Showing papers on "Heat transfer published in 1993"

Thermoelasticity without energy dissipation

Abstract: This paper deals with thermoelastic material behavior without energy dissipation; it deals with both nonlinear and linear theories, although emphasis is placed on the latter. In particular, the linearized theory of thermoelasticity discussed possesses the following properties: (a) the heat flow, in contrast to that in classical thermoelasticity characterized by the Fourier law, does not involve energy dissipation; (b) a constitutive equation for an entropy flux vector is determined by the same potential function which also determines the stress; and (c) it permits the transmission of heat as thermal waves at finite speed. Also, a general uniqueness theorem is proved which is appropriate for linear thermoelasticity without energy dissipation.

A water and heat management model for proton-exchange-membrane fuel cells

Abstract: Proper water and heat management are essential for obtaining high-power-density performance at high energy efficiency for proton-exchange-membrane fuel cells. A water and heat management model was developed and used to investigate the effectiveness of various humidification designs. The model accounts for water transport across the membrane by electro-osmosis and diffusion, heat transfer from the solid phase to the gas phase and latent heat associated with water evaporation and condensation in the flow channels. Results from the model showed that at high current (> 1A/cm[sup 2]) ohmic loss in the membrane accounts for a large fraction of the voltage loss in the cell and back diffusion of water from the cathode side of the membrane is insufficient to keep the membrane hydrated (i.e., conductive). Consequently, to minimize this ohmic loss the anode stream must be humidified, and when air is used instead of pure oxygen the cathode stream must also be humidified.

MFIX documentation theory guide

Abstract: This report describes the MFIX (Multiphase Flow with Interphase exchanges) computer model. MFIX is a general-purpose hydrodynamic model that describes chemical reactions and heat transfer in dense or dilute fluid-solids flows, flows typically occurring in energy conversion and chemical processing reactors. MFIX calculations give detailed information on pressure, temperature, composition, and velocity distributions in the reactors. With such information, the engineer can visualize the conditions in the reactor, conduct parametric studies and what-if experiments, and, thereby, assist in the design process. The MFIX model, developed at the Morgantown Energy Technology Center (METC), has the following capabilities: mass and momentum balance equations for gas and multiple solids phases; a gas phase and two solids phase energy equations; an arbitrary number of species balance equations for each of the phases; granular stress equations based on kinetic theory and frictional flow theory; a user-defined chemistry subroutine; three-dimensional Cartesian or cylindrical coordinate systems; nonuniform mesh size; impermeable and semi-permeable internal surfaces; user-friendly input data file; multiple, single-precision, binary, direct-access, output files that minimize disk storage and accelerate data retrieval; and extensive error reporting. This report, which is Volume 1 of the code documentation, describes the hydrodynamic theory used in the model: the conservation equations,more » constitutive relations, and the initial and boundary conditions. The literature on the hydrodynamic theory is briefly surveyed, and the bases for the different parts of the model are highlighted.« less

Heat transfer mechanisms during short-pulse laser heating of metals

Abstract: This work studies heat transfer mechanisms during ultrafast laser heating of metals from a microscopic point of view. The heating process is composed of three processes: the deposition of radiation energy on electrons, the transport of energy by electrons, and the heating of the material lattice through electron-lattice interactions. The Boltzmann transport equation is used to model the transport of electrons and electron lattice interactions. The scattering term of the Boltzmann equation is evaluated from quantum mechanical considerations, which shows the different contributions of the elastic and inelastic electron-lattice scattering processes on energy transport. By solving the Boltzmann equation, a hyperbolic two-step radiation heating model is rigorously established. It reveals the hyperbolic nature of energy flux carried by electrons and the nonequilibrium between electrons and the lattice during fast heating processes. Predictions from the current model agree with available experimental data during subpicosecond laser heating. 20 refs., 7 figs., 2 tabs.

Water and Thermal Management in Solid‐Polymer‐Electrolyte Fuel Cells

Abstract: A mathematical model of transport in a solid‐polymer‐electrolyte fuel cell is presented. A two‐dimensional membrane‐electrode assembly is considered. Water management, thermal management, and utilization of fuel are examined in detail. Because the equilibrium sorption of water between the gas phase and the polymer‐electrolyte depends strongly on temperature, water and thermal management are interrelated. The rate of heat removal is shown to be a critical parameter in the operation of these fuel cells.

Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part II—Transition and Turbulent Flows

Abstract: Thermal-hydraulic design correlations are developed to predict isothermal f and Nu for in-tube, turbulent flows with twisted-tape inserts. Experimental data taken for water and ethylene glycol are analyzed, and various mechanisms attributed to twisted tapes are identified. Tube blockage and tape-induced vortex mixing are the dominant phenomena that result in increased heat transfer and pressure drop; for loose- to snug-fitting tapes, the fin effects are insignificant. The limiting case of a straight tape insert correlates with the hydraulic-diameter-based smooth tube equation

Transient ballistic and diffusive phonon heat transport in thin films

Abstract: Ballistic and diffusive phonon transport under small time and spatial scales are important in fast‐switching electronic devices and pulsed‐laser processing of materials. The Fourier law represents only diffusive transport and yields an infinite speed for heat waves. Although the hyperbolic heat equation involves a finite heat wave speed, it cannot model ballistic phonon transport in short spatial scales, which under steady state follows the Casimir limit of phonon radiation. An equation of phonon radiative transfer (EPRT) is developed which shows the correct limiting behavior for both purely ballistic and diffusive transport. The solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation even for predominantly diffusive transport. For sudden temperature rise at one film boundary, the results show that the Fourier law and the hyperbolic heat equat...

One- and Two-Equation Models for Transient Diffusion Processes in Two-Phase Systems

Abstract: Publisher Summary The chapter examines the process of transient heat conduction for a two-phase system in terms of the method of volume averaging. Both one- and two-equation models are developed along with the relevant closure problems that allow determining theoretically the effective transport coefficients. Closure calculations are often carried out using symmetric unit cells. The chapter describes the numerical method used to solve the closure problems and presents selected results for stratified systems and uniform arrays of cylinders. Analytical results are given for stratified systems and a finite-volume numerical method is used for the nodular systems. Numerical results are provided for a two-dimensional array of cylinders embedded in a continuous phase. The chapter attempts to compare results from the two-equation model for transient heat conduction in a two-phase system with results from the one-equation model. Both models are exact in the sense that the effective transport coefficients are determined theoretically and this allows to draw some definite conclusions concerning the assumption of local thermal equilibrium.

Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part I—Laminar Flows

Abstract: Laminar flow correlations for f and Nu m are developed based on experimental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is considered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devise flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow flockage, longer flow path, and secondary fluid circulation; fin effects are found to be negligible is snug-to loose-fitting tapes

Physics of electrophotography

Abstract: Electrophotography is one means of arranging 100 million pigmented plastic particles on a sheet of paper to faithfully replicate an original. It is based on many diverse phenomena and employs many properties of matter. These include gaseous ionization in the charging step; photogeneration and charge transport through disordered solid-state materials in the latent-image-formation step; triboelectricity in the particle-charging step; mechanical, electrostatic, and magnetic forces to detach particles in the development and transfer steps; and the application and transfer of heat in the fixing step. In addition, it relies on a precise balance of thermorheological, chemical, and mechanical properties of large area films and small particles. This article reviews the physics of the latent-image formation and development steps.

Annual review of heat transfer

Abstract: The volumes of Annual Review of Heat Transfer published up to 2005 were edited by Professor Chang-Lin Tien. Chang-Lin had a long-lasting impact on the heat transfer community through his pioneering research. The current editors decided to use Volume XIV as a bridge between the past and the future by summarizing Chang-Lin's contributions and reviewing current and future research directions in areas in which Professor Tien made a significant impact. In this volume, his contributions are divided into six topical areas: radiation and combustion, micro/nanoscale heat transfer, phase change and heat pipes, porous media, materials processing and laser materials interactions, and energy systems. Previous volumes of Annual Review in Heat Transfer all aspects of heat transfer and fluid flow are examined by an array of the top international specialists in the field. Future volumes are being planned to include contemporary achievements in the thermal and fluids sciences.

