bluuebunny/crossref_metadata_2025 huggingface dataset:
Refractive Index Effects on Radiation in an Absorbing, Emitting, and Scattering Laminated Layer
Authors: Siegel R. , Spuckler C. M.
Published: May 2008

A simple set of equations is derived for predicting temperature distributions and radiative energy flow in a two-region semitransparent laminated layer in the limit of zero heat conduction. The composite is heated on its two sides by unequal amounts of incident radiation. The two layers of the composite have different refractive indices, and each material absorbs, emits, and isotropically scatters radiation. The interfaces are diffuse, and all interface reflections are included. To illustrate the thermal behavior that is readily calculated from the equations, typical results are given for various optical thicknesses and refractive indices of the layers. Internal reflections have a substantial effect on the temperature distribution and radiative heat flow.
Simulation of the Early Startup Period of High-Temperature Heat Pipes From the Frozen State by a Rarefied Vapor Self-Diffusion Model
Authors: Cao Y. , Faghri A.
Published: May 2008

The heat pipe startup process is described physically and is divided into five periods for convenience of analysis. The literature survey revealed that none of the previous attempts to simulate the heat pipe startup process numerically were successful, since the rarefied vapor flow in the heat pipe was not considered. Therefore, a rarefied vapor self-diffusion model is proposed, and the early startup periods, in which the rarefied vapor flow is dominant within the heat pipe, are first simulated numerically. The numerical results show that large vapor density gradients existed along the heat pipe length, and the vapor flow reaches supersonic velocities when the density is extremely low. The numerical results are compared with the experimental data of the early startup period with good agreement.
A Numerical Analysis of High-Temperature Heat Pipe Startup From the Frozen State
Authors: Cao Y. , Faghri A.
Published: May 2008

Continuum and rarefied vapor flows-co-exist along the heat pipe length for most of the startup period. A two-region model is proposed in which the vapor flow in the continuum region is modeled by the compressible Navier-Stokes equations, and the vapor flow in the rarefied region is simulated by a self-diffusion model. The two vapor regions are linked with appropriate boundary conditions, and heat pipe wall, wick, and vapor flow are solved as a conjugate problem. The numerical solutions for the entire heat pipe startup process from the frozen state are compared with the corresponding experimental data with good agreement.
Multidimensional Inverse Heat Conduction Using the Monte Carlo Method
Authors: Haji-Sheikh A. , Buckingham F. P.
Published: May 2008

The Monte Carlo method is used to solve inverse heat conduction problems when the surface temperature is spatial and time dependent. The standard random walk is modified to deal with curved boundaries. The proposed random walk has all the characteristics of the floating random walk, except its step size is small. This is a uniquely flexible method with excellent accuracy and it is computationally fast. The method is used to solve one- and three-dimensional heat conduction problems and the results are presented. A procedure is described to improve the accuracy of the solution, then used to calculate heat transfer from a cylindrical surface cooled by a stream of air.
Laser Beam Welding With Simultaneous Gaussian Laser Preheating
Authors: Liu Y.-N. , Kannatey-Asibu E.
Published: May 2008

An analytical solution of the dual, laser beam welding process is presented. It is based on a Gaussian distributed leading heat source for preheating, followed by a line source for the actual welding process. The effect of beam distribution parameters as well as interbeam spacing and relative power intensities on the resulting temperature distribution and cooling rate are presented. For a preheating Gaussian source of power 1550 W, the depth of region above 500°C is 2.25 mm, and that above 250°C is 3.5 mm. The cooling rate at the weld centerline without preheating for a temperature of 650° C, input power 1800 W, and welding velocity 20 mm/s is found to be 1004°C/s. Under the same conditions, the cooling rate with a 1550 W preheating Gaussian distributed heat source (beam distribution parameter 1 mm, and interbeam spacing 10 mm) is reduced to 570°C/s.
Maximum Temperatures in Diamond Heat Spreaders Using the Surface Element Method
Authors: Beck J. V. , Osman A. M. , Lu G.
Published: May 2008

Diamond heat spreaders are very attractive for lowering the temperature of laser diodes and computer chip components. The use of diamonds as heat spreaders is very advantageous because its thermal conductivity is so large, about four times that of copper. The diamond heat spreader is mounted on a semi-infinite heat sink. The purposes of this paper are to present (a) a surface element method for the analysis of such composite systems, (b) a set of convenient algebraic equations for the maximum temperatures, (c) optimal geometry conditions and (d) some accurate numerical results. The analysis method is an adaptation of the unsteady surface element method but is different because the present problem is a steady-state one. The surface element method with one node gives a relatively simple algebraic solution, which contains all the important dimensionless groups. The one-node solution is simple, accurate, and has a form that can give deep insight into the effects of various parameters; for example, it premits derivation of the optimal geometry corresponding to the minimal temperature at the hot spot. The study shows that the optimal geometry of the thickness to radius aspect ratio of the diamond heat spreaders is about 0.4. In addition to the single-node analysis, a multinode analysis is developed and very accurate results are presented. These results show that the single-node analysis is generally within 2 percent.
Local Heat Transfer Behavior and Its Impact on a Single-Row, Annularly Finned Tube Heat Exchanger
Authors: Hu X. , Jacobi A. M.
Published: May 2008

