Showing papers on "Transport phenomena published in 2007"

Advanced Transport Phenomena: Fluid Mechanics and Convective Transport Processes

Abstract: Advanced Transport Phenomena is ideal as a graduate textbook. It contains a detailed discussion of modern analytic methods for the solution of fluid mechanics and heat and mass transfer problems, focusing on approximations based on scaling and asymptotic methods, beginning with the derivation of basic equations and boundary conditions and concluding with linear stability theory. Also covered are unidirectional flows, lubrication and thin-film theory, creeping flows, boundary layer theory, and convective heat and mass transport at high and low Reynolds numbers. The emphasis is on basic physics, scaling and nondimensionalization, and approximations that can be used to obtain solutions that are due either to geometric simplifications, or large or small values of dimensionless parameters. The author emphasizes setting up problems and extracting as much information as possible short of obtaining detailed solutions of differential equations. The book also focuses on the solutions of representative problems. This reflects the book's goal of teaching readers to think about the solution of transport problems.

Transport phenomena during direct metal deposition

Abstract: The evolution of temperature and velocity fields during direct metal deposition with coaxial powder injection was simulated using a self-consistent three-dimensional model based on the solution of the equations of mass, momentum, energy conservation, and solute transport in the liquid pool. The basic physical phenomena, including heat transfer, phase changes, mass addition, fluid flow, and interactions between the laser beam and the coaxial powder flow, were considered in the model. The level-set method was implemented to track the evolution of the liquid/gas interface. The temperature and velocity fields, liquid/gas interface, and energy distribution at liquid/gas interface at different times were simulated. For verification purposes, the cladding depth and height were compared with experimental results.

Transport Phenomena in Micro Process Engineering

Abstract: Micro Process Engineering - An Interdisciplinary Approach: Introduction and Motivation.- Orientation of Micro Process Engineering.- The Role of Transport Processes.- Some Requirements for Successful Microstructures.- Scaling dimensions.- Actual Applications.- Barriers and Challenges. Fundamental Balances and Transport Processes: Introduction.- Unit Operations and Process Design.- Balances and Transport Equations.- Modeling, Calculation Methods, and Simulation.-Future Directions of Micro Process Engineering Research. Momentum Transfer: Momentum Transfer of Single-Phase Flow.- Convective Fluid Dynamics in Microchannels.- Multiphase Flow. Heat Transfer and Micro Heat Exchangers: Heat Transfer Fundamentals.- Microfluidic Networks for Heat Exchange.- Micro Heat Exchanger Devices. Diffusion, Mixing, and Mass Transfer Equipment: Mixing Processes and Their Characterization.- Diffuse Mass Transport and Concentration Distribution in Fluids.- Convective Mass Transport.- Characteristics of Convective Laminar Micromixers.- Mixing and Chaotic Advection.- Design and Fabrication of Silicon Micromixers.- High Throughput Mixing Devices with Microchannels. Chemical Reactions and Reactive Precipitation: Chemical Reactor Engineering.- Wall Mass Transfer and Surface Reactions in Multifluidic Systems.- Design Criteria for Microchannel Reactors.- Microreactor for Aerosol Generation.- Mixing and Defined Precipitation in Liquid Phase. Coupled Transport Processes: Thermodynamics of Irreversible Processes.- Thermoelectric Energy Conversion.- Electroosmotic and Electrokinetic Effects.- Thermodiffusion.- Pressure Diffusion. Conclusion and Final Remarks: Bibliography, Index

