Showing papers on "Transport phenomena published in 2003"

Transport phenomena in biological systems

Abstract: Previous Edition TOC 1 Introduction A INTRODUCTION TO PHYSIOLOGICAL FLUID MECHANICS 2 Conservation and Momentum Balances 3 Conservation Relations for Fluid Transport, Dimensional Analysis and Scaling 4 Macroscopic Form of Conservation Relations and Applications of Momentum Transport 5 Fluid Flow in the Circulation and Tissues B FUNDAMENTALS AND APPLICATIONS OF MASS TRANSPORT 6 Introduction to Mass Transport 7 Diffusion with Convection or Electrical Potentials 8 Transport in Porous Media 9 Transvascular Transport 10 Solvent and Solute Transport across the Kidney Glomerulus C THE EFFECT OF MASS TRANSPORT UPON BIOCHEMICAL INTERACTIONS 11 Mass Transport and Biochemical Interactions 12 Oxygen Transport from the Lungs to the Tissues 13 Ligand-Receptor Kinetics on the Cell Surface and Molecular Transport within Cells 14 Cell Adhesion and Cell Signaling 15 Transport of Drugs and Macromolecules in Tumors 16 Transport in Organs and Organisms 17 Heat Transfer in Biological Systems Appendix Relevant Mathematical Concepts

Transport phenomena in porous media

Abstract: Non-Newtonian flow in heterogeneous media is of enormous theoretical and industrial importance. This phenomenon is studied to reveal macroscopic effects that arise due to the interaction between the non-linear flow behaviour and the spatial variation of the medium through which it is forced to move. The heterogeneity is achieved by using porous granular media, which is naturally non-homogeneous. The non-Newtonian properties of the fluid may have many causes and is an intrinsic property of the fluid that is used: One way of achieving it is by studying dense slurries of neutral particles or naturally occurring magmatic flows. Another way is to study the case where the flow is dominated by its ionic content and where the double layer thickness (the effective size of the ionic entities) is of the order of magnitude of the pore size. All cases studied in this thesis pertain to slow flow (low Reynolds number), though the fluid may be compressible. The variations in the flow are calculated in first order and these turn out to be coupled to the spatial variations in the porous medium. In this way structure formation is predicted. The structures may be either aligned with or may be perpendicular to the mean flow direction. 'Experiments to decide on which regime is relevant have been conducted. The genesis of structure formation is studied as a temporal development by considering a compressible flow. The constitutive equation that is required to couple the compressibility to the flow parameters is investigated. Two possible mechanisms have been identified: compressibility coupled to the pressure field and compressibility associated with the fluctuations in the flow. Using linear analysis the structure formation patterns associated with these two mechanisms are established for the steady state. Flow of ionically laden fluids has also been studied. Experiments done at Loughborough University (Department of Chemical Engineering) on electrowashing of filter cakes has been used to prove a major macroscopic effect. This effect takes place when the ionic diameter (which is approximately twice the double layer thickness) is of the order of magnitude of the pore size. A phenomenological set of transport equations has been set up. These contain coefficients, such as transition probabilities and mean ionic flow rates, that can be obtained from experiments by doing a first order solution of the equations for short times. A more involved numerical solution is also supplied and confirms the initial analytical estimates.

Linear dynamics of double-porosity dual-permeability materials. II. Fluid transport equations.

Abstract: For the purpose of understanding the acoustic attenuation of double-porosity composites, the key macroscopic equations are those controlling the fluid transport. Two types of fluid transport are present in double-porosity dual-permeability materials: (1) a scalar transport that occurs entirely within each averaging volume and that accounts for the rate at which fluid is exchanged between porous phase 1 and porous phase 2 when there is a difference in the average fluid pressure between the two phases and (2) a vector transport that accounts for fluid flux across an averaging region when there are macroscopic fluid-pressure gradients present. The scalar transport that occurs between the two phases can produce large amounts of wave-induced attenuation. The scalar transport equation is derived using volume-averaging arguments and the frequency dependence of the transport coefficient is obtained. The dual-permeability vector Darcy law that is obtained allows for fluid flux across each phase individually and is shown to have a symmetric permeability matrix. The nature of the cross coupling between the flow in each phase is also discussed.

A fully coupled constitutive model for thermo‐hydro‐mechanical analysis in elastic media with double porosity

Abstract: [1] The partial differential equations governing non-isothermal flow and deformation in an elastic medium with double porosity are presented. The governing equations satisfy an effective stress concept, Darcy's law, Fourier' law and the equations of Hookean thermoelasticity. The equations are derived using a systematic macroscopic approach that satisfies conservation laws applicable to balance of linear momentum, mass and energy. The Thermo-Hydro-Mechanical coupling take into account processes associated with: thermal expansion, thermal convection by moving fluid, fluid flux due to temperature gradients, heat flux due to pressure gradients, fluid and heat exchange between the two pore system, and the heat of phase compression.

