Showing papers on "Transport phenomena published in 2000"

Nonequilibrium Molecular Dynamics Simulations of Steady-State Heat and Mass Transport in Condensation.

Abstract: Coupled transport phenomena across a 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...

Transport Phenomena and Droplet Formation During Pulsed Laser Interaction With Thin Films

Abstract: This work investigates transport phenomena and mechanisms of droplet formation during a pulsed laser interaction with thin films. The surface of the target material is altered through material flow in the molten phase induced by a tightly focused laser energy flux. Such a process is useful for developing a laser-based micromachining technique. Experimental and numerical investigations of the laser-induced fluid flow and topography variations are carried out for a better understanding of the physical phenomena involved in the process. As with many machining techniques, debris is often generated during laser-material interaction. Experimental parametric studies are carried out to correlate the laser parameters with the topography and droplet formations. It is found that a narrow range of operation parameters and target conditions exists for clean structures to be fabricated. The stop action photography technique is employed to capture the surface topography variation and the melting development with a nanosecond time resolution and a micrometer spatial resolution. Numerical simulations of the laser-induced surface deformation are also performed to obtain the transient field variables and to track the deforming surface. The comparison between the numerical and experimental work shows that, within the energy intensity range investigated in this work, the surface deformation and droplet formation are attributed to the surface-tension-driven flow, and the recoil pressure effect plays an insignificant role in the surface topography development.

Brownian motors: noisy transport far from equilibrium

Abstract: Transport phenomena in spatially periodic systems far from thermal equilibrium are considered. The main emphasize is put on directed transport in so-called Brownian motors (ratchets), i.e. a dissipative dynamics in the presence of thermal noise and some prototypical perturbation that drives the system out of equilibrium without introducing a priori an obvious bias into one or the other direction of motion. Symmetry conditions for the appearance (or not) of directed current, its inversion upon variation of certain parameters, and quantitative theoretical predictions for specific models are reviewed as well as a wide variety of experimental realizations and biological applications, especially the modeling of molecular motors. Extensions include quantum mechanical and collective effects, Hamiltonian ratchets, the influence of spatial disorder, and diffusive transport.

Introduction to Transport Phenomena

Abstract: I. MOLECULAR TRANSPORT. 1. The Nature of Transport Phenomena. 2. Transport Phenomena Laws. 3. One-Dimensional Molecular Energy Transport. 4. Molecular Mass Transport. 5. Molecular Momentum Transport. 6. The Transport Coefficients. 7. Similarity Analyses. II. CONVECTIVE TRANSPORT. 8. Convective Transport in Laminar Flow. 9. Turbulent Transport. 10. Transfer Coefficients. III. MACROSCOPIC CALCULATIONS. 11. Macroscopic Calculations: Momentum Transport. 12. Macroscopic Calculations: Energy Transport. 13. Macroscopic Calculations: Mass Transfer. Appendix A. Generalized Equations of Change. Appendix B. Using MATLAB ODE. Appendix C. Lennard-Jones Parameters and Collision Integrals. Appendix D. The Error Function. Appendix E: Viscosity, Thermal Conductivity Data. Appendix F. Conversion Factors. Index.

Density-of-states effective mass and scattering parameter measurements by transport phenomena in thin films

Abstract: A novel machine has been developed to measure transport coefficients in the temperature range of 50–350 K of thin films deposited on electrically insulating substrates. The measured coefficients—resistivity, Hall, Seebeck, and Nernst—are applied to solutions of the Boltzmann transport equation to give information about the film’s density-of-states effective mass, the Fermi energy level, and an energy-dependent scattering parameter. The machine is designed to eliminate or compensate for simultaneously occurring transport phenomena that would interfere with the desired measured quantity, while allowing for all four coefficients to be measured on the same sample. An average density-of-states effective mass value of 0.29±0.04me was measured on the transparent conductive oxide, cadmium stannate (CTO), over a carrier concentration range of 2–7×1020 cm−3. This effective mass value matched previous results obtained by optical and thermoelectric modeling. The measured scattering parameter indicates that neutral impurities or a mixture of scattering mechanisms may inhibit the transport of carriers in CTO.

