bluuebunny/crossref_metadata_2025 huggingface dataset:
Second Law Analysis in Assessing Constant Power Input Systems
Authors: Wilcoxon R. K. , Moutsoglou A.
Published: May 2008

A criterion for comparing the relative performance of various heat transfer augmentation methods used in constant power input systems is introduced. The analysis is based on the principle of minimizing the rate of total entropy generation. The heat transfer load (HTL), a parameter determined by the operating requirements of the heat dissipating process that indicates the difficulty of the heat transfer duty to be performed, is defined in the present study. By comparing the irreversibility distribution ratio (φ) of various configurations at a given heat transfer load, the most exergy efficient system can be selected. The data for three different types of fin configurations used in two constant power input applications (electronic equipment and internal turbine blade cooling) are utilized in demonstrating the technique. The results indicate which specific fin geometry of the particular configuration type analyzed will transfer the dissipated heat at the specified base surface temperature while requiring the least pumping power. Although the φ versus HTL criterion is applied to only fins in this study, the method can be extended to many other applications such as jet impingement cooling or mass transfer.
Transient Analysis on the Onset of Thermal Instability in the Thermal Entrance Region of a Horizontal Parallel Plate Channel
Authors: Lee F. S. , Hwang G. J.
Published: May 2008

A transient analysis on the onset of thermal instability in the thermal entrance region of a horizontal parallel plate channel, where the lower plate is heated isothermally, is presented. A novel time-dependent approach based on a three-dimensional linear stability theory is developed to determine the neutral stability position, where a disturbance neither grows nor decays with time. The range of parameters Pe=30, 40, 60, 80, and 100, and Pr=0.2, 0.7, 1.0, and 7.0 are covered in the study. One finds that increasing Prandtl number has a destabilizing effect on the flow for a fixed Peclet number along the streamwise direction. On the other hand, increasing Peclet number or Reynolds number tends to stabilize the flow. As the onset of thermal instability moves upstream, the critical Rayleigh number and the critical wave number are increased. The result shows a good agreement with the previous experimental investigations.
Buoyancy-Driven Heat Transfer and Flow Between a Wetted Heat Source and an Isothermal Cube
Authors: Close D. J. , Peck M. K. , White R. F. , Mahoney K. J.
Published: May 2008

This paper describes flow visualization and heat transfer experiments conducted with a heat source inside an isothermal cube filled with a saturated or near-saturated gas/vapor mixture. The mixture was formed by vaporizing liquid from the surface of the heat source, and allowing it to condense on the surfaces of the cube, which was initially filled with a noncondensing gas. Visualization studies showed that for air and ethanol below 35°C, and for air and water, the flow patterns were similar with the hot plume rising from the source. For air and ethanol above 35° C the flow pattern reversed with the hot plume flowing downward. For temperatures spanning 35° C, which is the zero buoyancy temperature for the ethanol/water azeotrope and air, no distinct pattern was observed. Using water, liquid droplets fell like rain throughout the cube. Using ethanol, a fog of droplets moved with the fluid flow. Heat transfer experiments were made with water and air, and conductances between plate and cube of around 580 W·m−2·K−1 measured. Agreement between the similarity theory developed for saturated gas/vapor mixtures, and correlations for single component fluids only, was very good. Together with qualitative support from the visualization experiments, the theory developed in a earlier paper deriving a similarity relationship between single fluids and gas/vapor mixtures has been validated.
A Numerical Solution of Variable Porosity Effects on Natural Convection in a Packed-Sphere Cavity
Authors: David E. , Lauriat G. , Cheng P.
Published: May 2008

The problem of natural convection in differentially heated vertical cavities filled with spherical particles saturated with Newtonian fluids is investigated numerically. The Brinkman–Darcy–Ergun equation is used as the momentum equation, and the wall effect on porosity variation is approximated by an exponential function. The effect of variable stagnant thermal conductivities is taken into consideration in the energy equation. The formulation of the problem shows that the flow and heat transfer characteristics depend on six dimensionless parameters, namely, the Rayleigh and Prandtl numbers of the fluid phase, the dimensionless particle diameter, the conductivity ratio of the two phases, the bulk porosity, and the aspect ratio of the cavity. The influences of these parameters on the heat transfer rate are thoroughly investigated. The predicted Nusselt numbers are compared with existing experimental results. It is found that the computed Nusselt numbers based on the present model compare the best with experimental data.
A Comparative Study of Methods for Computing the Diffuse Radiation Viewfactors for Complex Structures
Authors: Emery A. F. , Johansson O. , Lobo M. , Abrous A.
Published: May 2008

Several different numerical methods for calculating diffuse radiation viewfactors are described. Each is applied to a range of surface configurations, from almost completely unobstructed to a dense set of intersecting surfaces. The speed, accuracy, and unique characteristics are discussed in order to define optimal methods for different surface geometries.
Heat Transfer Characteristics of Porous Radiant Burners
Authors: Tong T. W. , Sathe S. B.
Published: May 2008