Lattice Boltzmann simulations for flow and heat/mass transfer problems in a three-dimensional porous structure

Abstract: SUMMARY The lattice Boltzmann method (LBM) for a binary miscible fluid mixture is applied to problems of transport phenomena in a three-dimensional porous structure. Boundary conditions for the particle distribution function of a diffusing component are described in detail. Flow characteristics and concentration profiles of diffusing species at a pore scale in the structure are obtained at various Reynolds numbers. At high Reynolds numbers, the concentration profiles are highly affected by the flow convection and become completely different from those at low Reynolds numbers. The Sherwood numbers are calculated and compared in good agreement with available experimental data. The results indicate that the present method is useful for the investigation of transport phenomena in porous structures. Copyright c � 2000 John Wiley & Sons, Ltd.

Multi-Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Abstract: Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

Modeling Wood Degradation by the Unreacted-Core-Shrinking Approximation

Abstract: The unreacted-core-shrinking approximation is used to model one-dimensional transport phenomena and finite-rate kinetics of thick wood pyrolysis. A quasi-steady-state formulation of the equations is suggested because the effects of process transients along the charred region are shown to be negligible. Temperature dynamics of the unreacted core are described with either a concentrated-parameter model or a distributed-parameter model combined with an integral solution method. Though both treatments produce qualitatively similar predictions, only in the latter case is good quantitative agreement obtained with measurements of volatile release characteristics (external radiative heat fluxes in the range 40−80 kW/m2). Depending on material properties and reaction kinetics, convective heat transfer may play an important role in the particle heating rate and conversion time. The model shows significant sensitivity to the activation energy of the pyrolysis reaction, the effective thermal conductivities of wood an...

A Numerical Model of a Liquid-Feed Solid Polymer Electrolyte DMFC and Its Experimental Validation

Abstract: A one-dimensional, biphasic, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymeric electrolyte membrane has been developed for a liquid-feed solid polymer electrolyte direct methanol fuel cell (SPE- DMFC). The model employs three important requisites: (i) implementation of analytical treatment of nonlinear terms to obtain a faster numerical solution as also to render the iterative scheme easier to converge, (ii) an appropriate description of two-phase transport phenomena in the diffusive region of the cell to account for flooding and water condensation/evaporation effects, and (iii) treatment of polarization effects due to methanol crossover. An improved numerical solution has been achieved by coupling analytical integration of kinetics and transport equations in the reaction layer, which explicitly include the effect of concentration and pressure gradient on cell polarization within the bulk catalyst layer. In particular, the integrated kinetic treatment explicitly accounts for the nonhomogeneous porous structure of the catalyst layer and the diffusion of reactants within and between the pores in the cathode. At the anode, the analytical integration of electrode kinetics has been obtained within the assumption of macrohomogeneous electrode porous structure, because methanol transport in a liquid-feed SPE- DMFC is essentially a single-phase process because of the high miscibility of methanol with water and its higher concentration in relation to gaseous reactants. A simple empirical model accounts for the effect of capillary forces on liquid-phase saturation in the diffusion layer. Consequently, diffusive and convective flow equations, comprising Nernst-Plank relation for solutes, Darcy law for liquid water, and Stefan-Maxwell equation for gaseous species, have been modified to include the capillary flow contribution to transport. To understand fully the role of model parameters in simulating the performance of the DMCF, we have carried out its parametric study. An experimental validation of model has also been carried out. (C) 2003 The Electrochemical Society.