Experimental studies of the local mass transfer characteristics of annularly finned tubes in crossflow are presented. Variations due to boundary layer development, forward-edge separation, the tube wake, horseshoe vortices, and tip vortices are discussed. In addition, regularly located local maxima in mass transfer rates associated with the horseshoe vortex system are found, and conjecture as to their mechanism is offered. Inferring heat transfer behavior from the mass transfer results, we find that the true fin efficiency is always less than that obtained with an assumed constant convective heat transfer coefficient. The difference is 3–7 percent for high-conductivity materials such as aluminum alloys, and 9–17 percent for low-conductivity materials such as mild steels.
Convective Heat Transfer Enhancement Due to Intermittency in an Impinging Jet
Authors: Zumbrunnen D. A. , Aziz M.
Published: May 2008

An experimental investigation has been performed to study the effect of flow intermittency on convective heat transfer to a planar water jet impinging on a constant heat flux surface. Enhanced heat transfer was achieved by periodically restarting an impinging flow and thereby forcing renewal of the hydrodynamic and thermal boundary layers. Although convective heat transfer was less effective during a short period when flow was interrupted, high heat transfer rates, which immediately follow initial wetting, prevailed above a threshold frequency, and a net enhancement occurred. Experiments with intermittent flows yielded enhancements in convective heat transfer coefficients of nearly a factor of two, and theoretical considerations suggest that higher enhancements can be achieved by increasing the frequency of the intermittency. Enhancements need not result in an increased pressure drop within a flow system, since flow interruptions can be induced beyond a nozzle exit. Experimental results are presented for both the steady and intermittent impinging jets at distances up to seven jet widths from the stagnation line. A theoretical model of the transient boundary layer response is used to reveal parameters that govern the measured enhancements. A useful correlation is also provided of local heat transfer results for steadily impinging jets.
A New Method for Thermal Analysis of Die Casting
Authors: Barone M. R. , Caulk D. A.
Published: May 2008

A new approach is developed for solving the initial value, steady periodic heat conduction problem in steady-state die casting. Three characteristics found in nearly all die casting processes are exploited directly: The casting is thin compared with its overall size, its thermal conductivity is high compared with that of the mold, and the cycle time is short compared with the start-up transient of the process. Under these conditions, it is reasonable to neglect the transverse temperature gradients in the casting and assume that all die temperatures below a certain depth from the cavity surface are independent of time. The transient die temperatures near the cavity surface are represented by a polynomial expansion in the depth coordinate, with time-varying coefficients determined by a Galerkin method. This leads to a set of ordinary differential equations on the cavity surface, which govern the transient interaction between the casting and the die. From the time-averaged solution of these equations, special conditions are derived that relate the transient solution near the cavity surface to the three-dimensional steady solution in the die interior. With these conditions, the steady temperatures in the bulk of the die can be determined independently of the explicit surface transients. This reduces the effort of solving a complex transient heat conduction problem to little more than finding a steady solution alone. The overall approach provides a general analytical tool, which is capable of predicting complex thermal interactions in large multicomponent dies.
Magnetically Damped Convection During Solidification of a Binary Metal Alloy
Authors: Prescott P. J. , Incropera F. P.
Published: May 2008

The transient transport of momentum, energy, and species during solidification of a Pb-19 percent Sn alloy is numerically simulated with and without magnetic damping. The system is contained in an axisymmetric, annular mold, which is cooled along its outer vertical wall. Since thermosolutal convection accompanies solidification and is responsible for final macrosegregation patterns, application of a steady magnetic field, which is parallel to the axis of the mold, has the potential to reduce macrosegregation by damping buoyancy-driven flow during solidification. Results show that, during early stages of solidification, the magnetic field significantly affects thermally driven flow in the melt, as well as interactions between thermally and solutally driven flows. However, interdendritic flows and macrosegregation patterns are not significantly altered by moderate magnetic fields. Scaling analysis reveals that extremely strong fields would be required to effectively dampen convection patterns that contribute to macrosegregation.
A Hybrid Transient Step-Heating Heat Transfer Measurement Technique Using Heater Foils and Liquid-Crystal Thermography
Authors: von Wolfersdorf J. , Hoecker R. , Sattelmayer T.
Published: May 2008

