bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixed Thermal Convection of Power-Law Fluids Past Bodies With Uniform Fluid Injection or Suction
Authors: Wang T.-Y. , Kleinstreuer C.
Published: May 2008

Steady laminar flow of power-law fluids past heated two-dimensional or axisym-metric bodies is strongly influenced by the rate of fluid mass transfer at the wall, the buoyancy force, and the characteristics of the power-law fluid. The effects of these parameters on the local skin friction group and heat transfer group are analyzed for wedges of different angles, horizontal cylinders, and spheres. The local heat transfer group, HTG~Nux, for wedge flows is greatly affected by the power-law viscosity index n, and by fluid withdrawal especially when the generalized Prandtl number is relatively high. Fluid suction at larger Prandtl numbers also generates high HTG values for cylinders and spheres. However, the local skin friction group, SFG~Cf, for cylinders or spheres increases with fluid injection and decreases with suction largely because of buoyancy effects.
Non-Darcian Boundary Layer Flow and Forced Convective Heat Transfer Over a Flat Plate in a Fluid-Saturated Porous Medium
Authors: Nakayama A. , Kokudai T. , Koyama H.
Published: May 2008

The local similarity solution procedure was successfully adopted to investigate non-Darcian flow and heat transfer through a boundary layer developed over a horizontal flat plate in a highly porous medium. The full boundary layer equations, which consider the effects of convective inertia, solid boundary, and porous inertia in addition to the Darcy flow resistance, were solved using novel transformed variables deduced from a scale analysis. The results from this local similarity solution are found to be in good agreement with those obtained from a finite difference method. The effects of the convective inertia term, boundary viscous term, and porous inertia term on the velocity and temperature fields were examined in detail. Furthermore, useful asymptotic expressions for the local Nusselt number were derived in consideration of possible physical limiting conditions.
A General Correlation for Saturated Two-Phase Flow Boiling Heat Transfer Inside Horizontal and Vertical Tubes
Authors: Kandlikar S. G.
Published: May 2008

A simple correlation was developed earlier by Kandlikar (1983) for predicting saturated flow boiling heat transfer coefficients inside horizontal and vertical tubes. It was based on a model utilizing the contributions due to nucleate boiling and convective mechanisms. It incorporated a fluid-dependent parameter Ffl in the nucleate boiling term. The predictive ability of the correlation for different refrigerants was confirmed by comparing it with the recent data on R-113 by Jensen and Bensler (1986) and Khanpara et al. (1986). In the present work, the earlier correlation is further refined by expanding the data base to 5246 data points from 24 experimental investigations with ten fluids. The proposed correlation, equations (4) and (5), along with the constants given in Tables 3 and 4, gives a mean deviation of 15.9 percent with water data, and 18.8 percent with all refrigerant data, and it also predicts the correct hTP versus x trend as verified with water and R-113 data. Additional testing with recent R-22 and R-113 data yielded the lowest mean deviations among correlations tested. The proposed correlation can be extended to other fluids by evaluating the fluid-dependent parameter Ffl for that fluid from its flow boiling or pool boiling data.
Heat Transfer Characterization of a Finned-Tube Heat Exchanger (With and Without Condensation)
Authors: Idem S. A. , Jacobi A. M. , Goldschmidt V. W.
Published: May 2008

The effects upon the performance of an air-to-water copper finned-tube crossflow heat exchanger due to condensation on the outer surface are considered. A four-tube, two-pass heat exchanger was tested over a Reynolds number range (based on hydraulic diameter) from 400 to 1500. The coil was operated both in overall parallel and overall counterflow configurations. Convective heat and mass transfer coefficients are presented as plots of Colburn j-factor versus Reynolds number. Pressure losses are, similarly, presented as plots of the friction factor versus Reynolds number. Enhancement of sensible heat transfer due to the presence of a condensate film is also considered.
Wind-Augmented Heat Transfer in an Open Thermosyphon Tube With Large Length-Diameter Ratios
Authors: Lock G. S. H. , Kirchner J. D.
Published: May 2008

The paper reports an experimental study of the open thermosyphon tube when its heat transfer performance is augmented by the prevailing wind flowing down the inside of an insert tube concentric with the thermosyphon tube. Data are presented for the large length-diameter ratios characteristic of geotechnical applications. A flow model is developed and used to interpret these and other results. Comparisons are made with the performance of liquid-filled thermosyphons.
The Coupling of Conduction With Forced Convection in Graetz Problems
Authors: Pozzi A. , Lupo M.
Published: May 2008

This paper presents an analytical solution of the energy equation for a coupled conduction-forced convection heat transfer problem in ducts. In order to compare the thermal field in plane and circular ducts, the solution is obtained in a form describing both flows. The method is based on an asymptotic expansion of the Laplace transform of the temperature and on an application of the stationary-phase method; it enabled us to write the solution, for any value of the coupling parameter p, in terms of confluent hypergeometric functions. Some simple and accurate expressions of the interface temperature, temperature at the axis, bulk temperature, and Nusselt number are given for small and high values of p. The accuracy of the results is proved by a comparison with those obtained through an expansion in terms of 120 eigenfunctions.
Heat Transfer Augmentation Through Wall-Shape-Induced Flow Destabilization
Authors: Greiner M. , Chen R.-F. , Wirtz R. A.
Published: May 2008

