Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1989"

Heat Transfer Measurements From a Surface With Uniform Heat Flux and an Impinging Jet

Abstract: There are numerous studies, mostly experimental, on the characteristics and heat transfer associated with jet impingement on surfaces. These studies have considered both single jets and multiple jets (i.e., arrays) and many different aspects of impinging jets including the effects of crossflow, jet orientation (oblique jets), jet temperature, rotating surfaces, and different surface shapes. The present study is concerned with the case of a single circular turbulent air jet at the ambient air temperature impinging on a flat stationary surface. One of the difficulties in comparing recent numerical work with previous experimental results is the lack of data on the jet characteristics and in some cases the mixed thermal boundary conditions at the surface. The present work provides some new experimental results that attempt to overcome this difficulty by using a fully developed jet and a well-controlled thermal boundary condition (i.e., a uniform heat flux). No other similar measurements were found in the literature.

Microelectronic Cooling by Enhanced Pool Boiling of a Dielectric Fluorocarbon Liquid

Abstract: An experimental study of boiling heat transfer from a simulated microelectronic component immersed in a stagnant pool of the dielectric Fluorinert (FC-72) is presented. Various enhancement surfaces were attached to an electrically heated copper calorimeter bar having a vertically oriented heat transfer surface area of 12.7 {times} 12.7 mm{sup 2}. A number of enhancement schemes aimed at a reduction of the incipience temperature overshoot were tested, employing various arrangement of fins, studs, grooves, and vapor-trapping cavities. Atmospheric pressure testing revealed a variation in the magnitude of boiling curve incipience temperature excursion as a function of both macro- and microcharacterization of the surface geometry and initial conditions (pressure and temperature history) prior to boiling. Increased incipience temperatures accompanied prolonged periods of nonboiling. It is assumed that this is due to vapor embryos within surface cavities collapsing to smaller radii. Large artificially created cavities (0.3 mm diameter) were found incapable of maintaining a stable vapor embryo for time periods greater than 10 min. In comparison to flat surfaces, low-profile surface geometries having a structure scale of the order of one bubble departure diameter resulted in significant enhancement of nucleate boiling while drilled surfaces had minimal effectiveness. Surface finish and artificial cavities hadmore » no effect on CHF, but levels of critical heat flux computed on base area were strongly dependent on macrogeometry, due in part to increased surface area.« less

Forced Convection in a Channel Filled With a Porous Medium: An Exact Solution

Abstract: In this paper fully developed forced convection in a porous channel bounded by parallel plates is considered based on the general flow model. Exact solutions are obtained and presented for both the velocity and the temperature fields. From these results the Nusselt number can be expressed in terms of the Darcy number and the inertia parameter. Finally, comparisons are made with the limiting case of no inertia and/or boundary effects. These results provide an in-depth insight into the underlying relationships between all of the pertinent variables. Furthermore, they can be used as strong candidates for bench marking of many numerical schemes.

Design of Cooling Towers by the Effectiveness-NTU Method

Abstract: Developpement de la methode NUT-efficacite pour la conception de tours de refroidissement: introduction de l'enthalpie de l'air sature en parametre et definition du nombre d'increments correspondant a la precision desiree Presentation de la methode LMED (difference d'enthalpie logarithmique moyenne) et comparaison des deux methodes sur des exemples

Framework for a Unified Model for Nucleate and Transition Pool Boiling

Abstract: An area and time-averaged model for saturated pool boiling heat fluxes has been developed. In the model, which is valid in the upper end of nucleate boiling and in transition boiling, the existence of stationary vapor stems at the wall is assumed. The energy from the wall is conducted into the liquid macro/micro thermal layer surrounding the stems and is utilized in evaporation at the stationary liquid-vapor interface. The heat transfer rate into the thermal layer and the temperature distribution in it are determined by solving a two-dimensional steady-state conduction equation. The evaporation rate is given by the kinetic theory. The heater surface area over which the vapor stems exist is taken to be dry. Employing experimentally observed void fractions, not only the nucleate and transition boiling heat fluxes but also the maximum and minimum heat fluxes are predicted from the model. The maximum heat fluxes obtained from the model are valid only for surfaces that are not well wetted and includes the contact angle as one of the parameters.