Transport phenomena in stochastic magnetic mirrors

Abstract: Parallel thermal conduction along stochastic magnetic field lines may be reduced because the heat conducting electrons become trapped and detrapped between regions of strong magnetic field (magnetic mirrors). The problem reduces to a simple but realistic model for diffusion of mono-energetic electrons based on the fact that when there is a reduction of diffusion, it is controlled by a subset of the mirrors, the principle mirrors. The diffusion reduction can be considered as equivalent to an enhancement of the pitch angle scattering rate. Therefore, in deriving the collision integral, we modify the pitch angle scattering term. We take into account the full perturbed electron-electron collision integral, as well as the electron-proton collision term. Finally, we obtain the four plasma transport coefficients and the effective thermal conductivity. We express them as reductions from the classical values. We present these reductions as functions of the ratio of the magnetic field decorrelation length to the electron mean free path at the thermal speed $V_T=\sqrt{2kT/m_e}$. We briefly discuss an application of our results to clusters of galaxies. Key words: magnetic fields: conduction --- magnetic fields: diffusion --- methods: analytical --- plasmas

Numerical simulation of the coupled turbulent flow and macroscopic solidification in continuous casting with Electromagnetic Brake

Abstract: A computer program has been developed for analyzing the three-dimensional, steady conservation equations for transport phenomena in a slab continuous casting process with Electromagnetic Brake (EMBr) to investigate the effect of EMBr on the turbulent melt-flow, temperature fields, and macroscopic solidification of the molten metal. The enthalpy-porosity relation was employed to suppress the velocity within a mushy region. A revised low-Reynolds number k-e turbulence model was used to consider the turbulent effects. The electromagnetic field was described by Maxwell equations. The application of EMBr to the mold region results in the decrease of the transfer of superheat to the narrow face, the increase of temperature in free-surface region and most part of the melt of submold region, and the higher temperature gradients near the solidifying shell. The increasing magnetic flux density has effect mainly on the surface temperature of the solidifying shell at the narrow face, hardly on that at the wide face. It is seen that in the presence of EMBr, a thicker solidifying shell is obtained at the narrow face of slab.

Modeling Hot Wire Electrochemistry. Coupled Heat and Mass Transport at a Directly and Continuously Heated Wire

Abstract: Microelectrode wires heated directly in situ by an electric current generate mass as well as heat energy transport phenomena. With continuous heating, a stationary surface temperature is establishe...

Insignificant role of hydrodynamic dispersion on bacterial transport

Abstract: Dispersion in porous media has been postulated as a mechanism that can increase particle trans- port, but the magnitude of increased transport predicted using analytical models of column experiments is a function of the boundary conditions chosen for the models. To date, insufficient attention has been paid to the implications of these boundary conditions for column or field simulations of particle transport. In order to assess the potential effect of particle dispersion on bacterial transport during bioremediation using bioaugmentation, we review the most frequently applied boundary conditions and models used in particle transport modeling. Using a finite-difference model written in MATLAB to simulate a column experiment, we demonstrate that for boundary conditions typically employed in short (;10 cm) laboratory columns, hydrodynamic dispersion in such systems does not appreciably alter the calculation of bacterial stickiness or overall transport. However, dispersion is known to increase with distance, resulting in dispersivities on the order of one to several hundred meters over distances of 100 m. Using boundary conditions appropriate for field tests (a semi-infinite domain), and increasing dispersivity in proportion to transport distance, it was similarly demonstrated that dispersion would not appreciably increase the distance of particle travel (defined as the distance prior to a 2-log reduction in bacteria concentration) in field situations. Thus, it is concluded that dispersion can be neglected in calculating particle collision efficiencies in both laboratory and field experiments.

Modeling of inclusion growth and dissolution in the weld pool

Abstract: The composition, size distribution, and number density of oxide inclusions in weld metal are critical factors in determining weldment properties. A computational model has been developed to understand these factors, considering fluid flow and the temperature field in the weld pool during submerged are (SA) welding of low-alloy steels. The equations of conservation of mass, momentum, and energy are solved in three dimensions to calculate the velocity and temperature fields in the weld pool. The loci and corresponding thermal cycles of thousands of oxide inclusions are numerically calculated in the weld pool. The inclusions undergo considerable recirculatory motion and experience strong temperature gyrations. The temperature-time history and the computed time-temperature-transformation (TTT) behavior of inclusions were then used to understand the growth and dissolution of oxide inclusions in the weld pool. The statistically meaningful characteristics of inclusion behavior in the weld pool, such as the residence time, number of temperature peaks, etc., were calculated for several thousand inclusions. The calculated trends agree with experimental observations and indicate that the inclusion formation can be described by combining thermodynamics and kinetics with the fundamentals of transport phenomena.