This paper reports a numerical study of the heat transfer characteristics of porous radiant burners, which have significant advantages over conventional burners. The heat transfer characteristics are investigated using a one-dimensional conduction, convection, and radiation model. The combustion phenomenon is modeled as spatially dependent heat generation. Nonlocal thermal equilibrium between the gas and solid phases is accounted for by using separate energy equations for the two phases. The solid matrix is assumed to emit, absorb, and scatter radiant energy. The spherical harmonics approximation is used to solve the radiative transfer equation. The coupled energy equations and the radiative transfer equations are solved using a numerical iterative procedure. The effects of the various factors on the performance of porous radiant burners are determined. It is revealed that for a given rate of heat generation, large optical thicknesses and high heat transfer coefficients between the solid and gas phases are desirable for maximizing radiant output. Also, low solid thermal conductivities, scattering albedos and flow velocities, and high inlet environment reflectivities produced high radiant output.
Markov Analysis of Radiative Transfer in Specular Enclosures
Authors: Billings R. L. , Barnes J. W. , Howell J. R. , Slotboom O. E.
Published: May 2008

This paper is directed at the use of Markov chains in the modeling of radiative heat transfer in specular enclosures containing nonparticipating gases. Following a brief review of previous literature in that area of investigation, a two-parameter state definition is given. The analysis of general infinitely long enclosures using that definition is then discussed, and the paper concludes with a report of selected implementation results.
On the Reverse Flow Ceiling Jet in Pool Fire-Ventilation Crossflow Interactions in a Simulated Aircraft Cabin Interior
Authors: Kwack E. Y. , Bankston C. P. , Shakkottai P. , Back L. H.
Published: May 2008

The behavior of the reverse flow ceiling jet against the ventilation flow from 0.58 to 0.87 m/s was investigated in a 1/3 scale model of a wide body aircraft interior. For all tests, strong reverse flow ceiling jets of hot gases were detected well upstream of the fire. The penetration lengths of the reverse flow ceiling jet for the tests agree fairly well with the relation, lr/H = 0.6 [F−5] where F is the fire-crossflow parameter, F = 2gHQ˙/ρcTsAua3. Both the thicknesses of the reverse flow ceiling jet and the smoke layer increased with the fire-crossflow parameter. The thickness of the smoke layer where the smoke flows along the main flow below the reverse flow ceiling jet was almost twice that of the reverse flow ceiling jet. Detailed spatial and time-varying temperatures of the gas in the test section were measured and velocity profiles were also measured using a temperature-compensated hot film.
A Comparison of Augmentation Techniques During In-Tube Evaporation of R-113
Authors: Reid R. S. , Pate M. B. , Bergles A. E.
Published: May 2008

An experimental study was conducted to determine the potential of three techniques for augmenting in-tube evaporation of refrigerants: high-fin tubes, microfin tubes, and twisted tape inserts. Five tubes with internal fins and one smooth tube with a twisted-tape insert were tested. Additionally, experiments were performed with two reference smooth tubes having diameters similar to the maximum inside diameters of the finned tubes. All experiments involved evaporating Refrigerant 113 (R-113) by direct electrical heating of the tube wall. Local evaporation heat transfer coefficients were measured as a function of quality for a range of mass fluxes and heat fluxes. Enhancement factors were calculated by forming ratios of the heat transfer coefficient for the augmented tube and a smooth tube of the same maximum inside diameter. Mass fluxes, pressure levels, and qualities were fixed when enhancement factors were calculated. For the internally finned tubes the enhancement factors varied from 1.1 to 2.8. An internally finned tube having helical spiral angles of 16 deg produced the largest enhancement of heat transfer. The tube with the twisted-tape insert typically had an enhancement factor of about 1.5. Pressure gradient ratios and enhancement performance ratios are also presented.
Geometry Effects on Critical Heat Flux for Subcooled Convective Boiling From an Array of Heated Elements
Authors: McGillis W. R. , Carey V. P. , Strom B. D.
Published: May 2008

The critical heat flux (CHF) condition was experimentally determined for subcooled flow boiling from an array of simulated microelectronic devices on one wall of a vertical rectangular passage. A test apparatus was used in these experiments that allowed visual observation of the boiling process while simultaneously measuring the heat flux and surface temperature for ten heat-dissipating elements. Using R-113 as the coolant, the CHF condition was determined for flush and slightly protruding heated elements. As expected, the element farthest downstream was found to reach the CHF condition first in all cases. For both the flush and slightly protruding elements, the trends in the CHF data are similar to those previously reported for subcooled flow boiling on an isolated element. At moderate flow velocities, the critical heat flux predicted by a proposed correlation for subcooled flow boiling from a single element was found to agree well with the multiple-flush-element data if the local fluid subcooling at the last element was used in the correlation. At lower velocities, however, the data deviated from the predicted values. The data for slightly protruding elements were also found to deviate from those for the flush elements at higher velocities. The apparent physical reasons for these trends are discussed in detail.
An Experimental Study of Laminar Film Condensation With Stefan Number Greater Than Unity
Authors: Mahajan R. L. , Chu T. Y. , Dickinson D. A.
Published: May 2008

