bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of a Hue Capturing Based Transient Liquid Crystal Method for High-Resolution Mapping of Convective Heat Transfer on Curved Surfaces
Authors: Camci C. , Kim K. , Hippensteele S. A. , Poinsatte P. E.
Published: May 2008

Accurate determination of convective heat transfer coefficients on complex curved surfaces is essential in the aerothermal design and analysis of propulsion system components. The heat transfer surfaces are geometrically very complex in most of the propulsion applications. This study focuses on the evaluation of a hue capturing technique for the heat transfer interpretation of liquid crystal images from a complex curved heat transfer surface. Impulsively starting heat transfer experiments in a square to rectangular transition duct are reported. The present technique is different from existing steady-state hue capturing studies. A real-time hue conversion process on a complex curved surface is adopted for a transient heat transfer technique with high spatial resolution. The study also focuses on the use of encapsulated liquid crystals with narrow color band in contrast to previous steady-state hue based techniques using wide band liquid crystals. Using a narrow band crystal improves the accuracy of the heat transfer technique. Estimated uncertainty for the heat transfer coefficient from the technique is about 5.9 percent. A complete heat transfer map of the bottom surface was possible using only seven liquid crystal image frames out of the 97 available frames during the transient experiment. Significant variations of heat transfer coefficients are quantitatively visualized on the curved surfaces of the transition duct.
Potential Systematic Errors in Droplet Temperatures Obtained by Fluorescence Methods
Authors: Zhang J. , Melton L. A.
Published: May 2008

Fluorescence methods have been used to measure droplet temperatures. In this work, a light scattering model is combined with a simplified heat transfer model in order to identify systematic errors that may be present in the inferred temperatures. Design of experiments that minimize these systematic errors is discussed.
Convective Heat Transfer From a Sphere Due to Acoustic Streaming
Authors: Gopinath A. , Mills A. F.
Published: May 2008

Convective heat transfer from a sphere due to acoustic streaming is examined for large streaming Reynolds numbers. Analytical and numerical solution techniques are used to obtain Nusselt number correlations for a wide range of Prandtl numbers with particular emphasis on the case of Pr~1. A simple experiment performed to confirm some of the predictions is described. The results obtained can be used for the thermal analysis of containerless materials processing in space using acoustic levitation.
Development of Oscillatory Asymmetric Recirculating Flow in Transient Laminar Opposing Mixed Convection in a Symmetrically Heated Vertical Channel
Authors: Lin Tsing-Fa , Chang Tsai-Shou , Chen Yu-Feng
Published: May 2008

Detailed flow and thermal characteristics in transient laminar opposing mixed convection in a vertical plane channel subject to a symmetric heat input are numerically investigated. First, a linear stability analysis was employed to evidence the occurrence of flow bifurcation. Then, the unsteady Navier–Stokes equations along with the continuity and energy equations were respectively integrated by a third-order upwind and power-law finite-difference scheme with the resulting matrices inverted by the Fast Fourier Transform and conjugated gradient methods. Reverse flow in the form of symmetric, elongated recirculating cells is initiated earlier and is stronger in a lower Prandtl number fluid with higher opposing buoyancy and Reynolds number and longer heated section length. At a high opposing buoyancy, sudden flow asymmetry and oscillation occur simultaneously in a nearly steady flow after the initial transient. Periodic flow and thermal evolution are noted in space and time. An empirical equation for the condition for inducing flow oscillation is proposed.
Thermal Analysis of Complex Crossflow Exchangers in Terms of Standard Configurations
Authors: Shah R. K. , Pignotti A.
Published: May 2008

An attempt is made in this paper to examine very complicated heat exchanger flow arrangements and relate them to simple forms for which either a solution exists or an approximate solution can be obtained. This is shown by an example of a tubular crossflow exchanger having seven possible flow arrangements that can be manufactured from the same six-row tube bundle. All but one are shown to be equivalent to standard crossflow configurations that have been solved in the literature, or to assemblies of such configurations. The remaining case can be bracketed between two such geometries. Values for the thermal effectiveness are obtained, and a comparative analysis is performed.
Rayleigh-Be´nard Convection in a Small Aspect Ratio Enclosure: Part II—Bifurcation to Chaos
Authors: Mukutmoni D. , Yang K. T.
Published: May 2008

The present numerical study documents bifurcation sequences for Rayleigh-Be´nard convection in a rectangular enclosure with insulated sidewalls. The aspect ratios are 3.5 and 2.1 and the Boussinesq fluid is water (average temperature of 70°C) with a Prandtl number of 2.5. The transition to chaos observed in the simulations and experiments is similar to the period-doubling (Feigenbaum) route to chaos. However, special symmetry conditions must be imposed numerically, otherwise the route to chaos is different (Ruelle-Takens-Newhouse). In particular, the Feigenbaum route to chaos can be realized only if the oscillating velocity and temperature field preserves the fourfold symmetry that is observed in the mean flow in the horizontal plane.
Measurements in Buoyancy-Assisting Separated Flow Behind a Vertical Backward-Facing Step
Authors: Baek B. J. , Armaly B. F. , Chen T. S.
Published: May 2008

