Showing papers on "Transport phenomena published in 2005"

Biomass Pyrolysis in a Fluidized Bed Reactor. Part 1: Literature Review and Model Simulations

Abstract: The literature on biomass pyrolysis regarding kinetics, models (single particle and reactor), and experimental results is reviewed from an engineering point of view. Predictions of existing single particle models derived from a detailed description of the transport phenomena and literature data on measured intrinsic chemical kinetics are presented. The main conclusions from the literature and modeling studies can be summarized as follows: (1) the available knowledge on kinetics and transport phenomena has not been integrated properly for reactor design, (2) complex two-dimensional single particle models do not provide more accurate, or otherwise better, information for engineering calculations than do the simple one-dimensional models, and (3) single particle models predict (for all available kinetics) that the influence of the particle size on the liquid yield is limited. This effect can be explained with the effective pyrolysis temperature, a parameter that represents the particle's average temperature at which the conversion is essentially taking place.

Heat transfer and fluid flow in laser microwelding

Abstract: The evolution of temperature and velocity fields during linear and spot Nd-yttrium aluminum garnet laser microwelding of 304 stainless steel was simulated using a well-tested, three-dimensional, numerical heat transfer and fluid flow model Dimensional analysis was used to understand both the importance of heat transfer by conduction and convection as well as the roles of various driving forces for convection in the weld pool Compared with large welds, smaller weld pool size for laser microwelding restricts the liquid velocities, but convection still remains an important mechanism of heat transfer On the other hand, the allowable range of laser power for laser microwelding is much narrower than that for macrowelding in order to avoid formation of a keyhole and significant contamination of the workpiece by metal vapors and particles The computed weld dimensions agreed well with the corresponding independent experimental data It was found that a particular weld attribute, such as the peak temperature or weld penetration, could be obtained via multiple paths involving different sets of welding variables Linear and spot laser microwelds were compared, showing differences in the temperature and velocity fields, thermal cycles, temperature gradients, solidification rates, and cooling rates It is shown that the temperature gradient in the liquid adjacent to the mushy zone and average cooling rate between 800 and 500 °C for laser spot microwelding are much higher than those in linear laser microwelding The results demonstrate that the application of numerical transport phenomena can significantly improve current understanding of both spot and linear laser microwelding

Transport at basin scales: 1. Theoretical framework

Abstract: . The paper describes the theoretical framework for a class of general continuous models of the hydrologic response including both flow and transport of reactive solutes. The approach orders theoretical results appeared in disparate fields into a coherent theoretical framework for both hydrologic flow and transport. In this paper we focus on the Lagrangian description of the carrier hydrologic runoff and of the processes embedding catchment-scale generation and transport of matter carried by runoff. The former defines travel time distributions, while the latter defines lifetime distributions, here thought of as contact times between mobile and immobile phases. Contact times are assumed to control mass transfer in a well-mixed approximation, appropriate in cases, like in basin-scale transport phenomena, where the characteristic size of the injection areas is much larger than that of heterogeneous features. As a result, we define general mass-response functions of catchments which extend to transport of matter geomorphologic theories of the hydrologic response. A set of examples is provided to clarify the theoretical results towards a computational framework for generalized applications, described in a companion paper.

Transport Phenomena in Polymer Electrolyte Membranes

Abstract: This paper presents a critical examination and analysis of classical and recently proposed models for transport phenomena in polymer electrolyte membranes. Key experimental observations related to membrane conductivity, membrane hydration, and sorption isotherms are first reviewed. Proton transport mechanisms in bulk water, and the influence of the membrane phase on these mechanisms, are examined. Finally, various formulations and underlying assumptions to account for macroscopic transport are reviewed, and an analysis of the binary friction model (BFM) and dusty fluid model (DFM) is performed to resolve an outstanding formulation issue. It is shown that the BFM provides a physically consistent modeling framework and implicitly accounts for viscous transport (i.e., Schloegl equation), whereas the dusty fluid model erroneously accounts twice for viscous transport. In Part II we apply the BFM framework to develop a general transport model for perfluorosulfonic acid membranes.