Experimental laminar condensation heat transfer data are reported for fluids with Stefan number up to 3.5. The fluid used is a member of a family of fluorinated fluids, which have been used extensively in the electronics industry for soldering, cooling, and testing applications. Experiments were performed by suddenly immersing cold copper spheres in the saturated vapor of this fluid, and heat transfer rates were calculated using the quasi-steady temperature response of the spheres. In these experiments, the difference between saturation and wall temperature varied from 0.5° C to 190°C. Over this range of temperature difference, the condensate properties vary significantly; viscosity of the condensate varies by a factor of nearly 50. Corrections for the temperature-dependent properties of the condensate therefore were incorporated in calculating the Nusselt number based on the average heat transfer coefficient. The results are discussed in light of past experimental data and theory for Stefan number less than unity. To the knowledge of the authors, this is the first reported study of condensation heat transfer examining the effects of Stefan number greater than unity.
Film Condensation of R-113 on In-Line Bundles of Horizontal Finned Tubes
Authors: Honda H. , Uchima B. , Nozu S. , Nakata H. , Torigoe E.
Published: May 2008

Film condensation of R-113 on in-line bundles of horizontal finned tubes with vertical vapor downflow was experimentally investigated. Two tubes with flat-sided annular fins and four tubes with three-dimensional fins were tested. The test sections were 3×15 tube bundles with and without two rows of inundation tubes at the top. Heat transfer measurements were carried out on a row-by-row basis. The heat transfer enhancement due to vapor shear was much less for a finned tube bundle than for a smooth tube bundle. The decrease in heat transfer due to condensate inundation was more marked for a three-dimensional fin tube than for a flat-sided fin tube. The predictions of the previous theoretical model for a bundle of flat-sided fin tubes agreed well with the measured data for low vapor velocity and a small to medium condensate inundation rate. Among the six tubes tested, the highest heat transfer performance was provided by the flat-sided fin tube with fin dimensions close to the theoretically determined optimum values.
Radiative Properties of Superconducting Y-Ba-Cu-O Thin Films
Authors: Phelan P. E. , Flik M. I. , Tien C. L.
Published: May 2008

Some applications of thin-film high-temperature superconductors, such as bolometers, radiation shielding, and space-cooled electronics, require knowledge of the superconducting-state radiative properties. At present, no general predictive model of the radiative properties has been presented. In this work, reflectance predictions based on two of the major theories of the superconducting state, the classical Drude–London theory and the quantum-mechanical Mattis–Bardeen theory, are compared with the available experimental data to determine the best method for predicting the radiative properties of thin-film Y-Ba-Cu-O. It is seen that the Mattis–Bardeen theory is more successful than the Drude–London theory in predicting the reflectance. Consequently, approximate formulae for the Mattis–Bardeen theory are developed, thus enhancing the theory’s usefulness for engineering calculations.
Thermal Coupling in Laminar Flow Double-Pipe Heat Exchangers
Authors: Pagliarini G. , Barozzi G. S.
Published: May 2008

Thermal interaction between the streams of laminar flow double-pipe heat exchangers is investigated theoretically by accounting for axial conduction along the wall separating the fluids. In a countercurrent arrangement, thermal coupling is demonstrated to have a definite influence on all the more important heat transfer parameters, such as the wall temperature, the heat flux density, the local entropy production rate, and the Nusselt number distributions. The overall performance of the device is considered under a second law point of view, and a complete parametric study is carried out.
An Analysis of Heat Transfer in Josephson Junction Devices
Authors: Lavine A. S. , Bai C.
Published: May 2008

Josephson junctions are electronic devices made from superconducting materials that cycle between resistive and nonresistive states. Heat generated in the resistive state causes a temperature rise, which may adversely affect electrical behavior, by reducing the critical Josephson current. In this work, temperature distributions and resulting reductions in critical current are calculated for Josephson junctions made from low and high-temperature superconductors. It is found that an unacceptable reduction in critical current may occur for junctions made from high-temperature materials. This problem can almost certainly be overcome, but perhaps at the expense of one advantage of Josephson junctions, namely compactness.
Electrostatic Cooling of a Horizontal Cylinder
Authors: Franke M. E. , Hogue L. E.
Published: May 2008

The effects of corona wind on the heat transfer rate from a heated horizontal cylinder are described. Corona discharge and a corona wind are developed when a high voltage is applied to an electrode (emitter) near a grounded surface. In this study the corona wind is directed toward the lower region of the grounded cylinder by placing either a positively charged single-wire emitter or multipoint emitter parallel to and directly below the heated cylinder. Heat transfer rates from the heated cylinder under free convection conditions with and without a corona wind are obtained by measuring the power input to the cylinder. Free convection rates are also obtained from interferometer photographs. Free convection heat transfer rates are increased several times by the corona wind.
Inverse Heat Conduction Applied to the Measurement of Heat Transfer Coefficient on a Cylinder: Comparison Between an Analytical and a Boundary Element Technique
Authors: Maillet D. , Degiovanni A. , Pasquetti R.
Published: May 2008