Measurements of velocity and temperature distributions in buoyancy-assisting laminar mixed convection boundary-layer flow over a vertical, two-dimensional backward-facing step are reported. The leading surface upstream of the step and the step itself were adiabatic, and the surface downstream of the step was heated and maintained at a uniform temperature. A laser-Doppler velocimeter and a cold-wire anemometer were utilized to measure simultaneously the velocity and the temperature distributions in the recirculation and the reattached region downstream of the step. Flow visualization was used to study the flow and to measure the reattachment length for different free-stream velocities (0.37 m/s ≤ u0 ≤ 0.72 m/s), wall temperature differences (10°C ≤ ΔT ≤ 30°C), and step heights (0.38 cm ≤ s ≤ 1 cm). Results show that for a given step height the reattachment length decreases as the buoyancy force parameter, Grs/Res2, increases. The Nusselt number at the heated wall downstream of the step increases and the location of its maximum value moves closer to the step as the buoyancy force parameter increases. For the present experimental range, it is found that the location of the maximum Nusselt number occurs downstream of the reattachment point and the distance between the reattachment point and the location of the maximum Nusselt number increases as the buoyancy force parameter increases. Predicted behavior agrees favorably with the measured results.
Radiative Properties of Flame-Generated Soot
Authors: Ko¨ylu¨ U¨. O¨. , Faeth G. M.
Published: May 2008

Approximate methods for estimating the optical properties of flame-generated soot aggregates were evaluated using existing computer simulations and measurements in the visible and near-infrared portions of the spectrum. The following approximate methods were evaluated for both individual aggregates and polydisperse aggregate populations: the Rayleigh scattering approximation, Mie scattering for an equivalent sphere, and Rayleigh-Debye-Gans (R-D-G) scattering for both given and fractal aggregates. Results of computer simulations involved both prescribed aggregate geometry and numerically generated aggregates by cluster-cluster aggregation; multiple scattering was considered exactly, considered using the mean-field approximation, and ignored using the R-D-G approximation. Measurements involved the angular scattering properties of soot in the postflame regions of both premixed and nonpremixed flames. The results show that available computer simulations and measurements of soot aggregate optical properties are not adequate to provide a definitive evaluation of the approximate prediction methods. The simulations involve either exact solutions for small aggregates where effects of multiple scattering are small, or approximate solutions of uncertain accuracy for the large aggregates of interest for practical flames. The measurements are limited to conditions where soot aggregate structure is not known, and for relatively large scattering angles where the various approximations yield similar results. Within these limitations (for aggregates larger than the Rayleigh scattering regime) the approximate theories performed as follows: Rayleigh scattering generally understimated scattering, Mie scattering for an equivalent sphere yielded unreliable results, while basic and fractal aggregate R-D-G scattering yielded best results for given and fractal aggregates, respectively. However, existing simulations suggest significant effects of multiple scattering for soot aggregates (except near soot inception conditions) that are not included in R-D-G scattering so that improved approximate optical theories for soot aggregates should be sought.
The Numerical and Experimental Study of a Power Plant Condenser
Authors: Zhang C. , Sousa A. C. M. , Venart J. E. S.
Published: May 2008

A numerical and experimental study to evaluate the performance of a power plant condenser has been carried out. Numerically, physically relevant effects are taken into consideration through a quasi-three-dimensional approach. The equations governing the conservation of mass, momentum, and air mass fraction are solved in primitive variable form using a semi-implicit consistent control-volume formulation in which a segregated pressure correction linked algorithm is employed. The modeling of the condenser geometry, including tube bundle and baffle plates, is carried out based on a porous medium concept using applicable flow, heat, and mass transfer resistances. The measurement program included determinations of the steam pressures on the tube bundle perimeter (96 points), steam temperatures (96 locations), inlet tube sheet water pressure distributions (26 measurements), outlet tube sheet flows and temperatures (26 points), hot well flow, and enthalpy in addition to all makeup and extraction flow rates as a function of load. The measurement program and its implementation are briefly described. One data set is compared with the numerical predictions.
Modeling of Temperature Distributions in the Workpiece During Abrasive Waterjet Machining
Authors: Ohadi M. M. , Cheng K. L.
Published: May 2008

Modeling of temperature distributions in a block-type workpiece during cutting with an abrasive waterjet (AWJ) was the subject of an analytical/experimental investigation in the present study. The experiments included measurement of detailed time-temperature distributions in the workpiece for selected AWJ/workpiece operational parameters. Mathematical modeling of the problem made use of a two-part process. In the first part, the measured experimental data were fed into an inverse heat conduction algorithm, which determined the corresponding heat flux in the workpiece. In the second part, this heat flux was fed into a two-dimensional transient heat conduction model that calculated the corresponding temperature distributions in the workpiece. It is demonstrated that the proposed model can serve as a useful thermal analysis tool for AWJ cutting processes so long as a quasi-steady-state condition can be established in the workpiece.
Thermal Gap Conductance of Conforming Surfaces in Contact
Authors: Song S. , Yovanovich M. M. , Goodman F. O.
Published: May 2008

