bluuebunny/crossref_metadata_2025 huggingface dataset:
Oscillatory Natural Convection of a Liquid Metal in Circular Cylinders
Authors: Kamotani Y. , Weng F.-B. , Ostrach S. , Platt J.
Published: May 2008

An experimental study is made of natural convection oscillations in gallium melts enclosed by right circular cylinders with differentially heated end walls. Cases heated from below are examined for angles of inclination (φ) ranging from 0 deg (vertical) to 75 deg with aspect ratios Ar (height/diameter) of 2, 3, and 4. Temperature measurements are made along the circumference of the cylinder to detect the oscillations, from which the oscillatory flow structures are inferred. The critical Rayleigh numbers and oscillation frequencies are determined. For Ar=3 and φ = 0 deg, 30 deg the supercritical flow structures are discussed in detail.
Criteria for Predicting the Transition to Turbulence in Natural Convection Along a Vertical Surface
Authors: Vitharana V. L. , Lykoudis P. S.
Published: May 2008

This paper evaluates the existing criteria for predicting the transition to turbulence in natural convection adjacent to a vertical surface. We have conducted an experiment that establishes the point of transition in mercury by means of both hot-film anemometry and local heat transfer measurements. These results confirm the claim of Bejan and Lage that the Grashof number correlates transition data better than the Rayleigh number. We have also concluded that the laminar-turbulent transition in liquid metals as well as regular fluids occurs in the vicinity of Grx=109. Furthermore, we show that the E parameter suggested by Gebhart and his co-workers for predicting the onset of transition, when appropriately modified, yields a criterion that depends on the Grashof number alone.
Electromagnetic Theory Predictions of the Directional Scattering From Triangular Surfaces
Authors: Dimenna R. A. , Buckius R. O.
Published: May 2008

Angular predictions of directional scattering distributions for metal and dielectric surfaces with length scales of the order of the wavelength are made from rigorous electromagnetic scattering theory. The theoretical and numerical formulation of the electromagnetic scattering solution based on the extinction theorem is presented. One-dimensional triangular surface profiles are generated using a Fourier series representation for various correlation lengths, deviations, and surface peak positions. Bidirectional reflection functions and directional emissivities are calculated for the surface geometry parameters above and various optical properties. Angular enhancements in bidirectional reflection and emissivity are quantified. Angular scattering and emissivity predictions have been extended beyond those previously reported to include surfaces with equivalent correlation length and deviation.
Radiation Heat Transfer in Fluidized Beds: A Comparison of Exact and Simplified Approaches
Authors: Flamant G. , Lu J. D. , Variot B.
Published: May 2008

Radiation heat transfer at heat exchanger walls in fluidized beds has never been examined through a complete formulation of the problem. In this paper a wall-to-bed heat transfer model is proposed to account for particle convection, gas convection, and radiation exchange in a variable porosity medium. Momentum, energy, and intensity equations are solved in order to determine the velocity, temperature, radiative heat flux profiles and heat transfer coefficients. The discrete-ordinates method is used to compute the radiative intensity equation and the radiative flux divergence in the energy equation. Both the gray and the non-gray assumptions are considered, as well as dependent and independent scattering. The exact solution obtained is compared with several simplified approaches. Large differences are shown for small particles at high temperature but the simplified solutions are valid for large particle beds. The dependency of radiative contribution on controlling parameters is discussed.
Critical Heat Flux Limits for a Heated Surface Impacted by a Stream of Liquid Droplets
Authors: Halvorson P. J. , Carson R. J. , Jeter S. M. , Abdel-Khalik S. I.
Published: May 2008

An experimental apparatus has been constructed to allow investigation of heat transfer from a horizontal, upward facing, heated surface impacted by streams of monodisperse water droplets of varying size and impact frequency. Droplet diameters between 2.3 and 3.8 mm were used, with drop frequencies varying from 2 to 15 droplets per second. The droplet impact velocity was 1.3 m/s. Critical heat flux, surface superheat, droplet size, and frequency were the primary measured data. Heat fluxes as high as 325 W/cm2 were achieved with wall superheats of only 24°C. The liquid film thickness produced upon droplet impact is shown to be a key factor in these experiments, and the importance of investigating the wetted area is highlighted. The effectiveness of droplet impact cooling using droplets with diameters on the order of millimeters is shown.
The Melting of an Ice Shell on a Heated Horizontal Cylinder
Authors: Vargas J. V. C. , Bejan A. , Dobrovicescu A.
Published: May 2008

This paper describes the fundamentals of melting when a shell of phase-change material rides on a heated horizontal cylinder. In the first part of the paper, contact melting theory is used to predict the history of the melting process and, in particular, the time when the remaining ice falls off the cylinder. It is shown that the melting process consists of two distinct regimes, first, an early regime when the cylinder is surrounded by ice and, second, a late regime when the cylinder cuts through the top of the ice shell. The second part describes laboratory measurements that validate the theory. The third part of the paper shows that in the complete cycle that starts with freezing the shell and ends with the contact-melting removal of the shell, there exists an optimal frozen shell thickness such that the cycle-averaged production of ice is maximized.
Transient Two-Dimensional Gas-Loaded Heat Pipe Analysis
Authors: Harley C. , Faghri A.
Published: May 2008