A new method using either an analytical or a boundary element inverse technique, is developed for measurement of local heat transfer coefficients. The direct model calculates the temperature field inside a cylindrical pipe. This is submitted to a given heat transfer coefficient angular profile on its outer radius and on an uniform temperature on its inner radius. Experimental temperature measurements inside the cylinder are processed by two techniques. Their results are very close and coherent with those of other authors. Variation of the cylinder conductivity with temperature, implemented by the boundary element technique, seems to show that the averaging of its value yields a regularization effect.
Solution for Transient Conjugated Forced Convection in the Thermal Entrance Region of a Duct With Periodically Varying Inlet Temperature
Authors: Travelho J. S. , Santos W. F. N.
Published: May 2008

This work presents an analytical solution of the transient conjugated laminar forced convection problem of a slug flow in the thermal entrance region inside a parallel plate duct. A solution in series form is already known for this kind of problem. This solution leads to a complex eigenvalue problem with transcendental equations. The present solution obtained by using the Laplace transform completely eliminates this problem. The amplitudes and phase lags with respect to the inlet conditions are determined for the complex wall temperature, fluid bulk temperature, and wall heat flux from this solution. The results are plotted for comparison with the results obtained with the series solution.
Starting Flow and Heat Transfer Downstream of a Backward-Facing Step
Authors: Tsou F. K. , Chen Shih-Jiun , Aung Win
Published: May 2008

Experiments are performed to study the starting process of heat transfer downstream of a backward-facing step. A Ludwieg tube wind tunnel is employed to produce the incompressible flow, which accelerates from a zero velocity to a steady state value with an accelerating period of 7 ms and a steady-state period of 12 ms. Hot-wire anemometry and heat flux gages are used to measure the flow and heat transfer history, respectively. The onset of transition in the free shear layer shows that the disturbance originates from the top corner of the step, then propagating to the free stream. The velocity and turbulence profiles in the free shear layer reach steady-state values after the leading edge disturbance traverses to the measurement locations. In regions upstream and far downstream of the step, heat flux history data suggest the transformation of the flow from laminar to transitional and finally to turbulent flow. Hot-wire anemometry measurements indicate high-frequency turbulence with a short characteristic time. In the recirculating region, however, a longer characteristic time is observed because of the existence of large-scale eddies. The dimensionless reattachment length (xr/H) is shown to increase with time from the bottom corner (xr/H = 0) in the laminar regime to a maximum value of 13.6 in the transitional regime, and decreases to a constant values of 7.6 in the turbulent regime. The steady-state flow field and heat transfer compare favorably with existing data obtained using steady-state techniques.
Numerical Simulation of Thermal Transport Associated With a Continuously Moving Flat Sheet in Materials Processing
Authors: Karwe M. V. , Jaluria Y.
Published: May 2008

The thermal transport that arises due to the continuous motion of a heated plate or sheet in manufacturing processes such as hot rolling, extrusion, continuous casting, and drawing is numerically investigated. The resulting temperature distribution in the solid is influenced by the associated flow in the ambient fluid, which is taken as stationary far from the moving surface, and is of particular interest in this work. A numerical study is carried out, assuming a two-dimensional, steady circumstance with laminar flow in the fluid. The full governing equations, including buoyancy effects, are solved, employing finite-difference techniques. The effect of various governing parameters, such as the Peclet number, Pe, the mixed convection parameter, Gr/Re2, and the conductivity parameter, Kf/Ks, which determine the temperature and flow fields, is studied in detail. Also, the effect of the boundary conditions, particularly at the location of the emergence of the plate, on the downstream thermal transport is investigated. The penetration of the conductive effects, upstream of the point of emergence, is found to be significant. The effect of buoyancy is found to be more prominent when the plate is moving vertically upward than when it is moving horizontally. The appropriate boundary conditions and their imposition in the numerical scheme are discussed for a variety of practical circumstances.
A Numerical Study of Developing Free Convection Between Isothermal Vertical Plates
Authors: Naylor D. , Floryan J. M. , Tarasuk J. D.
Published: May 2008

Steady two-dimensional laminar free convection between isothermal vertical plates including entrance flow effects has been numerically investigated. The full elliptic forms of the Navier-Stokes and energy equations are solved using novel inlet flow boundary conditions. Results are presented for Prandtl number Pr = 0.7, Grashof number range 50 ≤ Grb ≤ 5×104, and channel aspect ratios of L/b = 10, 17, 24. New phenomena, such as inlet flow separation, have been observed. The results cast doubt on the validity of previous elliptic solutions. Comparisons with the approximate boundary-layer results show that a full elliptic solution is necessary to get accurate local quantities near the channel entrance.
The Weighted-Sum-of-Gray-Gases Model for Arbitrary Solution Methods in Radiative Transfer
Authors: Modest M. F.
Published: May 2008

