CHF investigated.
Heat Transfer Enhancement in Narrow Channels Using Two and Three-Dimensional Mixing Devices
Authors: Garimella S. V. , Schlitz D. J.
Published: February 2008

The localized enhancement of forced convection heat transfer in a rectangular duct with very small ratio of height to width (0.017) was experimentally explored. The heat transfer from a discrete square section of the wall was enhanced by raising the heat source off the wall in the form of a protrusion. Further enhancement was effected through the use of large-scale, three-dimensional roughness elements installed in the duct upstream of the discrete heat source. Transverse ribs installed on the wall opposite the heat source provided even greater heat transfer enhancement. Heat transfer and pressure drop measurements were obtained for heat source length-based Reynolds numbers of 2600 to 40,000 with a perfluorinated organic liquid coolant, FC-77, of Prandtl number 25.3. Selected experiments were also performed in water (Prandtl number 6.97) for Reynolds numbers between 1300 and 83,000, primarily to determine the role of Prandtl number on the heat transfer process. Experimental uncertainties were carefully minimized and rigorously estimated. The greatest enhancement in heat transfer relative to the flush heat source was obtained when the roughness elements were used in combination with a single on the opposite wall. A peak enhancement of 100 percent was obtained at a Reynolds number of 11,000, which corresponds to a transitional flow regime. Predictive correlations valid over a range of Prandtl numbers are proposed.
Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer
Authors: Van Fossen G. J. , Simoneau R. J. , Ching C. Y.
Published: February 2008

This experiment investigated the effects of free-stream turbulence intensity, length scale, Reynolds number, and leading-edge velocity gradient on stagnation-region heat transfer. Heat transfer was measured in the stagnation region of four models with elliptical leading edges downstream of five turbulence-generating grids. Stagnation-region heat transfer augmentation increased with decreasing length scale but ann optimum scale was not found. A correlation was developed that fit heat transfer data for isotropic turbulence to within ±4 percent but did not predict data for anisotropic turbulence. Stagnation heat transfer augmentation caused by turbulence was unaffected by the velocity gradient. The data of other researchers compared well with the correlation. A method of predicting heat transfer downstream of the stagnation point was developed.
A Thermocapillary Convection Experiment in Microgravity
Authors: Kamotani Y. , Ostrach S. , Pline A.
Published: February 2008

Results are reported of the Surface Tension Driven Convection Experiment (STDCE) aboard the USML-1 Spacelab, which was launched on June 25, 1992. In the experiment, 10 cSt silicone oil was placed in an open 10-cm-dia circular container, which was 5 cm deep. The fluid was heated either by a cylinderical heater (1.11 cm diameter) located along the container centerline or by a CO2 laser beam to induce thermocapillary flow. Several thermistor probes were placed in the fluid to measure the temperature distribution. The temperature distribution along the liquid-free surface was measured by an infrared imager. Tests were conducted over a range of heating powers, laser-beam diameters, and free surface shapes. An extensive numerical modeling of the flow was conducted in conjunction with the experiments. Some results of the temperature measurements with flat free surfaces are presented in this paper and they are shown to agree well with the numerical predictions.
Buoyant Pulsating Exchange Flow Through a Vent
Authors: Conover T. A. , Kumar R. , Kapat J. S.
Published: February 2008

Buoyancy-driven bidirectional pulsating exchange flow through a vent in a horizontal partition is studied experimentally using a brine/water system. The associated transient and pulsating exchange flows were studied by densimetric measurements, flow visualization, and laser Doppler velocimetry (LDV) measurements for three different vent length-to-diameter ratios: 0.106, 0.0376, and 0.008. A time scale, based on the rate of decay of the density difference between the two compartments, is developed that collapses all experimental data regarding the decay of density in the top compartment into one curve. Flow visualization was used to understand the flow features contributing to the pulsating flow and to provide a quantitative measure of the major pulsation frequency. Interfacial instability between brine and water at the vent was found to contribute to the pulsation. The pulsation frequencies and their decay were determined from the power spectrum of LDV measurements. For the small length-to-diameter ratios (0.008 and 0.0376) there are two different frequencies that decay at different rates, suggesting multiple flow processes that contribute to flow pulsations.
Combined Buoyancy and Pressure-Driven Flow Through a Shallow, Horizontal, Circular Vent
Authors: Cooper L. Y.
Published: February 2008

Combined buoyancy and pressure-driven (i.e., forced) flow through a horizontal vent is considered where the vent-connected spaces are filled with fluids of different density in an unstable configuration (density of the top is larger than that of the bottom). With zero-to-moderate cross-vent pressure difference, Δp, the instability leads to bidirectional exchange flow between the two spaces, e.g., as in the emptying from the bottom of a liquid-filled can with a single vent opening. For relatively large Δp, the flow through the vent is unidirectional, from the high to the low-pressure space, e.g., as is the case when the can has a large enough second vent at the top. Problems of a commonly used unidirectional orifice vent flow model, with Bernoulli’s equation and a constant flow coefficient, CD are discussed. First, the orifice model does not predict bidirectional flows at zero-to-moderate Δp. Also, when Δp exceeds the critical value, ΔpFL, which defines the onset of unidirectional or “flooding” flow, there is a significant dependence of CD on the relative buoyancy of the upper and lower fluids (i.e., CD is not constant). Analysis of relevant boundary value problems and of available experimental data leads to a mathematical vent flow model, which removes the problems of the orifice flow model. The result is a general algorithm to calculate flow through shallow, horizontal, circular vents under high-Grashof-number conditions.