A Multi‐Scale Dynamic Mechanistic Model for the Transient Analysis of PEFCs

Abstract: In this paper we propose a new knowledge model to understand the dynamic behavior of a Membrane-Electrodes Assembly (MEA) of a Polymer Electrolyte Fuel Cell. The model, multiscale and mechanistic, is based on irreversible thermodynamics and electrodynamics, and depends on the internal physical parameters such as the specific catalytic area, the electric permittivity of the materials, the reaction kinetics and the diffusion coefficients of the reactant species. The model results of the coupling of a microscale transport phenomena description of charges (electrons and protons) through the electrode and the electrolyte thickness, a spatially distributed microscale model of the reactant diffusion (hydrogen and oxygen) through the Nafion ® layer covering the Pt/C particles, and a spatially distributed nanoscale description of the Nafion ® -Pt interface. This interfacial model is based on an internal description of the electrochemical double layer dynamics, coupling the transport phenomena in the diffuse layer and the electrochemical reactions and water adsorption in the compact layer. This multiscale model introduces many couplings between different physical domains, such as diffusion- migration transport and electrochemistry. A distributed-parameter Bond Graph description of the model has allowed to formulate it in a modular way, so that it could be adapted to other electrochemical systems (solid oxide fuel cells, Lithium-ion batteries…) or to be coupled witrh other physico-chemical phenomena (ageing mechanisms…). The numerical resolution allows to simulate the MEA dynamics, depending on current, temperature, reactant pressures and structural composition of the electrodes (Pt and Nafion ® loadings). Particularly, the model allows evaluating the sensitivity of impedance spectra to operating conditions and electrodes compositions, and it gives access to the contributions of the different phenomena and constitutive layers. A qualitative experimental validation of the model was carried out on a specific bench and on single cells with different platinum loadings.

Anomalous Transport Phenomena in Fermi Liquids with Strong Magnetic Fluctuations

Abstract: In many strongly correlated electron systems, remarkable violation of the relaxation time approximation (RTA) is observed. The most famous example would be high-Tc superconductors (HTSCs), and similar anomalous transport phenomena have been observed in metals near their antiferromagnetic (AF) quantum critical point (QCP). Here, we develop a transport theory involving resistivity and Hall coefficient on the basis of the microscopic Fermi liquid theory, by considering the current vertex correction (CVC). In nearly AF Fermi liquids, the CVC accounts for the significant enhancements in the Hall coefficient, magnetoresistance, thermoelectric power, and Nernst coefficient in nearly AF metals. According to the numerical study, aspects of anomalous transport phenomena in HTSC are explained in a unified way by considering the CVC, without introducing any fitting parameters; this strongly supports the idea that HTSCs are Fermi liquids with strong AF fluctuations. In addition, the striking \omega-dependence of the AC Hall coefficient and the remarkable effects of impurities on the transport coefficients in HTSCs appear to fit naturally into the present theory. The present theory also explains very similar anomalous transport phenomena occurring in CeCoIn5 and CeRhIn5, which is a heavy-fermion system near the AF QCP, and in the organic superconductor \kappa-(BEDT-TTF).

Nonequilibrium GW approach to quantum transport in nano-scale contacts.

Abstract: Correlation effects within the GW approximation have been incorporated into the Keldysh nonequilibrium transport formalism. We show that GW describes the Kondo effect and the zero-temperature transport properties of theAnderson model fairly well. Combining the GW scheme with density functional theory and a Wannier function basis set, we illustrate the impact of correlations by computing the I-V characteristics of a hydrogen molecule between two Pt chains. Our results indicate that self-consistency is fundamental for the calculated currents, but that it tends to wash out satellite structures in the spectral function. © 2007 American Institute of Physics. DOI: 10.1063/1.2565690 Electronic correlations are responsible for important transport phenomena such as Coulomb blockade and Kondo effects, 1 yet its significance for transport in nanoscale structures is not well understood nor has it been systematically studied. At present, the most popular approach to ab initio simulations of transport in nanocontacts combines a nonequilibrium Green’s function formalism with the single-particle Kohn-Sham KS scheme of density functional theory DFT. This approach works well for some systems, 2,3 but in other cases it fails to reproduce experimental data 4 indicating the need for computational transport schemes beyond the DFT level. 5‐7 A reliable description of transport through a molecular junction requires first of all a reliable description of the electronic structure of the molecule itself, i.e., its electron addition and removal energies. It is well known that the GW self-energy method yields quasiparticle properties of molecules 8,9 and solids 10,11 in good agreement with experiment improving drastically the DFT band structures. In view of this it seems tempting to extend the use of the GW approximation to transport calculations. It is clear, however, that this should not be implemented by shifting the molecular energy levels to their GW positions prior to coupling. The reason is, that when a confined interacting system is connected to external non-interacting leads, the electrons in the confined region become correlated with those in the leads. To capture these correlations, which are the origin of important many-body phenomena such as the Kondo effect, it is crucial that the self-energy be evaluated in the presence of coupling to the leads. Traditionally, correlation effects in transport have been studied on the basis of the Anderson and Kondo models by a variety of numerical and analytical techniques. Many of these techniques are, however, quite specific to the considered models and lack the generality needed to be combined with first-principles methods. In this paper we combine the GW approximation with the nonequilibrium Keldysh formalism to obtain a practical scheme for correlated quantum transport. We study the Anderson model out of equilibrium, and calculate the I-V characteristics of a molecular hydrogen contact using a Wannier function WF basis set. In both applications we emphasize the difference between self-consistent and nonselfconsistent evaluations of the GW self-energy. As a general model of a quantum conductor we consider a central region C connected to left L and right Rleads. The leads are kept at chemical potentials L and R, respectively. We construct the matrix hij

