Showing papers on "Heat transfer published in 1969"

Fundamentals of momentum, heat, and mass transfer

Abstract: Providing a unified treatment of momentum transfer (fluid mechanics), heat transfer and mass transfer. This new edition includes more modern applications of the basic material, and to provide many new homework exercises at the end of each chapter.

Rarefied Gas Dynamics

Abstract: Rarefied gas dynamics is concerned with flows at such low density that the molecular mean free path is not negligible. Under these conditions, the gas no longer behaves as a continuum. Important modifications in aerodynamic and heat transfer characteristics occur which are ascribable to the basic molecular structure of the gas.

Thermal Radiation Properties of Gases

Abstract: Publisher Summary This chapter aims to systematically develop the background information needed to formulate and evaluate thermal radiation properties of gases for engineering applications, and to review the literature of present works and approaches for future research in this area. The scope of the chapter is limited by the assumption that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect. The chapter introduces the general concepts concerning gaseous radiation and presents a review of the physics of atomic and molecular spectra. The radiation resulting from transitions of electronic, atomic, or molecular states has been discussed; they are line radiation, band radiation, and continuum radiation. The evaluation of total (engineering) emissivity and its applications to radiation from homogeneous gas bodies of complex geometry have been discussed. Consideration has been given to the appropriate absorption coefficients for use in the radiative transport calculations.

Hyperbolic Heat-Conduction Equation—A Solution for the Semi-Infinite Body Problem

Abstract: Heat finite propagation velocity effects on temperature distribution and heat flux for step temperature change at semiinfinite body surface

Evaluation of internal heat-transfer coefficients for impingement-cooled turbine airfoils.

Abstract: Although internal impingement cooling of the leading edge of gas-turbine airfoils has been shown to be effective, previously available heat-transfer data are not generally applicable to present-day turbine designs because of the unique geometry requirements. An experimental system which allows the ready acquisition of heat-transfer data necessary for thermal design of turbine airfoils is described. A cold-flow model is developed, and the measurement of local heat-transfer coefficients on a full-size model is accomplished by considering Joulean dissipation in very thin platinum strips bonded to the model. Heattransfer results are given which show the dependence of Nusselt number on Reynolds number, geometry, and chordwise location on the inside leading-edge region of the airfoil. Dimensionless correlations are presented which allow the designer to predict heat transfer for impingement cooling in these geometries for the range of parameters tested.

Deterioration in Heat Transfer to Fluids at Supercritical Pressure and High Heat Fluxes

Abstract: At slightly supercritical pressure and in the neighborhood of the pseudo-critical temperature (defined as the temperature corresponding to the peak in specific heat at the operating pressure), the heat transfer coefficient between fluid and tube wall is strongly dependent on the heat flux. For large heat fluxes, a marked deterioration takes place in the heat transfer coefficient in the region where the bulk fluid temperature is below and the wall temperature above the pseudo-critical temperature. An analysis has been developed, based on the integration of the transport equations, to predict the deterioration in heat transfer at high heat fluxes, and the results have been compared with the previously available experimental results for steam. Experiments have been performed with carbon dioxide for additional comparison. Limits of safe operation in terms of the allowable heat flux for a particular flow rate have been determined both theoretically and experimentally. Experiments with twisted tape inserted in the test section to generate swirl have shown that the heat transfer rates can be improved by this method. Qualitative visual observations have been made of the flow under varying conditions of heat flux and flow rate.

An Experimental Study of Turbulent Natural Convection Boundary Layers

Abstract: An experimental investigation on turbulent natural convection boundary layers has been conducted with water on a vertical plate of constant heat flux. Local heat transfer data are presented for laminar, transition, and turbulent natural convection, with the emphasis on the turbulent regime. The data extend to a modified Rayleigh number of 1016 for a threefold range in Prandtl number. The results indicate that natural transition occurs in the range 1012 < Ra* < 1014 ; i.e., fully developed turbulent flow occurs by Ra* = 104 . This latter value can be as low as 2 × 1013 with the use of a trip rod. The physical structure of the turbulent boundary-layer flow was studied using the combined time-streak marker hydrogen bubble method. Temperature data and temperature corrected velocity data obtained by hot-film sensors are presented for Ra* values between 8.7 × 1013 and 7.1 × 1014 . For the range of variables investigated, the major conclusions are (a) the local heat transfer coefficient exhibits a slight decrease with length, (b) confirmation that the vortex street layer in the transition region decays into a longitudinal-vortex-type structure, and (c) the outer portion of the thermal and velocity fields can be approximated by power profiles that fit almost all the data available to date.