A two-dimensional, transient mathematical model that accounts for diffusion and variable properties on the operation of a heat pipe is presented. The major advantage over previous models is that this model treats the noncondensable gas as a separate entity, which is described by mass transport phenomena. Also, the energy transport through the wall is coupled to the transient operation of the heat pipe through the use of a conjugate solution technique. The complete behavior of the heat pipe, along with the location and two-dimensional shape of the noncondensable gas front, are modeled from the initial continuum flow, liquid state startup to steady-state conditions. The proposed model predicted the existing experimental data for the operation of high-temperature heat pipes with and without noncondensable gases.
Forced Convective Heat Transfer From a Continuously Moving Heated Cylindrical Rod in Materials Processing
Authors: Roy Choudhury S. , Jaluria Y.
Published: May 2008

The flow and heat transfer associated with the forced convective cooling of an infinite heated cylindrical rod moving continuously along its axis in a circular channel has been numerically investigated. Two different flow circumstances, involving uniform flow at the inlet of the cooling channel in the same as well as in the opposite direction as the movement of the cylindrical rod, are considered in this study. This problem is of interest in several manufacturing processes such as hot rolling, continuous casting, extrusion, wire drawing, and glass fiber drawing. The transport processes are time dependent at the initial stages, following the onset of motion, and usually attain steady-state conditions at large time. The temperature distribution in the solid is of particular interest in materials processing. A detailed numerical study is carried out, assuming an axisymmetric, laminar flow, transient circumstance. The governing full elliptic equations are solved, employing a finite volume method. The transport in the solid material is coupled with that in the fluid through the boundary conditions. The effects of several physical and process parameters such as rod speed, material, free-stream velocity, channel dimensions, and fluid on the temperature and the flow field are investigated. For the opposing flow circumstance, some very interesting phenomena are observed. Notable among these is the appearance of a recirculating region within the fluid adjacent to the moving solid surface, causing a reduction in the heat transfer rate as compared to the aiding forced flow case. Validation of the numerical results is carried out by comparison with earlier experimental results, indicating very good agreement.
Convective Transport Phenomena and Macrosegregation During Solidification of a Binary Metal Alloy: I—Numerical Predictions
Authors: Prescott P. J. , Incropera F. P.
Published: May 2008

A continuum model is used to simulate transient convective transport phenomena numerically during solidification of a Pb-19 percent Sn alloy in an experimental test cell. Solidification occurs in an axisymmetric, annular mold of stainless steel, cooled along its outer vertical wall. Results show that, during early stages of solidification, double-diffusive convection and liquid exchange between melted and mushy zones are responsible for the formation of channels in the outer periphery of the ingot, which ultimately lead to a form of macrosegregation known as A-segregates. During intermediate stages of solidification, solutally driven natural convection spawns a cone segregate in the interior region of the ingot. The final macrosegregation pattern is characterized, in general, by increasing Sn concentration with increasing height throughout the ingot and by increasing Sn concentration with decreasing radius in the upper portion of the ingot.
A Study of High-Temperature Heat Pipes With Multiple Heat Sources and Sinks: Part I—Experimental Methodology and Frozen Startup Profiles
Authors: Faghri A. , Buchko M. , Cao Y.
Published: May 2008

A high-temperature sodium/stainless steel heat pipe with multiple heat sources and sinks was fabricated, processed, and tested. Experimental results from tests performed both under vacuum and in air are presented. The startup behavior of the heat pipe from the frozen state was investigated for various heat loads and input locations, with both low and high heat rejection rates at the condenser. No startup evaporator dryout failures were found, although the heat pipe was sonic-limited during startup in air.
A Study of High-Temperature Heat Pipes With Multiple Heat Sources and Sinks: Part II—Analysis of Continuum Transient and Steady-State Experimental Data With Numerical Predictions
Authors: Faghri A. , Buchko M. , Cao Y.
Published: May 2008

The experimental data presented in Part I were analyzed concerning the heat pipe performance characteristics and design. Postexperiment examination of the loosely wrapped screen wick revealed annular gaps both between the wick and the heat pipe wall and between adjacent screen layers, which greatly enhanced the maximum heat capacity of the heat pipe compared to the analytical capillary limit for a tightly wrapped screen wick. A numerical simulation for transient heat pipe performances including the vapor region, wick structure, and the heat pipe wall is given. Numerical results for continuum transient and steady-state operations with multiple heat sources were compared with experimental results and found to be in good agreement.
The Effect of Thermal Conductivity on the Singular Behavior of the Near-Tip Temperature Gradient
Authors: Tzou D. Y.
Published: May 2008

