bluuebunny/crossref_metadata_2025 huggingface dataset:
Exact Solutions for Radiative Heat Transfer in Box-Shaped Furnaces
Authors: Selc¸uk N.
Published: October 2009

Exact expressions for the distributions of the components of radiative flux density and the radiative energy source term in terms of wall and medium temperature distributions have been formulated for an emitting-absorbing medium of constant properties bounded by black walls of a box-shaped enclosure. The accuracy of numerical solutions has been tested on an idealized enclosure for which exact analytical solution of the expressions is possible and shown to have good accuracy. The exact expressions have then been solved numerically for an enclosure problem based on data reported previously on a large-scale experimental furnace. The principal feature of the data is highly nonuniform medium and wall temperature distributions which are typical of the conditions encountered in industrial furnaces. These data have been chosen because of their practical importance and the nonavailability of exact solutions for such data. The resulting exact solutions have been given in tabular form and are intended to serve in the future as standards for testing the accuracy of the approximate predictions produced using various three-dimensional flux models.
Measurements and Predictions of the Structure of Evaporating Sprays
Authors: Solomon A. S. P. , Shuen J-S. , Zhang Q-F. , Faeth G. M.
Published: October 2009

An experimental and theoretical study of turbulent evaporating sprays is described. Experiments considered round, Freon-11 sprays (Sauter mean diameters of 31 and 58 μm) produced by an air-atomizing injector directed vertically downward in still air. The following structure measurements were made: mean and fluctuating gas velocities, total concentration of Freon-11, drop size and velocity distributions, mean gas temperature, and liquid flux distributions. Three spray models were evaluated using the new measurements: (a) a locally homogeneous flow (LHF) model where interphase transport rates are assumed to be infinitely fast; (b) a deterministic separated flow (DSF) model where finite interphase transport rates are considered but drop-turbulence interactions are ignored; and (c) a stochastic separated flow (SSF) model where effects of finite interphase transport rates and drop-turbulence interactions are considered using random-walk computations for drop motion and transport. The LHF and DSF models performed poorly, since both finite interphase transport rates and drop-turbulence interactions were important for present test conditions. The SSF model gave best agreement between predictions and measurements and appears to be an encouraging approach for treating practical sprays.
Film Condensation Over a Horizontal Cylinder for Combined Gravity and Forced Flow
Authors: di Marzo M. , Casarella M. J.
Published: October 2009

The problem of laminar film condensation of a saturated vapor flowing over a cold horizontal cylinder is investigated. A rigorous formulation of the governing equations for the vapor boundary layer and the condensed liquid film, including both the gravity-driven body forces and the imposed pressure gradient caused by the vapor flow, is presented. A generalized transformation of the governing equations allows a wide range of Froude numbers to be investigated. A unique value of the Froude number is defined which allows a distinction between the gravity-dominated flow (Fr→0) and the forced flow (Fr→∞) and basically defines the overlap region for the two solution domains. Numerical solutions are obtained in the merging flow regions controlled by both driving forces. The effects of density/viscosity ratio at the liquid-vapor interface, Prandtl number, Jakob number, and Froude number on the heat transfer characteristics are presented.
Heat Transfer and Evaporation From Heated Water Bodies
Authors: Chattree M. , Sengupta S.
Published: October 2009

It is known that there is a sharp temperature gradient in a thin water layer at the air-water interface. Systematic measurements of this temperature difference and meteorological parameters were made at a cooling pond in East Mesa, CA. The water loss was measured directly, and the water temperatures were in the range of 30–46°C. Existing formulae which use the surface temperature difference and wind speeds to predict the net heat flux have been examined. Examination of the well-known Saunders’ equation shows that a parameter λ does not have a constant value. Nondimensional correlations have been developed to predict the net heat flux and the evaporative heat flux at the interface. A dimensional correlation which includes the effect of the surface temperature variation is found to give the most accurate prediction of the net heat flux.
A Model of Flow in a Closed-Loop Thermosyphon Including the Soret Effect
Authors: Hart J. E.
Published: October 2009

This theoretical study addresses the nature of convective motions in a toroidal loop of binary fluid oriented in the vertical plane and heated from below. The boundaries of the loop are impermeable, but gradients of the solute can be set up by Soret diffusion in the direction around the loop. The existence and stability of steady solutions are discussed over the Rayleigh number-Soret coefficient parameter plane. When the Soret coefficient is negative, periodic and chaotic oscillations analogous to those of thermohaline convection are predicted. When the Soret coefficient is positive, relaxation oscillations and low Rayleigh number chaotic motions are found. Both sets of phenomena are predicted to occur for realistic thermosyphon parameters.
Combined Heat Transfer and Fluid Dynamic Measurements Downstream of a Backward-Facing Step
Authors: Vogel J. C. , Eaton J. K.
Published: October 2009