Experiments on heat transfer augmentation in a rectangular cross-section water channel are reported. The channel geometry is designed to excite normally damped Tollmien-Schlichting modes in order to enhance mixing. In this experiment, a hydrodynamically fully developed flow encounters a test section where one channel boundary is a series of periodic, saw-tooth, transverse grooves. Free shear layers span the groove openings, separating the main channel flow from the recirculating vortices contained within each cavity. The periodicity length of the grooves is equal to one-half of the expected wavelength of the most unstable mode. The remaining channel walls are flat, and the channel has an aspect ratio of 10:1. Experiments are performed over the Reynolds number range of 300 to 15,000. Streakline flow visualization shows that the flow is steady at the entrance, but becomes oscillatory downstream of an onset location. This location moves upstream with increasing Reynolds numbers. Initially formed traveling waves are two dimensional with a wavelength equal to the predicted most unstable Tollmien-Schlichting mode. Waves become three dimensional with increasing Reynolds number and distance from onset. Some evidence of wave motion persists into the turbulent flow regime. Heat transfer measurements along the smooth channel boundary opposite the grooved wall show augmentation (65 percent) over the equivalent flat channel in the Reynolds number range 1200 to 4800. The degree of enhancement obtained is shown to depend on the channel Reynolds number, and increases with the distance from the onset location.
Nucleate Boiling With High Gravity and Large Subcooling
Authors: Ulucakli M. E. , Merte H.
Published: May 2008

Measurements of the heater surface temperature are presented for pool boiling of distilled water in an accelerating system with various subcoolings and levels of heat flux. The ranges of the experimental variables are: heat flux between 0.19 MW/m2 and 1.5 MW/m2, accelerations normal to the flat heating surface from 1 to 100 times earth gravity, and liquid subcoolings between 0 K and 89 K. Increasing sub-cooling first produces an increase and then a decrease in wall superheat, with the eventual cessation of nucleate boiling for certain combinations of conditions. The increase in wall superheat is particularly enhanced at 10g, reaching a maximum value of 9 K at 1.05 MW/m2 with 60 K subcooling. This type of behavior is attributed to the interactions between the fluid temperature distribution in the immediate vicinity of the heater surface as it is influenced by natural convection, the activation of nucleation sites, and the influence of increased buoyancy on the heat transfer associated with each departing bubble.
Mathematical Modeling and Analysis of Heat Pipe Start-Up From the Frozen State
Authors: Jang J. H. , Faghri A. , Chang W. S. , Mahefkey E. T.
Published: May 2008

The start-up process of a frozen heat pipe is described and a complete mathematical model for the start-up of the frozen heat pipe is developed based on the existing experimental data. The model is simplified and solved numerically. The two-dimensional transient model for the wall and wick is coupled with the one-dimensional transient model for the vapor flow when vaporization and condensation occur at the interface. A parametric study is performed to examine the effect of the boundary specification at the surface of the outer wall on the successful start-up from the frozen state. For successful start-up, the boundary specification at the outer wall surface must melt the working substance in the condenser before dryout takes place in the evaporator.
Theory of Heat Transfer From a Surface Covered With Hair
Authors: Bejan A.
Published: May 2008

This paper describes the fundamental mechanisms of heat transfer through a surface covered with perpendicular hair strands of uniform density. An air flow parallel to the skin seeps through the spaces created between the hair strands. It is shown that the total heat transfer rate from the surface is due to two contributions: (i) the heat conducted through the hair strands, which act as fins, and (ii) the heat convected from the bare portions of the skin. When the air flow is slow enough to conform to the Darcy regime, there exists an optimum hair strand diameter for which the total heat transfer rate is minimum. The optimum diameter increases as the square root of the length swept by the air flow, that is the linear size of the body of the animal covered with hair.
Effect of Modulation on the Onset of Convection in a Sparsely Packed Porous Layer
Authors: Rudraiah N. , Malashetty M. S.
Published: May 2008

The stability of a Boussinesq fluid-saturated horizontal porous layer heated from below is examined when the applied temperature gradient is the sum of a steady component and a time-dependent sinusoidal component. The Brinkman model is employed and only infinitesimal disturbances are considered. A perturbation solution as a function of the applied field is obtained. The critical Rayleigh number is obtained for several cases depending on the frequency of oscillations and it is found that it is possible to advance or delay the onset of convection by thermal modulation of the wall temperature. The Darcy limit and viscous flow limit are obtained as degenerate cases.
Electron Beam Deflection When Welding Dissimilar Metals
Authors: Wei P. S. , Lii T. W.
Published: May 2008

