Showing papers on "Transport phenomena published in 1996"

Combustion: Physical and Chemical Fundamentals, Modelling and Simulation, Experiments, Pollutant Formation

Abstract: Introduction * Fundamental Definitions and Phenomena * Experimental Investigation of Flames * Mathematical Description of Premixed Laminar Flat Flames * Thermodynamics of Combustion Processes * Transport Phenomena * Chemical Kinetics * Reaction Mechanisms * Laminar Prefixed Flames * Laminar Nonpremixed Flames * Ignition Processes * The Navier-Stokes Equations for Three-Dimensional Reacting Flows * Turbulent Reacting Flows * Turbulent Nonpremixed Flames * Turbulent Premixed Flames * Combustion of Liquid and Solid Fuels * Low-Temperature Oxidation, Engine Knock * Formation of Nitric Oxides * Formation of Hydrocarbons and Soot.

Introduction to thermodynamics and heat transfer

Abstract: Intro to Thermodynamics and Heat Transfer 2e 1 Introduction and Overview Part 1 Thermodynamics 2 Introduction and Basic Concepts 3 Energy, Energy Transfer, and General Energy Analysis 4 Properties of Pure Substances 5 Energy Analysis of Closed Systems 6 Mass and Energy Analysis of Control Volumes 7 The Second Law of Thermodynamics 8 Entropy Part 2 Heat Transfer 9 Mechanisms of Heat Transfer 10 Steady Heat Conduction 11 Transient Heat Conduction 12 External Forced Convection 13 Internal Forced Convection 14 Natural Convection 15 Radiation Heat Transfer 16 Heat Exchangers Appendix 1 Property Tables and Charts (SI Units) Appendix 2 Property Tables and Charts (English Units)

Equiaxed dendritic solidification with convection: Part I. Multiscale/multiphase modeling

Abstract: Equiaxed dendritic solidification in the presence of melt convection and solid-phase transport is investigated in a series of three articles. In part I, a multiphase model is developed to predict com-position and structure evolution in an alloy solidifying with an equiaxed morphology. The model accounts for the transport phenomena occurring on the macroscopic (system) scale, as well as the grain nucleation and growth mechanisms taking place over various microscopic length scales. The present model generalizes a previous multiscale/multiphase model by including liquid melt convec-tion and solid-phase transport. The macroscopic transport equations for the solid and the interdendritic and extradendritic liquid phases are derived using the volume averaging technique and closed by supplementary relations to describe the interfacial transfer terms. In part II, a numerical application of the model to equiaxed dendritic solidification of an Al-Cu alloy in a rectangular cavity is dem-onstrated. Limited experimental validation of the model using a NH4C1-H2O transparent model alloy is provided in part III.

Transport phenomena with drops and bubbles

Abstract: 1 Fundamental Principles and Definitions.- 1.1 Introduction.- 1.2 Mathematical Description.- 1.3 Heat and Mass Transfer.- References.- 2 Shape and Size of Fluid Particles.- 2.1 Introduction.- 2.2 Shapes of Static Particles.- 2.3 Shapes of Particles in Motion.- 2.4 Drop Size Distribution.- References.- 3 Transport at Low Reynolds Numbers.- 3.1 Introduction.- 3.2 Fluid Mechanics.- 3.3 Heat and Mass Transfer.- References.- 4 Transport at Intermediate and High Reynolds Numbers.- 4.1 Introduction.- 4.2 Fluid Mechanics.- 4.3 Heat and Mass Transfer.- References.- 5 Wall Interactions.- 5.1 Fluid Mechanics.- 5.2 Dropwise Condensation.- 5.3 Dropwise Evaporation.- References.- 6 Transport with a Spectrum of Fluid Particles.- 6.1 Introduction.- 6.2 Particle Sizes and Velocity Distributions.- 6.3 Transfer without Phase Change.- 6.4 Transfer with Phase Change.- References.- 7 Formation and Breakup of Bubbles and Drops.- 7.1 Introduction.- 7.2 Formation of Bubbles and Drops.- 7.3 Breakup of Bubbles and Drops.- References.- 8 Compound Drops and Bubbles.- 8.1 Introduction.- 8.2 Fluid Mechanics.- 8.3 Heat and Mass Transfer.- References.- 9 Special Topics.- 9.1 Transport in an Electric Field.- 9.2 Transport with a Slurry Fuel Droplet.- 9.3 Thermocapillary Phenomenon and Microgravity.- References.- Nomenclature.- Author Index.