Combined heat transfer and fluid dynamic measurements in a separated and reattaching boundary layer, with emphasis on the near-wall region, are presented. A constant heat-flux surface behind a single-sided sudden expansion is used to obtain Stanton number profiles as a function of Reynolds number and boundary-layer thickness at separation. Fluctuating skin-friction and temperature profiles demonstrate the importance of the near-wall region in controlling the heat transfer rate. The fluctuating skin friction controls the heat transfer rate near reattachment, while the conventional Reynolds analogy applies in the redeveloping boundary layer beginning two or three step heights downstream of reattachment.
The Exchange Factor Method: An Alternative Basis for Zonal Analysis of Radiating Enclosures
Authors: Larsen M. E. , Howell J. R.
Published: October 2009

In zonal analysis of an enclosure containing a radiatively participating medium, the volume and bounding surface are divided into volume and area elements which are assumed to be isothermal. For a system of M elements consisting of N surfaces and Γ volume elements (M = N + Γ), M2 relationships (direct- or total-exchange areas of conventional zonal analysis) define all possible radiant interchange between element pairs. Due to reciprocity there are at most M(M + 1)/2 unique direct-exchange areas. The use of symmetry may greatly reduce the number of unique exchange areas. In the alternative form presented, exchange factors are defined such that a gray medium’s gas-to-gas exchange factors describe only the redistribution of radiative source terms. Only surface-to-surface and surface-to-gas (gas-to-surface) relationships are required to wholly define the system in radiative equilibrium. That is, a system in radiative equilibrium may be characterized by as many as Γ(Γ + 1)/2 fewer factors. These same exchange factors may be augmented by gas-to-gas exchange factors to analyze media not in radiative equilibrium. In an isotropically scattering, gray medium, exchange factors are functions of system geometry and total extinction only. The degree of extinction due to scattering does not affect exchange factors. Transformations between direct-exchange areas and exchange factors are developed.
Spectral Emission Measurements From Planar Mixtures of Gas and Particulates
Authors: Skocypec R. D. , Walters D. V. , Buckius R. O.
Published: October 2009

An experimental system is developed that forms a hot layer of gas and particulates flowing through a test section with cooled walls. The test section forms a one-dimensional planar layer, allows intrusive probes to characterize the medium in terms of particle loading and temperature, and allows radiometric measurements of the normally directed spectral energy emitted from the medium. Gas flow, gas composition, and particle flow are controlled. An experimental investigation is undertaken yielding spectral normally directed emittance data obtained from a well-characterized layer containing gaseous constituents of carbon dioxide and nitrogen, and solid particles of BNi-2, a boron nickel alloy. Emittance data are presented and exhibit the effects of particulate scattering, including the extension of the 4.3 μm carbon dioxide band wings. Emittance data are compared to analytical predictions.
Transient Radiative Cooling of a Droplet-Filled Layer
Authors: Siegel R.
Published: October 2009

A proposed lightweight radiator system for waste heat dissipation in space would eject streams of coolant in the form of small, hot liquid droplets. The droplets would lose radiative energy by direct exposure to the very low-temperature environment of space, and would then be collected for reuse. The cooling behavior of a layer composed of many small droplets was studied by numerical solution of the radiative integral equations. Since there is mutual interference for radiative energy dissipation, an array droplet will cool more slowly than if each drop is exposed individually. Since liquid metal droplets may be used, the study includes results for conditions with high scattering. For optically thin regions, especially with high scattering, the temperature distribution is sufficiently uniform that the cooling can be computed using the approximation of a constant layer emittance. For optically thick layers starting at uniform temperature, the temperature distributions become nonuniform with time. It was found that the cooling process goes through a starting transient; a constant emittance condition is then achieved where the emittance is lower than that for a layer at uniform temperature.
Boiling Heat Transfer in a Shallow Fluidized Particulate Bed
Authors: Chuah Y. K. , Carey V. P.
Published: October 2009

Experimental data are presented which indicate the effects of a thin layer of unconfined particles on saturated pool boiling heat transfer from a horizontal surface. Results are presented for two different types of particles: (1) 0.275 and 0.475-mm-dia glass spheres which have low density and thermal conductivity, and (2) 0.100 and 0.200-mm-dia copper spheres which have high density and thermal conductivity. These two particle types are the extremes of particles found as corrosion products or contaminants in boiling systems. To ensure that the surface nucleation characteristics were well defined, polished chrome surfaces with a finite number of artificial nucleation sites were used. Experimental results are reported for heat fluxes between 20 kW/m2 and 100kW/m2 using water at 1 atm as a coolant. For both particle types, vapor was observed to move upward through chimneys in the particle layer, tending to fluidize the layer. Compared with ordinary pool boiling at the same surface heat flux level, the experiments indicate that addition of light, low-conductivity particles significantly increases the wall superheat, whereas addition of heavier, high-conductivity particles decreases wall superheat. Heat transfer coefficients measured in the experiments with a layer of copper particles were found to be as much as a factor of two larger than those measured for ordinary pool boiling at the same heat flux level. The results further indicate that at least for thin layers, the boiling curve is insensitive to layer thickness. These results are shown to be consistent with the expected effects of the particles on nucleation, fluid motion, and effective conductivity in the pool at or near the surface. The effect of surface nucleation site density on heat transfer with a particle layer present is also discussed.
Solidification Effects on the Fragmentation of Molten Metal Drops Behind a Pressure Shock Wave
Authors: Yang J. W. , Bankoff S. G.
Published: October 2009

