Showing papers on "Heat transfer published in 1975"

Analysis of Multidimensional Conduction Phase Change Via the Enthalpy Model

Abstract: The basis of the enthalpy model for multidimensional phase change problems in media having a distinct phase change temperature is demonstrated, and subsequent numerical applications of the model are carried out. It is shown that the mathematical representation of the enthalpy model is equivalent to the conventional conservation equations in the solid and liquid regions and at the solid-liquid interface. The model is employed in conjunction with a fully implicit finite-difference scheme to solve for solidification in a convectively cooled square container. The implicit scheme was selected because of its ability to accommodate a wide range of the Stefan number Ste. After its accuracy had been established, the solution method was used to obtain results for the local and surface-integrated heat transfer rates, boundary temperatures, solidified fraction, and interface position, all as functions of time. The results are presented with SteFo (Fo = Fourier number) as a correlating parameter, thereby facilitating their use for all Ste values in the range investigated. At low values of the Biot number, the surface-integrated heat transfer rate was relatively constant during the entire solidification period, which is a desirable characteristic for phase change thermal energy storage.

Analysis of multidimensional conduction phase change via the enthalpy model.

Abstract: The basis of the enthalpy model for multidimensional phase change problems in media having a distinct phase change temperature is demonstrated, and subsequent numerical applications of the model are carried out. It is shown that the mathematical representation of the enthalpy model is equivalent to the conventional conservation equations in the solid and liquid regions and at the solid-liquid interface. The model is employed in conjunction with a fully implicit finite-difference scheme to solve for solidification in a convectively cooled square container. The implicit scheme was selected because of its ability to accommodate a wide range of the Stefan number Ste. After its accuracy had been established, the solution method was used to obtain results for the local and surface-integrated heat transfer rates, boundary temperatures, solidified fraction, and interface position, all as functions of time. The results are presented with SteFo (Fo = Fourier number) as a correlating parameter, thereby facilitating their use for all Ste values in the range investigated. At low values of the Biot number, the surface-integrated heat transfer rate was relatively constant during the entire solidification period, which is a desirable characteristic for phase change thermal energy storage.

Radiation loss in the flash method for thermal diffusivity

Abstract: The boundary radiation heat loss method is investigated for the flash method in measuring thermal diffusivity. Present correction procedures for heat loss are tested experimentally using AXM−5Q (POCO) graphite. In addition, a new method for radiation loss correction is presented based solely on the heating portion of the temperature rise curve. Using the new method, corrections of 35% can be made within a ±3% error. The thermal diffusivity results calculated using the adiabatic expression for various portions of the fractional rise curve and for various heat losses are also presented.

A General Method of Obtaining Approximate Solutions to Laminar and Turbulent Free Convection Problems

Abstract: Publisher Summary The first part of the chapter focuses on determining equations for the local and mean rate of laminar heat transfer, which are approximately valid for different geometries By use of these equations, several new correlations are obtained for various laminar heat transfer problems, and the results compared with experiments The problems considered involve heat transfer (1) from a cylinder, (2) from a sphere, (3) between concentric cylinders, (4) between concentric and eccentric spheres, (5) between vertical plates, and (6) from a nonisothermal vertical plate Attention is then turned to turbulent free convection heat transfer where the heat transfer from inclined plates and between differentially heated plates is considered A method of solving problems involving both laminar and turbulent convection is then outlined The criterion developed for the regions of applicability of the laminar and turbulent equations is shown to accurately predict the experimentally determined onset of instability of the laminar flow for free convection from an isolated plate A recommendation is then made for correlating heat transfer results in a clearer and more convenient way

Thermal resistance of heterostructure lasers

Abstract: A GaAs−GaAlAs heterostructure laser is modeled as a stripe heat source embedded in a layered structure, and an analytic expression is given for the steady−state thermal resistance 〈R〉 of the model. Over the range of typical layer thicknesses and conductivities, and for heat generated uniformly in the active region, 〈R〉 varies between 14 and 31 K/W for a 12×375−μ active region. Four types of heat sinks are shown to contribute an additional 3 to 10 K/W. Design implications are drawn for various properties including layer thicknesses, heat−sink and bond parameters, and radiative heat transfer by spontaneous emission. In disagreement with the common tacit assumption of a unique active−region temperature, it is found that about 40% of the temperature drop within the laser occurs in the active region (center to edge).

