bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of Boiling Heat Transfer From a Sphere Embedded in a Liquid-Saturated Porous Medium
Authors: Tung V. X. , Dhir V. K.
Published: May 2008

Boiling heat transfer from a sphere embedded in a porous medium composed of nonheated glass particles was studied under steady-state and transient quenching conditions. In the experiments, the diameter of the nonheated glass particles forming the porous layers was varied parametrically. Freon-113 was used as the test liquid. Experimental results showed that the maximum heat flux increased monotonically with increasing glass particle diameter and approached an asymptotic value corresponding to the maximum heat flux obtained in a pool free of glass particles. It was also observed that the minimum heat flux was nearly insensitive to the particle size and the film boiling heat transfer coefficient increased slightly with decreasing particle size. In the nucleate boiling region, the heat transfer coefficient showed a much weaker dependence on wall superheat in the presence of particles. Transient data indicated that the surface temperature was not uniform during quenching. Therefore, different maximum heat fluxes were obtained depending on the location of the thermocouple whose temperature history was employed in recovering the transient boiling curve. However, for some applications, cooling rates predicted by imposing the steady-state boiling curve may not be in large error.
Heat Transfer Through Falling Film Evaporation and Boiling on Horizontal Tubes
Authors: Parken W. H. , Fletcher L. S. , Sernas V. , Han J. C.
Published: May 2008

Evaporation and boiling heat transfer coefficients are presented for thin, distilled water films flowing over the outside of horizontal, electrically heated brass tubes. Tests were conducted with a thin-slot water distribution system for 2.54- and 5.08-cm-dia smooth tubes. Both local and average heat transfer data were obtained for nonboiling and boiling conditions corresponding to feedwater temperatures ranging from 49 to 127°C and heat-flux values ranging from 30 to 80 kW/m2. Feed-water flow rates ranged from 0.135 to 0.366 kg/s per meter length per side of the tube. Both nonboiling and boiling correlations of the average heat transfer coefficients were developed and compared.
Critical Heat Flux Modeling in Pool Boiling for Steady-State and Power Transients
Authors: Pasamehmetoglu K. O. , Nelson R. A. , Gunnerson F. S.
Published: May 2008

Understanding and predicting critical heat flux. (CHF) behavior during steady-state and transient conditions is of fundamental interest in the design, operation, and safety of boiling and two-phase flow devices. The results of a comprehensive study specifically conducted to model transient CHF in pool boiling are presented in this paper. The model we developed includes the analysis of thermal energy conduction within the heater coupled with a macrolayer thinning model. Statistical variations in the vapor mass behavior also are incorporated into the model. The resultant model provides new insight into the basic physics of the CHF phenomenon and indicates favorable agreement with the experimental data from cylindrical heaters with small radii.
Approximate Solutions of Canonical Heat Conduction Equations
Authors: Vujanovic´ B. D. , Jones S. E.
Published: May 2008

We consider three analytical methods for finding the approximate solutions of one-dimensional, transient, and nonlinear heat conduction problems based upon the canonical equations of heat transfer. The first method can be considered as a generalization or refinement of the integral method. The second is an iterative method similar to that of Targ utilized in boundary layer theory. The third method is a variational procedure introduced in the spirit of Gauss’ variational principle of least constraint.
Investigation of Transient Heat Transfer Coefficients in Quenching Experiments
Authors: Osman A. M. , Beck J. V.
Published: May 2008

Methodological and experimental aspects of the estimation of transient heat transfer coefficients in quenching experiments, using inverse heat transfer methods, were addressed and investigated. Beck’s method was used for the estimation of the transient heat transfer coefficient history from interior transient temperature measurements during quenching. Experiments involved plunging a high-purity copper sphere into cooling baths without boiling. The sphere was instrumented with several interior thermocouples for measuring the transient temperature response during quenching. Water and ethylene glycol were investigated. The early transient values of the heat transfer coefficient history were found to be about 100–120 percent higher than the values predicted using well-known empirical correlations for free convection. The later time values were in good agreement with those predicted with empirical correlations. The transient inverse technique has the capability of estimating early transients and subsequent quasi-steady-state values of heat transfer coefficient in a single transient experiment.
Effect of Metallic Coatings on the Thermal Contact Conductance of Turned Surfaces
Authors: Kang T. K. , Peterson G. P. , Fletcher L. S.
Published: May 2008

An experimental investigation was conducted to determine the degree to which the thermal contact conductance at the interface of contacting Aluminum 6061 T6 surfaces could be enhanced through the use of vapor-deposited metallic coatings. Three different coating materials (lead, tin, and indium) were evaluated using four different thicknesses for each coating material. The results verified the existence of an optimum coating thickness, shown to be in the range of 2.0 to 3.0 μm for indium, 1.5 to 2.5 μm for lead, and 0.2 to 0.5 μm for tin. The enhancement factors for thermal contact conductance were found to be on the order of 700, 400, and 50 percent, respectively. Based upon the experimental data, the hardness of the coating materials appears to be the most significant parameter in ranking the substrate and coating material combinations; however, additional experimental data are needed to substantiate this hypothesis. Finally, it was apparent that the thermal contact conductance enhancement effect was greatest at low contact pressures and decreased significantly with increases in the contact pressure.
Cooling of a Multichip Electronic Module by Means of Confined Two-Dimensional Jets of Dielectric Liquid
Authors: Wadsworth D. C. , Mudawar I.
Published: May 2008