Transport and confinement in the Mega Ampère Spherical Tokamak (MAST) plasma

Abstract: A combination of recently installed state-of-the-art imaging and profile diagnostics, together with established plasma simulation codes, are providing for the first time on Mega Ampere Spherical Tokamak (MAST) the tools required for studying confinement and transport, from the core through to the plasma edge and scrape-off-layer (SOL). The H-mode edge transport barrier is now routinely turned on and off using a combination of poloidally localized fuelling and fine balancing of the X-points. Theory, supported by experiment, indicates that the edge radial electric field and toroidal flow velocity (thought to play an important role in H-mode access) are largest if gas fuelling is concentrated at the inboard side. H-mode plasmas show predominantly type III ELM characteristics, with confinement HH factor (w.r.t. scaling law IPB98[y, 2]) around ~1.0. Combining MAST H-mode data with the International Tokamak Physics Activities (ITPA) analyses, results in an L–H power threshold scaling proportional to plasma surface area (rather than PLH ~ R2). In addition, MAST favours an inverse aspect ratio scaling PLH ~ e0.5. Similarly, the introduction of type III ELMing H-mode data to the pedestal energy regression analysis introduces a scaling Wped ~ e−2.13 and modifies the exponents on R, BT and κ. Preliminary TRANSP simulations indicate that ion and electron thermal diffusivities in ELMing H-mode approach the ion-neoclassical level in the half-radius region of the plasma with momentum diffusivity a few times lower. Linear flux-tube ITG and ETG microstability calculations using GS2 offer explanations for the near-neoclassical ion diffusivity and significantly anomalous electron diffusivity seen on MAST. To complement the baseline quasi-steady-state H-mode, newly developed advanced regimes are being explored. In particular, 'broad' internal transport barriers (ITBs) have been formed using techniques developed at conventional aspect ratio. Electron and ion energy diffusivities are reduced towards the ion-neoclassical level in the ITB region of both co- and counter-injection NBI heated plasmas, with momentum diffusivity up to 10 times lower. Moving out to the edge and SOL, OSM2/EIRENE modelling is being used to extract edge perpendicular particle and heat diffusivities, results being consistent with the ballooning nature of power-flow seen in L-mode and reduction in outboard turbulence seen in ELM-free and inter-ELM H-mode. Modelling of parallel SOL transport requires the inclusion of the mirror force (~10 times higher in MAST than at the conventional aspect ratio) and B2SOLPS5.0 simulations show the edge electric field to be well modelled by neoclassical theory. Transient edge transport phenomena are being studied in detail using a variety of techniques (e.g. probability density function (PDF) and power spectrum analysis, differencing and rescaling methods). Intermittent transport is associated with a radial efflux at up to a tenth of the sound speed and up to 30 cm from the separatrix. Arguably, the most dramatic edge events seen in the plasma periphery are the ELMs. Recent results using fast, high-resolution visible imaging confirm the hypothesis that ELMs have both poloidal and toroidal structures (n ~ 10 at q = 4), consistent with recent theories of the non-linear evolution of ballooning modes.

Microfluid Flow in Circular Microchannel with Electrokinetic Effect and Navier's Slip Condition

Abstract: Electrokinetic transport phenomena and slippage of fluid at the surface are often considered independently for description of microchannel flow. Anomalous cases of experimental results disagreeing with theoretical prediction have frequently been reported. By combining the electrokinetic effect and Navier's slip condition, we illustrate a possible explanation and present an analytical solution for oscillating flow in a circular microchannel. The derived analytical solution is useful for more general time-dependent problems through superposition of time-harmonic solutions weighted by appropriate Fourier coefficients. Our parametric results suggest that these two surface phenomena can be important for a better understanding of microfluid flow with hydrophobic channel walls.

Variable sampling adaptive control of a distributed collector solar field

Abstract: Distributed collector solar fields are spatially distributed engineering systems, which aim at collecting and storing energy from solar radiation. They are formed by mirrors which concentrate direct incident sun light in a pipe where a fluid, able to accumulate thermic energy, flows. From the control point of view, the objective considered here consists of making the outlet oil temperature to track a reference signal by manipulating the oil flow, in the possible presence of fast disturbances caused by passing clouds. Although this plant may be successfully controlled by methods assuming it to be modeled as a "black box" lumped parameter system, the point of view advocated in this paper is that explicit consideration of its distributed character leads to an increased control performance. In this respect, the contributions of this paper are twofold: First, a controller relying on variable sampling is developed. This is derived from the partial differential equation describing the oil temperature evolution in time and space on the field and has the effect of linearizing the plant model. Second, the resulting performance is illustrated by means of experiments performed in an actual solar field. The experiments reported show that it is possible to make fast temperature setpoint changes, with reduced overshoot. The ideas presented are applicable to other types of industrial processes, involving transport phenomena.

Modeling of Electroosmotic Flow and Capillary Electrophoresis with the Joule Heating Effect: The Nernst−Planck Equation versus the Boltzmann Distribution

Abstract: Joule heating is present in electrokinetic transport phenomena, which are widely used in microfluidic systems. In this paper, a rigorous mathematical model is developed to describe the Joule heating and its effects on electroosmotic flow and mass species transport in microchannels. The proposed model includes the Poisson equation, the modified Navier−Stokes equation, and the conjugate energy equation (for the liquid solution and the capillary wall). Specifically, the ionic concentration distributions are modeled using (i) the general Nernst−Planck equation and (ii) the simple Boltzmann distribution. The relevant governing equations are coupled through the temperature-dependent solution dielectric constant, viscosity, and thermal conductivity, and, hence, they are numerically solved using a finite-volume-based CFD technique. The applicability of the Nernst−Planck equation and the Boltzmann distribution in the electroosmotic flow with Joule heating has been discussed. The results of the time and spatial dev...