Carbon nanotube, graphene, nanowire, and molecule-based electron and spin transport phenomena using the non-equilibrium Green function method at the level of first principles theory

Abstract: Based on density functional theory (DFT), we have developed algorithms and a program code to investigate the electron transport characteristics for a variety of nanometer scaled devices in the presence of an external bias voltage. We employed basis sets comprised of linear combinations of numerical type atomic orbitals and k-point sampling for the realistic modeling of the bulk electrode. The scheme coupled with the matrix version of the non-equilibrium Green function method enables determination of the transmission coefficients at a given energy and voltage in a self-consistent manner, as well as the corresponding current-voltage (I-V) characteristics. This scheme has advantages because it is applicable to large systems, easily transportable to different types of quantum chemistry packages, and extendable to describe time-dependent phenomena or inelastic scatterings. It has been applied to diverse types of practical electronic devices such as carbon nanotubes, graphene nano-ribbons, metallic nanowires, and molecular electronic devices. The quantum conductance phenomena for systems involving quantum point contacts and I-V curves are described for the dithiol-benzene molecule in contact with two Au electrodes using the k-point sampling method.

Heat Transfer and Fluid Flow during Gas-Metal-Arc Fillet Welding for Various Joint Configurations and Welding Positions

Abstract: Gas-metal-arc (GMA) fillet welding is one of the most commonly used welding processes in the industry. This welding process is characterized by the complex joint geometry, a deformable weld pool surface, and the addition of hot metal droplets. In this work, a three-dimensional numerical heat-transfer and fluid-flow model is developed to capture the effects of the tilt angle of the fillet joint and the welding positions, i.e., V, L, and other configurations on the temperature profiles, velocity fields, weld pool shape, weld pool free surface profile, thermal cycles, and cooling rates during GMA welding in spray mode. The governing equations of conservation of mass, momentum, and energy are solved using a boundary fitted curvilinear coordinate system. The weld pool free surface deformation is calculated by minimizing the total surface energy. A dimensional analysis is performed to understand the importance of heat transfer by conduction and convection and the role of various driving forces on convection in the liquid weld pool. The computed shape and size of the fusion zone, finger penetration characteristic of the GMA welds, and the solidified free surface profile are in fair agreement with the corresponding experimental results. The calculated cooling rates are also in good agreement with independent experimental data. The results reported here indicate a significant promise for understanding the effect of joint orientations and welding positions on weld pool shape, size, and the cooling rates based on fundamental principles of transport phenomena.