High-intensity electron beam deflection due to thermoelectric magnetism generated during the welding of dissimilar metals is systematically and analytically investigated. A simple thermoelectric model is proposed and the temperature field, penetration depth of the fusion zone, magnetic field, and motion of an electron are determined. Deviation of the fused zone from a joint is affected by the incident angle of the energy beam, the difference in Seebeck coefficients of workpieces, relative magnetic permeability, beam power, welding speed, thermal and electrical conductivities, and the effective electrical contact resistance. Their effects are clearly interpreted in this study. Analytical results for the deviation of the fused zone from the joint between the materials to be welded show good agreement with available experimental data.
Pressure Stratification Effects on Multiphase Transport Across a Vertical Slot Porous Insulation
Authors: Tien H. C. , Vafai K.
Published: May 2008

In this work, thermal stratification effects on heat and mass transfer in a porous insulation are analyzed. The vertical boundaries of the porous system are partially permeable for simulating holes or cracks in wall. Hydrostatic pressure variations are considered on the vertical boundaries and a set of realistic boundary conditions is imposed on the system under consideration. The transient intercoupled equations governing the complicated transport process along with the convective boundary conditions are solved by an efficient numerical scheme. The dependence of the Nusselt number and the field variables on several important parameters is investigated systematically. The results clearly show that infiltration can have a major effect on the overall heat transfer even for small pressure gradients across the insulation slab. Furthermore, it has been found that the opening locations have a pronounced effect on the heat transfer rate across the slab and the corresponding condensation rate. The qualitative information extracted from this investigation can be used to diminish the infiltration and condensation problems in the design of building insulation.
Critical Two-Phase Flow in Pipes for Subcooled Stagnation States With a Cavity Flooding Incipient Flashing Model
Authors: Lee S. Y. , Schrock V. E.
Published: May 2008

Analysis of loss of coolant accident (LOCA) scenarios in nuclear reactor safety evaluation depends on knowledge of many complex phenomena. A primary phenomenon controlling the sequence of events, by determining the residual coolant mass inventory within the primary system, is the critical flow process. Critical flow of a flashing liquid is complicated by marked departure from thermal equilibrium. Several complex models have been proposed to represent the non-equilibrium effects, including six-equation two-fluid models. Amos and Schrock (1983) developed a model based on the premise that the two-phase region is homogeneous and that thermal nonequilibrium is the dominant factor causing the departure from the homogeneous equilibrium idealization. Flashing inception was represented by a modification of the Alamgir-Lienhard (1981) pressure undershoot. Exponential relaxation of the metastable liquid was formulated as suggested by Bauer et al. (1976) and the critical flow criterion used the sound speed formulation of Kroeger (1976). Lee and Schrock (1988) extended the Amos-Schrock work by developing an improved correlation for the pressure undershoot correction factor in terms of Reynolds number and subcooling Jakob number. Improvements were also made in the relaxation constant and in the application of Kroeger’s formulation. In the present paper a new cavity flooding model is used for the evaluation of pressure undershoot at flashing inception. This model is similar to the one developed by Fabic (1964) for the evaluation of liquid superheat required for boiling on a surface subjected to transient heating. The model contains an experimentally deduced factor, which is correlated against stagnation subcooling using the experimental data of Amos and Schrock (1983, 1984), Jeandey et al. (1981), and the Marviken tests (Anon., 1979). The model was then tested against seven additional data sets and shown to be very accurate in predicted mass flux (standard deviation of 10.9 percent for all data). The cavity flooding model is thought to represent the true physics more correctly than does the earlier model, which had its origin in molecular fluctuation theory.
Optical Determination of Stagnation Temperature Behind a Gas Sampling Orifice
Authors: Herron J. R. , Peterson R. B.
Published: May 2008

A technique has been developed for measuring the temperature during a transient combustion event. It combines the features of atomic resonance absorption and direct sampling to produce a relatively simple, intrusive diagnostic technique to obtain time-resolved measurements. In this study, a propagating hydrogen/air flame was used to provide a rapid temperature increase. A small fraction of krypton was added to the reactants and the absorption of resonant radiation at 123.5 nm was recorded downstream of the sampling orifice within a flow tube. Conversion from absorption measurements to temperature values was performed using a computer model of the radiation source and the absorption by the sample. The model of the source was validated by comparing predicted and recorded spectra of hydrogen Lyman-α emissions, while the absorption model for the sampled gas was tested by comparing the temperatures predicted by krypton absorption measurements with those recorded at a range of known temperatures. The direct sampling atomic resonance technique minimizes time-history distortions inherent in other direct sampling techniques, and is capable of tracking local temperatures during the passage of a propagating flame front.
ITPE Technique Applications to Time-Varying Three-Dimensional Ground-Coupling Problems
Authors: Krarti M. , Claridge D. E. , Kreider J. F.
Published: May 2008