A transient heat transfer technique using a heating foil and liquid crystals is described. The basic idea is a step-heating technique, eliminating the local heat flux and the surface temperature during the data reduction. Nonuniformities in the heating pattern are allowed and calibration of the liquid crystals is no longer necessary. They are used as an indicator of an isotherm only. The heat transfer coefficient is deduced from two time measurements. The laminar and turbulent boundary layer flows over a flat plate were tested to verify the applicability and accuracy of the method.
Rayleigh-Be´nard Convection in a Small Aspect Ratio Enclosure: Part I—Bifurcation to Oscillatory Convection
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. Two transitions are documented numerically. The first transition is from steady-state to oscillatory flow and the second is a subharmonic bifurcation as the Rayleigh number is increased further. The dynamics of the flow and temperature field is analyzed in detail for the subcritical steady convection and the supercritical oscillatory convection. The numerical results compared well with experiments, both qualitatively and quantitatively.
Natural Convective Heat Transfer in a Divided Vertical Channel: Part II—Experimental Study
Authors: Naylor D. , Tarasuk J. D.
Published: May 2008

An interferometric study has been conducted on two-dimensional laminar natural convection heat transfer in an isothermal vertical divided channel. Interferograms were obtained for air and a plate-to-channel length ratio of Lp/Lc= 1/3. Data are presented for the dividing plate located at the bottom (Li/Lc = 0) and top of the channel (Li/Lc=2/3). Comparisons of local and average Nusselt numbers are made with the numerical predictions from Part I. Although the experimental average Nusselt numbers are typically about 10 percent lower than the numerical results, the general trends of the data are in good agreement. Average Nusselt number correlation equations are presented.
Heat Transfer Enhancement From a Vertical, Isothermal Channel Generated by the Chimney Effect
Authors: Straatman A. G. , Tarasuk J. D. , Floryan J. M.
Published: May 2008

A numerical and experimental investigation of free convection from vertical, isothermal, parallel-walled channels has been undertaken to explore the heat transfer enhancement obtained by adding adiabatic extensions of various sizes and shapes. Investigations were carried out for air (Pr= 0.7) over a wide range of wall heating conditions. In all cases, the adiabatic extensions were able to increase heat transfer. The increase varied from 2.5 at low Ra* to 1.5 at high Ra*. The experimental and numerical results are in excellent agreement. A single correlation accounting for the channel aspect ratio Lh/b, expansion ratio, B/b, modified Rayleigh number, Ra* and heated length ratio, Lh/L is presented.
Heat Transfer From Radiatively Heated Material in a Low Reynolds Number Microgravity Environment
Authors: Yamashita H. , Baum H. R. , Kushida G. , Nakabe K. , Kashiwagi T.
Published: May 2008

A mathematical model of the transient three-dimensional heat transfer between a slowly moving ambient gas stream and a thermally thick or thin flat surface heated by external radiation in a microgravity environment is presented. The problem is motivated in part by fire safety issues in spacecraft. The gas phase is represented by variable property convection-diffusion energy and mass conservation equations valid at low Reynolds numbers. The absence of gravity and low Reynolds number together permit the flow to be represented by a self-consistent velocity potential determined by the ambient velocity and the thermal expansion in the gas. The solid exchanges energy with the gas by conduction/convection and with the surroundings by surface absorption and re-emission of radiation. Heat conduction in the solid is assumed to be one dimensional at each point on the surface as a consequence of the limited times (of order of 10 seconds) of interest in these simulations. Despite the apparent simplicity of the model, the results show a complex thermally induced flow near the heated surface. The thermal exchange between the gas and solid produces an outward sourcelike flow upstream of the center of the irradiated area and a sinklike flow downstream. The responses of the temperature fields and the associated flows to changes in the intensity of the external radiation and the ambient velocity are discussed.
A New CHF Correlation Scheme Proposed for Vertical Rectangular Channels Heated From Both Sides in Nuclear Research Reactors
Authors: Sudo Y. , Kaminaga M.
Published: May 2008

In this study, an investigation was carried out to identify the important parameters affecting critical heat flux (CHF) in rectangular channels, focusing on the effects of flow direction, channel inlet subcooling from 1 to 213 K, the channel outlet condition extending from subcooling of 0–74 K to quality of 0–1.0, pressure of 0.1 to 4 MPa, water mass flux of −25,800 to + 6250 kg/m2s, and channel configuration. In particular, the effect of the outlet subcooling in upflow and downflow on the CHF was quantitatively investigated. As a result of this study, a new CHF scheme covering downflow, countercurrent flow, and upflow was established in the rectangular channels within the ranges of parameters investigated in this study.
Thermal Analysis of the Hot Dip-Coating Process
Authors: Zhang Hui , Moallemi M. Karim , Kumar Sunil
Published: May 2008

In this study a thermal analysis is performed on the hot dip-coating process where solidification of metal occurs on a bar moving through a finite molten bath. A continuum model is considered that accounts for important transport mechanisms such as axial heat diffusion, buoyancy, and shear-induced melt motion in the bath. A numerical solution procedure is developed, and its predictions are compared with those of an analytical approximate solution, as well as available experimental data. The predictions of the numerical scheme are in good agreement with the experimental data. The results of the approximate solution, however, exhibit significant disagreement with the data, which is attributed to the simplifying assumptions used in its development. Parametric effects of the bath geometry, and initial and boundary temperatures and solid velocity, as characterized by the Reynolds number, Grashof number, and Stefan numbers, are presented.
Thermal Waves Emanating From a Fast-Moving Heat Source With a Finite Dimension
Authors: Tzou D. Y. , Li J.
Published: May 2008