Boiling Heat Transfer in a Horizontal Small-Diameter Tube

Abstract: Results of a study on boiling heat transfer of refrigerant R-113 in a small-diameter (2.92 mm) tube are reported. Local heat transfer coefficients are measured for a range of heat flux (8.8-90.75 kW/m[sup 2]), mass flux (50-300 kg/m[sup 2]s), and equilibrium mass quality (0-0.9). The measured coefficients are used to evaluate 10 different heat transfer correlations, some of which have been developed specifically for refrigerants. High heat fluxes and low mass fluxes are inherent in small channels, and this combination results in high boiling numbers. In addition, based on a flow pattern map developed from adiabatic experiments with air-water mixtures, it has been shown that small-diameter channels produce a slug flow pattern over a large range of parameters when compared with larger-diameter channels. The effects of high boiling number and slug flow pattern lead to domination by a nucleation mechanism. As a result, the two-phase correlations that predicted this dominance also predicted the data the best when they properly modeled the physical parameters. The correlation of Lazarek and Black (1982) predicted the data very well. It is also shown that a simple form, suggested by Stephan and Abdelsalam (1980) for nucleate pool boiling, correlates the data equally well; both correlationsmore » are within a mean deviation of less than 13 percent. Results are applicable to boiling in compact heat exchangers. 28 refs., 11 figs., 3 tabs.« less

Correlating Equations for Impingement Cooling of Small Heat Sources With Multiple Circular Liquid Jets

Abstract: Experimental data have been obtained for liquid jet impingement cooling of small square heat sources resembling electronic integrated circuit chips. Both free-surface and submerged jet configurations have been studied for a range of velocities, nozzle diameters, and nozzle-to-heater separation distances, with water and a fluorocarbon liquid (3M FC-77) as coolants. Major trends in the data have been explained in terms of the underlying hydrodynamic and thermal phenomena. The data, obtained over parameter ranges applicable to the cooling of microelectronic chips, have been compared with the predictions of previously developed correlations for jet impingement heat transfer and substantial discrepancies between the data and the predictions have been noted. Based on the present data, two new correlating equations, one for free-surface and the other for submerged jet impingement, have been developed and presented.

Analysis of Convection and Secondary Reaction Effects Within Porous Solid Fuels Undergoing Pyrolysis

Abstract: A mathematical model of transport phenomena (heat, momentum and mass transfer) and chemical processes (primary and secondary reactions) of the thermal degradation of wood is presented. Implicit finite difference equations for energy, momentum and chemical species mass balances are formulated according to an operator-splitting technique and are numerically solved. The progress of the pyrolysis process along a wooden slab, radiatively heated on one side, is characterized by the following main processes: 1( a virgin wood region, crossed by a slow flow of pyrolysis products, where temperature and pressure values decrease as the non-irradiated boundary is approached; 2( a primary pyrolysis region where, due to the relatively low temperatures, secondary reactions are not active and 3( a char layer where volatile products of primary pyrolysis mainly flow and, temperature being higher, undergo secondary reactions. For low medium permeabilities, a peak in the gas overpressure is observed, separating the v...

On the molecular mechanism of thermal diffusion in liquids

Abstract: A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy. Simulations of isotope mixtures covering a range of densities and mass ratios show that the lighter component prefers the hot side of the system at stationary state. This implies a positive thermal diffusion factor in the definition we have adopted here. The molecular basis for the Soret effect is studied by analysing the energy flux through the system. In all cases we found that there is a difference in the relative contributions when we compare the hot and cold sides of the system. The contribution from the lighter component is predominantly flux of kinetic energy, and this contribution increases from the cold to the hot side. The contribution from the heavier component is predominantly energy transfer th...

Heat Transfer Phenomena in Lithium/Polymer‐Electrolyte Batteries for Electric Vehicle Application

Abstract: Mathematical modeling of heat generation and transport in lithium/polymer‐electrolyte batteries for electric vehicle applications has been conducted The results demonstrate that thermal management may not be a serious problem for batteries under low discharge rates However, under high discharge rates, the temperature of a battery may increase remarkably if the thickness of a cell stack exceeds a certain value Also, due to the low thermal conductivity of the polymer, the improvement of cooling conditions is not an effective means of improving heat removal for large‐stack systems For a required operational temperature range and a given discharge rate, model predictions can be used to design appropriate battery structures and to choose a suitable cooling scheme