The singular behavior of the temperature gradient in the vicinity of a macrocrack tip is studied in this work. The emphasis is placed on the effect of thermal conductivity on the power of singularity of the near-tip temperature gradient. Both the temperature dependency and the inhomogeneity of the thermal conductivity are investigated analytically. In the near-tip region, they are the sensitive factors influencing the singular behavior of the temperature gradient. The physical conditions under which their effect dominates over the geometric singularity at the crack tip are studied in detail. In some cases, their strong effects can even make the singularity of the temperature gradient diminished at the crack tip. Consequently, the thermal energy intensification disappears.
Optimization of Radiating Fin Array Including Mutual Irradiations Between Radiator Elements
Authors: Chung B. T. F. , Zhang B. X.
Published: May 2008

The present study develops a new approach to minimize the weight of a radiating straight fin array so that the whole system of the fin array is optimized by minimizing the weight of the individual fin, which is subjected to radiation interaction with adjacent fins and base. The obtained system has the minimum weight possible, and also yields the best shape of the individual fins for a given total heat dissipation and uniform base temperature. Based on the present analysis, the optimum number of fins in a fin array is obtained.
Influence of High Mainstream Turbulence on Leading Edge Heat Transfer
Authors: Mehendale A. B. , Han J. C. , Ou S.
Published: May 2008

The influence of high mainstream turbulence on leading edge heat transfer was studied. High mainstream turbulence was produced by a bar grid (Tu = 3.3–5.1 percent), passive grid (Tu = 7.6–9.7 percent), and jet grid (Tu = 12.9–15.2 percent). Experiments were performed using a blunt body with a semicylinder leading edge and flat sidewalls. The mainstream Reynolds numbers based on leading edge diameter were 25,000, 40,000, and 100,000. Spanwise and streamwise distributions of local heat transfer coefficients on the leading edge and flat sidewall were obtained. The results indicate that the leading edge heat transfer increases significantly with increasing mainstream turbulence intensity, but the effect diminishes at the end of the flat sidewall because of turbulence decay. Stagnation point heat transfer results for high turbulence intensity flows agree with the Lowery and Vachon correlation, but the overall heat transfer results for the leading edge quarter-cylinder region are higher than their overall correlation for the entire circular cylinder region. High mainstream turbulence tends not to shift the location of the separation-reattachment region. The reattachment heat transfer results are about the same regardless of mainstream turbulence levels and are much higher than the turbulent flat plate correlation.
Local Heat/Mass Transfer Distributions on the Surface of a Wall-Mounted Cube
Authors: Chyu M. K. , Natarajan V.
Published: May 2008

Local mass transfer from the surface of a wall-mounted cube is studied using the naphthalene sublimation technique. The streakline pattern on each face of the cube is visualized using the oil-graphite method. A horseshoe vortex system near the endwall, in conjunction with the separated shear layers initiated at sharp edges, determines the transport characteristics around the cube. As a direct influence of the flow field, the local mass transfer distributions reflect features of three-dimensional flow separation that are significantly different from their two-dimensional counterparts existing in the midsection of a long prism. According to several previous studies, average mass transfer over the rear surface of a square prism without end effects is the highest among all surfaces. However, it is the lowest for the present case with a wall-mounted cube. The cube side wall has the highest average mass transfer overall. This is a result of the elevated local mass transfer existing in the lower portion of the surface, near which a horseshoe vortex system prevails. In addition to local data, correlations of average Sherwood number with Reynolds number are also presented for various surfaces.
Surface Curvature Effect on Slot-Air-Jet Impingement Cooling Flow and Heat Transfer Process
Authors: Gau C. , Chung C. M.
Published: May 2008

Experiments are performed to study surface curvature effects on the impingement cooling flow and the heat transfer processes over a concave and a convex surface. A single air jet issuing from different size slots continuously impinges normally on the concave side or the convexside of a heated semicylindrical surface. An electrical resistance wire is used to generate smoke, which allows us to visualize the impinging flow structure. The local heat transfer Nusselt number along the surfaces is measured. For impingement on a convex surface, three-dimensional counterrotating vortices on the stagnation point are initiated, which result in the enhancement of the heat transfer process. For impingement on a concave surface, the heat transfer Nusselt number increases with increasing surface curvature, which suggests the initiation of Taylor–Go¨rtler vortices along the surface. In the experiment, the Reynolds number ranges from 6000 to 350,000, the slot-to-plate spacing from 2 to 16, and the diameter-to-slot-width ratio D/b from 8 to 45.7. Correlations of both the stagnation point and the average Nusselt number over the curved surface, which account for the surface curvature effect, are presented.
Heat Transfer in a Staggered Tube Array for a Gas-Solid Suspension Flow
Authors: Murray D. B. , Fitzpatrick J. A.
Published: May 2008

The crossflow heat transfer characteristics of a tube array subject to a dilute gas-solid suspension flow have been investigated. For a tube located in the first row of the array, the results indicate that an increase in the thermal capacity of the mixture is the dominant influence on heat transfer and a correlation for predicting the Nusselt number enhancement is developed. At the second row, the Nusselt number is influenced by thermal property changes and delayed boundary layer separation, whereas heat transfer in the third and later rows is determined by a combination of thermal capacity effects and changes in the flow structure and turbulence levels within the array.
The Aspect Ratio Effect on Natural Convection in an Enclosure With Protruding Heat Sources
Authors: Keyhani M. , Chen L. , Pitts D. R.
Published: May 2008