Experiments were performed to investigate single-phase heat transfer from a smooth 12.7 × 12.7 mm2 simulated chip to a two-dimensional jet of dielectric Fluorinert FC-72 liquid issuing from a thin rectangular slot into a channel confined between the chip surface and nozzle plate. The effects of jet width, confinement channel height, and impingement velocity have been examined. Channel height had a negligible effect on the heat transfer performance of the jet for the conditions of the present study. A correlation for the convective heat transfer coefficient is presented as a function of jet width, heater length, flow velocity, and fluid properties. A self-contained multichip cooling module consisting of a 3 × 3 array of heat sources confirmed the uniformity and predictability of cooling for each of the nine chips, and proved the cooling module is well suited for packaging large arrays of high-power density chips.
Evaluation of Turbulence Models for Predicting Buoyant Flows
Authors: Shabbir A. , Taulbee D. B.
Published: May 2008

Experimental data for the buoyant axisymmetric plume are used to validate certain closure hypotheses employed in turbulence model equations for calculating buoyant flows. Closure formulations for the turbulent transport of momentum, thermal energy, kinetic energy, and squared temperature used in the k–ε and algebraic stress models are investigated. Experimental data for the mean velocity, mean temperature, and kinetic energy are used in the closure formulation to obtain Reynolds stresses, heat fluxes, etc., which are then compared with their measured values.
An Approximate Analysis for Convective Heat Transfer on Thermally Nonuniform Surfaces
Authors: Park S. H. , Tien C. L.
Published: May 2008

This paper presents a new, simple, but powerful technique for nonsimilar natural and assisting mixed convection heat transfer problems in which thermal boundary conditions are specified arbitrarily even with step discontinuities. Temperature and velocity fields for natural convection over thermally nonuniform surfaces are formulated in terms of equivalent Grashof numbers defined by the superposition of surface heat fluxes and velocities obtained from similarity analyses for isoflux or isothermal surfaces. A local heat transfer rate for assisting mixed convection over thermally nonuniform surfaces is approximated using Nusselt numbers for pure forced and pure natural convection over such surfaces, which are obtained by the superposition method. Comparisons with existing similarity solutions, experimental results, and numerical solutions validate the use of this simple superposition method in many practical situations such as cooling configurations in electronic and manufacturing equipment.
Incipient Buoyant Thermal Convection in a Vertical Cylindrical Annulus
Authors: Littlefield D. L. , Desai P. V.
Published: May 2008

The incipient buoyant thermal convection in a vertical cylindrical annulus when heated from below is examined. The ends are assumed to be free, and the sidewalls perfectly conducting. The temperature needed to initiate fluid motion is expressed nondimensionally in terms of the Rayleigh number. The analytical conflict that arises for annuli of infinite aspects ratios due to insufficient independent boundary conditions is resolved. Calculations for the critical Rayleigh numbers are presented for a variety of geometries, and the corresponding velocity and temperature perturbations are also shown. The number of cells increases as the aspect and radius ratio decrease with a strong bias towards the development of azimuthally varying cells. These changes in cellular behavior are expected based on physical justifications and comparisons with previous studies.
A Note on Multiple and Unsteady Solutions in Two-Dimensional Convection in a Tall Cavity
Authors: Le Que´re´ P.
Published: May 2008

Two-dimensional natural convection in a vertical differentially heated air-filled cavity is investigated numerically from the onset of the multicellular flow structure to the return of the unicellular pattern. Several branches of solutions characterized by different numbers of cells in the flow structure are found. The return to the unicellular flow structure occurs through a gradual decrease in the number of cells and each change in the number of cells is characterized by hysteresis. Also, unsteady solutions are found.
Modified δ-M Scaling Results for Mie-Anisotropic Scattering Media
Authors: Kim T.-K. , Lee H. S.
Published: May 2008

A modified δ-M scaling method, which adjusts the δ-M scaled phase functions to be always positive, is applied to radiative transfer problems in two-dimensional square enclosures. The scaled anisotropic results are compared with the results obtained from an accurate model of the full anisotropic scattering problems using the S-N discrete ordinates method. The modified δ-M anisotropic scaling is shown to improve the isotropic scaled results of a collimated incidence problem, but the required number of terms increases as the phase function complexity and the asymmetry factor increase. For the diffuse incidence problems, even a low-order modified δ-M phase function significantly improves the accuracy of scaled solutions over the isotropic scaling. Significant savings in the computer times are observed when the modified δ-M method is applied.
Experimental Study of Natural Convection Heat Transfer Between a Cylindrical Envelope and an Internal Concentric Heated Octagonal Cylinder With or Without Slots
Authors: Zhang H. L. , Wu Q. J. , Tao W. Q.
Published: May 2008