The weighted-sum-of-gray-gases approach for radiative transfer in nongray participating media, first developed by Hottel in the context of the zonal method, has been shown to be applicable to the general radiative equation of transfer. Within the limits of the weighted-sum-of-gray-gases model (nonscattering media within a black-walled enclosure), any nongray radiation problem can be solved by any desired solution method after replacing the medium by an equivalent small number of gray media with constant absorption coefficients. Some examples are presented for isothermal media and media at radiative equilibrium, using the exact integral equations as well as the popular P-I approximation for the equivalent gray media solutions. The results demonstrate the equivalency of the method with the quadrature of spectral results, as well as the tremendous computer times savings (by a minimum of 95 percent) that are achieved.
Buoyancy, Soret, Dufour, and Variable Property Effects in Silicon Epitaxy
Authors: Mahajan R. L. , Wei C.
Published: May 2008

In most of the previous numerical and semi-analytical studies of silicon epitaxial deposition, a common practice has been to neglect the buoyancy flow, Dufour, Soret, and property variation effects. In this paper, we take a critical look at the validity of that approach and point out some fallacies. The geometric configuration studied is a horizontal reactor for the susceptor tilt angles of 0 and 2.9 deg. The full Navier-Stokes equations coupled with those for the energy and species transfer are solved numerically for a range of parameters typical of commercial silicon epitaxial deposition systems. The effects of ignoring terms due to buoyancy, Dufour, Soret, and variable properties on the mass transfer rate are systematically evaluated. The results indicate that for typical horizontal epitaxial deposition parameters, the buoyancy and Dufour effects have negligible effect on the mass transfer rate, while the Soret and property variation have a large impact. In light of this information, it is shown that the agreement reported in the past between the experimental and numerical/analytical studies is coincidental. The implication is that these assumptions must be critically examined for a given CVD system and not ignored a priori. Finally, the effects of important parameters—reactor height, inlet velocity, inlet concentration, and susceptor temperature—on the deposition characteristics are included to provide guidelines for controlling the epitaxial layer thickness and uniformity.
Evaporation of Water With Single and Multiple Impinging Air Jets
Authors: Trabold T. A. , Obot N. T.
Published: May 2008

An experimental investigation of impingement water evaporation under a single jet and arrays of circular jets was made. The parametric study included the effects of jet Reynolds number and standoff spacing for both single and multiple jets, as well as surface-to-nozzle diameter ratio and fractional nozzle open area for single and multiple jets, respectively. The nozzle exit temperature of the air jet, about the same as that of the laboratory, was 3–6° C higher than that of the evaporating water. Predictive equations are provided for mass transfer coefficient in terms of the flow and geometric conditions.
Homogeneous Bubble Nucleation Predicted by a Molecular Interaction Model
Authors: Kwak Ho-Young , Lee Sangbum
Published: May 2008

The homogeneous bubble nucleation of various hydrocarbons was estimated by the modified classical nucleation theory. In this modification, the kinetic formalism of the classical theory is retained while the surface energy needed for the bubble formation is calculated from the interaction energy between molecules. With a nucleation rate value of Jnc =1022 nuclei/cm3s, this modified model gives a very good prediction of the superheat limit of liquids. In another test of the model the complete evaporation time of a butane droplet at its superheat limit is compared with experiments and found to be in good agreement.
Critical Heat Flux in Saturated Forced Convective Boiling on a Heated Disk With Multiple Impinging Jets
Authors: Monde M. , Inoue T.
Published: May 2008

The existing data for critical heat flux (CHF) on a disk heater cooled by multiple impinging jets have been correlated successfully by deriving a generalized correlation that can predict the CHF on a disk heater cooled by a single impinging jet with high accuracy. The generalized correlation for the CHF for the single jet can be applied to predict the CHF for multiple jets with an accuracy of ±20 percent, in spite of a great difference in the flow situation on a disk between a single jet and multiple jets.
Analytical Modeling of Calcium Carbonate Deposition for Laminar Falling Films and Turbulent Flow in Annuli: Part I—Formulation and Single-Species Model
Authors: Chan S. H. , Ghassemi K. F.
Published: May 2008

This is a two-part series to model scaling of heat transfer surfaces by calcium carbonate based on the first conservation principle approach. In Part I, general physical and chemical processes in CaCO3 fouling are described and a single-species model is proposed whereby only Ca+2 species transport needs to be of concern. The deposition process is assumed to be controlled by the two processes of mass transport to and crystallization reaction on the heat transfer surface. The model is then used to solve for CaCo3 scaling in a laminar falling film system and to assess which of the two types of prevailing reaction rate expressions is more appropriate for the CaCO3 fouling analysis. The predicted deposition rate, scale thickness, and its profile are compared with experimental data. The comparison shows that the single-species model yields satisfactory predictions only for a limited range of concentrations. It reveals that the type of reaction rate expression used in the single-species model is less accurate than the other type used in the multispecies model. The latter is described in Part II of the series.
Analytical Modeling of Calcium Carbonate Deposition for Laminar Falling Films and Turbulent Flow in Annuli: Part II—Multispecies Model
Authors: Chan S. H. , Ghassemi K. F.
Published: May 2008

