Showing papers on "Transport phenomena published in 1985"

Introduction to Heat Transfer

Abstract: This book, designed for advanced undergraduate engineering majors, explains the physical concepts and methodologies of heat and mass transfer. It uses a systematic method for problem solving and discusses the relationship of heat and mass transfer to many important practical applications through examples and problems. A and significant contribution is the extensive use of the First Law of thermodynamics.

Fluid Mechanics and Transfer Processes

Abstract: This textbook deals with the fundamental principles of fluid dynamics, heat and mass transfer. The basic equations governing the convective transfer by fluid motion of matter, energy and momentum, and the transfer of the same properties by diffusion of molecular motion, are presented at the outset. These concepts are then applied systematically to the study of fluid dynamics in an engineering context and to the parallel investigation of heat and mass transfer processes. The influence of viscosity and the dominant role of turbulence in fluid motion are emphasised. Individual chapters are concerned with the important subjects of boundary layers, flow in pipes and ducts, gas dynamics, and flow in turbo-machinery and of a liquid with a free surface. Later chapters cover some of the special types of flow and transfer process encountered in chemical engineering applications, including two-phase flow, condensation, evaporation, flow in packed beds and fluidized solids.

Numerical Analysis of the Transport Phenomena in MOCVD Process

Abstract: The transport properties of the convective fluid motion in a MOCVD reactor are investigated using a numerical method. For the purpose of a uniform deposition rate on a substrate with a diameter of 3 inches, a combinatorial optimization was carried out concerning the pressure, the mass-flow rate and the reactor's geometry. Under the optimal condition, a deposition uniformity within ±0.6% is attainable. The results also reveal a variety of flow configurations, depending on the relative magnitudes of the forced and natural convections. When the volumetric velocity of the supplied gas decreases, a recirculating vortex appears on the upstream side of the deposition surface. The numerical results suggest that the appearance of the vortex is governed by Fr/Re when Re Rec.

Detailed computation of unsteady droplet dynamics

Abstract: In this study, a detailed time-dependent calculation of the lifetime of a fuel droplet is presented. The calculation includes fluid flow, heat transfer, and mass transfer in both the gas and liquid phases as the droplet radius changes with time. For a starting gas phase Reynolds number of 100, the transport processes are initially dominated by convection and then become conduction and diffusion controlled during the latter part of the droplet lifetime. A comparison between the calculated local drag and heat transfer and the estimates which would be obtained using availabe correlations shows that there is a very large deviation and, that the correlations will lead to incorrect results.

Solution of Premixed and Counterflow Diffusion Flame Problems by Adaptive Boundary Value Methods

Abstract: Combustion models that simulate pollutant formation and study chemically controlled extinction limits in flames often combine detailed chemical kinetics with complicated transport phenomena. Two of the simplest models in which these processes are studied are the premixed laminar flame and the counterflow diffusion flame. In both cases the flow is essentially one-dimensional and the governing equations can be reduced to a set of coupled nonlinear two-point boundary value problems with separated boundary conditions.

Analogy between mechanics and electricity

Abstract: The dissipative transport of energy is described in the momentum current picture. This picture provides a local-causes approach to mechanics whereby forces are considered as momentum currents. In this approach, friction, i.e. mechanical heat production, appears when a momentum current flows between two bodies of different velocities. The treatment of the transport and dissipation of energy follows the same rules in mechanics as in electricity. An 'Ohm's Law of momentum currents' is introduced in analogy to Ohm's Law in electricity. Newton's Third Law reduces to a simple statement about momentum conservation.

Irreversible thermodynamics of rarefied gases

Abstract: For transport phenomena in gases the Onsager scheme can be extended so as to include the quantities occurring in a complete set of linearized Burnett equations. To show this, the linearized Boltzmann equation is solved in the Chapman-Enskog manner, allowing for ∇∇T and ∇∇ν terms among the thermodynamic forces. Microscopic and macroscopic Burnett fluxes are introduced, and the phenomenological equations and formal expressions for the transport coefficients derived. The matrix of these coefficients is symmetric in the Onsager way, in agreement with an expression one can derive for the entropy density source. The scheme simplifies for the bulk part of plane flows between parallel plates, with their separation assumed to be greater than a few mean free paths. Boundary layers must be treated separately. The corresponding expression for the entropy source suggests another set of phenomenological equations, involving slip coefficients. The symmetry relations among the coefficients that were earlier obtained indirectly are thereby explained in Onsager's sense.