Heat and mass transport

Abstract: As mentioned in Chapter 1 the equations governing heat transfer with small temperature differences also govern transfer of small concentrations of other passive scalar contaminants. Particle-laden flows (Chapter 7) are excluded unless the particles are so small that they move with the flow and are negligibly affected by gravity. For convenience the discussion below is phrased in terms of heat transfer but translation to pollutant-transfer problems is straightforward. We use C, c for the mean and fluctuating parts of the concentration or temperature, for generality.

Theory, Measurement, and Application of Thermal Properties of Biomaterials

Abstract: Biomaterials are capable of heat transfer-energy transport by virtue of a temperature gradient. In the case of a living biomaterial, the heat transfer capability of the material can be especially important because the state of life may depend upon the maintenance of a specific temperature. All of the three basic mechanisms of heat transfer, that is, conduction, convection, and radiation, can occur in physical situations involving biomaterials, but of these three, conduction is usually most important in detcrmining the heat transfer within the biomaterial itself. The ability of a biomaterial to transport energy by conduction is best characterized in the steady state by its thermal conductivity, k, and in the nonsteady state by its thermal diffusivity, rx. Because of the complex structure of most biomaterials, there is no adequate microscopic theory, such as that for solids and to a lesser extent for liquids, that allows the direct determination of k and rx of a biomaterial from some fundamental property or properties. For this reason, it is usually necessary to

Theory of heat extraction from fractured hot dry rock

Abstract: A theory of heat extraction from fractured hot dry rock is presented, based on an infinite series of parallel vertical fractures of uniform aperture. Fractures are uniformly spaced and drain heat from blocks of homogeneous and isotropic impermeable rock. Cold water enters at the bottom of each fracture, and solutions are given in terms of dimensionless parameters from which the exiting water temperatures at the top of the fractures can be determined. An example of the application of the theory demonstrates how a multiply fractured system provides a more efficient mechanism for heat extraction than a single fracture in hot dry rock.

Upward turbulent fire spread and burning of fuel surface

Abstract: Two-dimensional upward flame spread and subsequent steady turbulent burning of a thermally thick vertical fuel surface is examined theoretically and experimentally. The upward spread rate for vertical PMM slabs is observed to increase exponentially with time. This result is predicted in terms of measured fuel thermophysical properties, flame heights and heat feedback to the fuel surface. The local steady burning rates established after completion of upward spread exhibit a minimum at a height of 18 cm from the bottom edge and increase continuously beyond this height, becoming 70% larger at a height of 140 cm. This increase is shown to be entirely attributable to increasing flame radiation. Individual measurements of the various energy transfer components during steady burning of the PMM slabs are obtained from radiant intensity measurements of (1) the surface alone and (2) flame plus surface. Above 76 cm flame radiation ranges from 75 to 80% of the total (radiation plus convection) heat transfer from the flames to the fuel surface. Surface heat transfer by convection decreases slightly with height.