Molten Wood’s metal drops falling through water fragment upon being overtaken by a shock wave, under conditions of simultaneous surface solidification. The study of this fragmentation is the subject of the present paper. If the solid crust thickness is sufficiently large by the time of the shock arrival, fine-scale fragmentation is prevented. A stability theory (Epstein, 1977) which includes the elastic crust stiffness in the pressure boundary conditions, and a nonlinear Taylor instability theory (Cooper and Dienes, 1977), are both surprisingly successful in predicting the threshold for fragmentation inhibition.
Flow Visualization Experiments on Secondary Flow Patterns in an Isothermally Heated Curved Pipe
Authors: Cheng K. C. , Yuen F. P.
Published: October 2009

Secondary flow patterns at the exit of a 180 deg bend (tube inside diameter d = 1.99 cm, radius of curvature Rc = 10.85 cm) are presented to illustrate the combined effects of centrifugal and buoyancy forces in hydrodynamically and thermally developing entrance region of an isothermally heated curved pipe with both parabolic and turbulent entrance velocity profiles. Three cases of upward, horizontal, and downward-curved pipe flows are studied for constant wall temperatures Tw=55–91°C, Dean number range K=22–1209 and ReRa=1.00×106–8.86×107. The flow visualization was realized by the smoke injection method. The secondary flow patterns shown are useful for future comparison with numerical predictions and confirming theoretical models. The results can be used to assess qualitatively the limit of the applicability of the existing correlation equations for laminar forced convection in isothermally heated curved pipes without buoyancy effects.
On the Problem of Heat Transfer in a Phase-Change Slab Initially Not at the Critical Temperature
Authors: Gutman L. N.
Published: October 2009

A one-dimensional heat transfer problem in the phase-change slab, one side of which is isothermal while the other is insulated, is considered. Both cases—fusion and solidification—are treated. Slab temperature at the intitial moment is assumed constant and not critical. The main goal of this paper is to find the additional time required for a total phase change, compared with the case of the critical initial temperature. By analogy with perturbation problems in hydrodynamics, an appropriate solution is constructed consisting of an inner and an outer solution. The evaluation of the maximum error of the integral heat balance equation of the slab is treated as an indirect evaluation of the accuracy of the solution obtained. This evaluation shows that the solution can provide sufficient accuracy only in cases in which at least one of the three nondimensional parameters of the problem is small.
A Convection Calibration Method for Local Heat Flux Gages
Authors: Borell G. J. , Diller T. E.
Published: October 2009

An apparatus for calibrating local heat flux gages in convective air flows is described. Heat transfer from a “hot” gage to a “cold” fluid was measured using a guarded hot-plate technique. The system was used to calibrate Gardon-type circular foil heat flux gages of 1/8 in. and 1/16 in. outer diameters. The results indicate that the calibration curves are nonlinear, which is different from the linear calibration obtained using the standard radiation technique. The degree of nonlinearity matches the analysis which accounts for the effect of the temperature distribution in the gage foil. The effect of this temperature distribution can be neglected in the standard radiation calibration but is often significant in convection applications. These results emphasize the importance of calibrating heat flux gages in thermal environments similar to those in which they will be used.
Second-Law-Based Thermoeconomic Optimization of Two-Phase Heat Exchangers
Authors: Zubair S. M. , Kadaba P. V. , Evans R. B.
Published: October 2009

This paper presents a closed-form analytical method for the second-law-based thermoeconomic optimization of two-phase heat exchangers used as condensers or evaporators. The concept of “internal economy” as a means of estimating the economic value of entropy generated (due to finite temperature difference heat transfer and pressure drops) has been proposed, thus permitting the engineer to trade the cost of entropy generation in the heat exchanger against its capital expenditure. Results are presented in terms of the optimum heat exchanger area as a function of the exit/inlet temperature ratio of the coolant, unit cost of energy dissipated, and the optimum overall heat transfer coefficient. The total heat transfer resistance represented by (1/U = C1 + C2 Re−n) in the present analysis is patterned after Wilson (1915) which accommodates the complexities associated with the determination of the two-phase heat transfer coefficient and the buildup of surface scaling resistances. The analysis of a water-cooled condenser and an air-cooled evaporator is presented with supporting numerical examples which are based on the thermoeconomic optimization procedure of this paper.
Experimental Investigation of the Augmentation of Forced Convection Heat Transfer in a Circular Tube Using Spiral Spring Inserts
Authors: Chiou J. P.
Published: October 2009

Results of the experimental investigation of a class of spiral spring coil used as a tube side heat transfer augmentative device for a single phase cooling mode operation are presented. SAE 10 engine oil flowing inside the tube is cooled by water flowing outside the tube. This spiral spring insert is inexpensive but it can increase the tube side heat transfer coefficient significantly. Thus its use as an augmentative device is effective. Application of this device in the design of oil coolers is discussed.
Heat Transfer From an Open- or Closed-Bore Cylinder Situated Longitudinal to a Freestream
Authors: Kang S. S. , Sparrow E. M.
Published: October 2009