The present study proposes a multispecies transport model to predict calcium carbonate deposition. The model has been applied to predict the deposition flux, the mean fouling layer thickness, and the profile of local fouling thickness along a heated plate of a laminar falling film. Good agreement is found when comparing with experimental data. Similarly, the model is applied to predict the fouling layer in a turbulent annulus flow system and a good agreement of the predicted results is also found with recent experimental data. Finally, solutions in dimensionless forms are presented to show the effects of various dimensionless parameters on calcium carbonate deposition.
Non-Darcian Effects in Open-Ended Cavities Filled With a Porous Medium
Authors: Ettefagh J. , Vafai K. , Kim S. J.
Published: May 2008

The importance and relevance of non-Darcian effects associated with the buoyancy driven convection in open-ended cavities filled with fluid-saturated porous medium is analyzed in this work. Several different flow models for porous media, such as Brinkman-extended Darcy, Forchheimer-extended Darcy, and generalized flow models, are considered. The significance of inertia and boundary effects, and their crucial influence on the prediction of buouancy-induced flow and heat transfer in open-ended cavities, are investigated. Analysis is made on the proper choice of parameters that can fully determine the criteria for the range of validity of Darcy’s law in this type of configuration. Critical values of the inertial parameter, Λcrit, below which, for any given modified Rayleigh number, the Darcy flow model breaks down, have been investigated. It is shown that the critical value of the inertial parameter depends on the modified Rayleigh number and that this critical value increases as Ra* increases. It is also observed that for higher modified Rayleigh number, the deviation from a Darcian formulation appears at Darcy numbers greater than 1×10−4. The Prandtl number effects on convective flow and heat transfer are shown to be quite significant for small values of Pr. The Prandtl number effects are reduced significantly for higher values of the Prandtl number.
Correlating Equations for Impingement Cooling of Small Heat Sources With Single Circular Liquid Jets
Authors: Womac D. J. , Ramadhyani S. , Incropera F. P.
Published: May 2008

Experimental data have been obtained for liquid jet impingement cooling of small square heat sources resembling electronic integrated circuit chips. Both free-surface and submerged jet configurations have been studied for a range of velocities, nozzle diameters, and nozzle-to-heater separation distances, with water and a fluorocarbon liquid (3M FC-77) as coolants. Major trends in the data have been explained in terms of the underlying hydrodynamic and thermal phenomena. The data, obtained over parameter ranges applicable to the cooling of microelectronic chips, have been compared with the predictions of previously developed correlations for jet impingement heat transfer and substantial discrepancies between the data and the predictions have been noted. Based on the present data, two new correlating equations, one for free-surface and the other for submerged jet impingement, have been developed and presented.
Thermal Drag and Critical Heat Flux for Natural Convection of Air in Vertical Parallel Plates
Authors: Guo Zeng-Yuan , Wu Xiao-Bo
Published: May 2008

Variable property effects on vertical channel natural convection in air are studied systematically. Numerical solutions of the governing equations show that both the mass flow rate and heat transfer in the channel are not only lower than the constant property results, but also show a nonmonotonic variation with increasing wall temperature or wall heat flux. This phenomenon, which seemingly conflicts with the conventional knowledge, has also been identified by experiments. For a vertical channel with a uniform heat flux boundary condition, the wall may experience a sharp rise in temperature up to damage of the channel if the wall heat flux is greater than the critical heat flux. This implies that the crisis phenomenon (or burnout) may occur in channel natural convection in gas as well as in the boiling process.
Natural Convection in a Partial Sector-Shaped Enclosure: Experimental Results
Authors: Al-ani N. , Nansteel M. W.
Published: May 2008

An experimental study of the buoyancy-induced motion and heat transfer in a differentially heated partial sector-shaped enclosure is carried out. Radial surfaces of the sector are heated and cooled isothermally while the circumferential surfaces are insulated. Water is used as the working fluid in experiments with sector angles α = 30, 45, and 90 deg for Rayleigh number in the range 4 × 108 ≲ Ra ≲ 3 × 109. Laser illumination/streak photography is used to visualize the flow. Steady laminar motion is observed, with flow structure qualitatively independent of sector angle except for the case α = 90 deg. Circulation intensity is found to increase modestly with sector angle. Convection energy transfer across the sector increases rapidly with respect to transport by conduction for increasing α. Total energy transport across the sector is a nonmonotonic function of sector angle due to the competing mechanisms of conduction and convective transport. The existence of a sector angle α*, which minimizes the cross-sector heat transfer rate, is discussed.
Investigation of Natural Convection Heat Transfer in Converging Channel Flows Using a Specklegram Technique
Authors: Kihm K. D. , Kim J. H. , Fletcher L. S.
Published: May 2008