Heat Transfer in Semitransparent Solids

Abstract: Publisher Summary The chapter presents an overview of the background information needed to formulate and analyze heat transfer in semitransparent materials systematically and then reviews the literature in some specific problem areas Primary emphasis is placed on semitransparent solids, although the principles presented are general and apply to any phase The chapter also discusses the wide variety of applications and involves nature of heat transfer phenomena in semitransparent condensed phases The radiation characteristics of semitransparent materials are not only dependent on surface but also volume phenomenon since some of the emitted radiation originates at considerable depths In short, the radiation characteristics depend on the spectral absorption coefficient and index of refraction, thickness, boundary conditions, and temperature distribution The transient heating of semitransparent materials under idealized conditions in which the emission of radiation from the material is neglected is also discussed This implies that the body is cold and the rate of emission of radiation per unit volume is negligible compared to absorption This simplification limits the applicability of analysis and results to those early stages of heating during which temperatures have not raised high enough to render the assumption invalid The chapter also presents an analysis to show the influence of physical parameters on the temperature field in a semitransparent solid irradiated from a high temperature source such as the sun The results intend to aid the designer of solar collectors in selecting the optimum material by indicating physical parameters, which determine maximum efficiency of solar energy conversion in different semitransparent solids

Detonation of fuel coolant explosions

Abstract: IN certain circumstances the mixing of a hot liquid and a cooler, vaporisable one leads to an explosive rate of vapour production. Such explosions have occured in foundries when molten metals and water mix1, and when liquid natural gas is spilt on to water2; they may also occur in liquid-cooled nuclear reactors under accident conditions. It seems likely3,4 that in these events an initial disturbance causes motions which fragment some of the material and so allow rapid heat transfer; this produces explosive expansion and further fragmentation, and so the reaction propagates through the medium. We give here a simple one-dimensional model of a system in which the two liquids are initially coarsely mixed, and show that there is the possibility of an extremely violent thermal explosion, the structure of which is analogous to that of a detonating chemical explosion.

Cavitation dynamics: II. Free pulsations and models for cavitation bubbles

Abstract: A mathematical formalism developed for investigating the dynamics of cavitation bubbles has been used to obtain numerical solutions describing the behavior of bubbles of different initial radii that are damped by heat conduction, viscosity, and compressibility. Calculations have been made to determine two measures of damping, the maximum temperatures, the maximum pressures, and the resonance frequencies of bubbles set into pulsations by a pressure pulse. In general, these quantities are controlled by heat conduction and viscosity at small amplitudes and mainly by compressibility at large amplitudes of motion. One measure of damping—the energy dissipation modulus—has a peak at a well‐defined maximum radius. This peak serves to define a transition radius; for pulsations with amplitudes greater than this transition radius, the fraction of available energy dissipated in a cycle decreases and hence very large internal energy densities may occur. A second transition occurs at a radius called the critical radius...

Circulation in Fractures, Hot Springs, and Convective Heat Transport on Mid-ocean Ridge Crests

Abstract: Summary Hot springs in continental geothermal areas, thermal springs which are not associated with known high-temperature areas, and hot springs associated with hydrothermal circulation on ocean ridge crests are described by models based on fluid flow in fractures. A steady state model shows that fractures of the order of a few millimetres wide can carry a substantial convective flow and that the convective flow depends upon the third power of the fracture width. The steady state model also furnishes estimates of conductive temperature losses in springs and gives estimates of the depths of circulation for thermal springs in the southeastern United States which are in good agreement with available field data. In many cases the temperature and flow rate of springs is non-stationary. This is particularly true of hot springs in high temperature geothermal areas and it is expected to be true of springs on mid-ocean ridge crests. Time-dependent models for springs show that the main effect of the circulation is to lower the regional geothermal gradient. Non-stationary convection controlled by fractures can explain the variability of heat flow data obtained near ocean ridge crests. A numerical example shows that convection in a block 3 km wide, containing fractures 3 mm wide and 5 km deep, and circulating for 104 years gives rise to a hot spring with a temperature of 125 °C and a flow rate of 0.14 kg m−1 s−1. Such a spring discharging at the sea floor would give rise to an unmeasurably small temperature anomaly in the sea water. The convective heat transfer due to such a circulation system is roughly 200 times greater than the heat transfer that would have been achieved by conduction alone.