Effect of defect structure on the electrical conduction mechanism in metallic thin films

Abstract: Thin metal films can be deposited in a number of different ways. As a result several types of defects or impurities are frozen in the film. In most practical cases films exhibit grain boundaries which play a decisive role in transport properties. This paper reviews the advances that have been made during the last five years in the field of theoretical description of electronic scattering at grain boundaries. Analytical expressions for the transport parameters (such as resistivity, temperature coefficient of resistivity and thermopower) of columnar, monocrystalline and polycrystalline films are derived. Care has been taken to give linearized equations for the transport phenomena. Methods for extracting grain parameters are outlined. Special attention is focused on correlated size effects. Imperfection or impurity effects on the film resistivity and thermopower are considered. Methods for determining the energetic parametersU andV and the componentS 1 of the thermopower associated with imperfections are proposed. Special emphasis is placed on procedures for identifying imperfections by simultaneous study of the restructuration processes induced by thermal ageing and of the changes in transport parameters on ageing.

Further development of a general-purpose, packed-bed model for analysis of underground coal gasification processes

Abstract: Further development of a theoretical modeling analysis for characterizing reacting flows through packed beds is presented. These flows are related to the underground coal gasification conditions in terms of combustion and multi-component chemical reactions taking place inside charring coal beds. Time-dependent, two-dimensional (including axisymmetrical) partial differential equations (PDE's) describing conservation of mass, species, momentum, and the thermal energy are formulated. These PDE's are then recast into a set of ordinary differential equations (ODE's) with time the independent variable. The resulting ODE's are solved by applying a method-of-lines (MOL) technique. The present formulation considers: the transport phenomena at the wall; various transient flow cases; reactions of gas and solid species; a wide range of options on the boundary conditions; temperature-dependent physical parameters; and rezoning capabilities. A numerical code called GSF has been developed, and computer runs have been performed to verify various aspects of the physical models as well as the numerical approach taken in the present analysis. These include favorable agreements with available analytical solutions for simple, one-dimensional flows and two-dimensional non-isotropic heat transfer to a wall. For more complicated flow situations for which there are no analytical solutions, good agreements between the results of the present method and thosemore » of alternative numerical methods have also been obtained. 27 refs., 13 figs.« less

Localization, Interaction, and Transport Phenomena in Impure Metals. An Introduction

Abstract: Transport properties of solids are determined by disorder due to the presence of impurities or imperfections of the lattice, by various interaction effects such as electron-electron, electron-phonon interaction and by spin-dependent processes due to spin-orbit and spin-spin coupling. At room temperature the details of these interactions are unimportant. They are usually incorporated into one of several mean free paths. As the temperature is lowered these effects become however more and more important. Thus the study of the transport properties of solids is a typical field of low-temperature physics.

Modeling of the transport phenomena in the anode region of high current arcs

Abstract: The coupled transport phenomena in the anode region of wall-stabilized high-current D.C. arcs are described by a set of differential conservation equations. These equations are solved numerically using experimentally derived conditions. Solutions are obtained for atmospheric pressure nitrogen and argon arcs operated at different arc currents. Considering the anode mechanism of wall-stabilized arcs without the influence of a superimposed plasma flow from the cathode, the results indicate that nitrogen arcs tend to form constricted anode arc roots in contrast to argon arcs.

The Hilbert theory of transport phenomena in multiple gas mixtures

Abstract: The Hilbert solution of the Boltzmann equation for a multiple gas mixture is presented. It is shown that the first approximation of the Hilbert theory yields the Euler equations for ideal flow. The Hilbert field equations of the second approximation are derived and the transport processes that appear in this approximation are discussed in a manner similar to that given in Chapman and Cowling. The Hilbert formulas for the transport coefficients of a multiple gas mixture are obtained and are seen to be precisely the same formulas of Chapman and Cowling obtained by the Chapman–Enskog method.