Kinetic and Heat Transfer Control in the Slow and Flash Pyrolysis of Solids

Abstract: The coupled effects of particle size and external heating conditions (reactor heating rate and final temperature) on cellulose pyrolysis are investigated by means of a computer model accounting for all main transport phenomena, variable thermophysical properties and primary, and secondary reaction processes. The dynamics of particle conversion are predicted, and final product distributions are favorably compared with experimental measurements. A map is constructed, in terms of particle size as a function of the reactor temperature, to identify the transition from a kinetically controlled conversion to a heat transfer controlled conversion (thermally thin and thermally thick regimes) and from flash to slow-conventional pyrolysis. Conditions for maximizing oil, gas, or char yields are also discussed.

Improved Glass Micromodel Methods for Studies of Flow and Transport in Fractured Porous Media

Abstract: Microscale experiments can provide mechanistic insights into larger-scale flow and transport phenomena. Studies of the microscale mechanics involved in preferential flow in general, and unsaturated fast flow paths in particular, require the development of new experimental techniques. A new method for constructing glass micromodels has been developed which permits direct visualization and quantification of flow and transport phenomena in fractured porous media. In the fracture-matrix micromodels a sequential etching procedure was developed in order to provide the necessary contrast of depths between matrix pores and fracture apertures. This high contrast in etching depths ensures that very different capillary properties are associated with micromodel "fractures" and "matrix" blocks. Improved techniques were also developed for reducing the pore sizes of the matrix to a natural fine-grained sandstone pore scale. The improved micromodel pattern designs allow for previously unachievable control of boundary conditions. Various saturated and unsaturated fracture flow and transport processes can be visually and quantitatively studied with these micromodels. A method for directly measuring pore-scale flow velocity distribution through tracing trajectories of suspended fluorescent microspheres was also developed. Examples of applications include measurements of velocity profiles in fractures, imbibition, fracture-matrix transient flow, and matrix diffusion. In general, the improved micromodel method provides a unique tool for exploring some of the previously unrecognized flow and transport processes in fractured porous media. This research is directed at providing microscale explanations to some currently unresolved flow and transport issues important in predicting the larger-scale flow processes.

Analytical Solutions for Two-Dimensional Transport Equation with Time-Dependent Dispersion Coefficients

Abstract: Analytical solutions to advection-dispersion equations are of continuous interest because they present benchmark solutions to problems in hydrogeology, chemical engineering, and fluid mechanics. In this paper, we examine solutions to two-dimensional advection-dispersion equation with time-dependent dispersion coefficients. The time- and space-dependent nature of the dispersion coefficient in subsurface contaminant transport problems has been demonstrated in the literature in both field and laboratory scale studies. Analytical solutions given in this paper could be used to mdoel the transport of solute in hydrogeologic systems characterized by dispersion coefficients that may vary as a function of travel time from the input source. In particular, in this paper we develop instantaneous and continuous point-source solutions for constant, linear, asymptotic, and exponentially varying dispersion coefficients. The relationship between the proposed general solution and the particular solutions given in the relevant literature are discussed. Examples are included to demonstrate the effect of time-dependent dispersion coefficients on solute transport.

Two‐Dimensional Model for Water, Heat, and Solute Transport in Furrow‐Irrigated Soil: I. Theory

Abstract: Because of salinity, many areas with irrigated farmland have suffered from reduced food and fiber production, especially where irrigation water or soil contains large amounts of soluble salts. In those areas, ridge-furrow tillage is a common practice and furrow irrigation is a popular irrigation method. Effects of water content, temperature, and solute concentration on transport phenomena in soil are hard to predict without complex models. The geometry of a ridge-furrow surface contributes to complexities in heat and vapor transport, and hence, in the distribution of water, temperature, and salt in soil. To improve understanding of water, heat, and solute transport in furrow-irrigated and salt-affected soil, we developed a two-dimensional mechanistic model using the Galerkin finite element method (FEM). The model was designed to consider interactive effects of water content, temperature, and solute concentration on water, heat, and solute transport. To simulate field conditions, an energy balance equation was applied to the ridge-furrow surface to provide boundary conditions for water and heat transport. To utilize a single FEM solver, equations governing transport of water, heat, and salt in soil were generalized as a diffusive-convective type equation. Numerical schemes for the model were tested by comparing simulated results with analytical or semianalytical solutions. Good agreements between FEM and analytically calculated soil water content, temperature, and solute concentration were obtained.