Green-function approach to transport phenomena in quantum pumps

Abstract: We present a general treatment based on nonequilibrium Green functions to study transport phenomena in systems described by tight-binding Hamiltonians coupled to reservoirs and with one or more time-periodic potentials. We apply this treatment to the study of transport phenomena in a double barrier structure with one and two ac potentials. Among other properties, we discuss the origin of the sign of the net current.

Properties of Glass-Forming Melts

Abstract: Glass-Forming Melts L.D. Pye Thermodynamic Properties K.H. Karlsson and R. Backman Redox Behavior and Electrochemical Behavior of Glass Melts C. Russel Transport Phenomena in Molten Glass: A Continuum Approach R.A. Murnane and R.R. Thomas Viscosity of Molten Glasses D. Martlew The Surface Tension of Glass-Forming Melts D.A. Weirauch, Jr. Density of Glass Melts O.V. Mazurin Heat Capacity of Glass Melts A.I. Priven and O.V. Mazurin Heat Transfer in Glass-Forming Melts M. K. Choudhary and R. M. Potter Electrical Conductivity of Glass Melts O.V. Mazurin and O. A. Prokhorenko How the Properties of Glass Melts Influence the Dissolution of Refractory Materials G.A. Pecoraro Nuclear Waste Glasses J.D. Vienna Solubility of Gases in Glass Melts F.W. Kramer Index

Transport and dynamics of liquid oxygen droplets in supercritical hydrogen streams

Abstract: The transport and dynamics of an isolated liquid oxygen (LOX) droplet in a supercritical hydrogen stream has been numerically studied based on the complete conservation equations in axisymmetric coordinates. The approach employs a unified treatment of general fluid thermodynamics and transport, and accommodates rapid variations of fluid properties in the transcritical regime. Surface tension of the droplet is ignored in consistency with the supercritical thermodynamic condition. The analysis allows for a systematic investigation into droplet behaviour over broad ranges of fluid thermodynamic states and ambient flow conditions. Detailed flow structures and transport phenomena are examined to reveal various key mechanisms underlying droplet vaporization in a supercritical forced convective environment. In addition, correlations of droplet lifetime and drag coefficient are established in terms of fluid properties, pressure, and free-stream Reynolds number.

Toward a unified theory of isotropic molecular transport phenomena

Abstract: Accurate models for multicomponent transport are a prerequisite for the design of many industrial processes and the interpretation of experiments. Present theories, stemming from the statistical-mechanics developments of Chapman-Enskog, Zhdanov-Kagan-Sazykin, and Bearman-Kirkwood are shown to apply only to systems with low shear. These theories are not able to describe gaseous counterdiffusion in capillaries, such as in the experiments of Remick and Geankoplis, and the salt diffusion experiment in a simple cylinder by Fick. A simple experiment shows that the irreversible thermodynamics approach by De Groot-Mazur and Hirschfelder-Curtiss-Bird provides inconsistencies. The cause for this is the common development of the theories as perturbations superposed on the mass-averaged velocity. A new solution to the Boltzmann equation is developed for monatomic dilute gases, based on nonequilibrium trial functions, in which the velocity distributions are centered around the averaged velocities of the individual species. In the resulting momentum balance, individual shear and convected momentum terms are present. Transport coefficients for pure monatomic gases are equal to those from classic theory; for mixtures of such gases new expressions are found that give an excellent description of experimental data. Based on these results, generalized versions of the transport equations are proposed, for dense media and liquids, and limit versions are presented. Important physical parameters are the partial viscosities. The applicability to the experimental situations above is demonstrated. The present theory offers the perspective of evaluating both concentration and velocity profiles, as well as temperature gradients, for individual species in three-dimensional space for molecular transport. Thus it provides a new basis for the modeling of multicomponent transport in a multitude of systems, such as in catalysts, adsorbents, membranes, CVD- and microreactors, but also for the classical problem of the circulation in Stefan tubes. In one of its limits, for long flat or cylindrical channels, it supports the earlier developed velocity profile model (VPM-1) for transport in pores. © 2004 American Institute of Chemical Engineers AIChE J, 51: 79-121, 2005

Quantum transport theory for semiconductor nanostructures: A density-matrix formulation