Characteristics of thermal waves emanating from a fast-moving heat source are studied in this paper. The heat source is assumed to occupy a finite region to model real situations in local heating. Analytical solutions are obtained respectively for a heat source moving in the subsonic, transonic, and supersonic regimes. The Green’s functions obtained previously from a point source are used to generate temperature distributions for all cases. The temperature jump across the shock surface is found to be weaker than the case with a point heat source, while the swinging phenomenon for the temperature variations with the thermal Mach number still exists.
Analytic Prediction of the Friction and Heat Transfer for Turbulent Flow in Axial Internal Fin Tubes
Authors: Kim Nae-Hyun , Webb R. L.
Published: May 2008

An analytic model is developed to predict the friction factors and Nusselt numbers for turbulent flow in axial internal fin tubes. The present model uses the Law of the Wall and applies the logarithmic universal velocity and temperature profile to the interfin and core regions of the flow. The fin shape is assumed trapezoidal, and the fin parameters such as fin height, fin root thickness and fin tip thickness are determined from the tube dimensional data. Theoretically based friction and heat transfer equations are developed for internally finned tubes in an algebraic form. The analytic model predicts Carnavos friction data for 11 axial internal fin tubes within ± 10 percent, and heat transfer data of air, water, and water-glycol within ± 15 percent when proper velocity and temperature profiles are used.
Internal Standing Waves in a Cylindrical Vessel and Their Near-Wall Features
Authors: Ushijima S. , Moriya S. , Tanaka N.
Published: May 2008

This paper describes numerical and experimental investigations on internal standing waves occurring in a cylindrical vessel and their near-wall behavior in the vicinity of an adiabatic cylinder located at the center of the vessel. A numerical prediction method was developed with a low-Reynolds-number turbulence model to stimulate the occurrence of the internal standing waves and their near-wall features. These features are characterized by the attenuation and phase shifts in temperature fluctuations as observed in the present experiments. The measured results were well predicted by the numerical simulation in terms of certain statistical values as well as qualitative internal wave motions and flow patterns.
Three-Dimensional Natural Convection in a Confined Fluid Overlying a Porous Layer
Authors: Singh A. K. , Leonardi E. , Thorpe G. R.
Published: May 2008

This paper presents a numerical study of three-dimensional, laminar natural convection in an enclosure containing a fluid layer overlying a porous layer saturated with the same fluid. The Brinkman-extended Darcy formulation is used to model fluid flow in the porous layer as this facilitates the imposition of a no-slip boundary condition at the fluid/porous layer interface. The enclosure is heated from one side wall and cooled from an opposite wall, while the remaining walls are adiabatic. The mathematical analysis is carried out in terms of a vorticity-vector potential formulation that ensures the conservation of mass. The governing equations in non-dimensional form are transformed into parabolic equations by means of a false transient method in order to facilitate a solution procedure by an alternating direction implicit method. Accuracy of the numerical solutions with respect to uniformly and nonuniformly spaced grid points has been tested by performing extensive numerical experiments. As expected, it is found that the intensity of free convection is much more profound in the fluid layer. The numerical results indicate that penetration of the fluid into the porous region depends strongly upon the Darcy and Rayleigh numbers. The effect of the ratio of thermal conductivities (porous to fluid regions) is to intensify the convection current in the fluid layer.
Droplet Vaporization in a High-Pressure Gas
Authors: Hartfield J. P. , Farrell P. V.
Published: May 2008

Evaporation of single, liquid droplets in a high-temperature, high-pressure gaseous environment has been investigated experimentally. The effects of gas temperature, pressure, and strength of naturally occurring convective flows were studied. Pure hydrocarbon (n-heptane) and trichlorotrifluoroethane (R-113) droplets were vaporized in a nitrogen atmosphere within a sealed chamber, which was developed to minimize forced convection. Experiments were carried out in normal and microgravity (~ 10−5 g) fields in order to examine the effect of natural convection. A single droplet was attached to the end of a quartz fiber. The gas temperature and pressure were raised quickly by a compressive process. The gas temperature and pressure were varied from 0.93<Tr<1.23 and 0.32<Pr<0.73. The droplet was located at the point of compressive symmetry. Droplet lifetime and instantaneous vaporization rate were determined from the data recorded by video camera. The results indicated that ambient gas temperature is a more significant parameter than ambient pressure for high-pressure droplet vaporization. This conclusion was based on comparisons of droplet vaporization rate for the range of temperatures and pressure tested. Ambient gas pressure was seen to have a weaker influence on vaporization rate. Removal of the gravity field during free-fall experiments resulted in an increase of droplet life time of about 30 percent for the case of R-113 liquid, and little change for the n-heptane droplets.
Heat Transfer From a Molten Phase to an Immersed Coal Particle During Devolatilization
Authors: Hurt R. H. , Fletcher T. H. , Sampaio R. S.
Published: May 2008