Wind tunnel experiments were performed to determine the local response of the heat transfer at the outer surface of a longitudinal cylinder to geometry-related differences in the pattern of fluid flow. Among the three investigated configurations, one was a cylinder with an open bore through which fluid could pass, while in the second the bore was closed at its downstream end, creating an upstream-open-ended cavity. In the third configuration, the upstream face of the cylinder was impenetrable and blunt. The Reynolds number ranged from 7700 to 47,000. For all cases, the axial distribution of the Nusselt number was characterized by an initial increase, followed by the attainment of a maximum and a monotonic decrease, reflecting the occurrence of flow separation and of post-reattachment boundary layer development. The magnitude and location of the maximum were configuration dependent, with that for the open configuration being highest and occurring first, and that for the blunt face configuration being lowest and occurring last; the cavity configuration gave intermediate results. Upstream of the maxima, the Nusselt numbers were arranged in the same order as at the maximum, with a configuration-related spread of 50 percent. Well downstream of the maxima, the ordering was reversed and the spread was in the 5 percent range. Tight, configuration-independent correlations were achieved both for the maximum Nusselt number and for the Nusselt numbers in the downstream region.
Effects of Non-Darcian and Nonuniform Porosity on Vertical-Plate Natural Convection in Porous Media
Authors: Hong J. T. , Yamada Y. , Tien C. L.
Published: October 2009

This work examines analytically the effects of non-Darcian and nonuniform permeability conditions on the natural convection from a vertical plate in porous media. The non-Darcian effects, which include the no-slip and inertia effects, decrease the flow and heat transfer rate, while the nonhomogeneity effect enhances the heat transfer. For packed spheres, in particular, the nonhomogeneity in permeability due to the packing of spheres near the solid wall results in a strong flow-channeling effect that significantly increases the heat transfer. The effect of transverse thermal dispersion is also examined. This dispersion effect causes an increase in the heat transfer.
Prandtl Number Dependence of Natural Convection in Porous Media
Authors: Jonsson T. , Catton I.
Published: October 2009

The effect of Prandtl number of a medium on heat transfer across a horizontal layer was measured. Stainless steel particles of diameters 1.6, 3.2, and 4.8 mm, glass particles of diameters 2.5 and 6.00 mm, and lead particles of diameter 0.95 mm were used with silicon oil, water, and mercury as working fluids. The bed height was varied from 2.5 to 12 cm. Experimental results are presented showing Nusselt number as a function of medium Rayleigh number with the effective Prandtl number, defined as the product of medium Prandtl number and Kozeny–Carmen constant, serving as a parameter. Correlations for Nusselt number are given for effective Prandtl number less than 0.1 and for effective Prandtl number greater than 0.1, which corresponds to an infinite effective Prandtl number. For the steel–water case the wavenumber is shown as a function of medium Rayleigh number.
The Effects of Side-Wall Conduction on Natural Convection in a Slot
Authors: Mallinson G. D.
Published: October 2009

A numerical model for the interaction between natural convection in a slot and conduction in the side walls that are parallel to the plane of the slot is described. Two-dimensional equations containing source terms which account for the viscous and thermal coupling between the fluid and the walls are solved by a finite difference method. The model neglects radiation effects. Solutions for a slot of square cross section filled with a high Prandtl number fluid and heated from below are compared with the results of a Galerkin analysis made by Frick [8] and with solutions obtained by a fully three-dimensional model. Solutions for a slot filled with air and heated from the side are also validated by comparison with three-dimensional solutions. The data produced by the model predict that the more conventional Hele Shaw analysis overestimates heat transfer when the slot aspect ratio is greater than 0.05. Perfectly conducting walls are shown to reduce the rate of heat transfer by the fluid but to increase the strength of the flow. Some effects of walls that are neither adiabatic nor perfectly conducting are assessed.
Combined Radiation and Convection in Absorbing, Emitting, Nongray Gas-Particulate Tube Flow
Authors: Tabanfar S. , Modest M. F.
Published: October 2009

The interaction of thermal radiation with conduction and convection in thermally developing absorbing, emitting, nongray gas–particulate turbulent suspension flow through a circular tube is investigated. The contribution of thermal radiation is obtained through evaluation of the total hemispherical emittance of the particulate cloud and through evaluation of single band absorptances for molecular gases, modified to account for the interaction with the particles. The governing differential equation is derived as a (nonlinear) energy equation, coupled with integral equations to find the thermal radiation contributions. The energy equation is solved numerically by an implicit finite difference method with an iterative procedure. Qualitative results for Nusselt numbers are shown for a variety and range of parameters, such as optical thickness of particulates and single molecular gas bands, relative gas band position and band width, and temperature ratios (heated as well as cooled suspension flows).
The Effect of Flow From Below on Dryout Heat Flux
Authors: Tsai T. P. , Catton I.
Published: October 2009