Heat transfer through gas layers of contact interfaces formed by two microscopically rough surfaces is studied. Rarefied gas conduction between smooth parallel plates is examined with data obtained from the literature. Two important dimensionless parameters are introduced; one representing the ratio of the rarefied gas resistance to the continuum gas resistance, and the other representing gas rarefaction effects. Effects of gas rarefaction and surface roughness are studied in relation to the parallel plates case. It is proposed that the effective gap thickness at light loads may be estimate by a roughness parameter, the maximum peak height Rp. Experiments were performed to measure gap conductance for a number of Stainless Steel 304 pairs and Nickel 200 pairs over a range of roughnesses and gas pressures. Three different types of gases, helium, argon, and nitrogen, were employed as the interstitial gas. The comparison between the theory and the measured values of gap conductance shows excellent agreement.
Kinetic Theory Analysis of Flow-Induced Particle Diffusion and Thermal Conduction in Granular Material Flows
Authors: Hsiau S. S. , Hunt M. L.
Published: May 2008

The present study on granular material flows develops analytical relations for the flow-induced particle diffusivity and thermal conductivity based on the kinetic theory of dense gases. The kinetic theory model assumes that the particles are smooth, identical, and nearly elastic spheres, and that the binary collisions between the particles are isotropically distributed throughout the flow. The particle diffusivity and effective thermal conductivity are found to increase with the square root of the granular temperature, a term that quantifies the kinetic energy of the flow. The theoretical particle diffusivity is used to predict diffusion in a granular-flow mixing layer, and to compare qualitatively with recent experimental measurements. The analytical expression for the effective thermal conductivity is used to define an apparent Prandtl number for a simple-shear flow; this result is also qualitatively compared with experimental measurements. The differences between the predictions and the measurements suggest limitations in applying kinetic theory concepts to actual granular material flows, and the need for more detailed experimental measurements.
Eddy Diffusivity Based Comparisons of Turbulent Prandtl Number for Boundary Layer and Free Jet Flows With Reference to Fluids of Very Low Prandtl Number
Authors: Bremhorst K. , Krebs L.
Published: May 2008

It is shown that turbulent Prandtl numbers for turbulent boundary layer and jet flows correlate well if compared on the basis of eddy diffusivity. Experimental data for liquid sodium (Pr= 0.0058) lead to a universal expression relating turbulent Prandtl number to eddy diffusivity of heat with the same expression applying to boundary layer and jet flows. This suggests that the length and velocity scale dependence of the turbulent Prandtl number is associated predominantly with the eddy diffusivity of momentum. Since turbulent flow codes generally include eddy diffusivity of momentum calculations, simple and accurate estimates of eddy diffusivity of heat are possible without reliance on a variety of turbulent Prandtl number functional relationships for prediction of the temperature field. Available results also indicate that turbulent Prandtl number can be set at 0.9–1.0 irrespective of molecular Prandtl number, provided that εM/ν > 3Pr−1 and the structures of the mean temperature gradient and mean shear are similar.
Local Heat Transfer Distribution in a Rotating Serpentine Rib-Roughened Flow Passage
Authors: Zhang N. , Chiou J. , Fann S. , Yang W.-J.
Published: May 2008

Experiments are performed to determine the local heat transfer performance in a rotating serpentine passage with rib-roughened surfaces. The ribs are placed on the trailing and leading walls in a corresponding posited arrangement with an angle of attack of 90 deg. The rib height-to-hydraulic diameter ratio, e/Dh, is 0.0787 and the rib pitch-to-height ratio, s/e, is 11. The throughflow Reynolds number is varied, typically at 23,000, 47,000, and 70,000 in the passage both at rest and in rotation. In the rotation cases, the rotation number is varied from 0.023 to 0.0594. Results for the rib-roughened serpentine passages are compared with those of smooth ones in the literature. Comparison is also made on results for the rib-roughened passages between the stationary and rotating cases. It is disclosed that a significant enhancement is achieved in the heat transfer in both the stationary and rotating cases resulting from an installation of the ribs. Both the rotation and Rayleigh numbers play important roles in the heat transfer performance on both the trailing and leading walls. Although the Reynolds number strongly influences the Nusselt numbers in the rib-roughened passage of both the stationary and rotating cases, Nuo and Nu, respectively, it has little effect on their ratio Nu/Nuo.
An Experimental Study on Heat/Mass Transfer and Pressure Drop Characteristics for Arrays of Nonuniform Plate Length Positioned Obliquely to the Flow Direction
Authors: Huang Huai-Zhang , Tao Wen-Quan
Published: May 2008