In several developmental and commercial processes, coal particles come into direct contact with a high-temperature molten phase. These processes include molten carbonate coal gasification and bath smelting for the production of iron. Recently, real-time X-ray fluoroscopic images have been published that show volatile matter evolving rapidly from coal particles immersed in molten phases, displacing the surrounding melt and producing a periodic cycle of formation, rise, and detachment of gas cavities. The present work makes use of these observations to develop a model of heat transfer from the melt to particles undergoing gas evolution. The model is developed for the general case and applied to predict melt-particle heat transfer coefficients under conditions relevant to bath smelting processes. The model shows that the presence of the gas film can actually increase the overall heat transfer rate under certain conditions.
Natural Convection Heat Transfer From Arrays of Isothermal Triangular Fins in Air
Authors: Karagiozis A. , Raithby G. D. , Hollands K. G. T.
Published: May 2008

Measurements of the heat transfer to air by natural convection from arrays of isothermal triangular fins on a vertical base plate are reported for several array geometries, for a large range of Rayleigh number, and for two orientations (vertical fins and horizontal fins). The data are believed to be the first available for this important geometry. A single equation is provided that correlates the measured Nusselt numbers for the vertical orientation with an rms error of 4.8 percent. The horizontal fin orientation was shown to have inferior heat transfer performance.
Significant Questions in Thin Liquid Film Heat Transfer
Authors: Bankoff S. G.
Published: May 2008

Thin liquid films appear in many contexts, such as the cooling of gas turbine blade tips, rocket engines, microelectronics arrays, and hot fuel element surfaces in hypothetical nuclear reactor accidents. Apart from these direct cooling applications of thin liquid layers, thin films form a crucial element in determining the allowable heat flux limits in boiling. This is because the last stages of dryout almost invariably involve the rupture of a residual liquid film, either as a microlayer underneath the bubbles, or a thin annular layer in a high-quality burnout scenario. The destabilization of these thin films under the combined actions of shear stress, evaporation, and thermocapillary effects is quite complex. The later stages of actual rupture to form dry regions, which then expand, resulting in possible overheating, are even more complex and less well understood. However, significant progress has been made in understanding the behavior of these thin films, which are subject to competing instabilities prior to actual rupture. This will be reviewed briefly. Recent work on the advance, or recession, of contact lines will also be described briefly, and significant questions that still remain to be answered will be discussed.
Numerical Modeling Using a Quasi-Three-Dimensional Procedure for Large Power Plant Condensers
Authors: Zhang C.
Published: May 2008

A quasi-three-dimensional numerical model is proposed to predict the performance of large power plant condensers. The proposed model is applied to a 350 MW power plant condenser under two different loading and operational conditions to demonstrate its predictive capability. The predictions are compared with the experimental data. The comparison is favorable. 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 the tube bundle and baffle plates, is carried out based on a porous media concept using applicable flow, heat, and mass transfer resistances.
Thermal Analysis of a Micro Heat Pipe
Authors: Khrustalev D. , Faghri A.
Published: May 2008

A detailed mathematical model is developed in which the heat and mass transfer processes in a micro heat pipe (MHP) are examined. The model describes the distribution of the liquid in a MHP and its thermal characteristics depending upon the liquid charge and the applied heat load. The liquid flow in the triangular-shaped corners of a MHP with polygonal cross section is considered by accounting for the variation of the curvature of the free liquid surface and the interfacial shear stresses due to a liquid-vapor frictional interaction. The predicted results obtained are compared to existing experimental data. The importance of the liquid fill, minimum wetting contact angle, and the shear stresses at the liquid-vapor interface in predicting the maximum heat transfer capacity and thermal resistance of the MHP is demonstrated.
Parametric Study of the Two-Dimensional Keyhole Model for High Power Density Welding Processes
Authors: Kim Charn-Jung , Kauh Sangken , Ro Sung Tack , Lee Joon Sik
Published: May 2008

This study presents a parametric study of the two-dimensional steady-state keyhole model for high power density welding processes. Keyhole formation is common to electron beam welding, laser welding, and plasma arc welding, all of which are important techniques for high-quality, high-precision welding. Computation was performed by adopting a recently developed concept of the position correction and modifying it suitably for the problem of interest. The dimensionless parameters pertaining to the model were identified and the influence of each parameter was investigated separately. Although the mathematical model employed here has been used in previous studies, a thorough investigation successfully revealed new features that have not been previously recognized in the literature.
Coupled Heat and Mass Transfer With One Discrete Sublimation Moving Interface and One Desorption Mushy Zone
Authors: Peng Shi-Wen , Chen Guo-Qian
Published: May 2008