Natural convection heat transfer characteristics in converging vertical channel flows were studied by nonintrusively measuring the wall temperature gradients using a laser specklegram technique. Local and average heat transfer coefficients were obtained for forty different configurations, including five different inclination angles from the vertical, γ = 0, 15, 30, 45 and 60 deg, with eight different channel exit openings for each inclination angle. Correlations for both local and average Nusselt numbers, based on the channel length L, were determined as functions of Grashof number, where the local Grashof number, based on the channel length L, ranged up to 7.16×106 and the overall Grashof number varied from 3.58×106 (γ = 60 deg) to 7.16×106 (γ = 0), depending upon the inclination angle. As the top opening was decreased, both local and average Nusselt numbers deviated from the single inclined plate theory and significant reductions in heat transfer resulted. The minimum opening ratio, at which the average Nusselt number started decreasing from that for the single plate, was determined as (b/L)min = 0.07, 0.1, 0.3, 0.35, and 0.4 for inclination angles of 0, 15, 30, 45 and 60 deg, respectively. For Ra* larger than 105, average Nusselt numbers, based on the channel opening b, approached the single-plate limit of the vertical channel flow theory, which was modified to incorporate the reduced gravity due to the inclination. When Ra* was smaller than 105, however, neither the single-plate limit nor the fully developed limit properly described the heat transfer characteristics in the converging channel.
Heat Transfer Characteristics of the Single-Phase, Elbow Thermosyphon
Authors: Lock G. S. H. , Ladoon D.
Published: May 2008

This paper describes the results of single-phase experiments on a right-angled, or elbow, thermosyphon with the cooled section upright and the heated section horizontal. For diameter-based Rayleigh numbers less than 107.6, the data indicate the existence of two flow regimes: fully mixed and impeded. A flow model is used to suggest how the cooled section and heated section flow patterns are coupled together. This model satisfactorily explains the effect of geometry on heat transfer, as revealed in the usual plots of Nusselt number versus Rayleigh number. Thermal performance was found to be comparable to that of the linear thermosyphon.
Transient Optical Transmission Measurement in Excimer-Laser Irradiation of Amorphous Silicon Films
Authors: Park H. K. , Xu X. , Grigoropoulos C. P. , Do N. , Klees L. , Leung P. T. , Tam A. C.
Published: May 2008

The transient temperature field development during heating of an amorphous silicon (a-Si) film, deposited on a fused quartz substrate, by pulsed excimer laser irradiation is studied. Static reflectivity and transmissivity measurements are used to obtain the thin film optical properties at elevated temperatures. Experimental in-situ, transient, optical transmission data are compared with heat transfer modeling results. The variation with temperature of the material complex refractive index across the thin film thickness is taken into account. The effects of the film thickness and thermal diffusivity, as well as of the laser pulse shape, are discussed.
Electron and Phonon Thermal Conduction in Epitaxial High-Tc Superconducting Films
Authors: Goodson K. E. , Flik M. I.
Published: May 2008

Electrons and phonons are the carriers of heat in the a-b plane of the high-Tc superconductor YBa2Cu3O7. In the absence of boundary scattering, the a-b plane thermal conductivity and the mean free path of each carrier type are calculated as functions of temperature using kinetic theory, the two-fluid model of the superconducting state, and experimental data for the thermal conductivity and electrical resistivity of a single crystal. The reduction by boundary scattering of the effective a-b plane thermal conductivity along an epitaxial YBa2Cu3O7 film is predicted as a function of temperature and film thickness. The size effect on the phonon conductivity dominates over the size effect on the electron conductivity. The predicted electron mean free path is limited by scattering on defects and is in very good agreement with experimental data from infrared spectroscopy.
Effects of Heating Surface Materials on a Liquid–Solid Contact State in a Sessile Drop-Boiling System
Authors: Inada S. , Yang W.-J.
Published: May 2008

Three experimental investigations are performed on the liquid–solid contact state at the instant when a liquid comes in contact with a heating surface in the transition-boiling regime between the maximum evaporation rate point and the spheroidal state: (1) measuring the piezoelectric potential by means of a quartz oscillator being placed on the heating surface, (2) monitoring the boiling sound generated at the instant of liquid–solid contact using a condenser microphone, and (3) observation of drop behavior, by means of high-speed photography. The information obtained from these three investigations is synthesized to quantify the liquid–solid contact state. The study has disclosed the roles of thermal properties and heating surface temperatures on the endurance time of the instantaneous liquid–solid contact state and the relationship between the critical bubble radius and the measured vapor–liquid interfacial pressure difference.
Magnetized Fiber Orientation Control in Solidifying Composites: Numerical Simulation
Authors: Dulikravich G. S. , Kosovic B. , Lee S.
Published: May 2008

This work deals with the development of a numerical algorithm for the prediction of magnetic force lines inside a flowing solidifying melt with the ultimate purpose of simulating and controlling alignment of short nickel-coated fibers during the curing process in composites. A complete mathematical model and an accompanying computer program have been developed for the simulation of a steady laminar flow of an incompressible fluid with strong heat transfer (involving solidification) and a strong superimposed magnetic field. An extended form of the Boussinesq approximation allowing for temperature-dependent physical properties of the fluid and the solid including latent heat of phase change was incorporated. This formulation simultaneously predicts detailed velocity, pressure, and temperature fields in the moving fluid while capturing the forming solid phase by using a single computer code. The same code can simulate the reverse process of thawing or melting of the solid phase. The computed sample configurations involve a two-dimensional closed container, a straight and a U-shaped channel, and a passage of an arbitrary shape. It was found that the presence of an external steady magnetic field: (a) diminishes flow field vorticity, (b) causes higher velocity gradients within the boundary layers, (c) is able to orient magnetized fibers along the lines of local magnetic lines of forces.
Heat Transfer Within a Concrete Slab Applying the Microwave Decontamination Process
Authors: Li W. , Ebadian M. A. , White T. L. , Grubb R. G.
Published: May 2008