In this paper, the results of an experimental study of laminar natural convection heat transfer and fluid flow in horizontal annuli between a cylindrical envelope and its inner concentric octagonal heated cylinder are presented. Two octagonal cylinders are investigated: one with a complete surface and the other with two horizontal slots on the top and bottom surfaces. The ratio of the slot width W to H is 0.072. Air is used as the working fluid. The range of Rayleigh number is 2.1×102–1.58×106 for the unslotted case and 1.2×102–1.5×106 for the slotted case. The average heat transfer correlations for the two cases are provided. The results show that the heat transfer intensity of the unslotted octagon is slightly weaker than that in a cylindrical annulus, while for the slotted case, the overall heat transfer enhancement may be as high as 74 percent. The smoke technique is used to visualize the flow patterns. A series of photographs of the flow patterns are provided, which enhances our understanding of the mechanism of heat transfer enhancement for the slotted octagonal case.
Natural Convection in Binary Gases Due to Horizontal Thermal and Solutal Gradients
Authors: Weaver J. A. , Viskanta R.
Published: May 2008

The influence of augmenting and opposing thermal and solutal buoyancy forces on natural convection of binary gases due to horizontal temperature and concentration gradients is examined through comparison of smoke flow visualization and measured temperature and concentration distributions with numerical predictions. The observed flow at the cold wall was unsteady for opposing body forces. The same basic flow structure was observed, but the unsteady flow intensifies as the opposing solutal buoyancy force increases as compared to the thermal buoyancy force. Comparison of predicted and measured temperatures and concentrations is fair overall, but the steady-state analytical model fails to predict the unsteady flow and heat and mass transport for opposing body forces.
Transient and Steady-State Combined Heat Transfer in Semi-Transparent Materials Subjected to a Pulse or a Step Irradiation
Authors: Heping Tan , Maestre B. , Lallemand M.
Published: May 2008

A numerical analysis based on the finite difference scheme and Hottel’s zonal method generalized by the ray tracing method is carried out to treat the one-dimensional transient and steady-state combined radiative-conductive heat transfer in non-gray Semi-Transparent Materials (STM), especially glasses, subjected to an external pulse or a step of irradiation. Coupling problems are studied for two optical boundary conditions: opaque and vitreous interfaces with specular reflections. The influence of slab thickness, spectral properties of the STM, wavelength of the incident radiation, and diffusivity coefficient are examined. It is pointed out that the application of the laser flash method in thermal metrology may give irrelevant results if the basic heat transfer model is not able to take into account radiative-conductive coupling phenomena. The effect of semi-transparency on the steady-state temperature distribution is also examined for materials ranging from fully opaque to fully transparent.
Normal Spectral Emission From Nonhomogeneous Mixtures of CO2 Gas and Al2O3 Particulate
Authors: Walters D. V. , Buckius R. O.
Published: May 2008

Measurements of spectral, normal intensity, accounting for nongray banded emission from CO2 gas and the influence of a highly scattering particulate, Al2O3, are presented. Detailed specifications of the temperatures, particle and gas properties, optical depths, and experimental uncertainties are given. The average temperatures are in the range 445 K to 600 K, pressures are approximately 1 atm, the layer depth is 0.5 cm, and particle optical depths are in the range 0 to 3. The intensity emitted from the medium is reported over a wavelength range of about 4.0 μm to 4.7 μm. Since the particle is virtually nonemitting, no continuum emission outside the 4.3 μm CO2 band is detected. The effect of the particle is seen in the spectrally dependent and extinction coefficient dependent changes in the band structure when scattering is present. Modeling of the nonisothermal temperature distribution of the layer emerges as a critical factor in determining the magnitude of the emitted intensity. Uncertainty in the magnitude of the temperature accounts for most of the estimated uncertainty in the spectral intensity. A comparison of the experimental band intensities with values computed using a nonhomogeneous scattering model (which rigorously incorporates wide-band modeling) indicates excellent agreement for most runs.
Simultaneous Fog Formation and Thermophoretic Droplet Deposition in a Turbulent Pipe Flow
Authors: Epstein M. , Hauser G. M.
Published: May 2008

Simultaneous aerosol formation by equilibrium condensation and the migration of the resulting droplets to the cold surface by thermophoresis is studied theoretically for a turbulent pipe flow. The problem is one in which a mixture of a vapor and noncondensable gas flows into a section of pipe where the pipe wall is cooled far below the dew point of the vapor. Because the temperature gradient at the pipe wall decays to zero once the gas travels far enough into the pipe, only some fraction of the droplets formed will deposit on the pipe wall. The equations of energy and diffusion suggest that turbulence leads to a discontinuity in the aerosol (fog) concentration at the boundary between the fog and clear regions. Numerical solutions are obtained for CsOH fog formation and deposition in steam flow—a particular case of current practical interest in water reactor safety. The axial and radial variations of the aerosol and vapor concentrations are displayed graphically, as are the location of the fog boundary as a function of axial distance and the efficiency of deposition as a function of the pipe wall temperature.
Thermal Rectification in Similar and Dissimilar Metal Contacts
Authors: Stevenson P. F. , Peterson G. P. , Fletcher L. S.
Published: May 2008