In this paper, heat/mass transfer and pressure drop characteristics for arrays of nonuniform plate length, aligned at an angle of 25 deg to the flow direction, are investigated experimentally via a naphthalene sublimation technique. The measurements of cyclic average Sherwood numbers and friction factors in the fully developed regime are conducted for nine geometric configurations. The following parameter ranges are studied: length ratio of successive plates 1.5–2.5; ratio of the transverse pitch to the longitudinal pitch 0.381–0.8, and Reynolds number based on short plate length 1.98×102 to 1.66×103. Comparisons with the results for arrays with uniform plate length are conducted. Two constraints are used, identical pumping power and identical pressure drop. It is found that for most cases studied, the thermal performance of the array with a nonuniform plate length is better than that of the array with a uniform plate length.
Combined Heat and Mass Transfer by Natural Convection With Opposing Buoyancies
Authors: Mahajan R. L. , Angirasa D.
Published: May 2008

A numerical study is presented for combined heat and mass transfer by natural convection from a vertical surface with opposing buoyancy effects. A comparison with similarity solutions shows that boundary layer analysis is suitable only when the two buoyant forces aid each other. For opposing flows the boundary layer analysis does not predict the transport rates accurately. A detailed comparison with experimental data with opposing buoyancies shows good agreement between the data and the numerical solutions. The heat and mass transfer rates follow complex trends depending on the buoyancy ratio and the Prandtl and Schmidt numbers. Comprehensive Nusselt and Sherwood number data are presented for a wide range of thermal Grashof number, buoyancy ratio, and Prandtl and Schmidt numbers.
Analysis of a Transient Asymmetrically Heated/Cooled Open Thermosyphon
Authors: Jones G. F. , Cai J.
Published: May 2008

We present a numerical study of transient natural convection in a rectangular open thermosyphon having asymmetric thermal boundary conditions. One vertical wall of the thermosyphon is either heated by constant heat flux (“warmup”) or cooled by convection to the surroundings (“cooldown”). The top of the thermosyphon is open to a large reservoir of fluid at constant temperature. The vorticity, energy, and stream-function equations are solved by finite differences on graded mesh. The ADI method and iteration with overrelaxation are used. We find that the thermosyphon performs quite differently during cooldown compared with warmup. In cooldown, flows are mainly confined to the thermosyphon with little momentum and heat exchange with the reservoir. For warmup, the circulation resembles that for a symmetrically heated thermosyphon where there is a large exchange with the reservoir. The difference is explained by the temperature distributions. For cooldown, the fluid becomes stratified and the resulting stability reduces motion. In contrast, the transient temperature for warmup does not become stratified but generally exhibits the behavior of a uniformly heated vertical plate. For cooldown and Ra > 104, time-dependent heat transfer is predicted by a closed-form expression for one-dimensional conduction, which shows that Nu → Bi1/2/A in the steady-state limit. For warmup, transient heat transfer behaves as one-dimensional conduction for early times and at steady state and for Ra* ≥ 105, can be approximated as that for a uniformly heated vertical plate.
Three-Dimensional Free Convection in a Long Rotating Porous Box: Analytical Solution
Authors: Vadasz P.
Published: May 2008

A three-dimensional analytical solution to the steady-state free convection problem in a long rotating porous box is presented for large values of the porous media Ekman number. The convection results from differential heating of the horizontal walls leading to temperature gradients orthogonal to the centrifugal body force. The solution to the nonlinear set of partial differential equations was obtained through an asymptotic expansion of the dependent variables in terms of two small parameters representing the reciprocal Ekman number in porous media and the aspect ratio of the domain. The results are focused towards the Coriolis effect on the flow. Secondary circulation was obtained in a plane orthogonal to the leading free convection plane. The results show that the Coriolis effect on free convection is controlled by a combined dimensionless group representing the ratio of the centrifugal Rayleigh number to the porous media Ekman number.
Effect of Surface Wettability on Active Nucleation Site Density During Pool Boiling of Water on a Vertical Surface
Authors: Wang C. H. , Dhir V. K.
Published: May 2008

Pool boiling of saturated water at 1 atm pressure has been investigated. In the experiments, copper surfaces prepared by following a well-defined procedure were used. The cumulative number density of the cavities and their shapes were determined with an optical microscope. The surface had a mirror finish and had a surface Ra (centerline average) value of less than 0.02 μm. The wettability of the surface was changed by controlling the degree of oxidation of the surface. In the experiments with the primary surface, the wall heat flux and superheat were determined with the help of thermocouples embedded in the test block. The density, spatial distribution, local distribution, and nearest-neighbor distance distribution of active nucleation sites in partial and fully developed nucleate boiling were determined from still pictures.
Forced Convective Boiling of Nonazeotropic Refrigerant Mixtures Inside Tubes
Authors: Murata K. , Hashizume K.
Published: May 2008

Forced convective boiling of nonazeotropic mixtures inside horizontal tubes was investigated experimentally. The heat transfer coefficient and pressure drop of pure refrigerant R123 and a mixture of R123 and R134a were measured in both a smooth tube and a spirally grooved tube. The heat transfer coefficient for the mixture was found to be lower than that for an equivalent pure refrigerant with the same phsycial properties, not only in the boiling-dominant region but also in the convection-dominant region. On the basis of this experiment, correlations were proposed for heat transfer coefficients in smooth and grooved tubes; the reduction in heat transfer coefficient for the mixture is attributed to the mixture effects on nucleate boiling and to the heat transfer resistance in the vapor phase. This heat transfer resistance is caused by the sensible heating of the vapor phase accompanying the rise in saturation temperature. These correlations are able to predict the heat transfer data within ± 20 percent
Direct-Contact Condensation of Low-Density Steam on Seawater at High Inlet Noncondensable Concentrations
Authors: Zangrando F. , Bharathan D.
Published: May 2008