The aspect ratio effect on natural convection heat transfer in a rectangular enclosure with protruding heat sources has been experimentally investigated. Five protruding heaters were mounted with uniform vertical spacing on one vertical wall. The vertical wall opposite to the wall on which heated sections were mounted was movable so that the enclosure width could be adjusted to the desired value. The top surface of the test enclosure was an isothermal heat sink. All other surfaces except the two end vertical surfaces were insulated. The five heaters were identical with each having horizontal protuberance of L3 = 9 mm and vertical height of L1 = 15 mm. The vertical spacing between the heaters was L2 = 15 mm. The enclosure width was varied in experiments from W = 13.5 mm to 45 mm. The experiments were conducted for six values of cavity width resulting in variations in the cavity height-to-width ratios (aspect ratios) and cavity width-to-protruding heater height ratios of 3.67 to 12.22 and 1.5 to 5.0, respectively. Ethylene glycol was used as the convective medium. Flow visualization pictures and heat transfer data indicate that the starting point of core flow directly affects the local heat transfer coefficient of the bottom heater, while the secondary flow cell between the top heated section and the top sink surface influences the heat transfer coefficient of the top heater. Cavity width variation influences the heat transfer process mainly through altered flow patterns. This influence is weak when the ratio of width-to-protruding height is 4.0 and negligible when this ratio is 5.0 or more. Based on the local height length scale (measured from the bottom of the cavity) the data for all the cavity widths are correlated and an explicit relation for the aspect ratio effect on local Nusselt number is reported. The correlation of local Nusselt number versus local modified Rayleigh number is independent of the number of heaters in the vertical array, cavity width-to-heater protrusion height ratio, W/L3, and vertical height location of the heaters. This conclusion is based on the present results and previously reported data and is valid for the following conditions: 1.5 ≤ W/L3 ≤ 5.0; 3.67 ≤ aspect ratio ≤ 1.22; vertical height of heater from 8 to 15 mm; and number of heated sections from 5 to 10.
Natural Convection in a Square Cavity With Thin Porous Layers on Its Vertical Walls
Authors: Le Breton P. , Caltagirone J. P. , Arquis E.
Published: May 2008

Natural convection in a square cavity in which differentially heated vertical walls are covered with thin porous layers is studied by using a control volume formulation and a SIMPLER algorithm for pressure-velocity coupling. Comparisons with benchmark solutions for natural convection in fluid-filled cavities are first presented for Rayleigh numbers up to 108. The problem of the square cavity with thin porous layers on its vertical walls is then studied by using a modified form of the Navier-Stokes equations by addition of a Darcy term. It is shown that the main effect of the introduction of porous layers is to produce a large decrease of the overall Nusselt number when the permeability is reduced. The higher the Rayleigh number is, the stronger is the decrease, and obviously the decrease also increases with the layer thickness. Moreover, porous layers having a thickness of the order of the boundary layer thickness are sufficient, and taking thicker ones only induces a small decrease of the heat transfer. The main effect of porous layers is to reduce the upwind flow and then to decrease the convective heat transfer.
On the Low Rayleigh Number Asymptote for Natural Convection Through an Isothermal, Parallel-Plate Channel
Authors: Martin L. , Raithby G. D. , Yovanovich M. M.
Published: May 2008

The problem of natural convection through a channel formed by isothermal, parallel plates forms a cornerstone of our understanding of a class of natural convection flows. Following the pioneering study of Elenbaas, it is widely accepted that there is a fully developed re´gime, at low Rayleigh number, in which the Nusselt number becomes directly proportional to the Rayleigh number. This paper gives a detailed analysis of heat transfer in this re´gime. It is concluded that the previous numerical studies, which appeared to confirm this asymptote, used inappropriate boundary conditions, and that the asymptotic behavior should, in fact, not be expected except under very special conditions.
Turbulent Natural Convection in a Horizontal Layer of Small-Prandtl-Number Fluid
Authors: Cheung F. B. , Shiah S. W. , Cho D. H. , Baker L.
Published: May 2008

Turbulent natural convection in a horizontal layer of liquid metal confined between two infinite rigid plates is studied theoretically. The layer, with uniformly distributed energy sources in the fluid, is heated from below and cooled from above. An approximate analysis of the Boussinesq equations of motion is performed for the case of small-Prandtl-number fluids to determine the temperature profiles in three different thermal regions of the layer. By matching these profiles in the regions of overlap, analytical expressions are derived for the lower and upper surface Nusselt numbers and the dimensionless turbulent core temperature as functions of the internal and external Rayleigh numbers defined respectively in terms of the volumetric heating rate and surface-to-surface temperature difference of the layer. Comparison of the present results with heat transfer data for liquid mercury is made and found to be good.
Simultaneous Hydrodynamic and Thermal Development in Mixed Convection in a Vertical Annulus With Fluid Property Variations
Authors: Aung W. , Moghadam H. E. , Tsou F. K.
Published: May 2008