Abstract: A general density-matrix formulation of quantum transport phenomena in semiconductor nanostructures is presented More specifically, contrary to the conventional single-particle correlation expansion, we shall investigate separately the effects of the adiabatic or Markov limit and of the reduction procedure Our fully operatorial approach allows us to better identify the general properties of the scattering superoperators entering our effective quantum transport theory at various description levels, eg, $N$ electrons-plus-quasiparticles, $N$ electrons only, and single-particle picture In addition to coherent transport phenomena characterizing the transient response of the system, the proposed theoretical description allows us to study scattering induced phase coherence in steady-state conditions As a prototypical example, we shall investigate polaronic effects in strongly biased semiconductor superlattices

Variational and Extremum Principles in Macroscopic Systems

Abstract: Part I: Theory Variational Principles in Macroscopic Systems Part II: Applications Statistical Physics and Thermodynamics, Hydrodynamics and Continuum Mechanics, Transport Phenomena and Energy Conversion, Ecology, Selforganization and Econophysics Glossary of principal symbols Index

Measurement of turbulence decorrelation during transport barrier evolution in a high-temperature fusion plasma.

Abstract: A low power polychromatic beam of microwaves is used to diagnose the behavior of turbulent fluctuations in the core of the JT-60U tokamak during the evolution of the internal transport barrier. A continuous reduction in the size of turbulent structures is observed concomitant with the reduction of the density scale length during the evolution of the internal transport barrier. The density correlation length decreases to the order of the ion gyroradius, in contrast to the much longer scale lengths observed earlier in the discharge, while the density fluctuation level remain similar to the level before transport barrier formation.

Shear viscosity in a CFL quark star

Abstract: We compute the mean free path and shear viscosity in the color-flavor locked (CFL) phase of dense quark matter at low temperature T, when the contributions of mesons, quarks and gluons to the transport coefficients are Boltzmann suppressed. CFL quark matter displays superfluid properties, and transport phenomena in such cold regime are dominated by phonon-phonon scattering. We study superfluid phonons within thermal field theory and compute the mean free path associated to their most relevant collision processes. Small-angle processes turn out to be more efficient in slowing transport phenomena in the CFL matter, while the mean free path relevant for the shear viscosity is less sensitive to collinear scattering due to the presence of zero modes in the Boltzmann equation. In analogy with superfluid He4, we find the same T power law for the superfluid phonon damping rate and mean free path. Our results are relevant for the study of rotational properties of compact stars, and correct wrong estimates existing in the literature.

On the Lagrangian formulations of reactive solute transport in the hydrologic response

Abstract: [1] We address Lagrangian dispersion of reactive solutes in the framework of the formulation of transport by travel time distributions, specifically aiming at models of basin-scale, nonpoint transport applicable to complex geomorphological settings. We revisit existing exact solutions of the reactive transport problem derived in the convective stochastic framework and extend them to the case of transport of mass arbitrarily distributed ( in time and space) within the immobile phase, a situation which is arguably suited to better describe nonpoint source solute transport driven by the hydrologic response. The initial conditions and, particularly, the mass initially stored in immobile rather than mobile phases bear a pronounced effect on the spatial and temporal moments of the solute plume. We also show that in many nonpoint source cases of interest ( typically when heterogeneous conditions prevail) a simpler model of reaction kinetics, where spatial gradients in the immobile concentration are neglected, does well. Such a class of models, termed mass response functions, is known from the literature and has the property, beside being simplified in the mass exchange terms, of embedding unsteady flow forcing of the type typically employed in the theory of the hydrologic response. Thus, in the range of cases where the well-mixed assumption proves meaningful, we suggest a natural extension of current geomorphological models of the hydrologic response to generic transport phenomena.