Transport phenomena and materials processing

Abstract: INTRODUCTION TO TRANSPORT PHENOMENA. Introduction to Fluid Flow. Introduction to Heat Transfer. Introduction to Mass Transfer. Fluid Flow, Heat Transfer, and Mass Transfer: Similarities and Coupling. Boundary Conditions at Interfaces. APPLICATIONS OF TRANSPORT PHENOMENA IN MATERIALS PROCESSING. Selected Materials Processing Technologies. Fluid Flow in Materials Processing. Heat Transfer in Materials Processing. Mass Transfer in Materials Processing. Appendices. Index.

Drying with internal heat generation: Theoretical aspects and application to microwave heating

Abstract: A model of simultaneous heat and mass transfer presented describes drying with internal heat generation. Since a liquid expulsion phase is observed, a numerical procedure was developed to account for saturated and unsaturated zones and to model the liquid expulsion. The model was validated by a drainage experiment. An experimental rig was built to conduct microwave drying experiments in well-controlled conditions using capillary porous body (light concrete) as test material. Two types of drying (high and low power) were distinguished, depending on whether or not boiling occurred in the sample. The heat source term in the medium was determined from the experimental results. The numerical results agree with the experimental observations in terms of drying kinetics and transfer mechanisms. This allows a very accurate description of the transport phenomena and the liquid expulsion phase associated with high-power drying.

A model for macrosegregation and its application to Al-Cu castings

Abstract: A macrosegregation model has been developed to evaluate solute redistribution during solidification of casting alloys. The continuum formulations were used to describe the macroscopic transports of mass, energy, and momentum, associated with the microscopic transport phenomena, for two-phase systems. It was assumed that liquid flow is driven by thermal and solutal buoyancy, as well as by solidification contraction. The movement of free surface was also considered to ensure volume con-servation. In numerical calculations, the solidification event was divided into two stages. In the first stage, the liquid containing freely moving equiaxed grains was described through the relative vis-cosity concept. In the second stage, when a fixed dendritic network formed after dendrite coherency, the mushy zone was treated as a porous medium. After validation of the proposed model for the case of segregation in a bottom-chilled unidirectionally solidified casting of Al-Cu alloys, the nu-merical model was applied to the study of three different castings with simple geometry. It was found that solutal convection tends to decrease the macrosegregation generated by thermal convec-tion. When shrinkage-driven convection was also considered, segregation was again increased, with highly segregated areas forming away from the riser and next to the mold wall. It was demonstrated that solidification contraction has a stronger effect on the liquid flow in the mushy region than buoyancy. The model also was applied to assess the probability of pore formation based on the pressure drop concept. While in the absence of experimental data for the critical pressure drop it was not possible to uniquely predict the formation of porosity, it was possible to indicate the regions where porosity may form preferentially.

Transport Phenomena During Resistance Spot Welding

Abstract: Unsteady, axisymmetric transport of mass, momentum, energy, species, and magnetic field intensity with a mushy-zone phase change in workpieces and temperature, and magnetic fields in electrodes during resistance spot welding, are systematically investigated. Electromagnetic force, joule heat, heat generation at the electrode–workpiece interface and faying surface between workpieces, different properties between phases, and geometries of electrodes are taken into account. The computed results show consistencies with observed nugget growth, electrical current, and temperature fields. The effects of the face radius and cone angle of the electrode, parameters governing welding current, electrical contact resistance, magnetic Prandtl number, electrical conductivity ratio, and workpiece thickness on transport phenomena are clearly provided.

Hard dense loops in a cold non-Abelian plasma.

Abstract: Classical transport theory is used to study the response of a non-Abelian plasma at zero temperature and high chemical potential to weak color electromagnetic fields. In this article the parallelism between the transport phenomena occurring in a non-Abelian plasma at high temperature and high density is stressed. In particular, it is shown that at high densities it is also possible to relate the transport equations to the zero-curvature condition of a Chern-Simons theory in three dimensions, even when quarks are not considered ultrarelativistic. The induced color current in the cold plasma can be expressed as an average over angles, which represent the directions of the velocity vectors of quarks having Fermi energy. From this color current it is possible to compute $n$-point gluonic amplitudes, with arbitrary $n$. It is argued that these amplitudes are the same as the ones computed in the high chemical potential limit of QCD, which are then called hard dense loops. The agreement between the two different formalisms is checked by computing the polarization tensor of QED due to finite density effects in the high density limit.