A experimental investigation of dryout heat flux in a saturated porous medium with forced flow from below has been conducted. Freon-113, methanol, and water were used as test fluids. Particle sizes were 0.59–0.79 mm, 1.6 mm, 3.2 mm, and 4.8 mm. The dryout heat flux increases as the inlet mass flux increases, and asymptotically approaches the total evaporation energy of the inlet flow. The pressure drop across the bed changes rapidly near the dryout point due to the formation of a dry zone.
Boiling Heat Transfer in Horizontal and Inclined Rectangular Channels
Authors: Morcos S. M. , Mobarak A. , Hilal M. , Mohareb M. R.
Published: October 2009

The present experimental investigation is concerned with boiling heat transfer of water inside both horizontal and inclined rectangular channels under a relatively low heat flux. These configurations simulate the absorber channel of line-focus solar concentrations under boiling conditions. The experimental facility includes electrically heated aluminum rectangular channels with aspect ratios of 2.67 and 0.37. The experimental results of the two-phase Nusselt number for the two aspect ratios and for the inclination angles 0, 15, 30, and 45 deg were correlated in terms of a ratio of the two-phase to the liquid-phase Reynolds number for the forced-convection vaporization region. The proposed correlations agree well with previous investigations. In the present work, classifications of the various flow patterns were made by direct observation through a glass window at the end of the test section.
Fouling From Vaporizing Kerosine
Authors: Crittenden B. D. , Khater E. M. H.
Published: October 2009

Fouling rate data have been obtained by passing kerosine at a low flow rate through a small horizontal tubular furnace. Substantial circumferential variations in surface temperature and fouling rate occurred during varporization, the highest rates being at the bottom of the tube where surface temperatures were the lowest. Fouling rates generally increased as pressure increased from 1 to 2.5 bar, but were reduced markedly either by decreasing the feedstock oxygen content or by raising the wall superheat. All the observations are consistent with fouling occurring predominantly via autoxidation and cracking-synthesis mechanisms for the liquid and vapor phases, respectively.
Numerical Determination of Heat Transfer and Pressure Drop Characteristics for a Converging–Diverging Flow Channel
Authors: Faghri M. , Asako Y.
Published: October 2009

A finite difference scheme was utilized to predict periodic fully developed heat transfer and fluid flow characteristics in a converging–diverging flow channel. The basis of the method is an algebraic nonorthogonal coordinate transformation which maps the complex fluid domain onto a rectangle. This transformation avoids the task of numerically generating boundary-fitted coordinates. The transformed equations and the entire discretization procedure were documented in an earlier paper which dealt with a general class of nonperiodic problems. Its adaptation to a periodic sample problem of converging–diverging flow channel will be illustrated in this work. Representative results were carried out for laminar flow, Prandtl number of 0.7, in the Reynolds number range from 90 to 1635, for various taper angles of converging–diverging flow channel, and for three ratios of maximum/minimum height of the flow channel. Moderate enhancement in the Nusselt number results occurred, at higher values of Reynolds number for most cases, when compared with corresponding values for straight ducts.
Fluid Motion and Heat Transfer of a High-Viscosity Fluid in a Rectangular Tank on a Ship With Oscillating Motion
Authors: Akagi S. , Uchida K.
Published: October 2009

Fluid motion and heat transfer of a high-viscosity fluid contained in a two-dimensional rectangular ship’s tank subjected to oscillating motion are investigated by a finite difference technique. The study is motivated by the thermal design of the heating system of oil tanks on a tanker which is moving in a wavy sea. The bottom of the tank is heated and its side walls are cooled. The motion of the tank is assumed to be a simple harmonic rolling motion. The isotherms and flow velocity vectors are determined by numerical solutions of the basic equations describing the convection flows in a tank with oscillating motion. The heat transfer rates to the tank walls are predicted. The influence of the frequency of the oscillating motion on the heat transfer rate is examined.
Boiling Characteristics of Small Multitube Bundles
Authors: Chan A. M. C. , Shoukri M.
Published: October 2009

Boiling characteristics of multitube bundles have been investigated experimentally. Small bundles of up to nine rows were used. Void fraction profiles in the test vessel, tube surface temperatures, power input to individual tubes, and critical heat fluxes were measured for different bundle arrangements and boiling conditions. The data were used to study the system hydrodynamics, bundle heat transfer coefficients, and bundle critical heat flux. The data showed that for lower heat fluxes, the heat transfer characteristics are affected by the system hydrodynamics resulting in higher heat transfer coefficients, whereas at higher heat fluxes nucleate boiling is the dominant mechanism. The data also showed that within a tube bundle, the vapor rising from lower tubes enhances the CHF characteristics of the upper tubes.
The Use of Organic Coatings to Promote Dropwise Condensation of Steam
Authors: Holden K. M. , Wanniarachchi A. S. , Marto P. J. , Boone D. H. , Rose J. W.
Published: October 2009