Approximate analytical solutions for the three-dimensional heat transfer between slab-on-grade floors and rectangular basements under steady-periodic conditions are developed using the Interzone Temperature Profile Estimation (ITPE) method. The slab-on-grade solution is the first analytical slab-on-grade solution that treats the presence of insulation on/under the floor, while the basement solution is the first analytical solution of the time-dependent three-dimensional problem for basements. Solutions are given for the temperature field and expressions are derived for the annual heat loss. Parametric analysis is used to emphasize the effect of geometric dimensions on the magnitude and phase of heat loss relative to ambient temperature. The results obtained are compared with those from the two-dimensional model, and the three-dimensional characteristics of heat flow from slabs and basements are examined.
Constriction Resistance Through Bolted and Riveted Joints
Authors: Fletcher L. S. , Peterson G. P. , Madhusudana C. V. , Groll E.
Published: May 2008

A correlation relating nondimensional macroscopic thermal resistance to the geometric variables of a bolted joint has been developed. The variables considered include the upper and lower plate thickness, the plate radius, the contact zone radius, and the bolt radius. Experiments were conducted using an electrolytic analog method. The investigation was conducted over a range of variables that include the plate thicknesses, 0.95 to 2.22 cm; the bolt radii, 0.32 to 1.27 cm; the plate radii, 5.1 to 10.2 cm; and the contact zone radii, 1.27 to 8.89 cm. A numerical technique was developed that accurately predicted the experimental results to within ±8 percent. Based on the analysis and experimental data, a correlation was developed that may be used to predict the macroscopic thermal resistance in bolted joints.
Size Effect on the Thermal Conductivity of High-Tc Thin-Film Superconductors
Authors: Flik M. I. , Tien C. L.
Published: May 2008

Using the kinetic theory approximation and reported data, this study shows that at low temperatures, the phonon mean free path in polycrystalline ceramic YBa2Cu3O7 can be of the order of the thickness of thin-film superconductors. In this case, boundary scattering reduces the thermal conductivity with decreasing film thickness. A simple method accounts for the size effect on conduction in thin films. This analysis rests solely on geometric arguments and does not consider the effect of grain boundaries. For conduction along the film, this model approximates well an analytical solution of the Boltzmann transport equation, and is in good agreement with experimental data for thin lead films. The model is also employed to analyze the size effect on conduction across the film and the influence of anisotropy.
Local Convective Heat Transfer From a Heated Surface to a Planar Jet of Water With a Nonuniform Velocity Profile
Authors: Wolf D. H. , Viskanta R. , Incropera F. P.
Published: May 2008

Experiments have been conducted on a planar, free surface jet of water to investigate the effects of a nonuniform velocity profile on the local convection coefficient for a uniform heat flux surface. Heat transfer coefficient distributions were measured for heat fluxes ranging from 0.24 to 1.47 MW/m2 and for Reynolds numbers (based on the average nozzle velocity and nozzle width) from 15,000 to 54,000. This range of flow conditions yielded turbulent velocity profiles similar to those of channel flow. Results have been obtained for both single-phase convection and nucleate boiling. Relative to results for a uniform velocity profile, the nonuniform profile was found to enhance heat transfer significantly. However, enhancement is attributed primarily to increased levels of turbulence and only secondarily to changes in the velocity profile.
Heat Transfer Across Turbulent Boundary Layers With Pressure Gradients
Authors: Sucec J. , Lu Y.
Published: May 2008

The problem being addressed is steady, constant property, turbulent, thin boundary layer flow over a body with a pressure gradient. To find the local Stanton number distribution, the integral energy equation, cast in inner variables, is solved for the thermal boundary layer thickness. The needed velocity distributions are given by the inner law and by the combined law of the wall and wake. Approximate temperature profiles are based upon thermal inner and outer laws, except for the thermal superlayer, which is modeled by a polynomial distribution of temperature. Comparison of predictions is made with experimental Stanton numbers from the literature. Very good agreement is noted for zero, adverse, and favorable pressure gradients, including very strong accelerations tending toward laminarization, with less satisfactory agreement in regions “relaxing” from acceleration.
Unsteady Laminar Forced Convection in Ducts With Periodic Variation of Inlet Temperature
Authors: Kakac S. , Li W. , Cotta R. M.
Published: May 2008

A theoretical and experimental study of laminar forced convection in the thermal entrance region of a rectangular duct, subjected to a sinusoidally varying inlet temperature, is presented. A general boundary condition of the fifth kind that accounts for both external convection and wall thermal capacitance effects is considered, and an analytical solution is obtained through extending the generalized integral transform technique. The variations of amplitudes and phase lags of centerline and bulk temperatures are determined as functions of modified Biot number, fluid-to-wall thermal capacitance ratio, and dimensionless inlet frequency. An apparatus has been designed, built, and used for the experimental study to provide validation of the mathematical modeling employed. Good agreement is obtained when the nonuniform sinusoidally varying inlet temperature profile obtained by experiments is incorporated into the theoretical model.
Parametric Study of Mixed Convection in a Porous Medium Between Vertical Concentric Cylinders
Authors: Kwendakwema N. J. , Boehm R. F.
Published: May 2008