Improved modelling technique of electric glass melting

Abstract: Physical, mathematical and some other types of new modelling techniques for modelling phenomena occurring in convective heat transfer and electrical parameters of an electric melting furnace will be shown. The optimal connections of electrodes with the multiphase sources of power, from the point of view of minimum energy consumption, will be described. When modelling the electric melting process we simulate the flow of the glass melt occurring in a continuous melting furnace caused by the withdrawal of molten glass, or due to the convection currents involved as a result of heat transfer. The first type of flow, the primary working or “pull” flow can be regarded as free flow in an open channel under the influence of gravity. The other type of flow, the convection currents, can be traced back to the temperature differences existing in the furnace. When the furnace is in operation the convection currents combine with the primary flow - “the pull” - of the glass. Another group of parameters which has to be modelled is connected with the electrical energy which is introduced into the furnace by means of electrodes. Using a physical model it is possible to model the transport phenomena in the furnace i.e. the hydrodynamic flow and heat transfer, as well as the electrical parameters.

Turbulent heat and mass transfer

Abstract: Turbulent flows and the associated transport phenomena are discussed. Turbulent heat and mass transfer are shown to be connected inseparably with turbulent momentum transfer, with additional complications due to fluctuations of the temperature and concentration. The turbulent velocity fluctuations are the simplest to measure reliably, and many attempts have been made to develop analogies between momentum, heat and mass transport in turbulent flows, some of which are discussed. More complex (second-order) approaches to the modeling of turbulent heat and mass transfer are also introduced and illustrated for turbulent heat transfer in pipe flows.

Analogy between turbulent momentum and heat transfer under complex conditions

Abstract: The analogy between turbulent momentum and heat transfer under complex conditions, i.e., under the action of several perturbing factors on the flow, is extended for a broad range of variation of the Prandtl number.

Guidelines for Photoreactor Research

Abstract: Research on photochemical reaction engineering is still in its infancy even though very useful work on this subject began well over twenty years ago. Successful applications of chemical engineering principles have been achieved in the analysis and modelling of homogeneous photoreactors of simple geometry with ideal flow characteristics. The central problem in photoreactor modelling is focused on the analysis of light absorption/distribution in a reacting medium. The model equations and boundary conditions are dependent on the reactor geometry and the spatial relationship between the irradiating source and the reactor. The light absorption model equation is formulated by writing the constitutive equation for radiative transfer. In general, the equation is of integral-differential form, if no simplifications are applied to the model. Chemical engineers are usually less familiar with this additional conservation equation than with those for mass, energy and momentum transport. The high degree of interaction between the transport phenomena, reaction kinetics and light absorption leads to strong coupling of the conservation equations. Not only are the solutions to the model equations more difficult, the experimental verification of models is certainly not an easy task. No doubt this obstacle is a major cause for the limited experimental research on photoreactor modelling. In the case of heterogeneous photoreactors, the effect of scattering on light absorption due to the heterogeneous medium needs to be assessed. Some interesting theoretical analyses of the scattering phenomenon have been attempted, but again, the experimental testing of models has not been forthcoming. The bases of photochemical reaction engineering are therefore far from well established as are those of conventional reaction engineering.

Volume, Grain Boundary and Surface Diffusion of an Isotope in some Refractory Oxide Systems: A Comparison

Abstract: It is the purpose of this paper to compare some recent results obtained on volume, grain boundary diffusion and surface diffusion of 51Cr in refractory oxides such as MgO, A12O3 and MgA12O4. In these materials the concentration of intrinsic point defects is very small and diffusion transport phenomena are very sensitive to the concentration of aliovalent impurities. However, from the technological point of view the knowledge of transport phenomena in these materials is important to explain sintering, grain growth, mechanical, thermal and electrical properties. To avoid uncontrolled influence of impurities carefully doped materials can be used. The doping with aliovalent ions is particularly interesting because the presence of such ions determines the concentration of point defects and allows experiments to be performed with well defined materials.

Stochastic analysis of transport phenomena in heterogeneous tissue.

Abstract: In the classical analysis of mass transport phenomena, several basic assumptions must be made before the mechanics of the transport analysis can be implemented. The first is that the medium through which the mass is being transported is homogenous. Second is that the particle displacements in space from a given point are normally distributed. The third assumption is that each molecule or “particle” moves independently of all the other particles and has zero volume and mass. These conditions allow the formulation of the Green’s function (which is used in the solution of the mass transport equation) for the respective geometry and boundary conditions. The last basic assumption is that the classical transport process is Markov. This means that the events which occur at some future time depend only upon the present state of the system, and not on the past.