Showing papers on "Heat transfer published 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.

Numerical analysis of the Poiseuille and thermal transpiration flows between two parallel plates on the basis of the Boltzmann equation for hard‐sphere molecules

Abstract: The Poiseuille and thermal transpiration flows of a rarefied gas between two parallel plates are investigated on the basis of the linearized Boltzmann equation for hard‐sphere molecules and diffuse reflection boundary condition. The velocity distribution functions of the gas molecules as well as the gas velocity and heat flow profiles and mass fluxes are obtained for the whole range of the Knudsen number with good accuracy by the numerical method recently developed by the authors.

Temperature control apparatus

Abstract: Apparatus is disclosed for controlling the heating and cooling of a plurality of upright containers containing a mixture used for performing gene amplification. The apparatus includes a support rack comprising aluminum blocks which is partially submerged in a thermally conductive fluid such that at least the lower portions of the containers are submerged in the fluid with the upper portions engaging the aluminum blocks for efficient heat transfer. Heaters are disposed within the aluminum block for heating the block and a plurality of thermoelectric cooling cells are used to cool the block. A programmable microprocessor is used for controlling the heating and cooling cycles, thereby allowing repetitive heating and cooling of the mixture to produce the copies of the genetic material sought to be copied. A cam separates the support rack from the cooling cells during the heating portion of the process.

Topology of Convection beneath the Solar Surface

Abstract: It is shown that the topology of convection beneath the solar surface is dominated by effects of stratification. Convection in a strongly stratified medium has: (1) gentle expanding structureless warm upflows and (2) strong converging filamentary cool downdrafts. The horizontal flow topology is cellular, with a hierarchy of cell sizes. The small density scale height in the surface layers forces the formation of the solar granulation, which is a shallow surface phenomenon. Deeper layers support successively larger cells. The downflows of small cells close to the surface merge into filamentary downdrafts of larger cells at greater depths, and this process is likely to continue through most of the convection zone. Radiative cooling at the surface provides the entropy-deficient material which drives the circulation. 13 refs.

Process for the gas-phase polymerization of olefins in a fluidized-bed reactor

Abstract: The present invention relates to a process and apparatus for the gas-phase polymerization of olefins in a fluidized-bed reactor maintained at a temperature T1. A gaseous reaction mixture comprising the olefins to be polymerized passes through the reactor and is recycled to the reactor by means of a recycling line comprising successively a first heat transfer means, a compressor and a second heat transfer means. The present invention consists in introducing a readily volatile liquid hydrocarbon into the inlet of the first heat transfer means or into the recycling line, upstream and in the vicinity of the first heat transfer means. The first heat transfer means cools the gaseous reaction mixture to a temperature T2, below T1, while volatilizing the readily volatile hydrocarbon and without condensing a constituent of the gaseous reaction mixture. The second heat transfer means cools the gaseous reaction mixture to a temperature T3, below T2, for maintaining the fluidized-bed at the desired temperature T1.

Heat transfer to newtonian and non-newtonian fluids in rectangular ducts

Abstract: Publisher Summary This chapter provides an overview of the analytical and experimental hydrodynamics and heat transfer studies of Newtonian and non-Newtonian fluids in laminar and turbulent flow through rectangular tubes. The chapter in particular focuses on the rectangular duct geometry, with emphasis on the friction factor and heat transfer behavior of non-Newtonian fluids. It is recognized that non-Newtonian behavior is generally more complicated than Newtonian flow. In the case of non-Newtonian fluids, the theoretical predictions yield low estimates of the heat transfer under laminar flow conditions. The fact that the available experimental heat transfer measurements lie above the predictions could reflect an inadequacy in the analytical model. For non-Newtonians in turbulent flow through rectangular channels, the situation is even more complicated. Some non-Newtonian fluids act as pseudoplastics, showing some reduction in friction and heat transfer as compared with a Newtonian fluid. Other non-Newtonian fluids experience large reductions in the friction factor and in heat transfer under turbulent flow conditions.

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

A Mathematical Model of the Fluid Mechanics and Gas‐Phase Chemistry in a Rotating Disk Chemical Vapor Deposition Reactor

Abstract: We describe a mathematical model of the coupled fluid mechanics and gas‐phase chemical kinetics in a rotating disk chemical vapor deposition reactor. The analysis is for the flow between an infinite radius, heated nonporous rotating disk and a parallel infinite radius porous surface through which reactive fluid is injected normal to the disk. The analysis extends the usual von Karman transformation to allow specification of the normal velocity at the porous disk, and reduces to a stagnation point flow in the limit of zero rotating rate. The deposition of silicon from silane is used as an example system. A new reaction mechanism and set of rate constants are given for the thermal decomposition of silane. We present an RRKM analysis of several of the unimolecular reactions in the mechanism. Calculated velocity and temperature profiles, chemical species density profiles, and deposition rates as functions of susceptor temperature, spin rate, and inlet flow velocity are presented.

Flow and heat transfer in a viscoelastic fluid over a stretching sheet

Abstract: The paper discusses the flow of an incompressible second-order fluid due to stretching of a plane elastic surface in the approximation of boundary layer theory. An exact analytical solution of the non-linear equation governing the self-similar flow is given. The skin friction decreases with increase in the elastic parameter k1. The analysis of heat transfer in this flow reveals that when the wall and the ambient temperature are held constant, temperature at a point increases with increase in k1, for fixed Prandtl number σ.