A numerical study has been performed to evaluate mixed convection heat transfer in a porous medium between two vertical concentric cylinders for a constant-temperature outer and an insulated inner boundary conditions. In modeling the flow in the bed a finite difference technique was utilized to represent the governing equations with appropriate boundary layer assumptions. The effects of flow inertia, variable porosity and properties, and the Brinkman friction were all taken into account. The model simulated the condition where water was the fluid flowing through the porous material. In all flow simulations the Darcy law condition was obeyed, i.e., the Reynolds number based on the particle diameter was less than unity. Results obtained include radial and axial velocity and temperature profiles in the bed. The dependence of local Nusselt number on the axial distance for several Reynolds numbers was also obtained. Correlations of the average Nusselt number against the Grashof, Peclet, and Darcy numbers were obtained for various radius ratios. Comparisons of the heat transfer predictions to data and calculations of others for special situations showed excellent agreement.
Flow Instability and Bifurcation in Gas-Loaded Reflux Thermosyphons
Authors: Peterson P. F. , Elkouh N. , Lee K. W. , Tien C. L.
Published: May 2008

In vertical gas-loaded two-phase reflux thermosyphons, the temperature and concentration gradients between the active and shut-off regions can create double-diffusive mixed convection flows. When the noncondensable gas molecular weight is greater than that of the vapor, different steady and time-dependent gas recirculation patterns develop, similar to those observed during Rayleigh-Be´nard convection in vertical slots. Experimental results presented here show that bifurcations from axisymmetric to three-dimensional, then asymmetric, and finally multiple distinct gas-flow patterns occur as the Rayleigh number increases. Ultimately, a sudden transition to a large-amplitude oscillatory gas flow takes place. Detailed concentration maps and power-density spectra from point wet-bulb temperature measurements clearly chart these transitions. These results impact the design of many condensing systems where noncondensables are present.
Some Measurements in Subcooled Flow Boiling of Refrigerant-113
Authors: Hasan A. , Roy R. P. , Kalra S. P.
Published: May 2008

Measurements of local vapor phase residence time fraction, liquid phase temperature, and heated wall temperature were carried out in subcooled flow boiling of Refrigerant-113 through a vertical annular channel. Data are reported for two fluid mass velocities and two pressures over a range of wall heat flux. Estimates of typical vapor bubble size and velocity are given. Some comparisons with a one-dimensional two-fluid model of subcooled boiling flow are also presented.
Experimental and Computational Analysis of Laser Melting of Thin Silicon Films
Authors: Grigoropoulos C. P. , Dutcher W. E. , Emery A. F.
Published: May 2008

Recrystallization of thin semiconductor films can yield improved electrical and crystalline properties. The recrystallization is often effected by using a laser source to melt the semiconductor that has been deposited on an amorphous insulating substrate. This paper describes detailed experimental observations of the associated phase-change process. A computational conductive heat transfer model is presented. The numerical predictions are compared to the experimental results and good agreement is obtained.
Correlations for Natural Convection Between Heated Vertical Plates
Authors: Ramanathan S. , Kumar R.
Published: May 2008

This paper presents the numerical results of natural convective flows between two vertical, parallel plates within a large enclosure. A parametric study has been conducted for various Prandtl numbers and channel aspect ratios. The results are in good agreement with the reported results in the literature for air for large aspect ratios. However, for small aspect ratios, the present numerical results do not agree with the correlations given in the literature. The discrepancy is due to the fact that the published results were obtained for channels where the diffusion of thermal energy in the vertical direction is negligible. The results obtained in this paper indicate that vertical conduction should be considered for channel aspect ratios less than 10 for Pr = 0.7. Correlations are presented to predict the maximum temperature and the average Nusselt number on the plate as explicit functions of the channel Rayleigh number and the channel aspect ratio for air. The plate temperature is a weak function of Prandtl number for Prandtl numbers greater than 0.7, if the channel Rayleigh number is chosen as the correlating parameter. For Prandtl numbers less than 0.1, the plate temperature is a function of the channel Rayleigh number and the Prandtl number. A correlation for maximum temperature on the plate is presented to include the Prandtl number effect for large aspect ratio channels.
Thermal Analysis of In-Situ Thermoplastic-Matrix Composite Filament Winding
Authors: Ghasemi Nejhad M. N. , Cope R. D. , Gu¨c¸eri S. I.
Published: May 2008

In filament winding of thermoplastics, localized melting/solidification can reduce the residual stresses and allow for improved dimensional stability and performance. This paper presents a three-dimensional thermal analysis for melting and consolidating impregnated tows in the presence of a local heat source during filament winding of thermoplastic composites. The analysis is performed using an Eulerian approach. The anisotropy of the filament wound woven structure is modeled as an orthotropic domain employing the concept of angle-ply sublaminates. The effective orthotropic conductivity tensor incorporates the effect of winding angle. The governing equations are discretized in a nonuniform mesh domain and solved using a finite difference approach. The processing parameters, such as winding angle, winding speed, and heat input, as well as material properties, are incorporated into the analysis. The results show large thermal gradients in the vicinity of the consolidation point. The effects of winding speed and heat input are investigated, and the overall thermal characterization of the process is discussed. The accuracy of the numerical method is assessed by comparing the results of a test problem with an available analytical solution.
Two Principles of Differential Second Law Heat Exchanger Design
Authors: Evans R. B. , von Spakovsky M. R.
Published: May 2008