Computer models and experimental data are summarized to characterize the condensation of low-density steam in direct contact with a process fluid that is distributed over structured packings of various geometries. The one-dimensional models integrate the heat, mass, and momentum processes in the two streams, over the height of the contactor. Both cocurrent and countercurrent flows of the two streams were analyzed. The models were validated extensively with experimental data obtained with fresh water and with seawater, at high inlet noncondensable concentrations. The data obtained in these experiments provide a broad engineering data base to design and evaluate the performance of advanced heat exchangers for a number of applications, such as production of electrical power and/or potable water using low-density steam, as could be used in Ocean Thermal Energy Conversion systems and in the low-pressure stages of conventional power cycles.
Effects of Moisture on Temperature During Drying of Consolidated Porous Materials
Authors: Kallel F. , Galanis N. , Perrin B. , Javelas R.
Published: May 2008

A one-dimensional model for simultaneous heat and moisture transfer in consolidated porous materials is solved for homogeneous brick and mortar slabs. It is validated by comparing numerically predicted moisture content and temperature evolutions with corresponding measured values. It correctly predicts that initially saturated slabs at 20°C which are suddenly placed in contact with air at 20°C and a relative humidity of 50 percent undergo a rapid transient reduction of their temperature down to 13°C due to the evaporation of excess water content. The model is used to study the effets of the initial moisture content and convection transfer coefficients on the minimum temperature of the slabs and on the duration of the transient.
A Numerical Investigation of Premixed Combustion Within Porous Inert Media
Authors: Hsu P.-F. , Howell J. R. , Matthews R. D.
Published: May 2008

A numerical investigation of premixed combustion within a highly porous inert medium is reported. Specifically, results of a numerical model using detailed chemical kinetics and energy exchange between the flowing gas and the porous solid are presented. The current formulation differs from prior models of this type of combustion in that multistep kinetics is used and a better description of the thermophysical properties of the solid is applied in the present model. It was found that the preheating effect increases strongly with increasing convective heat transfer and with increasing effective thermal conductivity of the solid. The convective heat transfer is expected to increase with increasing number of cells per unit length of porous matrix but the absorption coefficient decreases with increasing cell size and decreasing cell density. Numerical simulations using baseline properties indicate that the lean limit can be extended to an equivalence ratio of about 0.36 for a methane–air flame and that the peak flame temperature is generally higher than the adiabatic flame temperature. The latter effect is predicted to be more pronounced at lower equivalence ratios.
Experimental Investigation of Micro Heat Pipes Fabricated in Silicon Wafers
Authors: Peterson G. P. , Duncan A. B. , Weichold M. H.
Published: May 2008

An experimental investigation was conducted to determine the thermal behavior of arrays of micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels 45 μm wide and 80 μm deep and the other that used an anisotropic etching process to produce triangular channels 120 μm wide and 80 μm deep. Once fabricated, a clear pyrex cover plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtained for a plain silicon wafer and indicated that incorporating an array of micro heat pipes as an integral part of semiconductor devices could significantly increase the effective thermal conductivity; decrease the temperature gradients occurring across the wafer; decrease the maximum wafer temperatures; and reduce the number and intensity of localized hot spots. At an input power of 4 W, reductions in the maximum chip temperature of 14.1°C and 24.9°C and increases in the effective thermal conductivity of 31 and 81 percent were measured for the machined rectangular and etched triangular heat pipe arrays, respectively. In addition to reducing the maximum wafer temperature and increasing the effective thermal conductivity, the incorporation of the micro heat pipe arrays was found to improve the transient thermal response of the silicon test wafers significantly.
Heat Transfer and Thermoelectric Voltage at Metallic Point Contacts
Authors: Phelan P. E. , Nakabeppu O. , Ito K. , Hijikata K. , Ohmori T. , Torikoshi K.
Published: May 2008

Metallic point contacts have been extensively studied from the viewpoint of their interesting, and often nonlinear, electrical properties. Their thermal characteristics, however, have largely been ignored, even though they show great potential as microscale temperature sensors. It has been previously demonstrated that when a temperature drop exists across a point contact consisting of two identical metals, a thermoelectric voltage can be generated, provided the mean contact radius is comparable in size to the electron mean free path. In the present experimental study, a point contact is formed by pressing a sharply etched Ag whisker against either an Ag or a Cu flat plate. In addition to confirming the previous results, the feasibility of using such a point contact is demonstrated by calibrating the voltage output against the nondimensionalized plate temperature. Furthermore, the thermoelectric voltage at an Ag–Cu point contact is also presented, showing that a point contact made from dissimiliar metals is even more promising than one made from identical materials. Finally, the point-contact thermal resistance is shown to depend nonlinearly on the electrical resistance, or contact area, and on the temperature drop.
Liquid Immersion Cooling of a Substrate-Mounted Protrusion in a Three-Dimensional Enclosure: The Effects of Geometry and Boundary Conditions
Authors: Wroblewski D. E. , Joshi Y.
Published: May 2008