This paper concerns a theoretical investigation of forced and mixed convection heat transfer in a vertical concentric annulus. An implicit finite difference technique is developed to study the effects of temperature-dependent fluid properties, which are represented by power law relations. The fluid under consideration in this study is air, which is an ideal gas with Pr = 0.72. Computations are made with a radius ratio of 0.25. The inner wall is heated at UHF, and the outer wall is heated at UHF or is insulated. The axial distribution of the wall-to-bulk temperature ratio is found to undergo a maximum or minimum, depending on whether the wall is heated or insulated. Fluid property variations enhance the local Nusselt numbers. It is shown that, depending on the length of the duct, heat transfer can be appreciably affected by fluid property variation. For the UHF cases studied here, free convection effects enhance the local Nusselt numbers at intermediate axial distances.
A Study of Thermal Radiation Transfer in a Solar Thruster
Authors: Venkateswaran S. , Thynell S. T. , Merkle C. L.
Published: May 2008

Combined convective and radiative heat transfer in an axisymmetric solar thruster is analyzed. In a solar thruster, focused solar energy is converted into thermal energy by volumetric absorption, resulting in a significant increase in the temperature of the propellant gas. The heated gas is then expanded through a propulsive nozzle in order to generate thrust. In the present theoretical analysis, submicron size particles are employed for providing the mechanism of solar energy absorption. The two-dimensional radiation problem is solved using both an exact integral method and the P1-approximation. The overall energy transfer is solved iteratively by numerical means. The computational model is used to perform parametric studies of the effects of Boltzmann number, optical dimensions of the medium, and wall emissivity. The overall performance of the solar thruster is assessed by determining the thrust levels and the specific impulses of the device under different operating conditions.
Nongray Radiative Gas Analyses Using the S-N Discrete Ordinates Method
Authors: Kim T. K. , Menart J. A. , Lee H. S.
Published: May 2008

The S-N discrete ordinates method is applied to analyze radiative heat transfer in nongray gases. Spectral correlation between the terms in the equation of transfer is considered for black or nearly nonreflecting walls. Formulations to apply the S-N method using a narrow-band or the exponential wide-band model are presented. The net radiative wall heat fluxes and the radiative source distributions are obtained for uniform, parabolic, and boundary layer type temperature profiles, as well as for a parabolic concentration profile. The narrow- and wide-band nongray solutions are compared with gray-band approximations using the same band models. The computational speed of the gray-band approximation is obtained at the expense of accuracy in the internal fluxes and radiative source distributions. The wall radiative flux predictions by the gray-band approximation are satisfactory.
Temperature Uniformity Across the Surface in Transition Boiling
Authors: Haramura Y.
Published: May 2008

The condition under which temperature is kept uniform across the surface in transition boiling is investigated theoretically and numerically. Analysis of three-dimensional heat conduction indicates that an unstable mode emerges depending on the gradient of a boiling curve and dimension and properties of a heater block in steady-state runs. A thinner heated block for the same diameter or larger diameter for the same thickness loses uniformity for a shallower boiling curve in transition boiling. Simulation of a transient run and another analysis indicates that the surface is always kept sufficiently uniform in transient runs without control of heat input. The analysis also reveals the rapid change in the surface temperature in transient runs.
Vapor Dynamics of Heat Pipe Start-Up
Authors: Issacci F. , Catton I. , Ghoniem N. M.
Published: May 2008

Vapor dynamics of heat pipes during the start-up phase of operation is analyzed. The vapor flow is modeled by a two-dimensional, compressible viscous flow in an enclosure with inflow and outflow boundary conditions. For high-input heat fluxes, a compression wave is created in the evaporator early in the operation. A nonlinear filtering technique, along with the centered difference scheme, is used to capture the shocklike wave and overcome the cell Reynolds number problem. Multiple wave reflections are observed in the evaporation and adiabatic regions. These wave reflections cause a significant increase in the local pressure and flow circulations, which grow with time. It is shown that the maximum and maximum-averaged pressure drops oscillate periodically because of the wave reflections. Although the pressure drops converge to a constant value at steady state, they are significantly higher than their steady-state value at the initiation of the process. The time for the vapor core to reach a steady-state condition was found to be on the order of seconds.
Three-Dimensional Natural Convection From Vertical Heated Plates With Adjoining Cool Surfaces
Authors: Webb B. W. , Bergman T. L.
Published: May 2008

Natural convection in an enclosure with a uniform heat flux on two vertical surfaces and constant temperature at the adjoining walls has been investigated both experimentally and theoretically. The thermal boundary conditions and enclosure geometry render the buoyancy-induced flow and heat transfer inherently three dimensional. The experimental measurements include temperature distributions of the isoflux walls obtained using an infrared thermal imaging technique, while the three-dimensional equations governing conservation of mass, momentum, and energy were solved using a control volume-based finite difference scheme. Measurements and predictions are in good agreement and the model predictions reveal strongly three-dimensional flow in the enclosure, as well as high local heat transfer rates at the edges of the isoflux wall. Predicted average heat transfer rates were correlated over a range of the relevant dimensionless parameters.
Natural Convection Heat Transfer From a Plate in a Semicircular Enclosure
Authors: Moore G. A. , Hollands K. G. T.
Published: May 2008