An investigation was conducted to verify experimentally the existence of thermal rectification and to determine the effect of surface roughness and material type. Four pairs of test specimens were evaluated: one with a smooth Nickel 200 surface in contact with a rough Nickel 200 surface, one with a smooth Stainless Steel 304 surface in contact with a rough Stainless Steel 304 surface, one with a smooth Nickel 200 surface in contact with a rough Stainless Steel 304 surface, and finally, one with a smooth Stainless Steel 304 surface in contact with a rough Nickel 200 surface. The thermal contact conductance was measured for heat flow from both the smooth to rough and rough to smooth configurations for all four pairs. The results indicate that thermal rectification is a function of surface characteristics, material type, and heat flow direction. For similar materials in contact, some thermal rectification was observed with heat flow from the rough surface to the smooth surface resulting in a higher value of contact conductance. For dissimilar materials, the thermal contact conductance was highest when the heat flow was from the Stainless Steel 304 to Nickel 200. In these cases, the surface roughness was shown to be of secondary importance.
A PCM/Forced Convection Conjugate Transient Analysis of Energy Storage Systems With Annular and Countercurrent Flows
Authors: Cao Y. , Faghri A. , Juhasz A.
Published: May 2008

Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the counter-current flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.
Bounds on Heat Transfer in a Periodic Graetz Problem
Authors: Cousins A. K.
Published: May 2008

It is proven that the heat transfer coefficient and rate of decay of temperature with distance in a fully developed Graetz problem with temporally periodic inlet temperature are greater than or equal to the corresponding quantities in the corresponding steady Graetz problem. The proof is valid for arbitrary duct cross-sectional shapes and for either constant temperature, constant heat flux, or linearized radiation boundary conditions. A numerical solution of the energy equation demonstrates the validity of the theorem. The utility of the result is discussed in the context of heat exchanger design for pulsed gas lasers.
Forced Laminar Convection in a Curved Isothermal Square Duct
Authors: Hwang G. J. , Chao Chung-Hsing
Published: May 2008

A numerical study is made to investigate the forced laminar convection in the hydrodynamically and thermally fully developed region of a curved isothermal square duct. Solutions with one and two pairs of vortices superimposed on the main flow are obtained. In the thermally fully developed region, a three-dimensional energy equation of elliptic type is reduced to a two-dimensional one with an eigenvalue, and the axial diffusion term is considered for a small value of the Peclet number. Flow characteristics for cases of dimensionless radius of curvature β = ∞, 50, 10, and 5 with the square of the Dean number ranging from 0 to 106 in a square duct are studied. In addition, heat transfer characteristics for large dimensionless radii of curvature, Pr=0.7 and 7.0, and Peclet number Pe=∞, 10, 5, and 1 are also examined.
Analytical Solutions and Sinc Function Approximations in Thermal Conduction With Nonlinear Heat Generation
Authors: Lippke A.
Published: May 2008

This paper is concerned with the numerical and analytical evaluation of multiple solutions of the steady-state radial temperature distribution in a solid m-dimensional sphere, which is caused by nonlinear heat generation. To achieve highly accurate solutions a Sinc function approximation of the differential operator is introduced.
Regional Heat Transfer in Two-Pass and Three-Pass Passages With 180-deg Sharp Turns
Authors: Chyu M. K.
Published: May 2008

The heat transfer distributions for flow passing through two-pass (one-turn) and three-pass (two-turn) passages with 180-deg sharp turns are studied by using the analogous naphthalene mass transfer technique. Both passages have square cross section and length-to-height ratio of 8. The passage surface, including top wall, side walls, and partition walls, is divided into 26 segments for the two-pass passage and 40 segments for the three-pass passage. Mass transfer results are presented for each segment along with regional and overall averages. The very nonuniform mass transfer coefficients measured around a sharp 180-deg turn exhibit the effects of flow separation, reattachment, and impingement, in addition to secondary flows. Results for the three-pass passage indicate that heat transfer characteristics around the second turn are virtually the same as those around the first turn. This may imply that, in a multiple-pass passage, heat transfer at the first turn has already reached the thermally developed (periodic) condition. Over the entire two-pass passage, the heat transfer enhancement induced by the single-turn is about 45 to 65 percent of the fully developed values in a straight channel. Such a heat transfer enhancement decreases with an increase in Reynolds number. In addition, overall heat transfer of the three-pass passage is approximately 15 percent higher than that of the two-pass one. This 15 percent increase appears to be Reynolds number independent. The pressure loss induced by the sharp turns is found to be very significant. Within the present testing range, the pressure loss coefficient for both passages is Reynolds number dependent.
Local Heat Transfer Coefficients Under an Axisymmetric, Single-Phase Liquid Jet
Authors: Stevens J. , Webb B. W.
Published: May 2008