Numerical Calculation of Bubble Growth in Nucleate Boiling From Inception Through Departure

Abstract: The relative contributions of the fundamental mechanisms accounting for theenhanced heat transfer in nucleate boiling are difficult to quantifyanalytically or experimentally. A comprehansive model was developed thatpermits some accurate insights into this problem. An essential feature involvedthe numerical mapping of the complicated geometry to a plane where the bubbleand wall boundaries lie along constant coordinate lines. The results show thatmicrolayer evaporation accounts for 87 percent of the enhanced wall heattransfer during saturated boiling of water at 1 atm and 8.5 K wall superheat.In contrast, enhanced convective effects were essentially nonexistent duringgrowth and minimal following depature. The analysis predicts an extremelynonuniform thermal boundary layer around the bubble, and shows that the wallthermal boundary layer regenerates almost immediately following departure.

A Detailed Examination of Gas and Liquid Phase Transient Processes in Convective Droplet Evaporation

Abstract: A finite volume numerical technique has been used to model the evaporation of an n-heptane droplet with an initial Reynolds number of 100 in air at 800 K, 1 atm. The effects of variable thermophysical properties, liquid phase motion and heating, and transient variations in droplet size and velocity are included in the analysis. With appropriate corrections for the effects of variable properties and liquid phase heating, quasi-steady correlations are shown to predict accurately the transient histories of the drag coefficient and Nusselt and Sherwood numbers. For the case investigated here, the transient effects of importance were the variation in droplet velocity, the decline in the liquid phase velocities, and the rise in the droplet surface and volume average temperatures. In spite of the transient rise in the droplet temperature, the nature of the liquid phase heating, as characterized by the liquid Nusselt number, was found to remain constant during most of the droplet lifetime.

Natural Convection Along a Vertical Wavy Surface With Uniform Heat Flux

Abstract: Laminar free convection along a semi-infinite vertical wavy surface has been studied by Yao (1983) for the case of uniform surface temperature. This is a model problem for the investigation of heat transfer from roughened surfaces in order to understand heat transfer enhancement. In many applications of practical importance, however, the surface temperature is nonuniform. In this note, the case of uniform surface heat flux rate, which is often approximated in practical applications and is easier to measure in a laboratory, has been investigated. Numerical results have been obtained for a sinusoidal wavy surface. The results show that the Nusselt number varies periodically along the wavy surface. The wavelength of the Nusselt number variation is half of that of the wavy surface, while the amplitude gradually decreases downstream where the boundary layer grows thick. It is hoped that experimental results will become available in the near future to verify the results of this investigation.

High Rayleigh Number Laminar Natural Convection in an Asymmetrically Heated Vertical Channel

Abstract: Experiments have been performed to determine local heat transfer data for the natural convective flow of air between vertical parallel plates heated asymmetrically. A uniform heat flux was imposed along one heated wall, with the opposing wall of the channel being thermally insulated. Local temperature data along both walls were collected for a wide range of heating rates and channel wall spacings corresponding to the high modified Rayleigh number natural convection regime. Laminar flow prevailed in all experiments. Correlations are presented for the local Nusselt number as a function of local Grashof number along the channel. The dependence of both average Nusselt number and the maximum heated wall temperature on the modified Rayleigh number is also explored. Results are compared to previous analytical and experimental work with good agreement.

Cavity Heat Transfer on a Transverse Grooved Wall in a Narrow Flow Channel

Abstract: Etude du transfert de chaleur par convection dans une cavite rectangulaire dans la paroi d'un canal etroit

On the Thermal Shock Wave Induced by a Moving Heat Source

Abstract: Analytical solutions for the temperature field around a moving heat source in a solid with finite speed of heat propagation are obtained via the method of Green's functions. When the speed of the moving heat source is equal to or faster than that of the thermal wave propagated in the solid, the thermal shock wave is shown to exist in the thermal field. The shock wave angle is obtained as sin{sup {minus}1} (1/M) for M {ge} 1. Orientation of crack initiation in the vicinity of the heat source is also estimated by considering the temperature gradient T,{sub {theta}} along the circumference of a continuum circle centered at the heat source. Such an orientation is established as a function of the thermal Mach number in the subsonic, transonic, and supersonic regimes, respectively.

Mixed Convection Along a Wavy Surface

Abstract: The results of a study of mixed-convection flow along a wavy surface are presented. The forced-convection component of the heat transfer contains two harmonics. The amplitude of the first harmonic is proportional to the amplitude of the wavy surface; the second harmonic is proportional to the square of this amplitude. Thus, for a slightly wavy surface, only the influence of the first harmonic can be detected. The natural-convection component is a second harmonic, with a frequency twice that of the wavy surface. Since natural convection has a cumulative effect, the second harmonic eventually becomes the dominant component far downstream from the leading edge where forced convection is the dominant heat transfer mode. The results also demonstrate that the total mixed-convection heat flux along a wavy surface is smaller than that of a flat surface.