Coupled fire dynamics and thermal response of complex building structures

Abstract: Simulation of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena, and the stress analysis in the load-bearing materials. The connection between the two is established primarily through the interaction of the radiative heat transfer between the solid and gas phases with the conduction of heat through the structural elements. This process is made difficult in large, geometrically complex buildings by the wide disparity in length and time scales that must be accounted for in the simulations. A procedure for overcoming these difficulties used in the analysis of the collapse of the World Trade Center towers is presented. The large scale temperature and other thermophysical properties in the gas phase are predicted using the NIST Fire Dynamics Simulator. Heat transfer to subgrid scale structural elements is calculated using a simple radiative transport model that assumes the compartment is locally divided into a hot, soot laden upper layer and a cool relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the results obtained from the Fire Dynamics Simulator. Explicit formulae for the heat flux are obtained as a function of temperature, hot layer depth, soot concentration, and orientation of each structural element. These formulae are used to generate realistic thermal boundary conditions for a coupled transient three-dimensional finite element code. This code is used to generate solutions for the heating of complex structural assemblies.

Two-Phase Transport in Porous Gas Diffusion Electrodes

Abstract: The accumulation of liquid water in electrodes can severely hinder the performance of PEMFCs. The accumulated water reduces the ability of reactant gas to reach the reaction zone. Current understanding of the phenomena involved is limited by the inaccessibility of PEMFC electrodes to in situ experimental measurements, and numerical models continue to gain acceptance as an essential tool to overcome this limitation. This Chapter provides a review of the transport phenomena in the electrodes of PEM fuel cells and of the physical characteristics of such electrodes. The review draws from the polymer electrolyte membrane fuel cell literature as well as relevant literature in a variety of fields. The focus is placed on two-phase flow regimes in porous media, with a discussion of the driving forces and the various flow regimes. Mathematical models ranging in complexity from multi-fluid, to mixture formulation, to porosity correction are summarized. The key parameters of each model are identified and, where possible, quantified, and an assessment of the capabilities, applicability to fuel cell simulations and limitations is provided for each approach. The needs for experimental characterization of porous electrode materials employed in PEMFCs are also highlighted.

Transport Phenomena in Polymer Electrolyte Membranes II. Binary Friction Membrane Model

Abstract: The insight gained from the analysis conducted in Part I (see the preceding article) is used in the development of a general transport model for water and protons in perfluorosulfonic acid membranes based on the binary friction model. As a tool for investigating the unknown parameters in the general membrane transport model, a simplified conductivity model is derived to represent conditions found in alternating current (ac) impedance conductivity measurements. This binary friction conductivity model (BFCM) is applied to 1100 equivalent weight (EW) Nafion, and compared to other established membrane models. It is shown to provide a more consistent fit to the data over the entire range of water contents and at different temperatures. The subset of transport coefficients in the BFCM is the same as in the general binary friction membrane model (BFM2), and thus with additional data on water transport, the BFM2 model and all its required parameters can be fully specified. The paper discusses possible experimental investigations and fundamental simulations to determine the model parameters required to apply the general BFM2 to predict coupled proton and water transport in PEM fuel cells.

Analysis of Intermediate Temperature Solid Oxide Fuel Cell Transport Processes and Performance

Abstract: A new trend in recent years is to reduce the solid oxide fuel cell (SOFC) operating temperature to an intermediate range by employing either a thin electrolyte, or new materials for the electrolyte and electrodes. In this paper, a numerical investigation is presented with focus on modeling and analysis of transport processes in planar intermediate temperature (IT, between 600 and 800 degrees C) SOFCs. Various transport phenomena occurring in an anode duct of an ITSOFC have been analyzed by a fully three-dimensional calculation method. In addition, a general model to evaluate the stack performance has been developed for the purpose of optimal design and/or configuration based on specified electrical power or fuel supply rate.

Non-local neoclassical transport simulation of geodesic acoustic mode

Abstract: Neoclassical transport simulation code (FORTEC-3D) applicable to both axisymmetric and non-axisymmetric configurations is developed to investigate non-local effects on neoclassical transport phenomena. The time evolution of the radial electric field is simulated in the full volume of the confinement region of tokamak and helical model plasmas. It is found that the damping rate of the geodesic-acoustic-mode (GAM) oscillation becomes faster than that predicted from a single-surface transport analysis. The time evolution of the radial electric field towards the ambipolar state shows a non-local behaviour, which indicates a coupling of GAM oscillation between the neighbouring two flux surfaces because of the finite-orbit-width effect.