The purpose of this investigation was to characterize local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface. The problem parameters investigated were jet Reynolds number Re, nozzle-to-plate spacing z, and jet diameter d. A region of near-constant Nusselt number was observed for the region bounded by 0≤r/d≤0.75, where r is the radial distance from the impingement point. The local Nusselt number profiles exhibited a sharp drop for r/d > 0.75, followed by an inflection and a slower decrease there-after. Increasing the nozzle-to-plate spacing generally decreased the heat transfer slightly. The local Nusselt number characteristics were found to be dependent on nozzle diameter. This was explained by the influence of the free-stream velocity gradient on local heat transfer, as predicted in the classical analysis of infinite jet stagnation flow and heat transfer. Correlations for local and average Nusselt numbers reveal an approximate Nusselt number dependence on Re1/3.
Effects of Vortices With Different Circulations on Heat Transfer and Injectant Downstream of a Row of Film-Cooling Holes in a Turbulent Boundary Layer
Authors: Ligrani P. M. , Subramanian C. S. , Craig D. W. , Kaisuwan P.
Published: May 2008

Results are presented that illustrate the effects of single embedded longitudinal vortices on heat transfer and injectant downstream of a row of film-cooling holes in a turbulent boundary layer. Attention is focused on the changes resulting as circulation magnitudes of the vortices are varied from 0.0 to 0.15 m2/s. Mean temperature results are presented that show how injectant is distorted and redistributed by vortices, along with heat transfer measurements and mean velocity surveys. Injection hole diameter is 0.952 cm to give a ratio of vortex core diameter to hole diameter of about 1.5–1.6. The free-stream velocity is maintained at 10 m/s, and the blowing ratio is approximately 0.5. Film-cooling holes are oriented 30 deg with respect to the test surface. Stanton numbers are measured on a constant heat flux surface with a nondimensional temperature parameter of about 1.5. Two different situations are studied: one where the middle injection hole is beneath the vortex downwash, and one where the middle injection hole is beneath the vortex upwash. For both cases, vortex centers pass within 2.9–3.4 vortex core diameters of the centerline of the middle injection hole. To quantify the influences of the vortices on the injectant, two new parameters are introduced. S is defined as the ratio of vortex circulation to injection hole diameter times mean injection velocity. S1 is similarly defined except vortex core diameter replaces injection hole diameter. The perturbation to film injectant and local heat transfer is determined by the magnitudes of S and S1. When S is greater than 1–1.5 or when S1 is greater than 0.7–1.0, injectant is swept into the vortex upwash and above the vortex core by secondary flows, and Stanton number data show evidence of injectant beneath the vortex core and downwash near the wall for x/d only up to about 17.5. For larger x/d, local hot spots are present, and the vortices cause local Stanton numbers to be augmented by as much as 25 percent in the film-cooled boundary layers. When S and S1 are less than these values, some injectant remains near the wall beneath the vortex core and downwash where it continues to provide some thermal protection. In some cases, the protection provided by film cooling is augmented because of vortex secondary flows, which cause extra injectant to accumulate near upwash regions.
Numerical Solution of One Case of Inverse Heat Conduction Problems
Authors: Kurpisz K.
Published: May 2008

This paper is concerned with a method for solving one case of inverse heat conduction problems. The solution is presented in terms of a series containing the derivatives of the temperature with respect to time and shows no explicit dependence on the initial temperature distribution. In order to solve the problem some temperature histories at the interior points should be known. The derivatives are calculated with the aid of the regularization method. The results of some test cases are presented. Errors due to data accuracy, time steps, Biot numbers, etc. are investigated. The paper focuses on problems that are numerically formulated.
Thermal Aspects of Grinding: Heat Transfer to Workpiece, Wheel, and Fluid
Authors: Lavine A. S. , Jen T.-C.
Published: May 2008

A model of heat transfer in grinding has been developed that considers heat removed from the grinding zone by the workpiece, abrasive grains, and grinding fluid. This model eliminates the need to specify the fraction of the total grinding power that enters the workpiece, or the convection coefficient due to the grinding fluid. The dependence of the workpiece temperature on the various grinding parameters has been explored.
Investigation of the Effects of Flow Swirl on Heat Transfer Inside a Cylindrical Cavity
Authors: Salce A. , Simon T. W.
Published: May 2008

Experiments were conducted to determine local heat transfer coefficients on the inside surfaces of a cylindrical cavity that is cooled by a swirling air flow. Temperature-sensitive liquid crystals were used as temperature sensors. Five blowing (cooling) modes were tested: three with swirl numbers of 0.36, 0.84, and 1.73; a fourth with no swirl (axial flow), and a fifth that was similar to the fourth but had the flow direction reversed. Flow visualization and static pressure measurements were performed to improve understanding of the situation. The smoke-wire technique was successfully used to picture the flow patterns. Plots of local Nusselt number along the cavity surfaces were obtained for the five blowing modes and for three different Reynolds numbers. The swirling cases had similar flow fields with higher heat transfer rates near the cavity top and lower rates near the cavity bottom (the opposite of the nonswirling cases). A tornadolike structure on the cavity bottom was observed in the swirling cases. This structure became stronger and more violent as the degree of swirl and the Reynolds number were increased. The Nusselt number curves for the two nonswirling cases were of similar shape, although the flow direction was reversed.
Transient Laminar Mixed Convective Heat Transfer in a Vertical Flat Duct
Authors: Lin T. F. , Yin C. P. , Yan W. M.
Published: May 2008