Transport Phenomena for Chemical Reactor Design

Abstract: Preface.PART I: ELEMENTARY TOPICS IN CHEMICAL REACTOR DESIGN.Multiple Chemical Reactions in Plug Flow Tubular Reactors and Continuous Stirred Tank Reactors.Start Up Behaviour of a Series Configuration of Continuous Stirred Tank Reactors.Adiabatic Plug-Flow Tubular Reactor That Produces Methanol Reversibly in the Gas Phase from Carbon Monoxide and Hydrogen.Coupled Heat and Mass Transfer in Nonisothermal Liquid-Phase Tubular Reactors with Strongly Exothermic Chemical Reactions.Multiple Stationary States in Continuous Stirred Tank Reactors.Coupled Heat and Mass Transfer with Chemical Reaction in Batch Reactors.Total Pressure Method of Reaction-Rate Data Analysis.PART II: TRANSPORT PHENOMENA: FUNDAMENTALS AND APPLICATIONS.Applications of the Equations of Change in Fluid Dynamics.Derivation of the Mass Transfer Equation.Dimensional Analysis of the Mass Transfer Equation.Laminar Boundary Layer Mass Transfer around Solid Spheres, Gas Bubbles, and Other Submerged Objects.Dimensional Analysis of the Equations of Change for Fluid Dynamic s Within the Mass Transfer Boundary Layer.Diffusion and Chemical Reaction Across Spherical Gas-Liquid Interfaces.PART III: KINETICS AND ELEMENTARY SURFACE SCIENCE.Kinetic Mechanisms and Rate Expressions for Heterogeneous Surface-Catalyzed Chemical Reactions.PART IV: MASS TRANSFER AND CHEMICAL REACTION IN ISOTHERMAL CATALYTIC PELLETS.Diffusion and Heterogeneous Chemical Reaction in Isothermal Catalytic Pellets.Complete Analytical Solutions for Diffusion and Zeroth-Order Chemical Reactions in Isothermal Catalytic Pellets.Complete Analytical Solutions for Diffusion and First-Order Chemical Reactions in Isothermal Catalytic Pellets.Numerical Solutions for Diffusion and nth-Order Chemical Reactions in Isothermal Catalytic Pellets.Numerical Solutions for Diffusion and Hougen-Watson Chemical Kinetics in Isothermal Catalytic Pellets.Internal Mass Transfer Limitations in Isothermal Catalytic Pellets.Diffusion Coefficients and Damkohler Numbers Within the Internal Pores of Catalytic Pellets.PART V: ISOTHERMAL CHEMICAL REACTOR DESIGN.Isothermal Design of Heterogeneous Packed Catalytic Reactors.Heterogeneous Catalytic Reactors with Metal Catalyst Coated on the Inner Walls of the Flow Channels.Designing a Multicomponent Isothermal Gas-Liquid CSTR for the Chlorination of Benzene to Produce Monochlorobenzene.PART VI: THERMODYNAMICS AND NONISOTHERMAL REACTOR DESIGN.Classical Irreversible Therodynamics of Multicomponent Mixtures.Molecular Flux of Thermal Energy in Binary and Multicomponent Mixtures Via the Formalism of Nonequilibrium Thermodynamics.Thermal Energy Balance in Multicomponent Mixtures and Nonisothermal Effectiveness Factors Via Coupled Heat and Mass Transfer in Porous Catalysts.Statistical Thermodynamics of Ideal Gases.Thermodynamic Stability Criteria for Single-Phase Homogeneous Mixtures.Coupled Heat and Mass Transfer in Packed Catalytic Tubular Reactors That Account for External Transport Limitations.References.Index.

Transport phenomena and microscopic structure in partially miscible binary fluids: A simulation study of the symmetrical Lennard-Jones mixture

Abstract: Static and dynamic structure factors and various transport coefficients are computed for a Lennard-Jones model of a binary fluid (A,B) with a symmetrical miscibility gap, varying both the temperature and relative concentration of the mixture. The model is first equilibrated by a semi-grandcanonical Monte Carlo method, choosing the temperature and chemical potential difference Δμ between the two species as the given independent variables. Varying for Δμ=0 the temperature and particle number N over a wide range, the location of the coexistence curve in the thermodynamic limit is estimated. Well-equilibrated configurations from these Monte Carlo runs are used as initial states for microcanonical molecular dynamics runs, in order to study the microscopic structure and the behavior of transport coefficients as well as dynamic correlation functions along the coexistence curve. Dynamic structure factors Sαβ(q,t) [and the corresponding static functions Sαβ(q)] are recorded (α,β∈A,B), q being the wave number and t...