In this paper, two fundamental principles of differential Second Law analysis are set forth for heat exchanger design. The first principle defines a Second Law temperature, while the second principle defines a Second Law temperature difference. The square of the ratio of the Second Law temperature difference to the Second Law temperature is shown always to be equal to the negative of the partial derivative of the rate of entropy generation (for heat transfer) with respect to the overall conductance of the heat exchanger. For the basic design of elementary heat exchangers, each of these two Second Law quantities is shown to take the form of a simple geometric average. Nonelementary considerations result in corrected geometric averages, which relate directly to the corrected log-mean temperature difference. Both the corrected log-mean temperature difference (nonelementary considerations) and the uncorrected or just log-mean temperature difference (elementary considerations) are widely used in heat exchanger analysis. The importance of these two principles in both exergy and essergy analysis is illustrated by a unified basic treatment of the optimum design of elementary heat exchangers. This results in a single optimization expression for all flow arrangements (i.e., counterflow, parallel flow, and certain crossflow cases).
Maximization of Exergy Gain in High Temperature Solar Thermal Receivers by Choice of Pipe Radius
Authors: Bannister P.
Published: May 2008

The problem of optimizing the radius of boiler tubes in a radiation-dominated environment such as a solar thermal power receiver is examined. The trade-off between heat transfer and pumping power is investigated, resulting in an explicit expression for the radius that maximizes the net exergy gain under turbulent flow conditions. The effect of the pumping power being generated on site is included, thus making the result particularly applicable to the design of stand-alone power systems. Examples using typical design values for small parabolic dish solar thermal collectors using water and steam as the working fluid are given to illustrate the characteristics of the problem.
Optimal Design of Crossflow Heat Exchangers
Authors: Van den Bulck E.
Published: May 2008

The design of plate-fin and tube-fin crossflow heat exchangers is discussed. The transfer surface area of crossflow heat exchangers is used ineffectively because of the nonuniform distribution of the heat transfer across the volume of the exchanger. The optimal distribution of the transfer surface area for maximum heat exchanger effectiveness and constant total surface area is determined. It is found that a Dirac delta distribution of the transfer surface aligned along the diagonal of the crossflow exchanger gives the best performance; equal to that of a counterflow device. Design guidelines for optimal area allocation within crossflow heat exchangers are established. Compared to conventional designs, designs following these guidelines may lead to either a higher exchanger effectiveness for equal pressure drops and surface area, reduced pressure drops for equal exchanger effectiveness, or reduced weight and a near cubic form of the exchanger core for equal pressure drops and effectiveness.
Heat Transfer Through a Pressure-Driven Three-Dimensional Boundary Layer
Authors: Abrahamson S. D. , Eaton J. K.
Published: May 2008

An experimental investigation of heat transfer through a three-dimensional boundary layer has been performed. An initially two-dimensional boundary layer was made three dimensional by a transverse pressure gradient caused by a wedge obstruction, which turned the boundary layer within the plane of the main flow. Two cases, with similar streamwise pressure gradients and different lateral gradients, were studied so that the effect of the lateral gradient on heat transfer could be deduced. The velocity flowfield agreed with previous hydrodynamic investigations of this flow. The outer parts of the mean velocity profiles were shown to agree with the Squire-Winter theorem for rapidly turned flows. Heat transfer data were collected using a constant heat flux surface with embedded thermocouples for measuring surface temperatures. Mean fluid temperatures were obtained using a thermocouple probe. The temperature profiles, when plotted in outer scalings, showed logarithmic behavior consistent with two-dimensional flows. An integral analysis of the boundary layer equations was used to obtain a vector formulation for the enthalpy thickness, HH≜∫0∞ρuisdyρ∞ii,o(u∞2+w∞2)1/2,0,∫0∞ρwisdyρ∞is,o(u∞2+w∞2)1/2 (where is is the stagnation enthalpy), which is consistent with the scalar formulation used for two-dimensional flows. Using the vector formulation, the heat transfer data agreed with standard two-dimensional correlations of the Stanton number and enthalpy thickness Reynolds number. It was concluded that although the heat transfer coefficient decreased faster than its two-dimensional counterpart, it was similar to the two-dimensional case. The vector form of the enthalpy thickness captured the rotation of the mean thermal energy flux away from the free-stream direction. Boundary layer three dimensionality increased with the strength of the transverse pressure gradient and the heat transfer coefficients were smaller for the stronger transverse gradient.
Natural Convection in a Vertical Enclosure With Internal Permeable Screen
Authors: Zhang Z. , Bejan A. , Lage J. L.
Published: May 2008