Numerical analysis of the shear and thermal creep flows of a rarefied gas over a plane wall on the basis of the linearized Boltzmann equation for hard-sphere molecules

Abstract: Shear flow and thermal creep flow (flow induced by the temperature gradient along the boundary wall) of a rarefied gas over a plane wall are considered on the basis of the linearized Boltzmann equation for hard‐sphere molecules and diffuse reflection boundary condition. These fundamental rarefied gas dynamic problems, typical half‐space boundary‐value problems of the linearized Boltzmann equation, are analyzed numerically by the finite‐difference method developed recently by the authors, and the velocity distribution functions, as well as the macroscopic variables, are obtained with good accuracy. From the results, the shear and thermal creep slip coefficients and their associated Knudsen layers of a slightly rarefied gas flow past a body are derived. The results for the slip coefficients and Knudsen layers are compared with experimental data and various results by the Boltzmann–Krook–Welander (BKW) equation, the modified BKW equation, and a direct simulation method.

Thermal Performance of a Heat Storage Module Using PCM’s With Different Melting Temperatures: Mathematical Modeling

Abstract: In the present study, the performance of a heat storage unit consisting of number of vertical cylindrical capsules filled with phase change materials, with air flowing across them for heat exchange has been analyzed. Earlier theoretical models did not consider temperature distribution in the radial direction within the capsules, an assumption that limits their applications for small diameter capsules. The mathematical model developed in this work is based on solving the heat conduction equation in both melt and solid phases in cylindrical coordinates, taking into account the radial temperature distribution in both phases. Heat flux was then evaluated at the surface of the first row of the capsules to determine the temperature of the air leaving that row by a simple heat balance. It was found that such computation may be carried out for every few rows rather than for a single row to minimize computer time. The simulation study showed a significant improvement in the rate of heat transfer during heat charge and discharge when phase change materials with different melting temperatures were used. Air must flow in the direction of decreasing melting temperature during heat charge, while it must be reversed during heat discharge.

Effects of heat release on the large-scale structure in turbulent mixing layers

Abstract: The effects of chemical heat release on the large-scale structure in a chemically reacting turbulent mixing layer have been studied using three-dimensional time-dependent simulations. Moderate heat release is found to slow the development of the large-scale structures and to shift their wavelengths to larger scales. The results suggest that previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers may be the result of vorticity generation by baroclinic torques.

Heat transfer in rotating passages with smooth walls and radial outward flow

Abstract: Experiments were conducted to determine the effects of rotation on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a smooth wall, large-scale heat transfer model. The objective was to obtain the heat transfer data base required to develop heat transfer correlations and to assess computational fluid dynamic techniques for rotating coolant passages. An analysis of the governing equations showed that four parameters influence the heat transfer in rotating passages (coolant density ratio, Rossby number, Reynolds number, and radius ratio). These four parameters were varied over ranges that exceed the ranges of current open literature results, but that are typical of current and advanced gas turbine engine operating conditions. Rotation affected the heat transfer coefficients differently for different locations in the coolant passage. For example, heat transfer at some locations increased with rotation, but decreased and then increased again at other locations. Heat transfer coefficients varied by as much as a factor of five between the leading and trailing surfaces for the same test condition and streamwise location. Comparisons with previous results are presented.

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.

Effects of heat release in a turbulent, reacting shear layer

Abstract: Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to 940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean temperature rise profiles; normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.

Determination of the Local Quench Curve for Spray-Cooled Metallic Surfaces

Abstract: An experimental study of heat transfer from hot metallic surfaces to water sprays was conducted in the single-phase, nucleate boiling, and transition boiling regimes of the quench curve for surface temperatures below 400° C. Heat transfer measurements were made locally in the spray field using a heater surface area of 0.5 cm2. The hydrodynamic properties of the sprays such as drop diameters, drop velocities, and volumetric spray flux were also measured independently at a position in the spray field identical to that of the heater. The test conditions included variations in volumetric spray flux, mass mean drop diameter, and mean drop velocity of 0.6 × 10-3 to 9.96 × 10-3 m3s-1/m2, 0.434 to 2.005 mm, and 10.6 to 26.5 m/s, respectively. Correlations are presented for water temperatures from 23 to 80° C. These correlations constitute a universal approach to the development of quench curves for industrial sprays commonly employed in materials processing.

Infiltration of fibrous preforms by a pure metal. Part I. Theory

Abstract: General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow.

Single-ion heat of transport in electrolyte solutions: a hydrodynamic theory

Abstract: Le transfert thermique et ionique dans une solution d'electrolyte est etudie en fonction de la viscosite et des proprietes dielectriques des solutions

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.

The thermal catastrophe model of substorms

Abstract: Equations are presented for the physical mechanisms involved in the resonant absorption of Alfven waves in the plasma sheet boundary layer. It is shown that energy absorbed by the plasma-sheet particles is a function of the central plasma sheet temperature. The heating curve, when coupled with convective transport, yielded an equation of state for the steady state plasma sheet, whose solution has the form of a mathematical catastrophe. The master equation includes dynamic terms describing the transition across the thermal catastrophe, making it possible to evaluate the time scale for the catastrophe to occur.