Mass Transfer: Principles and Applications

Abstract: In recent years, the subject of mass transfer has been treated as a minor player in the larger field of transport phenomena and taken a back seat to its more mature "brother," heat transfer. Yet mass transfer is sufficiently mature as a discipline and sufficiently distinct from other transport processes to merit a separate treatment, particularly o

Numerical simulation of microfluidic injection processes in crossing microchannels

Abstract: The design and operation control of microfluidic devices have drawn a great deal of attention over the last decade due to the emerging lab-on-a-chip applications. Cross-shaped microchannels connecting liquid reservoirs are typical configurations of the microfluidic chips. Normally the microchannels have a large length-to-width aspect ratio (typically 1500:1), therefore, the transport phenomena in these microchannels are essentially multiscale and multidimensional problems. There are no analytical solutions existing for such kind of problems and it has been found that effectively and efficiently simulating the transport phenomena in such microchannels is very difficult. A numerical model developed here uses the designed boundaries to truncate the physical domain to a small computation domain in order to concentrate computing power in the areas exhibiting multidimensional phenomena (such as intersections) and apply analytical functions in the areas of one-dimensionality (such as fully developed flow region). This model is employed to simulate the flow and mass transport processes in a planar glass chip with a cross-shaped microchannel, and the model predictions are compared to the experimental results. Agreement between the model predictions and experimental results verified that this newly developed model is capable of accurately and efficiently simulating the transport phenomena in microfluidic devices.

The influence of transport phenomena during high-pressure processing of packed food on the uniformity of enzyme inactivation.

Abstract: Here we deal with the influence of heat-transport effects on a high-pressure-induced enzyme inactivation in packed substances. Special attention is given to the influence of the geometrical scale and to the heat-transfer characteristics of the packaging material. The investigation is based on mathematical modeling and numerical simulation. The method accounts both for compression phase and holding phase. The model includes convective and conductive heat transfer, fluid motion as well as an enzyme transport equation with a first-order kinetic source term accounting for the inactivation. Three configurations with a total volume of 0.8 L, 6.3 L, and 50.3 L are considered. The pressure medium is water. The enzyme solution is B. subtilis α-amylase dissolved in a TRIS-HCl-buffer. The packaging material is polypropylene. The heat-transfer coefficient for conduction through the packaging material is varied to simulate both changes in the material properties as well as modifications of the packaging material thickness. It is found that the efficiency of the inactivation increases with increasing chamber volume as long as the kinetic inactivation constant is increasing with temperature. In the considered case the activity retention obtained in a 0.8 L volume is about 2.4 times larger than the one obtained for the same process carried out in a 50.3 L volume. Furthermore, it was found that the properties of the packaging material could induce a significant degree of nonuniformity (worst case = 69%). An appropriate choice of the material can lead to maximum inactivation and maximum process uniformity since advantage is taken from the slow heat exchange after the compression phase. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 725–735, 2003.

Spectroscopic analysis of liquid/liquid interfaces in multiphase microflows.

Abstract: Microscopic quasi-elastic laser scattering (μQELS) spectroscopy has been developed for analysis of interfacial phenomena at laminar multiphase microflow in a microchannel. Transport phenomena of a metal chelate through a water/toluene interface were measured, and transient adsorption of the chelate in the initial step of the transport was measured. A water/methanol miscible interface was also measured, and the interfacial free energy of a miscible interface was determined for the first time. The μQELS is expected to be very effective not only for physicochemical investigations of transport and mixing, but also for elemental process analysis of heterogeneous reactions.

Modeling a transport phenomena limited reactivity in short contact time catalytic partial oxidation reactors

Abstract: The present paper describes a computational study of the short contact time catalytic partial oxidation of methane in fixed-bed reactors employing a rhodium-based catalyst. A monodimensional model was developed, and its validation was carried out on the basis of two experimental sets of literature data. Transport phenomena limited conditions and a local equilibrium reactivity along the catalytic bed were assumed. The last assumption provided an acceptable simplification to describe the complex microkinetic reactions taking place on the catalyst surface. The model results were satisfactorily in agreement with the measured values of conversion, product selectivity, and solid-phase temperature profiles along the fixed bed, at different space velocities and catalyst particle sizes. The numerical analysis, which allowed one to highlight the principles governing the system, also pointed out the existence of a local solid−gas temperature difference occurring within the catalytic bed. This difference dropped to z...