The present work discusses coupled heat and mass transfer during freeze-drying of a rigid product, as well as accelerated freeze-drying where sublimation and desorption occur concurrently. A desorption mushy zone model was developed to describe the desorption drying. An exact solution was obtained for coupled heat and mass transfer with one discrete sublimation moving interface and one desorption mushy zone where mass transfer is controlled by both Fick and Darcy laws. The effects of several parameters on the sublimation and desorption are analyzed and discussed.
Heat Transfer From a Very High Temperature Laminar Gas Flow With Swirl to a Cooled Circular Tube and Nozzle
Authors: Back L. H. , Massier P. F.
Published: May 2008

An experimental investigation was carried out to appraise the effect of swirl on heat transfer in the laminar boundary layer development region in a highly cooled tube and nozzle. The ratio of gas-side wall-temperature-to-stagnation-temperature ranged from 0.095 to 0.135. In the swirling flow of argon with ratio of peak-tangential-velocity-to-axial velocity of 3.6 at the injection port, the level of heat transfer to the tube wall was increased from 200 to 60 percent above the level without swirl. In the swirling flows, the wall heat flux level was significantly higher in the tube than in the nozzle downstream. Because of the relatively high heat transfer to the wall, there were appreciable reductions in stagnation enthalpy in the flows that spanned a range of Reynolds numbers from about 360 to 500.
An Experimental Investigation on Forced Convection Heat Transfer From a Cylinder Embedded in a Packed Bed
Authors: Nasr K. , Ramadhyani S. , Viskanta R.
Published: May 2008

Forced convection heat transfer from a cylinder embedded in a packed bed of spherical particles was studied experimentally. With air as the working fluid, the effects of particle diameter and particle thermal conductivity were examined for a wide range of thermal conductivities (from 200 W/m K for aluminum to 0.23 W/m K for nylon) and three nominal particle sizes (3 mm, 6 mm, and 13 mm). In the presence of particles, the measured convective heat transfer coefficient was up to seven times higher than that for a bare tube in crossflow. It was found that higher heat transfer coefficients were obtained with smaller particles and higher thermal conductivity packing materials. The experimental data were compared against the predictions of a theory based on Darcy’s law and the boundary layer approximations. While the theoretical equation was moderately successful at predicting the data, improved correlating equations were developed by modifying the form of the theoretical equation to account better for particle diameter and conductivity variations.
Effect of Flow Angle-of-Attack on the Local Heat/Mass Transfer From a Wall-Mounted Cube
Authors: Natarajan V. , Chyu M. K.
Published: May 2008

An experimental study of the local mass transfer over the entire surface of a wall-mounted cube is performed with a particular emphasis on the effects of flow angles-of-attack (0 deg ≤ α ≤ 45 deg). Invoking an analogy between heat transfer and mass transfer, the presently obtained mass transfer results can be transformed into their heat transfer counterparts. Reynolds number based on the cube height and mean free-stream velocity varies between 3.1 × 104 and 1.1 × 105. To substantiate the mass transfer results, streakline patterns are visualized on the cube surfaces as well as the endwall using the oil-graphite technique. Significantly different flow regimes and local mass transfer characteristics are identified as the angle-of-attack varies. The overall convective transport is dominated by three-dimensional flow separation that includes multiple horseshoe vortex systems and an arch-shaped vortex wrapping around the rear portion of the cube. In addition to the local study, power correlations between the surface-resolved mass transfer Sherwood number and the Reynolds number are presented for all α values studied. Mass transfer averaged over the entire cube is compared with that of its two-dimensional counterpart with crossflow around a tall prism.
Leading Edge Film Cooling Heat Transfer Through One Row of Inclined Film Slots and Holes Including Mainstream Turbulence Effects
Authors: Ou Shichuan , Han J. C.
Published: May 2008

The effects of film opening shape and mainstream turbulence on the leading edge heat transfer coefficient and film effectiveness were experimentally investigated. The experiments were performed using test models with a semi-cylindrical leading edge and a flat afterbody. A bar grid (Tu = 5.07 percent) and a passive grid (Tu = 9.67 percent) produced two levels of mainstream turbulence. Two separate cases of one-row injection through film slots or holes located only at ±15 deg or only at ±40 deg from the stagnation line were studied for three blowing ratios of 0.4, 0.8, and 1.2 at the Reynolds number (ReD) of 100,000. The slots in each row were spaced three cross-sectional slot lengths (P = 3l) apart, while the holes were spaced four holes diameters (P = 4d) apart. Both geometries had equal cross-sectional area and pitch. The results show that the leading edge heat transfer coefficient increases and the film effectiveness decreases with increasing blowing ratio; however, B = 0.8 provides the highest film effectiveness for the film hole with ±40 deg injection. The heat transfer coefficient increases and the film effectiveness decreases with increasing mainstream turbulence level. However, the mainstream turbulence effect on the film effectiveness is reduced as the blowing ratio is increased. Slot geometry provides better film cooling performance than the hole geometry for all test cases at the lowest blowing ratio of 0.4. However, at higher blowing ratios of 0.8 and 1.2, the reverse is true for ±40 deg injection at mainstream turbulence of 0.75 and 9.67 percent.
Turbulent Heat Transfer Between a Series of Parallel Plates With Surface-Mounted Discrete Heat Sources
Authors: Kim S. H. , Anand N. K.
Published: May 2008

