Showing papers on "Heat transfer published in 1976"

Magnetic heat pumping near room temperature

Abstract: It is shown that magnetic heat pumping can be made practical at room temperature by using a ferromagnetic material with a Curie point at or near operating temperature and an appropriate regenerative thermodynamic cycle. Measurements are performed which show that gadolinium is a resonable working material and it is found that the application of a 7-T magnetic field to gadolinium at the Curie point (293 K) causes a heat release of 4 kJ/kg under isothermal conditions or a temperature rise of 14 K under adiabatic conditions. A regeneration technique can be used to lift the load of the lattice and electronic heat capacities off the magnetic system in order to span a reasonable temperature difference and to pump as much entropy per cycle as possible

Molecular Gas Band Radiation

Abstract: Publisher Summary Radiation heat transfer involving molecular gases is the desirable effect in fossil-fuel or waste-fired steam generators for power production or process steam supply and in industrial heat-treating, melting, or enameling furnaces Heat radiation from gases may be utilized as a means of heat transfer and also of information transfer Information on the location, extent, composition, and temperature of a volume of molecular gases can be inferred from thermal radiation measurements The information obtained can be used by process control, fire and air pollution warning and monitoring systems, and by weapon and brush-fire-suppressant targeting or guidance systems This chapter examines how to make a nongray radiation heat transfer analysis for problem situations involving molecular gases with infrared vibration rotation bands when the gaseous mixture is in a state corresponding to thermodynamic equilibrium at the local thermodynamic state variables: pressure, temperature, and composition The special circumstances under which total emissivity and total absorptivity appear as useful quantities are derived, and approximations extending their utility are discussed The chapter examines how the more general and difficult problem of a nonisothermal gas is treated Some particular solutions for the parallel-plate duct are described, both to illustrate nonisothermal gas analysis and to obtain wall-layer transmission factors to extend the utility of engineering models employing an isothermal gas volume concept The chapter provides a brief description of the experimental knowledge of molecular gas behavior and a review of the fundamental ideas of radiation transfer The spectral band nature of gas radiation is briefly described and correlations for working calculations of gas radiation properties are presented

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.

The hydrodynamic stability of rapidly evaporating liquids at reduced pressure

Abstract: The hydrodynamic stability of a pure liquid undergoing steady rapid evaporation at reduced pressure is examined using linear stability analysis. Results show that the rapidly evaporating liquid is unstable to local variations in evaporation rate, local surface depressions being produced by the force exerted on the surface by the rapidly departing vapour and sustained liquid flows being driven by the resultant shear exerted on the liquid surface by the vapour. The coupling of this ‘differential vapour recoil’ mechanism to the Marangoni effect is investigated and the importance of inertial heat transfer, fluid inertia and viscous dissipation at the interface to system stability is resolved.

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.

Studies of finite amplitude non‐Newtonian thermal convection with application to convection in the Earth's mantle

Abstract: Studies of non-Newtonian thermal convection have been made to determine the importance of non-Newtonian viscosity on flow in the earth's mantle. Finite difference solutions have been obtained with a viscosity law representing the sum of deformation rates due to diffusion and dislocation creep. Non-Newtonian flow structures differ only slightly from corresponding Newtonian flows. The principal effect of the non-Newtonian viscosity is to increase the effective Rayleigh number. An effective Rayleigh number based on a strain rate squared averaged viscosity provides a good correlation between Newtonian and non-Newtonian flows over a wide range of Rayleigh numbers.

The relation between heat flow, sediment thickness, and age in the eastern Pacific

Abstract: Heat flow measurements in the eastern Pacific now total over 800, a sufficient number to permit the analysis of their distribution within a wide range of age zones. Major results are that (1) the mid-ocean ridge crestal regions have lower heat flow than that expected from conductive cooling models of the lithospheric plate, (2) an increase in heat flow is observed some distance away from the crest in a zone which appears to delineate a transition from dominantly convective to conductive heat transfer, and (3) the heat flow in crust older than this transition zone closely approximates that predicted by conductive cooling models for oceanic lithospheric plates. The transition from low to high heat flow occurs when 150-200 m of sediment is deposited over basement, apparently preventing convective transfer of heat from the oceanic crust to seawater. Thus the general shape of the empirical heat flow versus age curve for the East Pacific Rise (EPR) agrees with that from the Galapagos Spreading Center, but the age of the transition zone occurs in 5- to 6-m.y.-old crust on the Galapagos Spreading Center, whereas it occurs in 10- to 15-m.y.-old sea floor on the EPR. Proximity to the equatorial high sedimentation regionmore » causes the deposition of a thick layer of sediment much more quickly on the Galapagos Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topographic relief.« less

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.