A three-dimensional computational study of steady natural convection cooling of a substrate-mounted protrusion (chip) in a rectangular enclosure filled with dielectric liquid is described. Energy is generated in the chip at a uniform rate Q. Conduction within the chip and substrate are accounted for in the model, as is the coupled natural convection in the surrounding liquid. The nondimensional governing equations with the appropriate boundary conditions have been solved in the primitive variable form for Ra = 108 using a fully implicit finite volume formulation. Baseline computations have been performed for a cubical enclosure with a centrally placed silicon chip on a vertical alumina substrate, which forms one enclosure wall. The cooling liquid was Fluorinert FC 75 resulting in Pr = 25. The effects of conductive spreading along the substrate were found to be quite pronounced due to the low thermal conductivity of the liquid. Effects of chip and enclosure sizes on the maximum chip temperatures displayed a strong dependence on the substrate to fluid thermal conductivity ratio, RS. Conditions for the validity of two-dimensional approximations were investigated for large and small RS. Two other thermal boundary conditions on the enclosure walls were also considered, with the smallest chip temperatures found for the top and one vertical sidewall cooled condition. For the baseline boundary conditions a numerical correlation for the maximum chip temperature was obtained.
Thermal Analysis of Heat-Generating Pools Bounded From Below by Curved Surfaces
Authors: Tan M. J. , Cho D. H. , Cheung F. B.
Published: May 2008

A computer code that features the use of a directional effective thermal conductivity in modeling natural convection in heat-generating pools has been developed to analyze heat transfer in such pools, which are bounded from below by curved surfaces. Illustrative calculations pertaining to two published experimental studies on convective heat transfer in water pools with uniformly distributed volumetric energy sources are carried out using the code. The water pools used in the two studies under consideration were cooled either from the top or from the bottom, but not from both. The utility as well as the limitations of the effective thermal conductivity approach in the context of addressing the issue of melt-pool coolability is demonstrated by comparisons of calculated results with the experimental data.
A Study of Natural Convection in a Rotating Enclosure
Authors: Hamady F. J. , Lloyd J. R. , Yang K. T. , Yang H. Q.
Published: May 2008

The local and mean natural convection heat transfer characteristics and flow fields were studied experimentally and numerically in an air-filled, differentially heated enclosure with a cross-sectional aspect ratio of one. The enclosure is rotated above its longitudinal horizontal axis. A Mach-Zehnder interferometer was employed to reveal the entire temperature field, which enable the measurement of the local and mean Nusselt numbers at the hot and cold surfaces. Laser sheet flow visualization was employed to observe the flow field. The result showed that the Coriolis and centrifugal buoyancy forces arising from rotation have a remarkable influence on the local heat transfer when compared with the nonrotating results. Local heat fluxes were obtained as a function of Taylor (Ta≤4×105) and Rayleigh numbers (104<Ra≤3×105), at different angular positions of the enclosure. In addition, a series of interferograms, stream function and isotherm plots demonstrated the strong effect of rotation on the flow field and heat transfer. A correlation of Nusselt number as a function of Taylor and Rayleigh numbers is presented.
Optical Properties of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times
Authors: Ko¨ylu¨ U¨. O¨. , Faeth G. M.
Published: May 2008

The optical properties of soot were studied for the fuel-lean (overfire) region of buoyant turbulent diffusion flames in still air. Results were limited to the long residence time regime where soot structure is independent of position in the overfire region and residence time for a particular fuel. Measurements included scattering, absorption, and extinction cross sections at 514.5 nm and extinction cross sections at 632.8 and 1152 nm for flames fueled with acetylene, propylene, ethylene, and propane. The measurements were used to evaluate scattering predictions based on the Rayleigh-Debye-Gans (RDG) approximation for randomly oriented poly disperse fractal aggregates of spherical primary soot particles having constant diameters. The present soot aggregates exhibited significant departures from Rayleigh-scattering behavior at 514.5 nm, with forward scattering roughly 100 times larger than wide-angle scattering and ratios of scattering to absorption cross sections in the range 0.22–0.41, increasing with increasing propensity of the fuel to soot. The approximate RDG theory generally provided an acceptable basis to treat the optical properties of the present overfire soot aggregates, although additional measurements in the Guinier (small angle) regime are needed for a definitive evaluation of model performance.
Rewetting Theory and the Dryout Heat Flux of Smooth and Grooved Plates With a Uniform Heating
Authors: Chan S. H. , Zhang W.
Published: May 2008