Fourteen polymer coatings were evaluated for their ability to promote and sustain dropwise condensation of steam. Nine of the coatings employed a fluoropolymer as a major constituent; four employed hydrocarbons and one a silicone. Each coating was applied to 25-mm-square by approximately 1-mm-thick metal substrates of brass, copper, copper–nickel, and titanium. While exposed to steam at atmospheric pressure, each coating was visually evaluated for its ability to promote dropwise condensation. Observations were also conducted over a period of 22,000 hr. Hardness and adhesion tests were performed on selected specimens. On the basis of sustained performance, six coatings were selected for application to the outside of 19-mm-dia copper tubes in order to perform a heat transfer evaluation. These tubes were mounted horizontally in a separate apparatus through which steam flowed vertically downward. Steam-side heat transfer coefficients were inferred from overall measurements. Test results indicate that the steam-side heat transfer coefficient can be increased by a factor of five to eight through the use of polymer coatings to promote dropwise condensation.
A Thermal Instability in the Laser-Driven Melting and Recrystallization of Thin Silicon Films on Glass Substrates
Authors: Grigoropoulos C. P. , Buckholz R. H. , Domoto G. A.
Published: October 2009

This paper develops a conductive heat transfer stability theory for the laser-driven melting and recrystallization of thin silicon films deposited on conductive (glass) substrates. The important parameters are: laser power, laser intensity distribution, and beam scanning speed. Basic state temperature distributions are obtained for straight phase boundaries. These calculated temperature distributions show the origin of instability. A linear perturbation analysis is used to obtain the leading order corrections to the basic-state temperature fields. The perturbation time rate of growth, as a function of the disturbance wavelength, is then predicted.
A Numerical Model of the Flow and Heat Transfer in a Rotating Disk Chemical Vapor Deposition Reactor
Authors: Evans G. , Greif R.
Published: October 2009

Steady, laminar, axisymmetric, and circumferentially uniform flow and heat transfer, including the effects of variable properties and buoyancy, have been modeled within a rotating disk chemical vapor deposition (CVD) reactor. The reactor is oriented vertically, with the hot, isothermal, spinning disk facing upward. The Navier–Stokes and energy equations have been solved for the carrier gas helium. The solutions have been obtained over a range of parameters, which is of importance in CVD applications. The primary parameters are the ratio of the disk temperature to the free stream temperature Tw/T∞, the disk Reynolds number Re = rd2ω/ν∞, a mixed convection parameter Gr/Re3/2 = g(ρ∞ − ρw)/(ρwωων∞), the dimensionless inlet velocity u∞/ων∞, and two geometric parameters ro/rd and L/rd. Results are obtained for the velocity and the temperature fields and for the heat flux at the surface of the rotating disk. Comparisons are made with the one-dimensional, variable-property (excluding buoyant effects), infinite rotating disk solutions of Pollard and Newman. Results are presented in terms of a local Nusselt number. The potential uniformity of CVD in this geometry can be inferred from the variation of the Nusselt number over the surface of the rotating disk. The effects of buoyancy and the finite size of the rotating disk within the cylindrical reactor are clearly evident in the present work.
Effect of Wall Conduction on Combined Free and Forced Laminar Convection in Horizontal Rectangular Channels
Authors: Hwang G. J. , Chou F. C.
Published: October 2009

This paper presents a numerical study of the effect of peripheral wall conduction on combined free and forced laminar convection in hydrodynamically and thermally fully developed flow in horizontal rectangular channels with uniform heat input axially, In addition to the Prandtl number, the Grashof number Gr+, and the aspect ratio γ, a parameter Kp indicating the significance of wall conduction plays an important role in heat transfer. A finite-difference method utilizing a power-law scheme is employed to solve the system of governing partial differential equations coupled with the equation for wall conduction. The numerical solution covers the parameters: Pr = 7.2 and 0.73, γ = 0.5, 1, and 2, Kp = 10−4–104, and Gr+ = 0–1.37×105. The flow patterns and isotherms, the wall temperature distribution, the friction factor, and the Nusselt number are presented. The results show a significant effect of the conduction parameter Kp.
Mixed Convection in Ducts With Asymmetric Wall Heat Fluxes
Authors: Aung Win , Worku G.
Published: October 2009

Results are presented of a numerical study dealing with combined free and forced laminar convection in a parallel plate vertical channel with asymmetric wall heating at uniform heat fluxes (UHF). The forced flow at the inlet is assumed to be spatially uniform and directed vertically upward. Quantitative information is provided pertaining to the effects of buoyancy and asymmetric heating on the hydrodynamic and thermal parameters. For values of Gr/Re up to 500 no flow reversal is predicted, in contrast to the case of uniform wall temperatures (UWT) recently reported. Other fundamental differences between UHF and UWT also are indicated.
Asymptotic Solution for Thermocapillary Flow at High and Low Prandtl Numbers Due to Concentrated Surface Heating
Authors: Chan C. L. , Chen M. M. , Mazumder J.
Published: October 2009