Photon-assisted electron transport in graphene : Scattering theory analysis

Abstract: Photon-assisted electron transport in ballistic graphene is analyzed using scattering theory. We show that the presence of an ac signal (applied to a gate electrode in a region of the system) has interesting consequences on electron transport in graphene, where the low energy dynamics is described by the Dirac equation. In particular, such a setup describes a feasible way to probe energy dependent transmission in graphene. This is of substantial interest because the energy dependence of transmission in mesoscopic graphene is the basis of many peculiar transport phenomena proposed in the recent literature. Furthermore, we discuss the relevance of our analysis of ac transport in graphene to the observability of zitterbewegung of electrons that behave as relativistic particles (but with a lower effective speed of light).

Numerical solutions for micropolar transport phenomena over a nonlinear stretching sheet

Abstract: We present a numerical study for the steady, coupled, hydrodynamic, heat and mass transfer of an incompressible micropolar fluid flowing over a nonlin ear stretching sheet. The governing differential equations are partially decoupled usin g a similarly transformation and then solved by two numerical techniques - the finite elem ent method and the finite difference method. The dimensionless translational velocity, microrotation (angular velocity), temperature and mass distribution function are computed for the different thermophysical parameters controlling the flow regime, viz the nonlinear (stretching) parameter, b, Grashof number, G and Schmidt number, Sc. All results are shown graphically. Additionally skin friction and Nusselt number, which provide an estimate of the surface shear stress and the rate of cooling of the surfac e, respectively, are also computed. Excellent agreement is obtained between both numerical methods. The dimensionless translational velocity (f ' ) for both micropolar and Newtonian fluids is shown to decrease with an increase in nonlinear parameter b. Dimensionless micro- rotation (angular velocity), g, generally increases with a rise in nonlinear parameter b (in particular in the vicinity of the wall) and decreases with a rise in convective pa rameter, G. The effects of other parameters on the flow variables are also discuss ed. The flow regime has significant applications in polymer processing technology and metallurgy.

Introduction to Chemical Transport in the Environment

Abstract: Prologue 1. The global perspective on environmental transport and fate 2. The diffusion equation 3. Diffusion coefficients 4. Mass, heat, and momentum transport analogies 5. Turbulent diffusion 6. Reactor mixing assumptions 7. Computational mass transport 8. Interfacial mass transfer 9. Air-water mass transfer in the field Appendices References.

Insight into Theories of Heat and Mass Transfer at the Solid-Fluid Interface Using Direct Numerical Simulation and Large Eddy Simulation

Abstract: Solid-fluid heat-transfer coefficients have an important role in the design of chemical processing equipment. The major resistance to heat transfer lies in a region very close to the wall, where experimental measurements are very difficult. The validity and accuracy of the models developed for the estimation of the heat- and mass-transfer coefficient still do not have general applicability for the entire range of Reynolds and Prandtl numbers, because of the limited knowledge of near-wall turbulence. There have been two approaches for such model development: one is an analytical approach, which considers the momentum, mass, and heat transfer to be analogous in nature and the understanding of one of these processes can be used to predict the other two; the other approach is heuristic, based on the visualization of the behavior of the coherent structures in the near-wall region. The continuous movement of fluid elements to and away from the wall (coherent structures) affects the transport phenomena. The models for the quantification of this behavior have been developed for the estimation of heat- and mass-transfer rates in the literature. However, both these approaches contain parameters fitted empirically to obtain good agreement with the experimental heat- and mass-transfer data. These models must be tested for their formulation and empirical constants on the basis of accurate solutions of governing equations of heat, mass, and momentum transfer. This is possible using direct numerical simulation (DNS) and large eddy simulation (LES), which can accurately predict the near-wall flow pattern. An attempt has been made to exploit the ability of DNS and LES to develop insight into hitherto used models, based on analogies and/or heuristic arguments.

Dynamics of a single cavitating and reacting bubble.