The evaporation and condensation of thin liquid films are of significant importance in a wide variety of problems ranging from specific applications in the heat pipe field to more general ones in the chemical, nuclear, and petrochemical industries. Although several investigations have been conducted to determine the rewetting characteristics of liquid films on heated rods, tubes, and flat plates, no solutions are yet available to describe the rewetting process of a hot plate subjected to a uniform heating. A model is presented to analyze the rewetting process of such plates with and without grooves. Approximate analytical solutions are presented for the prediction of the rewetting velocity and the transient temperature profiles of the plates. It is shown that the present rewetting velocity solution reduces correctly to the existing solution for the rewetting of an initially hot isothermal plate without heating from beneath the plate. Numerical solutions have also been obtained to validate the analytical solutions. Finally, a simple method is presented to predict the dryout heat flux of a liquid film flowing over a heated smooth or grooved plate. The results of the prediction are found to be in reasonable agreement with the existing experimental data.
Heat Transfer and Friction in Rectangular Channels With Ribbed or Ribbed-Grooved Walls
Authors: Zhang Y. M. , Gu W. Z. , Han J. C.
Published: May 2008

The effect of compound turbulators on friction factors and heat transfer coefficients in rectangular channels with two opposite ribbed-grooved walls was determined for a Reynolds number range of 10,000 to 50,000. The channel width-to-height ratio was 10. The fully developed heat transfer coefficients and friction factors on the ribbed-grooved and smooth side walls of each test channel were measured for six rib-groove spacings (p/e = 8, 10, 15, 20, 25, and 30). The fully developed friction and heat transfer in similar aspect ratio rectangular channels with two opposite ribbed walls with two rib spacings (p/e = 8.5 and 11.5) was also measured for comparison. The results show that the heat transfer performance of the rib-groove roughened duct is much better than the rib roughened duct. The rib-groove roughened wall enhances the heat transfer 3.4 times and pays 6 times the pressure drop penalty, whereas the rib roughened wall, with similar rib height and rib spacing, enhances the heat transfer 2.4 times and pays about the same pressure drop penalty. Semi-empirical friction and heat transfer correlations were obtained. Flow measurements show that the roughened ducts have flatter velocity profiles than the smooth duct and rib-groove roughened duct produces higher turbulence intensity than the rib roughened duct. The flatter velocity profile and higher turbulence intensity are responsible for producing higher heat transfer.
Wall Roughness Effects on Stagnation-Point Heat Transfer Beneath an Impinging Liquid Jet
Authors: Gabour L. A. , Lienhard J. H.
Published: May 2008

Jet impingement cooling applications often involve rough surfaces, yet few studies have examined the role of wall roughness. Surface protrusions can pierce the thermal sublayer in the stagnation region and increase the heat transfer. In this paper, the effect of surface roughness on the stagnation-point heat transfer of an impinging unsubmerged liquid jet is investigated. Experiments were performed in which a fully developed turbulent water jet struck a uniformly heated rough surface. Heat transfer measurements were made for jets of diameters 4.4–9.0 mm over a Reynolds number range of 20,000–84,000. The Prandtl number was held nearly constant at 8.2–9.1. Results are presented for nine well-characterized rough surfaces with root-mean-square average roughness heights ranging from 4.7 to 28.2 μm. Measured values of the local Nusselt number for the rough plates are compared with those for a smooth wall, and increases of as much as 50 percent are observed. Heat transfer in the stagnation zone is scaled with Reynolds number and a roughness parameter. For a given roughness height and jet diameter, the minimum Reynolds number required to increase heat transfer above that of a smooth plate is established. A correlation for smooth wall heat transfer is also given.
Effects of Interactions Between Adjoining Rows of Circular, Free-Surface Jets on Local Heat Transfer From the Impingement Surface
Authors: Slayzak S. J. , Viskanta R. , Incropera F. P.
Published: May 2008

Experiments have been conducted to obtain single-phase local heat transfer coefficient distributions associated with impingement of one or two rows of circular, free-surface water jets on a constant heat flux surface. The nozzle diameter, the centerline-to-centerline distance between nozzles in a row, and the nozzle-to-heater separation distance were fixed at 4.9, 6.3, and 89.7 mm, respectively. Two row-to-row separations (81 and 51 mm) were considered, and nozzle discharge Reynolds numbers were varied over the range from 16,800 to 30,400. The interaction zone created by opposing wall jets from adjacent rows is characterized by an upwelling of spent flow (an interaction fountain) for which local coefficients can approach those of the impingement zones. Interactions between wall jets associated with nozzles in one row can create sprays that impact the adjoining row with sufficient momentum to induce a dominant/subordinate row behavior. In this case the interaction zone is juxtaposed with the subordinate row, and local coefficients in the impingement and wall jet regions of the affected row may be significantly enhanced. This result contrasts with the deleterious effects of crossflow reported for submerged jets throughout the literature. Spray-induced enhancements, as well as interaction zone maxima, increase with decreasing row-to-row pitch and with increasing Reynolds number.
On the Wave Theory in Heat Conduction
Authors: O¨zis¸ik M. N. , Tzou D. Y.
Published: May 2008