In the subject geometry, a long thin plate at uniform temperature is contained coaxially and symmetrically in a long semicircular trough closed at the top and having a uniform but different temperature. Heat flows across the air-filled region between the two by both natural convection and gaseous conduction. The problem of characterizing the free convective component of this heat transfer—that is, the component caused by bulk fluid motion—is treated experimentally by using a heat balance technique, with the measurements being repeated at different pressures, in order to cover a wide Rayleigh number range, from Ra ≈ 10 to Ra ≈ 108. Nusselt number versus Rayleigh number plots are presented for each of several combinations of plate-to-trough spacing and tilt angle, and the plots are correlated by equations. The problem of characterizing the conductive component is treated by numerically solving the steady diffusion equation in the air-filled region, and the results are correlated as a function of the spacing and the plate thickness.
Mixed Convection Through Vertical Porous Annuli Locally Heated From the Inner Cylinder
Authors: Choi C. Y. , Kulacki F. A.
Published: May 2008

Mixed convection in a vertical annulus filled with a saturated porous medium is numerically and experimentally investigated. Calculations are carried out under the traditional Darcy assumptions and cover the ranges 10 ≤ Ra ≤ 200 and 0.01 ≤ Pe ≤ 200. Both numerical and experimental results show that the Nusselt number increases with either Ra or Pe when the imposed flow is in the same direction as the buoyancy-induced flow. When the imposed flow opposes buoyancy-induced flow, the Nusselt number first decreases with an increase of the Peclet number and reaches a minimum before increasing again. Under certain circumstances, the Nusselt number for a lower Rayleigh number may exceed that for larger value. Nusselt numbers are correlated by the parameter groups Nu/Pe1/2 and Ra/Pe3/2. Good agreement exists between measured and predicted Nusselt numbers, and the occurrence of a minimum Nusselt number in mean flow that opposes buoyancy is verified experimentally.
Convective Nucleate Boiling on a Heated Surface Cooled by an Impinging, Planar Jet of Water
Authors: Vader D. T. , Incropera F. P. , Viskanta R.
Published: May 2008

Convective nucleate boiling has been studied on a flat, upward facing, constant heat flux surface cooled by a planar, impinging water jet. Surface temperature distributions are presented for jet velocities between 1.8 and 4.5 m/s, fluid temperatures of 30, 40, and 50°C, and heat fluxes between 0.25 and 2.5 MW/m2. Although the critical Reynolds number, Rex*,c, is independent of heat flux for q” < q”ONB, boiling incipience strongly affects the transition to a turbulent boundary layer. As the heat flux increases, vapor bubbles of 1 mm diameter first appear at the point of maximum surface temperature, which also marks the onset of boundary layer turbulence. The leading edge of these bubbles moves toward the stagnation line and Rex*,c decreases with further increases in heat flux. Acceleration in the stagnation region stabilizes the flow, however, so that boundary layer turbulence is restricted to x/wj ≳ 1.6. With increasing heat flux, vigorous nucleate boiling covers more of the heater and surface temperature variations decrease.
Considerations in Predicting Burnout of Cylinders in Flow Boiling
Authors: Sadasivan P. , Lienhard J. H.
Published: May 2008

Previous investigations of the critical heat flux in flow boiling have resulted in widely different hydrodynamic mechanisms for the occurrence of burnout. Results of the present study indicate that existing models are not completely realistic representations of the process. The present study sorts out the influences of the far-wake bubble breakoff and vapor sheet characteristics, gravity, surface wettability, and heater surface temperature distribution on the peak heat flux in flow boiling on cylindrical heaters. The results indicate that burnout is dictated by near-surface effects. The controlling factor appears to be the vapor escape pattern close to the heater surface. It is also shown that a deficiency of liquid at the downstream end of the heater surface is not the cause of burnout.
Thermal Analysis of Droplet Spray Evaporation From a Heated Solid Surface
Authors: Tio K.-K. , Sadhal S. S.
Published: May 2008

This paper deals with the heat transfer aspects of droplet spray evaporation from a heated solid surface. The analysis is restricted to the low superheat regime, in which the impinging droplets stick onto the solid without bubble nucleations. In this limit of approximation, the heat flow is quasi-steady and conduction dominated. The difficulty arising from droplet thermal interactions is overcome with an approximate model of an evaporating droplet surrounded by a uniform surface heat flux. The size of the droplet changes with time, but the effect of the surrounding droplets, which are treated as a continuum producing the uniform surface flux, is taken into consideration in an average sense. In addition, the analysis also incorporates the effects of droplet contact angle, droplet concentration, and solid conductivity. A transcendental equation has been derived and solved by using the Newton-Raphson method to obtain the Nusselt number.
Thermal Analysis and Control for Sputtering Deposition of High-Tc Superconducting Films
Authors: Flik M. I. , Choi B. I. , Anderson A. C. , Westerheim A. C.
Published: May 2008