Two-dimensional turbulent heat transfer between a series of parallel plates with surface mounted discrete block heat sources was studied numerically. The computational domain was subjected to periodic conditions in the streamwise direction and repeated conditions in the cross-stream direction (Double Cyclic). The second source term was included in the energy equation to facilitate the correct prediction of a periodically fully developed temperature field. These channels resemble cooling passages in electronic equipment. The k–ε model was used for turbulent closure and calculations were made for a wide range of independent parameters (Re, Ks/Kf, s/w, d/w, and h/w). The governing equations were solved by using a finite volume technique. The numerical procedure and implementation of the k–ε model was validated by comparing numerical predictions with published experimental data (Wirtz and Chen, 1991; Sparrow et al., 1982) for a single channel with several surface mounted blocks. Computations were performed for a wide range of Reynolds numbers (5 × 104–4 × 105) and geometric parameters and for Pr = 0.7. Substrate conduction was found to reduce the block temperature by redistributing the heat flux and to reduce the overall thermal resistance of the module. It was also found that the increase in the Reynolds number decreased the thermal resistance. The study showed that the substrate conduction can be an important parameter in the design and analysis of cooling channels of electronic equipment. Finally, correlations for the friction factor (f) and average thermal resistance (R) in terms of independent parameters were developed.
Generation of Longitudinal Streamwise Vortices—A Device for Improving Heat Exchanger Design
Authors: Biswas G. , Deb P. , Biswas S.
Published: May 2008

Laminar flow and heat transfer characteristics in a rectangular channel, containing built-in vortex generators of both the slender delta-wing and winglet-pair type, have been analyzed by means of solution of the full Navier–Stokes and energy equations. Each wing or winglet pair induces the creation of streamwise longitudinal vortices behind it. The spiraling flow of these vortices serves to entrain fluid from their outside into their core. These vortices also disrupt the growth of the thermal boundary layer and serve ultimately to bring about the enhancement of heat transfer between the fluid and the channel walls. The geometric configurations considered in the study are representative of single elements of either a compact gas-liquid fin-tube crossflow heat exchanger or a plate-fin crossflow heat exchanger. Physically, these vortex generators can be mounted on the flat surfaces of the above-mentioned heat exchangers by punching or embossing the flat surfaces. They can also act as spacers for the plate fins. Because of the favorable pressure gradient in the channel, the longitudinal vortices are stable and their influence persists over an area many times the area of the slender vortex generators. From a heat transfer point of view, the delta-wing generator is found to be more effective than the winglet-pair. However, most convective heat transfer processes encounter two types of loss, namely, losses due to fluid friction and those due to heat transfer across finite temperature gradient. Because these two phenomena are manifestations of irreversibility, an evaluation of the augmentation techniques is also made from a thermodynamic viewpoint. Conclusions that are drawn thus include discussion about the influence of vortex generators (wings/winglets) on irreversibility.
An Analytical Solution and Sensitivity Study of Sublimation-Dehydration Within a Porous Medium With Volumetric Heating
Authors: Scott E. P.
Published: May 2008

A study of sublimation-dehydration within a porous medium as a result of volumetric heating, such as that associated with microwave heating, is presented in this paper. A semi-infinite frozen porous medium with constant thermal properties subject to a sublimation-dehydration process involving a volumetric heat source is considered. One-dimensional analytical solutions for dimensionless temperature, vapor concentration, and pressure were obtained. A sensitivity study was also conducted in which the effects of the material properties inherent in these solutions were analyzed. Finally, some of the assumptions used in the formulation of the problem were analyzed, and predicted drying curves were found to compare reasonably with previously published experimental results.
A One-Dimensional Model of a Micro Heat Pipe During Steady-State Operation
Authors: Longtin J. P. , Badran B. , Gerner F. M.
Published: May 2008

Micro heat pipes are small structures that will be used to cool microscale devices. They function much like their conventional counterparts, with a few exceptions, most notably the absence of a wick. It is expected that water-filled micro heat pipes will be able to dissipate heat fluxes on the order of 10–15 W/cm2 (100,000–150,000 W/m2). This work addresses the modeling of a micro heat pipe operating under steady-state conditions. A one-dimensional model of the evaporator and adiabatic sections is developed and solved numerically to yield pressure, velocity, and film thickness information along the length of the pipe. Interfacial and vapor shear stress terms have been included in the model. Convection and body force terms have also been included in the momentum equation, although numerical experiments have shown them to be negligible. Pressure, velocity, and film thickness results are presented along with the maximum heat load dependence on pipe length and width. Both simple scaling and the model results show that the maximum heat transport capability of a micro heat pipe varies with the inverse of its length and the cube of its hydraulic diameter, implying the largest, shortest pipes possible should be used.
Convective Transport Phenomena and Macrosegregation During Solidification of a Binary Metal Alloy: II—Experiments and Comparisons With Numerical Predictions
Authors: Prescott P. J. , Incropera F. P. , Gaskell D. R.
Published: May 2008