Decontamination of a radioactive contaminated concrete surface is a new technology for the treatment of radioactive waste. In this paper, concrete decontamination using microwave technology is investigated theoretically. A plane wave assumption of microwave propagation has been employed to estimate the microwave field and power dissipation within the concrete. A one-dimensional, unsteady heat conduction model with microwave heat dissipation resulting from microwave-material interaction has been used to evaluate frequency, steel reinforcement within the concrete, and thermal boundary conditions are also considered in the present model. Four commonly used microwave frequencies of 0.896, 2.45, 10.6, and 18.0 GHz have been utilized in the analysis. The results revealed that as the microwave frequency increases to, or higher than 10.6 GHz, the microwave power dissipation shifts toward the front surface of the concrete. Furthermore, it was observed that use of a higher frequency microwave could reduce power intensity requirements needed to raise the temperature difference or thermal stress to the same value in the same period of time. It was found that the presence of reinforcing steel mesh causes part of the microwave energy to be blocked and reflected. Thus, the temperature or thermal stress of the concrete increases before the reinforcement, and decreases after the reinforcement.
Heat Transfer and Friction Factor Measurements in Ducts With Staggered and In-Line Ribs
Authors: Hong Ying-Jong , Hsieh Shou-Shing
Published: May 2008

The combined effects of rib alignment and channel aspect ratio on the distributions of the local heat transfer coefficient and on the friction factors for developing and fully developed flow in short square and rectangular channels (L/DH = 13.5–18) with a pair of opposite rib-roughened walls were determined for Reynolds numbers ranging from 13,000 to 130,000. The channel aspect ratios are 1/2 and 1 and the rib alignment configurations are arranged as staggered and in-line types, respectively. The pitch to rib height ratio is 5.31 for all test channels. The local heat transfer distributions on the bottom rib-roughened wall from the channel entrance to the downstream region are presented and discussed. Semi-empirical heat transfer and friction correlations are developed, and the results are compared with those of previous investigations for similarly configured channels, which were roughened by regularly spaced transverse ribs.
Optimal Arrays of Pin Fins and Plate Fins in Laminar Forced Convection
Authors: Bejan A. , Morega A. M.
Published: May 2008

This paper reports the optimal geometry of an array of fins that minimizes the thermal resistance between the substrate and the flow forced through the fins. The flow regime is laminar. Two fin types are considered: round pin fins, and staggered parallel-plate fins. The optimization of each array proceeds in two steps: The optimal fin thickness is selected in the first step, and the optimal thickness of the fluid channel is selected in the second. The pin-fin array is modeled as a Darcy-flow porous medium. The flow past each plate fin is in the boundary layer regime. The optimal design of each array is described in terms of dimensionless groups. It is shown that the minimum thermal resistance of plate-fin arrays is approximately half of the minimum thermal resistance of heat sinks with continuous fins and fully developed laminar flow in the channels.
Effects of Sodium and Potassium Chlorides on Silica Fouling
Authors: Chan S. H. , Chen Z. J. , He P.
Published: May 2008

The effects of NaCl and KCl salts in aqueous solutions on silica fouling of heat exchangers under turbulent flow conditions were examined experimentally over a range of pH, velocity, and salt concentration levels. The fundamental attachment process of silica fouling from those aqueous solutions was described. The silica fouling rate in NaCl solutions was found to be greater than that in KCl solutions. In all cases, the fouling rate increased with the salt concentration. When both salts were present simultaneously, the fouling rate found to be closer to that of the KCl system than that of the NaCl system. Effects of salinity on equilibrium solubility of silica at 165° C were also reported. Finally, the plausible theory of polymerization and the fundamental ion-cloud theory were used to describe the silica scale attachment process and the possible causes of salt effects on silica fouling of heat transfer surfaces.
A Perturbation Analysis of Transient Freezing of a Laminar Liquid Flow in a Cooled Two-Dimensional Channel
Authors: Weigand B. , Ho¨hn W. , Beer H.
Published: May 2008

The paper shows the applicability of a regular perturbation method for predicting the transient development of the ice layer thickness inside a cooled planar channel subjected to laminar flow. Applying the perturbation expansion to the conservation equations, closed-form solutions for the velocity and temperature distributions in the fluid for an arbitrarily shaped channel could be derived under the assumption that the axial variation in solid layer thickness is small. The distributions obtained for the steady-state ice layer thickness and the velocity were checked by numerical calculations and compared with the measurements of Kikuchi et al. (1986) and a generally good agreement was found.