Finite Analytic Solution of Convective Heat Transfer for Tube Arrays in Crossflow: Part II—Heat Transfer Analysis

Abstract: The convective heat transfer and pressure drop in flow past two types of tube array are solved numerically by the Finite Analytic Method in this investigation. The tube arrays considered are an in-line tube array and a staggared tube array with longitudinal and transverse pitch of two. The flow field solution is obtained and analyzed in Part 1 of the study. In the present Part 2, the temperature field, heat transfer characteristics, and pressure drop are investigated. The fluid and tubes are considered to be at the same temperature, except for the array tube, which is heated or cooled. The solution domain covers three pitches of tube arrays in order to simulate accurately the non-periodic behavior of the temperature field downsream of the heated tube. In general, the heat transfer in a staggered array of tubes is found to be higher than that in an in-lined array of tubes. However, the pressure drop in a staggered array arrangement is also higher. Local heat transfer varies between in-line and staggered tube arrays and depends on Reynolds number and Prandtl number. At high Reynolds number, the local heat transfer tends to peak at the upstream surface of the heated tube but awaymore » from the stagnation point and becomes minimum at the separation point. Heat transfer in recirculation zones are, in general, small.« less

Transfer Functions for Efficient Calculation of Multidimensional Transient Heat Transfer

Abstract: Finite difference or finite element methods reduce transient multidimensional heat transfer problems into a set of first-order differential equations when thermal physical properties are time invariant and the heat transfer processes are linear. This paper presents a method for determining the exact solution to a set of first-order differential equations when the inputs are modeled by a continuous, piecewise linear curve. For long-time solutions, the method presented is more efficient than Euler, Crank-Nicolson, or other classical techniques.

Void Fraction Measurements During Saturated Pool Boiling of Water on Partially Wetted Vertical Surfaces

Abstract: Void fraction profiles adjacent to a vertical wall 6.3 cm wide and 10.3 cm high were measured during nucleate boling. The experiments were conducted in saturated water at 1 atm pressure. In the experiments, the wettability of the surface was varied by controlling the degree of oxidation of the surface. Static contact angle was used as an indicator of the surface wettability. The void fraction was measured with a gamma densitometer. The experimental results show that the maximum void fraction occurs about 1--1.5 mm away from the heater surface. The wall void fraction, the maximum void fraction, and the thickness of the void layer increase with wall heat flux. It is found that for a given heat flux, the wall void fraction increases as the surface becomes less wettable, whereas the maximum heat flux decreases with increase in contact angle.

Numerical Investigation of Near-Wall Turbulent Heat Transfer Taking Into Account the Unsteady Heat Conduction in the Solid Wall

Abstract: The deterministic near-wall turbulence model developed by Kasagi et al. (1984b) is used in a numerical analysis of turbulent heat transfer, in which the unsteady heat conduction inside the wall associated with the turbulent flow unsteadiness is taken into account. Unlike the typical methodology based on Reynolds decomposition, the algebraic expressions for the three fluctuating velocities given by the model are directly introduced into the governing energy equation. From the numerical results of the unsteady conjugate heat transfer, the statistical quantities, such as temperature variance, turbulent heat flux, and turbulent Prandtl number, are obtained for fluids of various Prandtl numbers. It is demonstrated that the near-wall behavior of these quantities is strongly influenced by the thermal properties and thickness of the wall. In addition, the budget of the temperature variance associated with coherent turbulence structure is calculated and, except for dissipation, each budget term is in qualitative agreement with the experiment.

Turbulent Heat Transfer and Friction in Pin Fin Channels With Lateral Flow Ejection

Abstract: Experiments were conducted to study the effects of lateral flow ejection on the overall heat transfer and pressure drops for turbulent flow through pin fin channels. The two test sections of the investigation were rectangular channels with staggered arrays of six and eight streamwise rows of pins, respectively. The pin length-to-diameter ratio was one and both the streamwise and spanwise pin spacings were 2.5 times the pin diameter. Heat transfer and friction data were obtained for various ejection exit geometries, for ejection ratios between 0 and 1, and for Reynolds numbers between 6,000 and 60,000. The results of the study show that, for any given ejection ratio, the overall Nusselt number increases with increasing Reynolds number. However, the overall Nusselt number is reduced by as much as 25% as the ejection ratio is increased from 0 to 1 over the range of Reynolds number studied. The Nu-Re-{epsilon} relationship, which is insensitive to varying the ejection exit geometry, can be correlated by the equation (Nu/Nu{sub 0}) = (Nu{sub 1}/Nu{sub 0}){sup {epsilon}}, where Nu{sub 0} = c{sub 0}Re{sup m} and Nu{sub 1} = c{sub 1}Re{sup n} are the overall Nusselt numbers in the 0 and 100% lateral flow ejection cases, respectively. Themore » results also show that the overall friction factor is independent of the flow Reynolds number over the range of Reynolds number studied. However, the friction factor is strongly dependent on the ejection ratio as well as the geometries of the straight flow exit and lateral ejection flow exit.« less