Abstract: Some of the studies on the dynamics of cavitating bubbles often consider simplified submodels assuming uniform fluid properties within the gas bubbles, ignoring chemical reactions, or suppressing fluid transport phenomena across the bubble interface. Another group of works, to which the present contribution belongs, includes the radial dependence of the fluid variables. Important fluid processes that occur inside the gas bubble, such as chemical reactions, and across the bubble interface, such as heat and mass transfer phenomena, are here considered also. As a consequence, this model should yield more realistic results. In particular, it is found that water evaporation and condensation are fundamental transport phenomena in estimating the dissociation reactions of water into OH. The thermal and mass boundary layers and the radial variation of the chemical concentrations also seem essential for accurate predictions.

The Role of Thermal Properties in Periodic Time-Varying Phenomena.

Abstract: The role played by physical parameters governing the transport of heat in periodical time-varying phenomena within solids is discussed. Starting with a brief look at the conduction heat transport mechanism, the equations governing heat conduction under static, stationary and non-stationary conditions, and the physical parameters involved, are described. Special emphasis is given to the phenomenon of diffusion of heat in the presence of periodical heat sources and the related (still unknown for the majority of people) thermal effusivity concept.

Magnetic filter operation in hydrogen plasmas

Abstract: A fluid-plasma model description of the operation of a magnetic filter for electron cooling in gas-discharge plasmas is presented in the study. Directed to the use of weak magnetic fields in the sources of negative hydrogen ion beams for additional heating of fusion plasmas, hydrogen discharges have been considered. The numerical results obtained within a 2D-model are stressed. The 1D-model presented aims at showing the main trends whereas the results obtained within the 3D-model, also developed, confirm the 2D-model description. The models outline the importance of the transport phenomena: electron-energy and charged-particle fluxes. Reduction of the thermal flux across the magnetic field together with thermal diffusion and diffusion, acting in combination, is the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the (E × B)-drift and the diamagnetic drift form a fine spatial structure of the plasma-parameter variations.

Lattice Boltzmann method for multi-component, non-continuum mass diffusion

Abstract: Recently, there has been a great deal of interest in extending the lattice Boltzmann method (LBM) to model transport phenomena in the non-continuum regime. Most of these studies have focused on single-component flows through simple geometries. This work examines an ad hoc extension of a recently developed LBM model for multi-component mass diffusion (Joshi et al 2007 J. Phys. D: Appl. Phys. 40 2961) to model mass diffusion in the non-continuum regime. In order to validate the method, LBM results for ternary diffusion in a two-dimensional channel are compared with predictions of the dusty gas model (DGM) over a range of Knudsen numbers. A calibration factor based on the DGM is used in the LBM to correlate Knudsen diffusivity to pore size. Results indicate that the LBM can be a useful tool for predicting non-continuum mass diffusion (Kn > 0.001), but additional research is needed to extend the range of applicability of the algorithm for a larger parameter space. Guidelines are given on using the methodology described in this work to model non-continuum mass transport in more complex geometries where the DGM is not easily applicable. In addition, the non-continuum LBM methodology can be extended to three-dimensions. An envisioned application of this technique is to model non-continuum mass transport in porous solid oxide fuel cell electrodes.

Convection and Diffusion in Charged Hydrated Soft Tissues: A Mixture Theory Approach

Abstract: The extracellular matrix of cartilage is a charged porous fibrous material Transport phenomena in such a medium are very complex In this study, solute diffusive flux and convective flux in porous fibrous media were investigated using a continuum mixture theory approach The intrinsic diffusion coefficient of solute in the mixture was defined and its relation to drag coefficients was presented The effect of mechanical loading on solute diffusion in cartilage under unconfined compression with a frictionless boundary condition was analyzed numerically using the model developed Both strain-dependent hydraulic permeability and diffusivity were considered Analyses and results show that (1) In porous media, the convective velocity for each solute phase is different (2) The solute convection in tissue is governed by the relative convective velocity (ie, relative to solid velocity) (3) Under the assumption that all the frictional interactions among solutes are negligible, the relative convective velocity for α-solute phase is equal to the relative solvent velocity multiplied by its convective coefficient (H α) which is also known as the hindrance factor in the literature The relationship between the convective coefficient and the relative diffusivity of solute is presented (4) Solute concentration profile within the cartilage sample depends on the phase of dynamic compression