Unsteady laminar aiding and opposing mixed convection heat transfer in a vertical flat duct is numerically investigated for an initially fully developed flow. Results indicate that unsteady heat transfer characteristics in the flow are principally determined by wall-to-fluid heat capacity ratios. Effects of the buoyancy and degree of asymmetric heating or cooling are rather insignificant. Correlation equations for the time variations of local Nusselt numbers with wall-to-fluid heat capacity ratios are proposed.
A Three-Dimensional Analysis of the Flow and Heat Transfer for the Modified Chemical Vapor Deposition Process Including Buoyancy, Variable Properties, and Tube Rotation
Authors: Lin Y. T. , Choi M. , Greif R.
Published: May 2008

A study has been made of the heat transfer, flow, and particle deposition relative to the modified chemical vapor deposition (MCVD) process. The effects of variable properties, buoyancy, and tube rotation have been included in the study. The resulting three-dimensional temperature and velocity fields have been obtained for a range of conditions. The effects of buoyancy result in asymmetric temperature and axial velocity profiles with respect to the tube axis. Variable properties cause significant variations in the axial velocity along the tube and in the secondary flow in the region near the torch. Particle trajectories are shown to be strongly dependent on the tube rotation and are helices for large rotational speeds. The component of secondary flow in the radial direction is compared to the thermophoretic velocity, which is the primary cause of particle deposition in the MCVD process. Over the central portion of the tube the radial component of the secondary flow is most important in determining the motion of the particles.
An Experimental Investigation of Transition Boiling in Subcooled Freon-113 Forced Flow
Authors: Passos J. C. , Gentile D.
Published: May 2008

An experimental study of subcooled boiling in a Freon-113 forced flow is presented. The test section is a short tube (length 50 mm, inner diameter 8 mm, and wall thickness 0.3 mm) heated by the Joule effect. Wall temperature profiles along the tube are presented for different operating points and discussed in terms of the upstream propagation of a temperature front separating regions of nucleate and film boiling. This study emphasizes the effect of axial heat conduction on the boiling processes.
Fractal Network Model for Contact Conductance
Authors: Majumdar A. , Tien C. L.
Published: May 2008

The topography of rough surfaces strongly influences the conduction of heat and electricity between two surfaces in contact. Roughness measurements on a variety of surfaces have shown that their structure follows a fractal geometry whereby similar images of the surface appear under repeated magnification. Such a structure is characterized by the fractal dimension D, which lies between 2 and 3 for a surface and between 1 and 2 for a surface profile. This paper uses the fractal characterization of surface roughness to develop a new network model for analyzing heat conduction between two contacting rough surfaces. The analysis yields the simple result that the contact conductance h and the real area of contact At are related as h ~ AtD/2 where D is the fractal dimension of the surface profile. Contact mechanics of fractal surfaces has shown that At varies with the load F as At ~ Fη where η ranges from 1 to 1.33 depending on the value of D. This proves that the conductance and load are related as h ~ FηD/2 and resolves the anomaly in previous investigations, which theoretically and experimentally obtained different values for the load exponent. The analytical results agreed well with previous experiments although there is a tendency for overprediction.
Heat Transfer and Turbulent Flow Characteristics of Isolated Three-Dimensional Protrusions in Channels
Authors: Roeller P. T. , Stevens J. , Webb B. W.
Published: May 2008

The flow structure and average heat transfer characteristics of single, isolated three-dimensional protrusions in a flow channel have been investigated experimentally. This configuration has relevance in the electronics industry. The study was designed to identify the influence of the three-dimensional flow around a heated protrusion on its average heat transfer. Heated protrusions varying in width between 0.12 and 1.0 channel widths for a fixed protrusion height and streamwise length were studied in the channel Reynolds number range 500≤Re≤10,000. The channel wall spacing was also varied parametrically between 1.25 and 2.5 streamwise protrusion lengths. The study included both average heat transfer measurements, and detailed local velocity and turbulent flow structure measurements made using laser-Doppler velocimetry. The experimental results show that the Nusselt number increases with both decreasing channel wall spacing and decreasing protrusion width. The increase in heat transfer with decreasing wall spacing is explained by the accelerated flow due to the protrusion-obstructed channel. Increasing Nusselt number with decreasing protrusion width is a result of increased three-dimensional flow and associated turbulent mixing. Both of these flow-related phenomena are illustrated with local mean velocity and turbulence intensity measurements. The presence of recirculation zones both upstream and downstream of the module is revealed. The flow acceleration around the heated protrusions, and three dimensionality of the flow and heat transfer are competing mechanisms; the higher heat transfer due to flow acceleration around the protrusions for larger protrusions goes counter to the trend for higher heat transfer due to increased three-dimensional flow and transport for smaller protrusions. A Nusselt number correlation is developed as a function of channel Reynolds number and protrusion and channel geometric parameters, which describes the tradeoffs discussed.
Factors Affecting Nugget Growth With Mushy-Zone Phase Change During Resistance Spot Welding
Authors: Wei P. S. , Yeh F. B.
Published: May 2008