Single- and Two-Phase Convective Heat Transfer From Smooth and Enhanced Microelectronic Heat Sources in a Rectangular Channel

Abstract: Experiments have been performed to assess the feasibility of cooling microelectronic components by means of single-phase and two-phase forced convection. Tests were conducted using a single heat source flush mounted to one wall of a vertical rectangular channel. An inert fluorocarbon liquid (FC-72) was circulated upward through the channel at velocities up to 4.1 m/s and with subcooling up to 46 {degree}C. The simulated microelectronic heat sources tested in this study include a smooth surface and three low-profile microstud surfaces of varying stud height, each having a base area of 12.7{times}12.7 mm{sup 2}. Correlations were developed for the single-phase convective heat transfer coefficient over the Reynolds number range from 2800 to 1.5{times}10{sup 5}, where Reynolds number is based on the length of the heater. The results demonstrate that the low thermal resistances required for cooling of microelectronic heat sources may be achieved with single-phase forced convection by using high fluid velocity coupled with surface enhancement. Experiments were also performed to understand better the parametric trends of boiling heat transfer from the simulated microelectronic heat source. It was found that increased velocity and subcooling and the use of microstud surfaces enhance nucleate boiling, increase the critical heat flux, and reduce themore » magnitude of temperature overshoot upon the inception of nucleation.« less

Effect of Solid Subcooling on Natural Convection Melting of a Pure Metal

Abstract: A combined experimental and numerical study is reported of melting of a pure metal inside a vertical rectangular enclosure with natural convection in the liquid and con­ duction in the solid. The numerical model is successfully verified by conducting a series of experiments covering a wide range of hot and cold wall temperatures. It is found that solid subcooling significantly reduces the melting rate when compared to melting with the solid at the fusion temperature. Because the cooled wall is held below the fusion temperature of the metal, the solid/liquid interface eventually reaches a stationary position. For moderate values of the subcooling parameter the steady-state interface is almost vertical and parallel to the cold wall. Strong subcool­ ing results in an early termination of the melting process, such that natural convec­ tion in the relatively small liquid region cannot fully develop. For moderate subcool­ ing, correlations have been derived for the steady-state volume and heat transfer rates. While many aspects of melting with solid subcooling appear to be similar to ordinary nonmetallic solids, important differences in the local flow structures and heat transfer mechanisms are observed.

Finite Analytic Solution of Convective Heat Transfer for Tube Arrays in Crossflow: Part I—Flow Field Analysis

Abstract: The convective motion in two types of tube array is solved numerically by the Finite Analytic Method. The Finite Analytic Method utilizes the local analytic solution of governing differential equations in obtaining its discretized algebraic representation. Both in-line tube arrays and staggered tube arrays with longitudinal and transverse pitches of 2 are studied. The geometries are expressed in boundary-fitted coordinates on which the Navier-Stokes equations and energy equation are solved. Solutions for Reynolds numbers of 40, 120, 400, and 800 are obtained. Differences in stream function, vorticity function, and location of separation and reattachment for flow past in-line tube arrays and staggered tube array are predicted and compared. The zone of separation for both arrays tends to increase with increasing Reynolds number. The predicted results on flow field and heat transfer are shown to agree with available experimental measurements.

Heat Transfer Characteristics in Two-Phase Closed Conventional and Concentric Annular Thermosyphons

Abstract: The heat transfer in the condenser sections of conventional and annular two-phase closed thermosyphon tubes has been studied experimentally and analytically. In addition, the results of a series of experiments on the flooding phenomena of the same thermosyphons are reported. Freon 113 and acetone were used as working fluids. An improved correlation was developed to predict the performance limits of conventional thermosyphons using the present and previously existing experimental data for flooding with different working fluids. The prediction of the theoretical Nusselt number for the situations associated with measured heat transfer coefficients in the condenser section indicated that the effect of interfacial shear on the film flow is small. The increase of the experimental reflux condensation heat transfer coefficients over theoretical predictions is attributed to waves at the vapor-liquid interface.