Heat or thermal energy storage structure

Abstract: A heat or thermal energy storage structure comprising a crosslinked polymeric resinous matrix having a plurality of substantially unconnected small closed cavities and a heat sink material encapsulated within the cavities. The structure is characterized in that the heat sink material forms an essentially stable dispersion in the uncured polymeric resinous matrix when mixed therewith before the matrix is crosslinked. The structure can beneficially be used in conjunction with low level heat or thermal energy collector means and heat transfer means to provide a space heating or cooling apparatus. The storage structure may also be effectively used to provide an additional function of a building component in a building construction such as a wall structure.

Heat Transfer at Low Temperatures

Abstract: 1: Introduction- I Conduction and Convection Heat Transfer- 2: Conductive Heat Transfer- 3: Convective Heat Transfer to Low-Temperature Fluids- II Two-Phase Phenomena- 4: Terminology and Physical Description of Two-Phase Flow- 5: Nucleate Pool Boiling- 6: Critical Heat Flux- 7: Film Boiling- 8: Minimum Film Boiling Heat Flux- 9: Vapor-Liquid Condensation on Cryogenic Surfaces- 10: Vapor-Solid Condensation- 11: Pressure Drop and Compressible Flow of Cryogenic Liquid-Vapor Mixtures- 12: Forced Convection Heat Transfer with Two-Phase Flow- 13: Transient Conditions in Boiling and Two-Phase Discharge- III Radiation and Helium II Heat Transport- 14: Radiative Properties- 15: Heat Transport in Liquid Helium II

Heat transfer pipe

Abstract: A heat transfer pipe for use in a heat exchanger such as air conditioner, freezer and boiler, wherein grooves are formed in the inner wall surface of the pipe, which are by far finer in size than the grooves that have been provided for the purpose of increasing the heat transfer area in general, and slanting relative to the axis of pipe, to thereby improve the heat transfer rate without increasing the pressure loss caused to the fluid flowing through the pipe.

Direct energy conversion device

Abstract: A method and apparatus are described for directly converting heat to electricity within an array of thermoelements and heat pipes. The thermoelectric generator contains two conduits for two fluid streams, and a plurality of thermal bridges connecting points in both fluid streams, placing them in an overall counterflow heat exchange relationship. Each thermal bridge comprises at least one heat exchange surface with the first fluid stream, at least one heat exchange surface with the second fluid stream and at least one heat pipe, delivering heat to and/or from the thermoelectric element. The rapid heat delivery capability of the heat pipes, in combination with the counterflow heat exchange relationship between the two fluid streams within the generator, are responsible for the simultaneous considerable improvement of efficiency and power density of the generator.

Temperature and Concentration Dependence of Mutual Diffusion Coefficients of Some Binary Liquid Systems

Abstract: Mutual diffusion coefficients are measured for ethanol-water, acetone-water, and acetone-chloroform systems by means of a three-compartment cell. In each case, the results cover the complete concentration range and temperatures from 25°C to around the normal boiling point. The diffusion coefficients ai the terminal concentrations and those at the normal boiling point are obtained by extrapolation of the experimental values. The uncertainty of the experimental diffusion coefficients is estimated to be f2.6%. These results are compared with the results computed from various prediction correlations. Most mass transfer and heat transfer calculations in chemical engineering employ mutual diffusion coefficients at the ambient and at the higher temperatures. These data are scarce because of experimental difficulties of measurement, particularly at the higher temperatures.

Multihole Cooling Film Effectiveness and Heat Transfer

Abstract: Adiabatic film effectiveness and heat transfer measurements with injection of secondary air through arrays of holes in a flat plate into a turbulent boundary layer are presented. Measurements were taken both within and downstream of the multihole pattern for different hole pitch-to-diameter ratios and blowing rates. The hole configurations considered were regular patterns of staggered holes. The holes were angled 30 deg to the plate’s surface and 45 deg to the mainstream. A comparison between the measured adiabatic film effectiveness and a model based on the superposition of point heat sinks is made.