Experiments involving the solidification of a Pb-19 percent Sn alloy in an axisymmetric, annular mold of stainless steel are performed, and results are compared with numerical predictions. Agreement between measured and predicted cooling curves is reasonable, although undercooling and recalescence, which occur in the experiments, are not predicted by the model. Measured temperatures also indicate that dendrite fragments are transported throughout the mold cavity during early stages of solidification, and convection patterns similar to those predicted by numerical simulation can be inferred from the measurements. While there is general agreement between measured and predicted trends for macrosegregation, the numerical simulation, which assumes axial symmetry, cannot predict the inherently three-dimensional nature of solute redistribution. Conclusions drawn from temperature and composition measurements are supported by metallographic examinations of experimental ingot specimens.
Correlation of Thermal Conductivities of Unidirectional Fibrous Composites Using Local Fractal Techniques
Authors: Pitchumani R. , Yao S. C.
Published: May 2008

The arrangement of fibers strongly influences heat conduction in a composite. Traditional approaches using unit cells to describe the fiber arrangements work well in the case of ordered arrays, but are not useful in the context of disordered arrays, which have been analyzed in the literature by statistical means. This work presents a unified treatment using the tool of local fractal dimensions (although, strictly speaking, a composite cross section may not be an exact fractal) to reduce the geometric complexity of the relative fiber arrangement in the composite. The local fractal dimensions of a fibrous composite cross section are the fractal dimensions that it exhibits over a certain small range of length scales. A generalized unit cell is constructed based on the fiber volume fraction and local fractal dimensions along directions parallel and transverse to the heat flow direction. The thermal model resulting from a simplified analysis of this unit cell is shown to be very effective in predicting the conductivities of composites with both ordered as well as disordered arrangement of fibers. For the case of square packing arrays, the theoretical result of the present analysis is identical to that of Springer and Tsai (1967).
Effect of Interfacial Roughness on Phonon Radiative Heat Conduction
Authors: Majumdar A.
Published: May 2008

The interface between a solid and a liquid or two dissimilar solids poses a resistance to heat transport by phonons, which are quanta of lattice vibrational energy. The classical theory explains that the resistance R arises due to a mismatch in acoustic impedances of the two media and predicts R to vary with temperature T as R ~ T−3. Experiments with copper and liquid 3He have shown that first, although RT3 is a constant below 0.1 K, its value is much less than that predicted by the classical theory. Second, at higher temperatures the resistance behaves as R ~ T−n where n > 3. This study explains these observations in the temperature range of 0.06 K−1 K and 3He pressure range from zero atmospheres in liquid state to solid state, by including the effects of surface roughness and phonon attenuation in solids by electrons or dislocations. Roughness measurements have shown that solid surfaces have a fractal structure and are characterized by a fractal dimension D lying between 2 and 3 for a surface. Using the fractal characteristics, the thermal boundary resistance is shown to follow the relation RT3 ~ (1 + ηTβ)−1 where β is a function of the dimension D and η is a scaling constant. The predictions are in excellent agreement with the experimental observations, indicating that, in addition to other surface effects, the fractal surface structure could have a strong influence on thermal boundary resistance.
Analysis of Matrix Heat Exchanger Performance
Authors: Venkatarathnam G. , Sarangi S.
Published: May 2008

Matrix heat exchangers, invented forty years ago, have found widespread applications during the last two decades. Because of their unique construction, they satisfy the diverse requirements of high compactness, high transfer coefficient, low axial conductivity, and uniform flow distribution. The heat transfer mechanism in these exchangers is quite complex. Convective heat transfer takes place in the pores as well as in the front and rear faces of the plates. Conduction heat transfer takes place in two directions: between the streams through the perforated plates, and along the separating wall in the axial direction. The five sources of heat transfer are strongly coupled with each other. The governing equations have been derived and simplified using well-justified assumptions. The discrete structure of the exchanger helps in reducing the partial differential equations to sets of algebraic and ordinary differential equations. A numerical scheme is presented for solving these equations. Its use is illustrated with two examples.
Simultaneously Developing, Laminar Flow, Forced Convection in the Entrance Region of Parallel Plates
Authors: Nguyen T. V. , Maclaine-cross I. L.
Published: May 2008

This paper reports on a numerical study of simultaneously developing flow in the entrance region of a cascade of thin parallel horizontal plates with a uniform flow at upstream infinity. Results obtained by ADI solution of finite difference equations for the Navier-Stokes and energy partial differential equations are extrapolated to zero mesh size with extended Richardson extrapolation. Nusselt numbers, incremental heat transfer number, and thermal entrance length are presented for Reynolds numbers between 40 and 2000; Pr = 0.2, 0.7, 2, 7, 10 and 100; and constant wall temperature and heat flux boundary conditions.