For the preparation of high-quality films of high-Tc superconductors (HTSC) on crystalline substrates, it is necessary to control the substrate temperature accurately during deposition. This study shows that thermal radiation heat transfer in the deposition chamber governs the substrate temperature. The application of thin-film optics yields the emittance of the substrate holder-substrate-film composite as a function of the thickness of the growing film. In a single-target off-axis sputtering system, the substrate temperature is measured during film deposition using a novel method for the attachment of a thermocouple to the substrate front surface. For constant heater power, the measurements show a decrease of the substrate temperature, in agreement with the theoretical prediction. Based on the substrate emittance variation determined in this work, a pyrometric in-situ temperature measurement technique can be developed.
Thermal Analysis of Electron-Beam Absorption in Low-Temperature Superconducting Films
Authors: Flik M. I. , Goodson K. E.
Published: May 2008

The absorption of an electron beam in a superconducting microbridge reduces its critical current, the maximum d-c electric current it can carry without resistance. A two-dimensional heat conduction analysis determines numerically the temperature field in the film caused by electron-beam heating, considering the nonlinear thermal boundary resistance between film and substrate. The method of Intrinsic Thermal Stability yields the critical current for this temperature field. The critical current predictions agree with experimental data from low-temperature scanning electron microscopy (LTSEM) with superconducting lead microbridges. The method developed in this study permits the quantitative prediction of LTSEM experiments, enhancing the value of this technique for the local characterization of superconducting films.
Moving Front Fixing in Thin Film Laser Annealing
Authors: Grigoropoulos C. P. , Dutcher W. E.
Published: May 2008

The process of laser melting and recrystallization of thin silicon films, which are deposited on amorphous substrates, is capable of improving the semiconductor electrical and crystalline properties. The process is controlled by the intensity of the laser beam, the material translation speed, and the thermal and radiative properties of the semiconductor layer and the encapsulating structure. Accurate theoretical modeling of the associated phase change process is essential for optimal material processing. This paper presents a numerical model implementing a front-fixing approach and body-fitted curvilinear grids to analyze the heat transfer and the induced crystallization rates in thin film laser annealing. The results are compared to experimental data and reasonable agreement is obtained. Further improvements depend upon knowledge of thin film thermal and optical properties.
Differential Methods for the Performance Prediction of Multistream Plate-Fin Heat Exchangers
Authors: Prasad B. S. V. , Gurukul S. M. K. A.
Published: May 2008

Use of traditional methods of rating can prove inaccurate or inadequate for many plate-fin heat exchanger applications. The superiority in practical situations of differential methods, based on dividing the heat exchanger into several sections and a step-wise integration of the heat transfer and pressure loss functions, is discussed. Differential methods are developed for counterflow, crossflow, and cross-counter-flow heat exchangers. The methods developed also avoid iterations at the section level calculations. Design of computer algorithms based on these methods is outlined. Two computer programs developed using the methods are presented and the results for a few typical cases are discussed.
Temperature Distribution Near a Heat Exchanger Wall Immersed in High-Temperature Packed and Fluidized Beds
Authors: Flamant G. , Fatah N. , Olalde G. , Hernandez D.
Published: May 2008

Experimental results dealing with both particle and gas temperature distribution in the vicinity of a water-cooled wall immersed in fixed and fluidized beds are presented. The measurement of particle temperature is based on the use of a mobile optical fiber connected to a two-color radiometer. The gas temperature is obtained on the basis of the indications of a bare thermocouple. Particle and gas temperature fields are compared in fixed and fluidized beds for alumina and silicon carbide particles. In the fixed bed, temperature differences as large as 300°C between the gas and the solid are measured. In the fluidized bed, temperature decreases of both solid and gas phase are shown for large particle at incipient fluidization. The temperature variation reaches more than 100°C for corundum particles and 200°C in the gas. The temperature distribution in the solid phase is shown to be dependent on the thermophysical properties of the particles (thermal conductivity and emissivity).
Turbulent Heat Transfer Augmentation and Friction in Periodic Fully Developed Channel Flows
Authors: Liou T.-M. , Hwang J.-J.
Published: May 2008

Measurements are presented of the distribution of average friction factors (f) as well as local and average (Nu) heat transfer coefficients for fully developed channel flows with two rib-roughened opposite walls. The temperature measurements were made by using both a laser holographic interferometer and thermocouples. In addition, the reattachment length was determined by flow visualization. The Reynolds number (Re) was varied from 5.0 × 103 to 5.4 × 104; the rib pitch-to-height ratios (Pi/H) were 10, 15, and 20; and the rib height-to-hydraulic diameter ratios (H/De) were 0.063, 0.081, and 0.106. The detailed results allowed the peaks of heat transfer augmentation and the regions susceptible to hot spots to be located and allowed the relative contribution of the rib surface and the channel wall to the heat transfer augmentation to be determined. Moreover, relative to a smooth duct, the enhancement of both Nu and f at various Re, Pi/H, and H/De was documented in detail. Furthermore, compact correlations in terms of Re, Pi/H, and H/De were developed for both Nu and f.
Influence of Jet-Grid Turbulence on Flat Plate Turbulent Boundary Layer Flow and Heat Transfer
Authors: Young C. D. , Han J. C. , Huang Y. , Rivir R. B.
Published: May 2008