Analytic Solution for Charge Transport and Chemical‐Potential Variation in Single‐Layer and Multilayer Devices of Different Mixed‐Conducting Oxides

Abstract: Transport phenomena are described involving one mobile ionic species (oxygen ions) and electronic carriers (electrons and holes) in single- and multilayer devices of different mixed-conducting oxides under the influence of an external load and an oxygen-chemical-potential gradient. The analysis utilizes the intrinsic ionic and electronic transport properties of the oxide layers and explicitly computes the ionic and electronic fluxes, the potential drop across the layers, and the chemical-potential variation in the layers. Some limiting cases are solved to illustrate the generality of the analysis. Based on this analysis, the accuracy, limitations, and physical significance of using an equivalent circuit to represent transport in such devices are discussed. The engineering implications of the analysis, in terms of designing efficient, stable devices with layered structures for fuel cells, sensors, separation membranes, and batteries, are also provided.

Modeling Low Velocity/High Dispersion Flow in Water Distribution Systems

Abstract: Under most circumstances, dispersive processes can be neglected in water distribution systems due to high velocities and low dispersion coefficients that ensure that advective transport dominates constituent spreading. However, during periods of low flow, dispersive effects may become important if the velocities are significantly decreased (e.g., during the night). Thus, the question that arises during a 24-h simulation is whether dispersive processes may dominate the transport of chlorine in a water distribution system. Given that advective transport models (e.g., EPANET, PICCOLO, NET, and DWQM) cannot account for dispersive transport, it is likely that these models would underpredict the required concentration of chlorine at locations behind the advective front and overpredict the required concentration at locations in advance of the advective front. There­ fore, when is a purely advective transport model an appropriate solution for contaminant transport in a water distribution system? Two constraining equations are presented to aid the user in assessing the applicability of an advective transport model.

Thermodynamic theory of transport processes in semiconductors

Abstract: Important energetic processes in semiconductors are analyzed from the point of view of equilibrium and irreversible thermodynamics. After carefully defining the necessary local variables and current densities, the continuity equations for particles, energy, and entropy are derived, leading to an expression for entropy generation. From this, the conjugate fluxes and affinities are directly obtained and the transport equations written using these quantities. The Onsager relations are applied to the kinetic coefficients defined in the transport equations and, after further manipulations, the experimental transport parameters may be found in terms of these quantities. Particular care is taken to ensure that the thermal conductivity is correctly defined. A new thermoinjection coefficient is found which describes the transport of heat by electrons and holes under conditions of zero total electric current and zero temperature gradient. The heat dissipation is derived in a number of different ways and compared with formula proposed by other authors. Expressions for the generation of useful external work using both photovoltaic and thermoelectric conversion are also found and related to the difference between free energy input and entropy generation. The equations presented form a suitable basis for the improved design of energy conversion devices. In two Appendixes, the thermodynamic methods used in the main text are compared with those based on the Boltzmann transport equations for electrons, holes, and phonons. Theoretical expressions are derived in these Appendixes for the kinetic coefficients. Issues relating to the definition of internal energy and chemical potential are analyzed in a third Appendix.

Transport Phenomena in Picoliter Size Solder Droplet Dispension

Abstract: This paper presents a study of the fluid dynamics and heat transfer phenomena occurring during the impingement of a picoliter size liquid solder droplet upon a multilayer, composite substrate. The theoretical model, based on the Lagrangian formulation, is solved numerically with the finite element method. A deforming mesh is utilized to accurately simulate the large deformations, as well as the domain nonuniformities characteristic of the spreading process. The occurrences of droplet recoiling and mass accumulation around the deposit periphery are features of the numerical simulations and yield a nonmonotonic dependence of the maximum radius on time. The results also document the transient temperature fields developing in both the solder droplet and the substrate during the impingement process. Convection effects on the temperature field development in a deforming droplet are found to be important for the entire history of spreading. The work is directly applicable to the miniature solder droplet dispension technology for the mounting of microscopic electronic components on various substrates under development at MicroFab Inc. The results of the numerical simulations are used to explain the shape of solidified microscopic solder bumps.