An unsteady, axisymmetric model is first proposed to investigate extensively effects of the physical, thermal, and metallurgical properties and welding conditions on nugget growths with mushy-zone phase change during resistance spot welding. The electromagnetic force, joule heat and interfacial heat generation, and cooling effects of electrodes are taken into account. Fluid patterns, temperature fields, and solute distributions in the liquid, solid, and mushy zones are determined. Results show that the computed nugget growths and temperature fields are consistent with experimental data. Variations of properties strongly affect the nugget growth. The maximum velocity in the weld nugget is found to be small and around 5 mm/s.
Radiative Phenomena in CW Laser Annealing
Authors: Grigoropoulos C. P. , Dutcher W. E. , Barclay K. E.
Published: May 2008

Recrystallization of thin semiconductor films can yield improved electrical and crystalline properties. Recrystallization is often effected by using a laser source to melt the semiconductor film, which has been deposited on an amorphous insulating substrate. Although temperature measurement data would be valuable for the processing of materials on a microscopic scale, very few such measurements have been presented. It is the intent of this paper to demonstrate work toward the development of completely noninvasive experimental methods for in situ quantitative analysis of the laser annealing process, based on the acquisition of surface radiative data.
Multiple Reflection Effects on Evaporative Cutting With a Moving CW Laser
Authors: Bang S. Y. , Modest M. F.
Published: May 2008

Multiple reflection and beam guiding effects during laser machining are analyzed numerically. The surface of the treated material is considered to reflect the laser irradiation in a fully diffuse fashion, limiting the analysis to bodies that have a rough surface during laser evaporation (e.g., micro-explosive removal, violent surface boiling, etc.). For local irradiation calculations the material surface is divided into a number of triangular elements with linear interpolation functions. The net radiative flux for these elements is obtained from standard view factor theory. The irradiation calculations are combined with a simple integral method governing conduction losses into the medium, and the resulting groove shape and depth are found through an iterative procedure. Considering multiple reflection results in increased material removal rates and deeper grooves, accompanied by a flatter profile near the centerline and steeper slopes in the other parts of the groove cross section.
Heat Transfer With Vaporization of a Liquid by Direct Contact in Another Immiscible Liquid: Experimental and Numerical Study
Authors: Tadrist L. , Sun J. , Santini R. , Pantaloni J.
Published: May 2008

An experimental setup was designed to study direct-contact evaporators using a liquid dispersed in another immiscible liquid. The study was carried out on an n-pentane–water system to determine the influence of different parameters on these systems, and consequently to construct a model for this type of evaporator. An optical probe was used to measure the local void fraction. At different column abscissas along a selected diameter, the local void fraction variations were determined. The shape of the curves can be attributed to the different processes occurring in the spray column. A one-dimensional heat transfer model in the spray column was established. Simplifying assumptions were used to establish and resolve the set of equations governing heat transfer and two-phase flow. The vaporization process induces a volumetric expansion of the two-phase mixture. A theoretical model was used, in which the coalescence between the spherical fluid particles is taken into account. Different coalescence laws dependent on particle density were introduced into the theoretical model and then tested. The numerical results are discussed and compared with the experimental data obtained for the n-pentane–water system.
Experimental and Numerical Analysis of Low-Temperature Heat Pipes With Multiple Heat Sources
Authors: Faghri A. , Buchko M.
Published: May 2008

A numerical analysis and experimental verification of the effects of heat load distribution on the vapor temperature, wall temperature, and the heat transport capacity for heat pipes with multiple heat sources is presented. A numerical solution of the elliptic conjugate mass, momentum and energy equations in conjunction with the thermodynamic equilibrium relations and appropriate boundary conditions for the vapor region, wick structure, and the heat pipe wall are given. The experimental testing of a copper-water heat pipe with multiple heat sources was also made showing excellent agreement with the numerical results. An optimization of the heat distribution for such heat pipes was performed and it was concluded that by redistribution of the heat load, the heat capacity can be increased.
Drying of Porous Materials in a Medium With Variable Potentials
Authors: Liu Jen Y.
Published: May 2008

This paper presents an application of the Luikov system of heat and mass transfer equations in dimensionless form to predict the temperature and moisture distributions in a slab of capillary-porous material during drying. The heat and mass potentials of the external medium in the boundary conditions are assumed to vary linearly with time. The method of solution is illustrated by considering the drying of a slab of lumber. Numerical results based on the estimated thermophysical properties of spruce are presented.
Conjugate Heat Transfer From a Heated Disk to a Thin Liquid Film Formed by a Controlled Impinging Jet
Authors: Faghri A. , Thomas S. , Rahman M. M.
Published: May 2008