Thermal stratification in lakes: Analytical and laboratory studies

Abstract: Theories are developed for the time dependent vertical temperature distribution in a deep lake during the yearly cycle of solar heating and cooling. A portion of the incoming solar radiation is assumed to be absorbed at the water surface, whereas the remainder is absorbed exponentially beneath the surface. Heat is also conducted downward by molecular diffusion. A heat flux balance at the water surface, which accounts for back radiation and evaporative heat loss, is formulated as a boundary condition. The linear, second-order heat transfer equation is solved by superposition of solutions for the surface absorbed radiation and the internally absorbed radiation. Analytical solutions are given for three different assumptions regarding the time dependence of the incoming radiation and the surface heat losses. At certain times, the resulting temperature and density distribution in the epilimnion are unstable. Under these conditions convective mixing and turbulent diffusion are accounted for by generating a surface mixed layer of uniform temperature. The depth of the surface mixed layer is determined by a thermal energy balance. The theory shows good agreement with field observations of temperature distributions in Lake Tahoe. Experiments are performed using artificial insolation (mercury vapor and infrared lamps) on a laboratory tank. We conclude that it is possible to simulate the development of thermal stratification under laboratory conditions.

Effective thermal conductivity of packed beds

Abstract: Numerical solutions were obtained by the relaxation method to the combined conduction and radiation heat transfer equation in the cubic (e = o.476) and orthorhombic (e=O.395) lattice models of spheres, respectively. The calculated results are shown in chart as ke/kf vs. ks/kf (=10-3104) with parameter of hrDp/ks (-0-1). Effective thermal conductivities were experimentally determined and found in good agreement with the theory. The formula for radiation heat transfer coefficient hr is also presented.

Convection Heat Transfer in Rotating Systems

Abstract: Publisher Summary Because the convective heat transfer phenomena in rotating systems are intimately related to the flow characteristics, they too are quite complex and offer challenges to theoreticians as well as experimenters. Heat transfer by convection to or from bodies of revolution spinning about their axes of symmetry in an otherwise undisturbed fluid has been studied analytically and experimentally by numerous authors. This chapter summarizes results of more recent investigations. The flow and heat transfer characteristics of spinning bodies of revolution in a forced flow field are important for projectiles or re-entry missiles with spin as well as for certain other engineering problems. The fluid-mechanical phenomena of enclosed rotating disks and of parallel corotating disks are distinctly different from those of disks rotating in an infinite environment. In general, the results for convection heat transfer of a disk rotating in an infinite environment can, therefore, not be applied to shrouded or corotating disks. When a quiescent horizontal layer of fluid is heated from below, the fluid at the bottom becomes lighter than the fluid at the top and convection currents are set into motion. The chapter deals with convection in fluids that are set in motion principally by rotating bounding surfaces, but that do not have an independent axial flow. It also deals with heat transfer by convection in systems in which the fluids also have an independent axial motion superimposed on the rotating motion.

Variational Calculation of the Thermal Conductivity of Germanium

Abstract: The thermal conductivity of germanium has been calculated using an anharmonic isotropic continuum model. This model has been extended so as to incorporate umklapp processes by introducing a pseudo-reciprocal-lattice translation vector of magnitude equal to the diameter of the Debye sphere, in such a way that the wave vectors of the three phonons involved are coplanar. Boundary and mass-difference scattering are also included, and the calculations, besides giving good agreement with experimental values of thermal conductivity, provide information on the relative contributions of the different scattering processes to the thermal resistivity and of the different polarization branches to the conductivity. In particular, it is shown that almost all the heat transport is by transverse phonons.

Impingement Cooling of Concave Surfaces With Lines of Circular Air Jets

Abstract: An experimental study of the heat transfer characteristics between single lines of circular jets and concave cylindrical surfaces is presented. It is intended to model a practically important class of impingement cooling configurations for which existing heat transfer correlations are not obviously applicable. The results clarify the present uncertain position with regard to the optimum spacing between the jet nozzle and the heat transfer surface and with regard to the center-to-center spacing between the jets. Some limited results for a two-dimensional jet impinging on the concave surfaces are also presented.

Real Gas Effects in Carbon Dioxide Cycles

Abstract: The potential performance of carbon dioxide as working fluid is recognized to be similar to that of steam, which justifies thorough thermodynamic analysis of possible cycles.The substantially better results achievable with CO2 with respect to other gases are due to the real gas behaviour in the vicinity of the Andrews curve. Simple cycles benefit from the reduced compression work, but their efficiency is compromised by significant losses caused by irreversible heat transfer. Their economy, however, is appreciably better than that of perfect gas cycles. More complex cycle arrangements, six of which are proposed and analyzed in detail, reduce heat transfer losses while maintaining the advantage of low compression work and raise cycle efficiency to values attained only by the best steam practice.Some of the cycles presented were conceived to give a good efficiency at moderate pressure which is of particular value in direct-cycle nuclear applications.The favourable influence on heat transfer coefficients of the combined variation with pressure of mechanical, thermal and transport properties, due to real gas effects, is illustrated. Technical aspects as turbo-machines dimensions and heat transfer surfaces needed for regeneration are also considered.Cooling water requirements are found to be not much more stringent than in steam stations.Copyright © 1969 by ASME