Mathematical modeling of monolithic catalysts

Abstract: Mathematical models are developed which account for simultaneous heat transfer, mass transfer, and chemical reaction in the oxidation of carbon monoxide over platinum containing monoliths. A two-dimensional model is shown to predict unusual behavior of the Nusselt number in the presence of rapid reaction. However, a simpler one-dimensional model is adequate for predicting monolith behavior.

Heat transfer mechanism for integrated circuit package

Abstract: Heat is removed from silicon devices in an integrated circuit package by means of a thermal liquid material contained in a film mounted on the underside of a cover enclosing the integrated circuit device. The film is electrically non-conductive and the film with the enclosed thermal liquid material form a formable pillow such that after the chip/substrate are assembled, the cover with the film containing the thermal liquid material is placed over the substrate and sealed thereto in a manner such that the film comes into direct contact with the top of the chips mounted on the substrate. This provides a direct heat transfer from the chip through the film to the thermal liquid material out to the cover, which may be formed as a heat radiator.

An evaluation of heat exchange in small birds

Abstract: Equations are derived that quantify the component thermal resistances to heat transfer in small birds and account explicity for the effects of variation in these resistances. Heat transfer theory is used to quantify external resistances, and an experiment was conducted to estimate body resistance (rb, plumage plus tissues) as a function of external temperature and wind speed. The value ofrb decreased with wind speed, and decreased as air temperature approached 0°C. Heat transfer from small birds is shown to be relatively independent of external resistances and mainly dependent onrb. The predictions of the new theoretical equations are shown to agree well with existing empirical data.

Thermoconvective instabilities in a porous medium bounded by two concentric horizontal cylinders

Abstract: The study of natural convection in a saturated porous medium bounded by two concentric, horizontal, isothermal cylinders reveals different types of evolution according to the experimental conditions and the geometrical configuration of the model. At small Rayleigh numbers the state of the system corresponds to a regime of pseudo-conduction. The isotherms are coaxial with the cylinders. At larger Rayleigh numbers a regime of steady two-dimensional convection sets in between the two cylinders. Finally, for Rayleigh numbers above the critical Rayleigh number Ra*c the phenomena become three-dimensional and fluctuating. The appearance of these different regimes depends, moreover, on the geometry considered and, in particular, on two numbers: R, the ratio of the radii of the cylinders, and A, the ratio of the length of the cylinders to the radius of the inner one. In order to approach these experimental observations and to obtain realistic theoretical models, several methods of solving the equations have been used.The perturbation method yields information about the thermal field and the heat transfer between the cylinders under conditions close to the equilibrium state.A numerical two-dimensional model enables us to extend the range of investigation and to represent properly the phenomena when steady convection appreciably modifies the temperature distribution and the velocities within the porous layer.Neither of these models allows account to be taken of the instabilities observed experimentally above a critical Rayleigh number Ra*c. For this reason, a study of stability has been carried out using a Galerkin method based on equations corresponding to an initial state of steady convection. The results obtained show the importance of three-dimensional effects for the onset of fluctuating convection. The critical transition Rayleigh number Ra*c is thus determined in terms of the ratio of the radii R by solving an eigenvalue problem.A numerical three-dimensional model based on the method of finite elements has thus been developed in order to point out the different types of evolution with time. Steady two-dimensional convection and fluctuating three-dimensional convection have been actually found by calculation. The solution of the system of equations by the method of finite elements is briefly described.The experimental and theoretical results are then compared and a general physical interpretation is given.

Nonlinear energy transfer and the energy balance of the internal wave field in the deep ocean

Abstract: The source function describing the energy transfer between the components of the internal wave spectrum due to nonlinear interactions is derived from the Lagrangian of the fluid motion and evaluated numerically for the spectral models of Garrett & Munk (1972a, 1975). The characteristic time scales of the transfer are found to be typically of the order of some days, so that nonlinear interactions will play an important role in the energy balance of the wave field. Thus implications of the nonlinear transfer within the spectrum for generation and dissipation processes are considered.

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.