Thermocapillary convection due to nonuniform surface heating is the dominant form of fluid motion in many materials processing operations. The velocity and temperature distributions for the region adjacent to the area of peak surface heating are analyzed for the limiting cases of large and small Prandtl numbers. For a melt pool whose depth and width are large relative to the thermal and viscous boundary layers, it is shown that the most important parameter is the curvature (i.e., ∇2q) of the surface heat flux distribution. The solutions of the temperature and stream functions are presented, some of which are in closed form. Simple, explicit expressions for the velocity and maximum temperature are presented. These results are found to be quite accurate for realistic Prandtl number ranges, in comparison with exact solutions for finite Prandtl numbers. Besides being more concise than exact results, the asymptotic results also display the Prandtl number dependence more clearly in the respective ranges.
Mixed Convection in a Liquid-Saturated Porous Medium
Authors: Reda D. C.
Published: October 2009

An experimental and numerical investigation of mixed convection phenomena about a finite-length, vertical, cylindrical heat source in a uniform, liquid-saturated, porous medium was conducted. Buoyancy-induced upflow about the heat source was systematically altered by the superposition of vertical, pressure-driven flows which opposed the buoyancy-induced fluid motion. The evolution of the mixed convection velocity and thermal fields with increasing magnitude of the imposed-flow Peclet number are reported. The ratio of the natural convection Rayleigh number Ra to the imposed-flow Peclet number Pe is shown to be the nondimensional parameter that characterizes the relative influence of buoyancy-induced to pressure-driven fluid motion. Using total disappearance of buoyancy-induced upflow as the criterion, the transition from mixed to forced convection, for opposing flows, is numerically predicted to occur for |Ra/Pe| ≈ 1/2, independent of the heat source length or power input.
Group Combustion of a Cylindrical Cloud of Char/Carbon Particles
Authors: Annamalai K. , Ramalingam S. , Dahdah T. , Chi D.
Published: October 2009

Extensive experiments were carried out in the past in order to obtain kinetics data on the pyrolysis of coal particles and the char reactions. The literature survey distinctively reveals two kinds of studies: (i) Individual Particle Combustion (IPC) and (ii) Combustion of Particle Streams or Clouds. The experimental data obtained with particle streams are normally interpreted using IPC models with the a priori assumption that the cloud is dilute. But the term “dilute” is rarely quantified and justified considering the collective behavior of a cloud of particles. The group combustion model accounts for the reduction in burning rate due to the collective behavior of a large number of particles. While the spherical group combustion model may be employed for coal/char spray combustion modeling, the cylindrical group combustion model is more useful in interpreting the experimental data obtained with a monosized stream of particles. Hence a cylindrical group combustion model is presented here. As in the case of spherical group combustion models, there exist three modes of combustion: (i) Individual Particle Combustion (IPC), (ii) Group Combustion (GC), and (iii) Sheath Combustion (SC). Within the range of parameters studied, it appears that the cylindrical and spherical cloud combustion models yield similar results on nondimensional cloud burning rates and on the combustion modes of a cloud of particles. The results from group theory are then used to identify the mode of combustion (IPC, GC, or SC) and to interpret the experimental data.
An Experimental Study of Convective Boiling in a Partially Heated Horizontal Channel With Offset Strip Fins
Authors: Mandrusiak G. D. , Carey V. P. , Xu X.
Published: October 2009

Measured local heat transfer data and visually observed two-phase flow behavior are reported for convective boiling of methanol and water in a partially heated horizontal channel with offset strip fins. In this study, a special test section was used that permitted direct visual observation of the two-phase flow while simultaneously measuring the convective boiling heat transfer coefficient at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for top heating and bottom heating of the horizontal channel. For the conditions examined here, the two-phase flow was in the stratified, wavy, or annular flow regime over almost the entire length of the channel. It was visually observed that virtually no nucleate boiling is present when the flow is in these regimes. A flow regime map is presented, which summarizes the observed two-phase flow characteristics. At the same quality and mass flux levels, the measured heat transfer coefficients for top heating and bottom heating were found to be virtually the same for the conditions examined here. In addition for the same quality and mass flux, the convective boiling heat transfer coefficients for top heating and bottom heating agree well with the results for comparable experiments with the channel oriented vertically. Hence, prior to dryout of the liquid film, the boiling heat transfer performance appears to be insensitive to channel orientation. Observed differences in the dryout conditions for top heating and bottom heating are also discussed.
Heat Transfer-Based Reconstruction of the Concepts and Laws of Classical Thermodynamics
Authors: Bejan A.
Published: October 2009

As an alternative to the mechanistic point of view expressed in Carathe´odory’s axioms, it is shown that the laws and concepts of thermodynamics are covered also by two statements made from a purely heat transfer perspective: Axiom I′—The heat transfer is the same in all zero-work processes that take a system from a given initial state to a given final state. Axiom II′—In the immediate neighborhood of every state of a system there are other states that cannot be reached from the first via a zero-work process. The primary concepts of this formulation are heat transfer, temperature, entropy, and zero-work boundary. Axiom I′ is used to define the property “energy,” and to deduce the secondary (derived) concept of “work transfer.” Axiom II′ is used to define the thermodynamic properties of “volume” and “pressure.” In this new heat transfer-based scheme, the analog of the Kelvin–Planck statement of the second law is: “∮δW < 0 is impossible” for an integral number of cycles executed by a closed system while in communication with no more than one pressure reservoir.
1986 Max Jakob Memorial Award Lecture: Heat Transfer Research for Ocean Thermal Energy Conversion
Authors: Kreith F. , Bharathan D.
Published: October 2009