The influence of high mainstream turbulence on turbulent boundary layer flow and heat transfer is experimentally investigated for length Reynolds numbers between 4 × 104 and 1.5 × 106. The high mainstream turbulence is produced by a round tube grid with uniform jet injection. Injected air is blown in either an upwind or downwind direction at a controllable flow rate. A flat plate test section instrumented with foil thermocouples is located downstream from the jet grid. The turbulence intensity decay and length scale growth along the test plate, the mean velocity and temperature profiles across the boundary layer, and surface heat transfer distribution are measured. The results show that the grid with downwind injection produces a slightly higher turbulence intensity and a smaller length scale than the grid with upwind injection. A higher turbulence intensity and a smaller length scale further enhance the surface heat transfer coefficient. The jet-induced high turbulence does not alter the downstream velocity and temperature profiles in their logarithmic regions, but the wake regions are lower than the zero turbulence profiles. The Reynolds analogy factor, the augmented friction factor, and the augmented Stanton number are higher than those from existing correlations when the jet grid turbulence intensity is greater than 6 percent.
Measurements of the Free Surface Flow Structure Under an Impinging, Free Liquid Jet
Authors: Stevens J. , Webb B. W.
Published: May 2008

The objective of this research was to characterize the flow structure under an impinging liquid jet striking a flat, normally oriented surface. The approach was the measurement of the free surface velocities of the jet prior to impingement and the surface velocities of the radially spreading liquid layer. A novel laser-Doppler velocimetry technique was used. The LDV system was configured such that the measurement volume would span the time-dependent fluctuations of the free surface, with the surface velocity being measured. The mean and fluctuating components of a single direction of the velocity vector were measured. It was found that the radial liquid layer data collapsed well over the range of jet Reynolds numbers 16,000 < Re < 47,000 if plotted in dimensionless coordinates, where the measured velocity was normalized by the average jet exit velocity and the radial coordinate was normalized by the nozzle diameter. Mean liquid layer depths were inferred from the velocity measurements by assuming a velocity profile across the layer, and were reported. Pre-impingement jet measurements suggest that the flow development is nearly complete two diameters from the nozzle exit.
Experimental Study of Laminar Natural Convection in Cells With Various Convex and Concave Bottoms
Authors: Lewandowski W. M. , Khubeiz M. J.
Published: May 2008

Heat transfer and free convective motion in limited space from the bottoms of different hemispherical convex or concave shapes have been studied experimentally. The ratio of the diameter of the hemisphere (d) to the diameter of the bottom (D) (0 < d/D < 1) has been tested for a range of Rayleigh numbers (105 < Ra <107). In comparison with a flat bottom (d/D = 0), about 40 percent inhibition or about 50 percent intensification depending on the bottom configuration (d/D) have been observed. The mechanism of the phenomenon based on dead space, local overheating, and shape influence effects has been proposed.
Free Two-Dimensional Convective Bifurcation in a Horizontal Annulus
Authors: Cheddadi A. , Caltagirone J. P. , Mojtabi A. , Vafai K.
Published: May 2008

Natural convection is investigated numerically and experimentally in a cylindrical annulus. The governing equations based on primitive variables are solved using Chorin’s method. In addition to the unicellular flows reported in the literature, depending on initial conditions, bicellular flows are observed for high Rayleigh numbers. The bifurcation point is determined numerically. The velocity field for unicellular flows is measured by laser-Doppler anemometry in an air-filled annulus. A perturbation solution is also presented. The experimental results are in good agreement with numerical predictions and the perturbation solution.
Transient Heat Transfer in a Conducting Particle With Internal Radiant Absorption
Authors: Tuntomo A. , Tien C. L.
Published: May 2008

The objective of the present work is to analyze rigorously the transient heat transfer of an irradiated particle by treating the radiant absorption on a local basis. A new conduction-to-radiation parameter is introduced to characterize the relative importance of heat transfer by conduction as compared with that by radiation. The study on the transient temperature field as a function of conduction-to-radiation parameter establishes a criterion identifying the circumstances where heat transfer by radiation is so predominant that conduction is negligible. The current effort is also directed at developing a convenient method for predicting the transient local maximum temperature and explosion time delay of an intensely irradiated liquid droplet.
Cross-Correlation Velocimetry for Measurement of Velocity and Temperature Profiles in Low-Speed, Turbulent, Nonisothermal Flows
Authors: Motevalli V. , Marks C. H. , McCaffrey B. J.
Published: May 2008

A technique utilizing thermocouple pairs as sensors to measure velocity and temperature profiles in low-speed, turbulent, nonisothermal flows is described here. In this technique, Cross-Correlation Velocimetry (CCV), the temperature-time records from a pair of thermocouples, one downstream of the other, are cross-correlated to determine the flow’s preferred mean velocity while temperature is measured directly. The velocity measurements have undergone extensive verification using hotwire, pitot tube, and Laser-Doppler Velocimetry to determine the degree of confidence in this technique. This work demonstrates that the CCV technique is quite reliable and can measure the mean preferred component of the convective velocity with better than ±5 percent certainty. Application of this technique to the measurement of velocities in a ceiling jet induced by a fire plume is briefly presented here.