Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1976"

A Comprehensive Model for Nucleate Pool Boiling Heat Transfer Including Microlayer Evaporation

Abstract: The results of an experimental investigation are presented in which dichloromethane (methylene chloride) boiling on a glass surface was studied using laser interferometry and high-speed photography. New data for active site density, frequency of bubble emission, and bubble departure radius were obtained in conjunction with measurements of the volume of microlayer evaporated from the film underlying the base of each bubble for various combinations of heat flux and subcooling. These results were used to support a model for predicting boiling heat flux incorporating microlayer evaporation, natural convection, and nucleate boiling mechanisms. Microlayer evaporation heat transfer is shown to represent a significant proportion of the total heat transfer for the range of heat flux and subcooling investigated.

Natural Convection in Enclosed Spaces—A Review of Application to Solar Energy Collection

Abstract: A useful solar-thermal converter requires effective control of heat losses from the hot absorber to the cooler surroundings. Based upon the theory and some experimental measurements it is shown that the spacing between the tilted hot solar absorber and successive glass covers should be in the range 4 to 8 cm to assure minimum gap conductance. Poor choice of spacing can significantly affect thermal conversion efficiency, particularly when the efficiency is low or when selective black absorbers are used. Recommended data for gap Nusselt number are presented as a function of the Rayleigh number for the high aspect ratios of interest in solar collector designs. It is also shown that a rectangular cell structure placed over a solar absorber is an effective device to suppress natural convection, if designed with the proper cell spacing d, height to spacing ratio L/d and width to spacing ratio W/d needed to give a cell Rayleigh number less than the critical value.

The Effect of Heater Size, Location, Aspect Ratio, and Boundary Conditions on Two-Dimensional, Laminar, Natural Convection in Rectangular Channels

Abstract: The effect of localized heating in rectangular channels was studied by solving the partial differential equations for the conservation of mass, momentum, and energy numerically using an unsteady state formulation and the alternating-direction-implicit method. The heating element was a long, horizontal, isothermal strip located in one, otherwise-insulated vertical wall. The opposing wall was maintained at a lower uniform temperature and the upper and lower surfaces were insulated or maintained at the lower temperature. Computations were carried out for Pr = 0.7, 0 less than or equal to Ra less than or equal to 10/sup 5/, a complete range of heater widths and locations and a wide range of aspect ratios. Flow visualization studies and comparison with prior computed results for a limiting case confirm the validity of the computed values. The computed rates of heat transfer and circulation provide guidance for locating heaters or coolers.

Melting Heat Transfer in Steady Laminar Flow Over a Flat Plate

Abstract: Exact similarity solutions for melting heat transfer in steady laminar flow over a flat plate were collected from the diffusional mass transfer literature to provide solutions for Prandtl numbers from 0.001 to 100. An exact closed-form solution is introduced for the case of melting in porous media. The results are presented and discussed in such a way as to emphasize the similarity between melting and diffusional mass transfer.

An Experimental Investigation of the Thermally Induced Flow Oscillations in Two-Phase Systems

Abstract: An experimental study on the onset of thermally induced two-phase flow oscillations has been carried out in a uniformly heated boiling channel using Freon-113 as the operating fluid. The effects of inlet subcooling, system pressure, inlet and exit restrictions, and inlet velocity have been studied. The experimental data have been compared with the equilibrium as well as the nonequilibrium theory including the effect of subcooled boiling. It has been found that the effect of thermal nonequilibrium should be included in a theoretical model for accurate prediction of the onset and the frequency of thermally induced flow oscillations. A simplified stability criterion has also been presented and compared with the experimental data.

Onset of Convection in a Porous Medium With Internal Heat Generation

Abstract: The conditions leading to the onset of thermal convection in a horizontal porous layer are determined analytically using the method of linear stability of small disturbances. The lower boundary is treated as a rigid surface and the upper boundary as a free surface. The critical internal and external Rayleigh numbers are determined for both stabilizing and destabilizing boundary temperatures. The predicted critical external Rayleigh number in the limit of no heat generation is in agreement with the critical number predicted for a porous medium heated from below. (16 refs.)

Natural Convection From a Vertical Surface to a Thermally Stratified Fluid

Abstract: Results are presented of a study of natural convection from an isothermal finite plate immersed in a stable thermally stratified fluid. An analytical solution to the problem is obtained by using the local nonsimilarity method. Theoretical local and overall heat transfer coefficients are given for Pr = 0.7 and 6.0. Velocity and temperature profiles are given for Pr = 6.0. The actual experimental configuration was a vertical copper cylinder enclosed in a cube with rigid walls. Heat transfer data are correlated with the measured ambient thermal gradient. Visual studies of the flow field are also discussed. Excellent agreement is achieved between analysis and experiment.