Cross-flow drying of wheat. A simulation program with a diffusion-based deep-bed model and a kinetic equation for viability loss estimations

Abstract: A mathematical model of heat and mass transfer for fixed beds was developed according to the modern theory of process simulation and standard laws of thermodynamics and transport phenomena. The mass transfer grain-air was predicted with simplified diffusional expressions together with an equation for the static equilibrium moisture content. Four differential equations were obtained for a grain layer and they were integrated along the bed depth and time with second and a fourth-order methods, respectively. The model was validated by comparing drying time predictions with experimental values, being the average error of 6%. The model was extended into a program for continuous cross-flow drying-cooling

Heat transfer and fluid dynamics in the process of spray deposition

Abstract: Publisher Summary This chapter presents a review of the existing knowledge base of the process of spray deposition, focusing on issues in which transport phenomena are relevant. It is apparent that in addition to the materials-science community, the heat-transfer and fluid-dynamics communities can have a significant impact in improving the understanding of the fundamental mechanisms occurring in this important process. The chapter discusses that to exemplify, even the single liquid-metal-droplet impact process is not well understood, although progress has been made to this end for axisymmetric splats. This process involves complex fluid-dynamics phenomena occurring within a severely deforming domain in the presence of conjugate convection-conduction heat transfer and solidification. Three-dimensional phenomena and splat breakup, which often occur in cases involving high-impact velocities, have yet to be modeled. The same is true regarding the rigorous modeling of more realistic situations involving droplet groups and sprays. In all, heat-transfer and fluid-dynamics researchers are strongly encouraged to focus their attention on the various challenges offered by the process of spray deposition.

Statistical mechanics of transport phenomena: Polymeric liquid mixtures

Abstract: A summary is given of the kinetic theory of flexible macromolecules, represented by bead-spring models of arbitrary connectivity. The formal theory is applicable to polydisperse systems, multicomponent mixtures, dilute or concentrated solutions, and fluids with concentration, temperature, and/or velocity gradients. Formal expressions are given for the momentum-flux (stress) tensor, the mass-flux vector, and the heat-flux vector; these can be combined with stochastic simulations to solve problems in rheology, diffusion, and heat conduction. Care is taken to point out where empiricisms are introduced, so that these can be modified or eliminated in the future. Other topics included are: the equation of change for angular momentum, the elastic terms in the equation of change for energy, the effect of velocity gradients on thermal conduction, the uniqueness of the molecular expression for the stress tensor, the relation between the mass flux and the stress tensor, thermal diffusion, and the Onsager receiprocal relations.

Nonequilibrium molecular dynamics simulations of steady-state heat and mass transport in distillation

Abstract: Coupled transport phenomena across gas/liquid interface, relevant for distillation, were studied by nonequilibrium molecular dynamics simulations. The simulations were set in the context of bulk irreversible thermodynamics. It was then shown that mole fraction profiles in the liquid phase and the gas phase of ideal isotope mixtures are linear. For nonideal mixtures, Fick`s law cannot be applied in the interface region, because the activity coefficients change dramatically across the interface. Fourier`s law has a constant heat conductivity for both types of liquid mixtures but not for gas mixtures. The coupling between heat and mass transfer becomes negligible for distillation in the special case of ideal mixtures with constant molal overflow. In all other cases, the heat of transfer contributes significantly to the heat flux and causes deviations from Fourier`s law in the gas phase. This all means that coupled flux equations are needed to describe distillation and that the properties of the surface are important for a description of the heat and mass fluxes involved. The value of the heat of transfer has a bearing on the calculation of the number of theoretical stages in the column. When considered as a function of distance from the surface, the local entropymore » production rate has a peak or a shoulder (depending on the conditions) slightly into the vapor. The entropy production rate in the liquid cannot be neglected compared to that of the gas. The second law efficiency of distillation was quantified from this knowledge.« less

Bed-Load Transport. II: Stochastic Characteristics

Abstract: This paper deals with the stochastic characteristics of the saltation of bed-load transport. The trajectories of particles with different specific gravity are measured by high-speed photographic technique, and the experiments on bed-load transport processes also have been conducted. It is concluded that the probability density distributions of dimensionless saltation height and length follow the Γ-distributions and the dimensionless salation velocity follows the Gaussian distribution. Moreover, the probability density distributions of the concentration in the bed-load layer and bed-load transport rate also are formulated, and they follow the Γ-distributions approximately. Measured data in natural rivers and experimental flume agree with these distributions very well.