Self-similar piston problems with radiative heat transfer

Abstract: A differential approximation for the equations of radiative transfer in a grey gas is applied in a study of the effects of thermal radiation upon the classical problem of the compressive action of a plane, cylindrical or spherical piston. The ambient gas ahead of the precursor shock wave is supposed cool and the shock wave transparent, whilst the piston is taken to be neither an emittor nor reflector of radiative energy. It is shown that self-similar flow patterns may arise if the ambient density and piston speed are both non-uniform with variations linked to the absorption coefficient which is assumed to be density and temperature dependent. Detailed flow patterns are obtained in the case of general opacity and also in the transparent limit from which it is deduced that under certain conditions the approximation provided by the latter may be rather dubious.

Transport of heat and mass between liquids and spherical particles in an agitated tank

Abstract: Data are reported for heat transfer from water to melting ice spheres and for mass transfer in the case of dissolving spheres of pivalic acid suspended in water agitated in a stirred vessel. The transport coefficients are found to depend on agitator power input but not on agitator design, in agreement with the Kolmogoroff theory. These experimental results are used with others in the literature to develop a correlation involving Nusselt and Prandtl or Schmidt numbers together with a dimensionless group involving agitation power. The correlation is essentially independent of solid-liquid density ratio in the range 0.8 to 1.25, and in this range the gravity group also appears to be unimportant.

Heat transfer in the top millimeter of the ocean

Abstract: Different mechanisms of heat transfer to the ocean surface dominate within different depth regions Under suitable weather conditions, radiation dominates within the upper micron of depth Turbulence is dominant at greater depths, but the evidence indicates that, with wind speeds less than 10 m/sec, it dominates only at depths greater than 05 mm A region in which heat is transferred almost entirely by conduction lies between Radiometric measurements of the total heat flow to the ocean surface may be made in this region

Effects of ultrasonic vibrations on heat transfer to liquids by natural convection and by boiling

Abstract: The effects of ultrasonic vibrations on heat transfer to water and methanol by natural convection and by boiling were measured at three ultrasonic energy levels with frequency ranging from 20.6 to 306 kcycles/sec., using electrically heated platinum wires of diameters 0.007 and 0.010 in. Up to an eight-fold increase in heat transfer coefficient was obtained in natural convection, but the effects diminished with increased temperature difference and became negligible in the well-developed nucleate boiling region. High-speed photographs showed that the increase was due to the motion of cavitation bubbles on the wire surface. The heat transfer results were correlated by local cavitation activity values measured by a technique developed for this work.

Relaxation Model for Heat Conduction in Metals

Abstract: A time‐dependent relaxation model for the heat flux in metals is derived from the quantum mechanical form of the Boltzmann transport equation. In the derivation, the manipulation of the nonlinear integral term of the Boltzmann equation is simplified by assuming that the phonons are in thermal equilibrium at all times and by using the Lorentz approximation to treat free electron‐phonon interactions. The relaxation model for the heat flux is found to yield a damped wave equation for the temperature and as a result, the speed of propagation for heat is shown to be finite instead of infinite as implied by the Fourier model for the heat flux. Approximate expressions for the thermal conductivity and the isothermal electrical conductivity are derived in the appendices in order to estimate the magnitude of the relaxation times and to obtain an expression for the Lorentz number. In general it is found that the thermal and electrical relaxation times are not equal although they are estimated to be at the same order‐of‐magnitude, 10−14 sec for the common monovalent metals. The Lorentz number is found to be a function of the ratio of the relaxation times and as a consequence the difference in the relaxation times may account, at least in part, for the derivation of the experimental Lorentz number from the usual theoretical value based upon equal relaxation times.

The rotating heat pipe - A wickless, hollow shaft for transferring high heat fluxes.

Abstract: Rotating wickless hollow shaft heat pipe utilizing centrifugal acceleration for return pumping of condensate and transferring high heat fluxes

Thermal super insulation

Abstract: Thermal super insulation is formed by utilizing highly heat reflective, thin layers on bases under vacuum conditions to increase radiation reflection and decrease radiation heat transfer through the insulation. Thermal super insulation preferably comprises a plurality of extremely small spheres of a hard, low thermal conductivity material grouped together in point contact with adjacent spheres. The spheres are individually coated with a thin layer of a highly heat reflective, low emissivity material. Interstitial spaces between the spheres are maintained in a vacuum. The spheres are preferably hollow and formed of an inorganic insulating material with a thin metallic heat reflective surface layer. In some cases, fibers rather than spheres are used and are coated with the heat reflective surface layer preferably by a sputtering technique. The thermal super insulation material is used to form a heat barrier by positioning a plurality of spheres or fibers in a compact mass over a surface to be insulated. The spaces formed between the spheres or fibers are then maintained in a vacuum to obtain good heat insulation values when used in extremely high temperature applications.