In this lecture an overview of the heat and mass-transfer phenomena of importance in ocean thermal energy conversion (OTEC) is presented with particular emphasis on open-cycle OTEC systems. Also included is a short historical review of OTEC developments in the past century and a comparison of open and closed-cycle thermodynamics. Finally, results of system analyses, showing the effect of plant size on cost and the near-term potential of using OTEC for combined power production and desalination systems, are briefly discussed.
The Effect of Condensible Impurities in the Working Fluid on the Performance of Cryogenic Regenerators
Authors: Sahoo R. K. , Sarangi S.
Published: October 2009

In many cryogenic regenerator applications the working fluid contains a small quantity of condensible impurity, which undergoes condensation/evaporation along its sublimation curve due to pressure cycling. The governing differential equations for such a system have been formulated in terms of conventional dimensionless parameters. Numerical solution of these equations is presented for several combinations of fluid flow parameters using nitrogen as the working fluid and carbon dioxide as the impurity. The model indicates that: (a) as long as there is no net accumulation of frost a higher pressure ratio has a negligible effect on the thermal performance of the regenerator; (b) a larger reduced period affects the thermal performance of the regenerator in the presence of a condensible impurity to a greater extent than in the absence of it; (c) the matrix temperature profile undergoes a much larger swing in the presence of a condensible component; and (d) since the exit impurity concentration is a function of reduced length, reduced period, and inlet composition, arbitrarily chosen regenerator parameters cannot be used to purify a warm process stream.
Local Heat/Mass Transfer Distributions Around Sharp 180 deg Turns in Two-Pass Smooth and Rib-Roughened Channels
Authors: Han J. C. , Chandra P. R. , Lau S. C.
Published: October 2009

The detailed mass transfer distributions around the sharp 180 deg turns in a two-pass, square, smooth channel and in an identical channel with two rib-roughened opposite walls were determined via the napthalene sublimation technique. The top, bottom, inner (divider), and outer walls of the test channel were napthalene-coated surfaces. For the ribbed channel tests, square, transverse, brass ribs were attached to the top and bottom walls of the channel in alignment. The rib height-to-hydraulic diameter ratios (e/D) were 0.063 and 0.094; the rib pitch-to-height ratios (P/e) were 10 and 20. Experiments were conducted for three Reynolds numbers of 15,000, 30,000, and 60,000. Results show that the Sherwood numbers on the top, outer, and inner walls around the turn in the rib-roughened channel are higher than the corresponding Sherwood numbers around the turn in the smooth channel. For both the smooth and the ribbed channels, the Sherwood numbers after the sharp turn are higher than those before the turn. The regional averages of the local Sherwood numbers are correlated and compared with published heat transfer data.
Steady-State Temperatures in an Infinite Medium Split by a Pair of Coplanar Cracks
Authors: Chang Shangchow
Published: October 2009

This article presents a study on the steady-state heat conduction in an infinite medium containing two coplanar cracks. Using an integral transform technique, formal temperature solutions have first been worked out for both the fundamental symmetric and antisymmetric cases. The explicit and exact expressions for temperatures are then developed via both the conventional inversion transform approach and an analytical continuation method proposed in this paper. Numerical results prepared from analytic and numerical methods are presented in graphic form for temperatures on the horizontal crack plane and on a plane slant to the cracks. The relative merit of various possible solution methods is also discussed.
Conjugated Heat Transfer From a Hot-Film Probe for Transient Air Flow
Authors: Cole K. D. , Beck J. V.
Published: October 2009

A numerical heat transfer model of a flush-mounted hot-film probe is presented for transient air flow. The model geometry involves simultaneous heat flow in both the air and the adjacent slab wall. Two cases are considered, steady laminar flow and laminar flow with a low-frequency oscillating component. In the steady-flow case, the numerical results compare favorably with steady calibration data from the literature. In the unsteady flow case, transfer function results are consistent with literature experimental data. In both cases, agreement with the literature data is obtained by calibrating the numerical model at zero flow.
Variable Property Effects of Fully Developed Laminar Flow in Concentric Annuli
Authors: Herwig H. , Klemp K.
Published: October 2009

By means of a linear perturbation theory, the influences of density, viscosity, thermal conductivity, and specific heat capacity, all varying with temperature, are taken into account. The wall heat flux is assumed to be constant at the inner and outer wall, with an arbitrary ratio between these two. Even for variable properties the problem can be reduced to solving a set of ordinary differential equations with three parameters: heat flux ratio, diameter ratio, and Prandtl number. Skin friction and heat transfer results are given for specific numbers of the parameters, including the limiting cases of pipe and channel flows.