This paper documents the thermal insulation effect of a screen installed inside a vertical rectangular enclosure (e.g., double-glazed window). The screen is a venetian blind system made out of horizontal strips that can be rotated. The focus is on the “closed” position, where the strips almost touch. The effect of this permeable screen on the temperature field, the flow field, and the overall heat transfer rate is determined numerically. The study shows that there exists a ceiling (critical) conductance for the air leakage through the screen, above which the screen does not cause a significant drop in the overall heat transfer rate. A numerical example shows how this critical conductance can be used to calculate the critical spacing that can be tolerated between two consecutive strips in the screen.
Internal Distribution of Radiant Absorption in a Spherical Particle
Authors: Tuntomo A. , Tien C. L. , Park S. H.
Published: May 2008

This paper applies electromagnetic wave theory for the study of the internal radiant absorption field of a small spherical particle, particularly to determine the optimum combination of size-to-wavelength parameter and complex refractive index for maximum local peak absorption. A map is devised to illustrate the general pattern of the internal field, which can be divided into three main regimes: uniform, front-concentrated, and back-concentrated absorption. In addition, the current study employs geometrical optics to investigate the internal field of radiant absorption. A comparison between the results from the geometrical optics approach to those from electromagnetic wave theory shows that the error involved in the geometrical optics approach increases sharply with the real part of the complex refractive index. A criterion is established to define the region of the applicability of geometrical optics.
Infrared Radiation Statistics of Nonluminous Turbulent Diffusion Flames
Authors: Kounalakis M. E. , Sivathanu Y. R. , Faeth G. M.
Published: May 2008

Mixture fraction and radiation statistics were studied for radiation paths through turbulent carbon monoxide/hydrogen diffusion flames burning in still air. Measurements included Mie scattering for mixture fraction statistics and fast-response infrared spectroscopy for radiation statistics. Measured mixture fraction statistics also were used to predict radiation statistics based on stochastic time series methods, the laminar flamelet approximation, and a narrow-band radiation model. Measured intensities of radiation fluctuations were in the range 10–40 percent, which causes mean radiation levels to be 1.1–4.2 times larger than estimates based on mean scalar properties in the flames. In contrast, stochastic predictions of mean and fluctuating radiation properties were generally in excellent agreement with measurements. An exception was the temporal integral scales of radiation fluctuations, where differential diffusion errors of the Mie scattering measurements were identified as the source of the discrepancies.
Convective Heat and Mass Transfer in the Stagnation Region of a Laminar Planar Jet Impinging on a Moving Surface
Authors: Zumbrunnen D. A.
Published: May 2008

Manufacturing processes frequently employ impinging jets to cool or dry a material. Materials are often in motion since many manufacturing processes are designed to produce large quantities of a product. In some cases, the surface velocity can exceed or be comparable to the jet impingement velocity. In this study, the stagnation region of a laminar, planar jet is considered where surface motion is directed perpendicular to the jet plane. A similarity solution to the Navier-Stokes equations is formulated to determine the flow velocity in the stagnation region. Heat and mass transfer distributions are determined from numerical solutions to the conservation equations for energy and species, where velocity components are calculated from the similarity solution. Restrictions regarding the use of heat and mass transfer correlations, which are commonly developed with experimental apparatuses where the impingement surface is stationary, are provided.
Convective Heat Transfer by Impingement of Circular Liquid Jets
Authors: Liu X. , Lienhard J. H. , Lombara J. S.
Published: May 2008

The impingement of circular, liquid jets provides a convenient method of cooling surfaces. Here, jet impingement cooling of uniformly heated surfaces is investigated analytically and experimentally for stable, unsubmerged, uniform velocity laminar jets in the absence of phase change. Analytical and numerical predictions are developed for a laminar radial film flow. Experiments using undisturbed laminar jets were performed to determine local Nusselt numbers from the stagnation point to radii of up to 40 diameters. Turbulent transition in the film flow is observed experimentally at a certain radius. Beyond this transition radius, a separate turbulent analysis is constructed. Integral method results are compared to numerical results, and Prandtl number effects are investigated. The predictions are found to agree well with the measurements for both laminar and turbulent flow. Predictive formulae are recommended for the entire range of radii.
Augmented Heat Transfer in Square Channels With Parallel, Crossed, and V-Shaped Angled Ribs
Authors: Han J. C. , Zhang Y. M. , Lee C. P.
Published: May 2008

The effect of the rib angle orientation on the local heat transfer distributions and pressure drop in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 90,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio of 20; the rib height-to-hydraulic diameter ratio is 0.0625; the rib pitch-to-height ratio equals 10. Nine rib configurations were studied: 90 deg rib, 60 and 45 deg parallel ribs, 60 and 45 deg crossed ribs, 60 and 45 deg ∨-shaped ribs, and 60 and 45 deg ∧-shaped ribs. The results show that the 60 deg (or 45 deg) ∨-shaped rib performs better than the 60 deg (or 45 deg) parallel rib and, subsequently, better than the 60 deg (or 45 deg) crossed rib and the 90 deg rib. The ∨-shaped rib produces the highest heat transfer augmentation, while the ∧-shaped rib generates the greatest pressure drop. The crossed rib has the lowest heat transfer enhancement and the smallest pressure drop penalty.
An Experimental Study of Convective Heat Transfer in Radially Rotating Rectangular Ducts
Authors: Soong C. Y. , Lin S. T. , Hwang G. J.
Published: May 2008