An Upper Bound for the Critical Boiling Heat Flux

Abstract: The great value of boiling and condensation in process heat transfer is that they yield the highest known heat transfer coefficients. People who have to transfer a great deal of energy rapidly, under fairly low driving temperature differences, usually turn to these processes. The authors therefore constantly face the question. What is the upper limit on these heat fluxes - how far can they be driven.

On the Nondimensionalization of Constriction Resistance for Semi-infinite Heat Flux Tubes

Abstract: The insulated semi-infinite cylinder with heat supplied uniformly through a coaxial contact area is an important unit cell in the theory of contact resistance. In the past, several investigators have examined the problem of the thermal constriction resistance of a circular contact area on an insulated semi-infinite, coaxial circular cylinder. Recently the case of a circular contact on a square cylinder has been investigated. By using a new approximate technique as well as double-infinite series solution to Laplace's equation in Cartesian coordinates, the case of a square contact area on a square cylinder can also be solved. Previously the nondimensionalization technique employed for the results has varied with different configurations and researchers. However, the similarity of these three configurations in terms of their integrated parameter, the thermal constriction resistance, will only be seen when nondimensionalization is made using a characteristic dimension that best describes these geometries.

Improved Liquid-Crystal Thermometry Excluding Human Color Sensation

Abstract: A new liquid-crystal thermometry method is described to determine an isothermal map on a heat transfer surface coated with a cholesteric liquid-crystal layer that changes color according to temperature. This method is based on the use of a set of sharp band-pass optical filters, one of which is attached to a black-and-white video camera to take a monochromatic image having a specified color. From the image, and isothermal line was drawn with the aid of a digital image processing techniques that excludes human color sensation. The authors obtain as many isothermal lines as band-pass filters and can determine an isothermal map. An experiment is presented as an application of the present method to measure temperature distributions on a heated surface cooled by air flow and disturbed by a short attached cylinder.

Performance Characteristics of a Concentric Annular Heat Pipe: Part I—Experimental Prediction and Analysis of the Capillary Limit

Abstract: This paper describes the design, testing, and theoretical capillary limit prediction of a new heat pipe configuration, which is the concentric annular heat pipe. The concentric annular vapor space. With this arrangement, capillary wicks can be placed on both the inside of the outer pipe and the outside of the inner pipe. This design heat pipes, since the cross-sectional area of the wick as well as the surface area for heating and cooling are increased. The heat pipe was tested for the temperature distribution in the three sections of the heat pipe under various tilt angles and heating loads through the inner and outer pipes in the evaporator section. A simple analysis for the prediction of the capillary limitation of the concentric annular heat pipe is presented.

Performance Characteristics of a Concentric Annular Heat Pipe: Part II—Vapor Flow Analysis

Abstract: The solutions of the equations of fluid motion for compressible and incompressible flow in a concentric annular heat pipe have been analyzed In addition, a similarity solution is presented that can predict the pressure losses in all the segments of the concentric annular heat pipe as well as conventional heat pipes A theoretical analysis to predict the sonic limit for this new pipe is also presented

Multipass plate heat exchangers -- Effectiveness-NTU results and guidelines for selecting pass arrangements

Abstract: Plate heat exchangers are classified on the basis of number of passes on each side and the flow arrangement in each channel, taking into account the end plate effects. This results in four configurations each for the 1-1 (1 Pass-1 Pass), 2-1, 2-2, 3-3, 4-1, 4-2, and 4-4 arrangements, and six configurations for the 3-1 arrangement. These arrangements are analyzed using the Gauss-Seidel iterative finite difference method; the plate arrangement that yields the highest effectiveness in each pass configuration is identified. Comprehensive results are presented in tabular form for the temperature effectiveness P{sub 1} and log-mean temperature difference correction factor F as functions of the number of transfer units NTU{sub 1}, the heat capacity rate ratio R{sub 1}, and the total number of thermal plates. On the basis of these results, specific guidelines are outlined for the selection of appropriate plate heat exchanger configurations.

An Inverse Convection Problem

Abstract: The nature of inverse problems in convective environments is investigated. The illposed quality inherent in inverse problems is verified for free convection laminar flow in a vertical channel. A sequential function specification algorithm is adapted for the semiparabolic system of equations that governs the flow and heat transfer in the channel. The procedure works very well in alleviating the ill-posed symptoms of inverse problems. The performance of a simple smoothing routine is also tested for the prescribed conditions.