An experimental and numerical study of the heat transfer from a heated horizontal disk to a thin film of liquid is described. The liquid was delivered to the disk by a collar arrangement such that the film thickness and radial velocity were known at the outer radius of the collar. This method of delivery is termed as a controlled impinging jet. Flow visualization tests were performed and heat transfer data were collected along the radius of the disk for different volumetric flow rates and inlet temperatures in the supercritical and subcritical regions. The heat transfer coefficient was found to increase with flow rate when both supercritical and subcritical regions were present on the heated surface. A numerical simulation of this free surface problem was performed, which included the effects of conjugate heat transfer within the heated disk and the liquid. The numerical predictions agree with the experimental results and show that conjugate heat transfer has a significant effect on the local wall temperature and heat transfer coefficient.
Natural Convection From L-Shaped Corners With Adiabatic and Cold Isothermal Horizontal Walls
Authors: Angirasa D. , Mahajan R. L.
Published: May 2008

A numerical study of two-dimensional fluid flow and heat transfer by natural convection from L-shaped corners is reported. The vertical side is hot isothermal, and the horizontal side is either adiabatic or cold isothermal at the ambient temperature. The effect of changing the aspect ratio (length of the horizontal side/height of the vertical side) on the transport from the vertical side is studied in detail. It is shown that when the length of the horizontal wall is of the order of the boundary layer thickness on the vertical side, the entrainment flow as well as the boundary layer flow are influenced significantly by a change in the length of the horizontal surface. The heat transfer rate from the vertical side also decreases with increasing length. For values of the aspect ratio > 0.3 (Pr = 0.7), the Nusselt number for the vertical side of the L-shaped body is about 10 percent less than that for the vertical plate. As the length of the horizontal plate increases further, the flow in the outer region undergoes a significant change, but the heat transfer from the vertical heated leg remains unaffected. As the aspect ratio approaches = 2.0, increasing the length of the horizontal side ceases to have any further influence on the entire flow field. Comparison of Nusselt number with past experimental data for air shows good agreement. Finally, Nusselt number correlations in the range of Rayleigh number from 105 to 109 are presented.
Natural Convection Between Two Horizontal Cylinders in an Adiabatic Circular Enclosure
Authors: Ho C. J. , Chang W. S. , Wang C. C.
Published: May 2008

A numerical study of natural convection flow structure and heat transfer has been undertaken for air around two horizontal, differentially heated cylinders confined to an adiabatic circular enclosure. Parametric simulations were performed to assess the effects of gap width between cylinders as well as the inclination angle of the enclosure with respect to gravity. Results clearly indicate that the fluid flow complexity and heat transfer characteristics of air amid the cylinders and enclosure wall are strongly affected by the Rayleigh number, the inclination angle, and the gap width between the cylinders. With the exception of the vertical orientation, heat exchange between the differentially heated cylinders is predominantly controlled by a counterclockwise recirculating flow enclosing them. In addition, flow visualization experiments were conducted for the physical configuration under consideration, and a generally good agreement for the flow pattern was observed between the predictions and the experiments, further validating the present numerical simulation.
Natural Convection in the Inclined, Cranked Thermosyphon
Authors: Lock G. S. H. , Fu J.
Published: May 2008

Consideration is given to natural convection in a cranked, tubular thermosyphon oriented in various ways with respect to the vertical. Experiments on a small-bore, water-filled tube are used to investigate three orientational effects: by tilting in a vertical plane containing the tubes; by incremental rotation about the axis of the horizontal connecting tube; and by incremental rotation about a horizontal axis parallel to the thermosyphon tube axes. The results reveal similarities between the cranked and linear thermosyphons. In general, horizontal alignments are found to produce higher heat transfer rates.
Discrete Ordinates Solutions of Nongray Radiative Transfer With Diffusely Reflecting Walls
Authors: Menart J. A. , Lee HaeOk Skarda , Kim Tae-Kuk
Published: May 2008

Nongray gas radiation in a plane parallel slab bounded by gray, diffusely reflecting walls is studied using the discrete ordinates method. The spectral equation of transfer is averaged over a narrow wavenumber interval preserving the spectral correlation effect. The governing equations are derived by considering the history of multiple reflections between two reflecting walls. A closure approximation is applied so that only a finite number of reflections have to be explicitly included. The closure solutions express the physics of the problem to a very high degree and show relatively little error. Numerical solutions are obtained by applying a statistical narrow-band model for gas properties and a discrete ordinates code. The net radiative wall heat fluxes and the radiative source distributions are obtained for different temperature profiles. A zeroth-degree formulation, where no wall reflection is handled explicitly, is sufficient to predict the radiative transfer accurately for most cases considered, when compared with increasingly accurate solutions based on explicitly tracing a larger number of wall reflections without any closure approximation applied.