A simple dynamic model for solid transport in rotary dryers

Abstract: The solid particle movement in a rotary drum plays an important role in drying processes. The solid distribution in the drum affects the amount of contact surface between the solid and the gas. The retention time of solids influences the time particles can stay in contact with the gas in order to transfer heat and mass. Any heat and mass transfer model for a solid particle dryer must be able to predict solid flowrate and solid hold-up. There have been several reports in the literature regarding the modelling aspects of solid transport in dryers. If the model is developed for model-based control, it must be simple and yet represent dynamics of the system accurately. This paper addresses solid motion modelling and the effects of different variables involved in solid transport phenomena. Sugar drying process is the case study in this work. A steady state semi-empirical model was modified to predict solid hold-up and flowrate in rotary dryers. This model was incorporated into a heat and mass transfer model to predict solid moisture and temperature for inferential and model-based control purposes. Results of several experiments that have been used to investigate dynamics of the system in terms of solid motion and to validate the model are also presented. The approach advocated in this paper is directly applicable to the transport of other solids in rotary drum equipment and can thus be regarded as a generalized model.

Friction factor and Nusselt number for thermoacoustic transport phenomena in a tube

Abstract: The transport phenomena for a viscous compressible oscillating flow (with a zero mean velocity) in a tube subjected to a prescribed cycle-steady axial temperature gradient are analyzed The governing equations are linearized under the conditions of high oscillating frequencies, small amplitudes, and in a tube with a high length-to-radius ratio Based on a linearized theory, an analytical expression is obtained for the local friction factor, which depends on the prescribed cycle-steady axial temperature and independent of time The local friction factor is shown to be a complex number indicating a phase shift between the cross-sectional averaged velocity and the local pressure gradient Closed-form analytical expressions are also obtained for the temperature distribution and for the Nusselt number of solid/fluid interfacial heat transfer by solving the energy equations of the fluid and solid phases The Nusselt number is also a complex number indicating a phase shift between the heat flux and the temperature difference between the wall and the oscillating fluid The magnitude of the Nusselt number is also dependent on the prescribed axial cycle-steady temperature and independent of time, and is related to dimensionless thermal property parameters, dimensionless geometrical parameter, and dimensionless operation conditions parameters To understand the momentum transport and energy transport characteristics, the radial distributions of axial velocity and temperature of the fluid are presented for different ratios of the inner radius with respect to fluid's viscous penetration depth Particular attention is given to the transport phenomena in the following two limiting cases: 1) a viscous compressible oscillating flow in a porous medium based on a capillary-tube model, and 2) a viscous compressible oscillating flow in a resonant tube of a thermoacoustic refrigerator or in a pulse tube of a Stirling-type pulse-tube refrigerator

Mass balance in eulerian-lagrangian transport simulations in estuaries

Abstract: This paper investigates the generation of flow mass errors in finite-element shallow water models and the effect of these errors in the mass conservation of Eulerian-Lagrangian transport simulations Flow mass errors are shown to be similar for several primitive and wave equation formulations These errors occur primarily in areas of steep bathymetric gradients and near complex boundaries Forcing Eulerian-Lagrangian transport simulations with nonconservative flow fields generates important mass imbalances, which can be mitigated by refining the flow grid Comparatively, refining the transport grid only reduces marginally the mass errors

A method whereby Onsager coefficients may be evaluated

Abstract: Much of our understanding of transport phenomena is based on linear irreversible thermodynamics formalized most notably by Onsager in 1931. As with any theory based on an a priori linear assumption, coefficients appear that may not be calculated from within the theory. In the case of transport these coefficients are usually found empirically. To correctly apply a linear theory the criteria for linearity should be evaluated. In this paper, a general theory of nonequilibrium thermodynamics, statistical rate theory, is used to outline a method whereby Onsager coefficients may be evaluated for a given circumstance. For a particular osmotic transport example, expressions for the Onsager coefficients are found which are shown to satisfy Onsager’s reciprocity hypothesis. Most importantly, criteria for linearity are explicitly formulated.