This work contains three major components: a thorough review on the research emphasizing engineering applications of the thermal wave theory, special features in thermal wave propagation, and the thermal wave model in relation to the microscopic two-step model. For the sake of convenience, the research works are classified according to their individual emphases. Special features in thermal wave propagation include the sharp wavefront and rate effects, the thermal shock phenomenon, the thermal resonance phenomenon, and reflections and refractions of thermal waves across a material interface. By employing the dual-phase-lag concept, we show that the energy equation may be reduced to that governing the heat transport through the metal lattice in the microscopic two-step model. The dual-phase-lag concept can thus capture the microscopic mechanisms in some limiting cases.
Application of a Complex Nusselt Number to Heat Transfer During Compression and Expansion
Authors: Kornhauser A. A. , Smith J. L.
Published: May 2008

Heat transfer during compression and expansion can be out of phase with bulk gas-wall temperature difference. An ordinary convective heat transfer model is incapable of predicting this phenomenon. Expressions for compression/expansion heat transfer developed from simple conduction models use a complex heat transfer coefficient. Thus, heat flux consists of one part proportional to temperature difference plus a second part proportional to rate of change of temperature. Surface-averaged heat flux was calculated from experimental pressure-volume data for piston-cylinder gas springs over a range of speeds, pressures, gases, and geometries. The complex Nusselt number model proved capable of correlating both magnitude and phase of the measured heat transfer as functions of an oscillation Peclet number.
Analysis of Heat Transfer and Fluid Flow Through a Spirally Fluted Tube Using a Porous Substrate Approach
Authors: Srinivasan Vijayaragham , Vafai Kambiz , Christensen Richard N.
Published: May 2008

An innovative approach was opted for modeling the flow and heat transfer through spirally fluted tubes. The model divided the flow domain into two regions. The flutes were modeled as a porous substrate with direction-dependent permeabilities. This enabled modeling the swirl component in the fluted tube. The properties of the porous substrate such as its thickness, porosity, and ratio of the direction-dependent permeabilities were obtained from the geometry of the fluted tube. Experimental data on laminar Nusselt numbers and friction factors for different types of fluted tubes representing a broad range of flute geometry were available. Experimental data from a few of the tubes tested were used to propose a relationship between the permeability of the porous substrate and the flute parameters, particularly the flute spacing. The governing equations were discretized using the Finite Element Method. The model was verified and applied to the other tubes in the test matrix. Very good agreement was found between the numerical predictions and the experimental data.
Convective Heat Transfer to a Confined Impinging Array of Air Jets With Spent Air Exits
Authors: Huber A. M. , Viskanta R.
Published: May 2008

This investigation has examined the influence of spent air exits located between the jets on the magnitude and uniformity of the local heat transfer coefficient for a confined 3×3 square array of axisymmetric air jets impinging normally to a heated surface. The heat transfer coefficient was measured using a 0.025-mm-thick stainless steel impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected wavelength of light from the liquid crystal with the use of bandpass filters and an electronic digitizer board. The effect of small nozzle-to-plate spacings (0.25 and 1.0 diameters) commonly used in material processing applications was also considered. Average Nusselt numbers are presented for a Reynolds number range of 3500 to 20,400 along with radial distributions of the local Nusselt number. The local Nusselt number distributions illustrate the uniformity of the convective heat transfer coefficient and contribute to understanding the variations in the magnitude of the average Nusselt number. Results have shown that the addition of spent air exits increased the convective heat transfer coefficient and changed the location of the optimal separation distance. In addition, significant enhancement of the uniformity and magnitude of the heat transfer coefficient was observed at the 0.25 and 1.0 jet diameter nozzle-to-plate spacings when compared to a 6.0 diameter spacing.
Heat Transfer Enhancement Under Various Orientations Resulting From Attraction Mode Induction Electrohydrodynamic Pumping
Authors: Margo B. D. , Seyed-Yagoobi J.
Published: May 2008

An induction electrohydrodynamic (EHD) pump in an axisymmetric configuration was designed and built to examine the heat transfer enhancement that accompanies fluid pumping. The apparatus was operated at several tilt angles between 0 (horizontal) and 90 while the augmentation of heat transfer was measured and compared to typical forced pumping and natural circulation. The increase in the overall conductance value for EHD pumping compared to typical forced pumping under turbulent flow conditions was as high as 71 percent. This corresponds to a much higher increase in the inside convection coefficient, which is the only resistance to heat transfer being affected by induction pumping.
Natural Convection in a Porous, Horizontal Cylindrical Annulus
Authors: Barbosa Mota J. P. , Saatdjian E.
Published: May 2008

Natural convection in a porous medium bounded by two horizontal cylinders is studied by solving the two-dimensional Boussinesq equations numerically. An accurate second-order finite difference scheme using an alternating direction method and successive underrelaxation is applied to a very fine grid. For a radius ratio above 1.7 and for Rayleigh numbers above a critical value, a closed hysteresis loop (indicating two possible solutions depending on initial conditions) is observed. For a radius ratio below 1.7 and as the Rayleigh number is increased, the number of cells in the annulus increases without bifurcation, and no hysteresis behavior is observed. Multicellular regimes and hysteresis loops have also been reported for fluid layers of same geometry but several differences between these two cases exist.