The paper presents an experimental study of convective heat transfer in radially rotating isothermal rectangular ducts with various height and width aspect ratios. The convective heat transfer is affected by secondary flows resulting from Coriolis force and the buoyancy flow, which is in turn due to the centrifugal force in the duct. The growth and strength of the secondary flow depend on the rotational Reynolds number; the effect of the buoyancy flow is characterized by the rotational Rayleigh number. The aspect ratio of the duct may affect the secondary flow and the buoyancy flow, and therefore is also a critical parameter in the heat transfer mechanism. In the present work the effects of the main flow, the rotational speed, and the aspect ratio γ on heat transfer are subjects of major interest. Ducts of aspect ratios γ=5, 2, 1, 0.5, and 0.2 at rotational speed up to 3000 rpm are studied. The main flow Reynolds number ranges from 700 to 20,000 to cover the laminar, transitional, and turbulent flow regimes in the duct flow. Test data and discussion are presented.
Axial Transport Effects on Natural Convection Inside of an Open-Ended Annulus
Authors: Vafai K. , Ettefagh J.
Published: May 2008

The present work centers around a numerical three-dimensional transient investigation of the effects of axial convection on flow and temperature fields inside an open-ended annulus. The transient behavior of the flow field through the formation of a three-dimensional flow field and its subsequent effect on the temperature distribution at different axial locations within the annulus were analyzed by both finite difference and finite element methods. The results show that the axial convection has a distinctly different influence on the temperature and velocity fields. It is found that in the midportion of the annulus a two-dimensional assumption with respect to the temperature distribution can lead to satisfactory results for Ra<10,000. However, such an assumption is improper with respect to the flow field. Furthermore, it is shown that generally the errors for a two-dimensional assumption in the midportion of the annulus are less at earlier times (t<50Δt) during the transient development of the flow and temperature fields.
Transient Natural Convection in Enclosures at High Rayleigh Number
Authors: Olson D. A. , Glicksman L. R.
Published: May 2008

Transient natural convection at Rayleigh numbers of 1010 was studied experimentally in two enclosures of aspect ratio 1/3, one a 1:5.5 physical scale model containing the dense refrigerant gas R114, and the second a full-scale room containing air. In one type of transient the vertical endwall temperature was suddenly changed, while in a second type of transient the isothermal, well-mixed gas was suddenly exposed to hot and cold vertical endwalls. The experiments indicated that the dominant time constant was a convective one. Comparisons between the scale model and full scale show that R114 gas can simulate the transient behavior of air-filled enclosures.
Heat Transfer to a Thin Solid Combustible in Flame Spreading at Microgravity
Authors: Bhattacharjee S. , Altenkirch R. A. , Olson S. L. , Sotos R. G.
Published: May 2008

The heat transfer rate to a thin solid combustible from an attached diffusion flame, spreading across the surface of the combustible in a quiescent, microgravity environment, was determined from measurements made in the drop tower facility at NASA-Lewis Research Center. With first-order Arrhenius pyrolysis kinetics, the solid-phase mass and energy equations along with the measured spread rate and surface temperature profiles were used to calculate the net heat flux to the surface. Results of the measurements are compared to numerical solution of the complete set of coupled differential equations that describes the temperature, species, and velocity fields in the gas and solid phases. The theory and experiment agree on the major qualitative features of the heat transfer. Some fundamental differences are attributed to the neglect of radiation in the theoretical model. A scale analysis is developed that makes use of the experimental data at different ambient conditions to support the notion that radiation is important and to investigate the effect of pressure on the spread rate.
Group Ignition of a Cloud of Coal Particles
Authors: Ryan W. , Annamalai K.
Published: May 2008

Ignition of an isolated single coal particle is known to occur either heterogeneously or homogeneously. While single-particle studies may be useful for dilute coal sprays, their application to burners is limited since ignition occurs in the vicinity of the burners where the spray is dense. Rather than considering an isolated particle, one must consider a collection of particles in order to determine the change in ignition characteristics resulting from particle interactions. Thus, group combustion models have been developed essentially to predict the ignition and combustion characteristics of a larger number of interacting drops/particles. This paper presents results of the ignition characteristics of a spherical cloud of uniformly distributed coal particles in quiescent surroundings using a simple group combustion model. For the conditions studied, the results are as follows: (1) Ignition is heterogeneous if the cloud is dilute and homogeneous if the cloud is dense under the same ambient conditions; (2) there is a minimum ignition time for a given set of initial conditions corresponding to a certain cloud denseness; (3) ignition time is less sensitive to the denseness of the cloud at higher ambient temperatures; and (4) decreased proximate volatile matter can result in either increased or decreased ignition time depending on the cloud denseness (ignition mode). Qualitative comparisons to experimental data are given; however, these comparisons should be approached with caution since the experimental conditions and geometries may be vastly different than those used in the numerical study presented here.