A full 3-dimensional adaptive finite volume scheme for transport and phase-change processes, part i: formulation and validation

Abstract: A full three-dimensional (3-D) numerical formulation for accurate simulation of transport and phase-change processes is presented. These processes are characterized by a variety of flow and heat transfer mechanisms in irregular domains with or without the movement of phase-change interfaces and free surfaces. A generalized 3-D nonorthogonal curvilinear finite volume formulation is developed in conjunction with a robust mesh generation scheme known as multizone adaptive grid generation (MAGG) to tackle such problems. The coupling between the interfacial dynamics and transport phenomena in the bulk of the phases is inherent in this formulation. A 3-D k-epsilon model is also incorporated to tackle the turbulent flows in these applications. The unified numerical model is validated against classical 3-D problems such as turbulent natural convection in a differentially heated cube, solidification in a cavity, and so on. In a companion paper, Part II (see this issue), application of this formulation to 3-D simul...

Transport Phenomena in Turbulent Falling Films

Abstract: This study was concerned with transport phenomena in turbulent falling films, with negligible interfacial shear. Emphasis was on the development of general expressions for eddy diffusivities, which could then be utilized to predict transport coefficients for various momentum and heat- and mass transfer processes. Experiments were pursued to obtain new data for heat transfer during film evaporation, a phenomenon that involves significant boundary layer resistance at both the wall and the free interface. These new data extended the existing database by an order of magnitude in the Prandtl number. When predictions of the new model are assessed against both new and existing experimental information, good agreement was found for film thickness, sensible heat transfer at the wall, mass transfer at the free surface, and film evaporation.

Modelling Transport Phenomena and Performance of Direct Methanol Fuel Cell Stacks

Abstract: A prototype direct methanol fuel cell (DMFC) stack has been designed and built at Newcastle University, based on a flow bed design developed with the aid of a flow visualization study and fluid flow modelling. In addition, a series of engineering models have been developed that predict the stack voltage, fluid distribution from the stack manifolds, the overall system pressure, the chemical equilibrium in both anode and cathode flow beds and the thermal management of the stack. The results of this work are presented in terms of an overall engineering model that incorporates all the aforementioned models. The initial steady state performance data of the prototype stack presented was obtained as a result of our experience of scaling up the system to achieve the designed power outputs.

Simulation of Transport Phenomena in Shallow Aquatic Environment

Abstract: In the framework of the present study, 2D and 3D finite-difference procedures have been developed and applied with regard to flow, heat, and contaminant transport in the marine environment, where a shallow aquatic environment can be assumed. The common characteristic of such a type of environment is that the water depth is much smaller than a typical length in the horizontal direction. Conditions of this environment allow 2D or \Ishallow water\N 3D numerical models to be applied instead of a more expensive and time consuming full 3D solution. such an approach is almost obtained without loss of accuracy. By reference to a practical case of a wind dominated environment, the requirements and possible use of the 2D or 3D numerical codes are identified and analyzed. It is shown that the applicability of the 2D model mainly depends on the wind direction. Considerations are made with regard to recirculation of thermal effluent into the inlet of a power station. It is shown that, even in an extremely complicated geometry of the simulated domain, the 2D and 3D simulations can provide the same order of magnitude of results for some wind directions, particularly in the case of wind blowing parallel to the shoreline. On the other hand, for a wind direction nearly perpendicular to the shoreline, the 2D model produces unreliable or simply nonphysical results, whereas the 3D model provides reliable results. These conjectures are of great practical importance because most of the previous thermal effluent studies have been carried out by using the 2D modeling. It is considered that the performance of the 2D and shallow water 3D simulations provide a better description and understanding of basic transport phenomena in the simulated domain than can be provided by the performance of either 2D or 3D simulation.

Transport Phenomena: Equations and Numerical Solutions

Abstract: Preface TRANSPORT PHENOMENA Conservation Equations Incompressible Fluid Dynamics Conduction Heat Transfer Forced Convection Natural Convection Radiation Heat Transfer Mass Transfer NUMERICAL SOLUTION Finite Differences Elliptic Equations Finite Volume Method Equations incurvilinear coordinates Vector analysis An introduction to tensor analysis Prediction of transport properties Laplace transforms The solution of Bessel's equation Radiative transfer in a cloud of particles The Runge-Kutta method Integration using Gaussian quadrature Index