Packed Bed Thermal Storage Models for Solar Air Heating and Cooling Systems

Abstract: In order to simulate solar heating systems where air is the transfer fluid, an adequate model of the packed gravel bed energy store is required. One model of a packed bed thermal store can be obtained by solving the partial differential equations of the Schumann Model as described by Jakob (1957). However, when these equations are solved as part of a long term simulation, the computing costs become unacceptably high. This paper describes the development of a simple model, whose validity is demonstrated by comparing the long term behavior of a system incorporating both the complex and simplified packed bed models. (WDM)

Natural Convection in a Porous Medium: Effects of Confinement, Variable Permeability, and Thermal Boundary Conditions

Abstract: Two-dimensional numerical calculations are reported for natural convection of a fluid in a porous, horizontal layer heated from below. Effects of the following parameters are examined: rigid (impermeable) and constant-pressure (permeable) upper boundaries; isothermal and uniform heat flux at the lower boundary; and permeabilities which are constant, or which vary with depth to simulate compaction of a porous medium or property variations of real fluids within the medium. Steady-state results are presented for the heat flux distribution on the upper surface, as well as for flow and temperature fields in the interior.

Thermal Aspects of Cryosurgery

Abstract: Tissue reactions to cryosurgical procedures depend on temperature variations and on rates of temperature variations induced by freezing probes. In this paper thermal responses of biological systems undergoing freezing are obtained through the application of the finite element method to the solution of the nonlinear bio-equation. This model allows realistic predictions of isotherm fields and of rates of freezing in practically any cryosurgical procedure.

Experimental and Analytical Study of Axial Turbulent Flows in an Interior Subchannel of a Bare Rod Bundle

Abstract: Reactor fuel elements generally consist of rod bundles with the coolant flowing axially through the bundles in the space between the rods. Heat transfer calculations form an important part in the design of such elements, which can only be carried out if information of the velocity field is available. A one-equation statistical model of turbulence is applied to compute the detailed description of velocity field (axial and secondary flows) and the wall shear stress distribution of steady, fully developed turbulent flows with incompressible, temperature-independent fluid, flowing through triangular arrays of rods with different aspect ratios (P/D). Also experimental measurements of the distributions of the axial velocity, turbulence kinetic energy, and Reynolds stresses were performed using a laser Doppler anemometer (LDA), operating in a ''fringe'' mode with forward scattering, in a simulated interior subchannel of a triangular rod array with P/D = 1.123 and L/D/sub H/ = 77. From the experimental results, a new mixing length distribution is proposed. Comparisons between the analytical results and the results of this experiment as well as other experimental data available in the literature are presented. The results are in good agreement.

Numerical Prediction of the Flow Field and Impingement Heat Transfer Caused by a Laminar Slot Jet

Abstract: The effects of uniform suction and nozzle exit velocity profile on the flow and heat-transfer characteristics of a semiconfined laminar impinging slot jet were investigated numerically. The full Navier-Stokes and energy equations were solved using a hybrid or upwind finite-difference representation of the equations cast into their vorticity-stream-function form. The importance of the nozzle exit profile is shown by comparison of the computed heat-transfer distribution with the available experimental data in the laminar range. Application of suction at the impingement surface is shown to enhance the local heat-transfer rates by a constant amount. The nondimensional heat-transfer coefficient and skin friction at the plate are computed as functions of the nozzle Reynolds number, the suction rate, and the nozzle velocity profile. The effect of temperature-dependent physical properties is included in the analysis.

Calculations of Combined Radiation and Convection Heat Transfer in Rod Bundles Under Emergency Cooling Conditions

Abstract: A model has been developed to calculate the heat transfer coefficients from the fuel rods to the steam-droplet mixture typical of Boiling Water Reactors under Emergency Core Cooling System (ECCS) operation conditions during a postulated loss-of-coolant accident. The model includes the heat transfer by convection to the vapor, the radiation from the surfaces to both the water droplets and the vapor, and the effects of droplet evaporation. The combined convection and radiation heat transfer coefficient can be evaluated with respect to the characteristic droplet size. Calculations of the heat transfer coefficient based on the droplet sizes obtained from the existing literature are consistent with those determined empirically from the Full-Length-Emergency-Cooling-Heat-Transfer (FLECHT) program. The present model can also be used to assess the effects of geometrical distortions (or deviations from nominal dimensions) on the heat transfer to the cooling medium in a rod bundle.

Heat and Mass Transfer From Freely Falling Drops

Abstract: When a drop breaks free from a liquid film or feeding orifice and falls through an atmosphere of lower temperature it experiences a transient heat and mass transfer process involving acceleration, the development of hydrodynamic, thermal, and concentration boundary layers in the gas, oscillation of the drop shape, and the development of internal circulation within the drop. This problem, which is of importance in evaporative cooling systems, has been studied experimentally for water drops 3--6 mm in diameter falling through air. Study of a simplified set of governing equations indicates that similitude does not exist in this problem. However, it has been found that for this size range an approximate procedure based on the assumption of negligible internal thermal resistance and an empirical transient correction factor applied to the Ranz- Marshall correlation could describe the data very well.