Showing papers on "Transport phenomena published in 2002"

Introduction to Methodology

Abstract: This chapter explores various apparently unrelated problems and formulating them in the simplest form such that in most cases an analytical solution to the governing equations can be obtained. Lessons of general applicability are extracted from the solutions of the problems considered. The concept that the subjects of transport phenomena and of thermodynamics are strongly intertwined is strictly emphasized. All problems in engineering science are formulated on the basis of two types of equations: balance and constitutive. A balance equation can be written either for a quantity for which a general principle of conservation exists, or for a quantity for which no such principle exists provided its rate of generation is included in the balance equation. A constitutive equation is one that assigns the value of F ( X , t )—the flux of the quantity considered—in terms of C ( X , t )—the amount of the quantity considered per unit volume—so that the problem becomes a mathematically well posed one. The two types of equations are discussed by illustrating several classic problems such as the plug flow reactor with diffusion, shock waves in gases, and stefan problem.

Transport processes in concrete

Abstract: Preface 1. Physical and Chemical Processes in Concrete 2. Fundamentals of Transport Phenomena in Chemically Reacting Mixtures 3. Models of Heat, Moisture and Chemical Compounds Transport in Porous Materials 4. Modeling Transport Processes in Concrete 5. Experimental Methods for Determination of Field Variables and of Material Properties of Concrete 6. Examples of Practical Applications of Computational Models of Heat, Moisture and Salt Transport in the Design of Concrete Structures Appendix 1: Basic Mathematical Relations Appendix 2: Recommended Data for Material Parameters of Concrete. Subject Index

Toward a better understanding of friction and heat/mass transfer in microchannels-- a literature review

Abstract: A critical review of published results is presented, to provide a better understanding of microchannel transport phenomena, together with the framework for future research. The main conclusions are (1) the onset of transition to turbulent flow in smooth microchannels does not occur if the Reynolds number is h 1,000; (2) the Nusselt number varies as the square root of the Reynolds number in laminar flow; and (3) satisfactory estimates of transfer coefficients, within the accuracy of experimental errors, can be obtained by using either experimental results for smooth channels with large hydraulic diameter or conventional correlations.

Modeling intra- and extra-particle processes of wood fast pyrolysis

Abstract: A detailed single-particle model, including a description of transport phenomena and a global reaction mechanism, is coupled with a plug-flow assumption for extraparticle processes of tar cracking, in order to predict the fast pyrolysis of wood in fluid-bed reactors for liquid-fuel production. Good agreement is obtained between predictions and measurements of product yields (liquids, char, and gases) as functions of temperature. Particle dynamics are very affected by the convective transport of volatile products. The average heating rates are on the order of 450–455 K/s, whereas reaction temperatures vary between 770 and 640 K (particle sizes of 0.1–6 mm and a reactor temperature of 800 K). The effects of several factors, such as size, shape, and shrinkage of wood particles, and external heat-transfer conditions are also examined.

Biological and Bioenvironmental Heat and Mass Transfer

Abstract: Contents ENERGY TRANSFER Equilibrium, Energy Conservation, and Temperature Modes of Heat Transfer Governing Equation and Boundary Conditions of Heat Transfer Conduction Heat Transfer: Steady-State Conduction Heat Transfer: Unsteady-State Convection Heat Transfer Heat Transfer with Change of Phase Radiative Energy Transfer MASS TRANSFER Equilibrium, Mass Conservation, and Kinetics Modes of Mass Transfer Governing Equations and Boundary Conditions of Mass Transfer Diffusion Mass Transfer: Steady-State Diffusion Mass Transfer: Unsteady-State Convection-Dispersion and Convection-Diffusion Mass Transfer APPENDIX Summary of Processes and Equations Physical Constants, Unit Conversions, and Mathematical Functions Heat Transfer and Related Properties Mass Transfer Properties Miscellaneous Environmental Data Equations of Motion in Various Coordinate Systems Some Useful Mathematical Background Index

Introduction to Thermal Systems Engineering: Thermodynamics, Fluid Mechanics, and Heat Transfer

Abstract: 1. What is Thermal Systems Engineering? 2. Getting Started in Thermodynamics: Introductory Concepts & Definitions. 3. Using Energy and the First Law of Thermodynamics. 4. Evaluating Properties. 5. Control Volume Analysis Using Energy. 6. The Second Law of Thermodynamics. 7. Using Entropy. 8. Vapor Power and Refrigeration Systems. 9. Gas Power Systems. 10. Psychrometric Applications (CD only). 11. Getting Started in Fluid Mechanics: Fluid Statics. 12. The Momentum and Mechanical Energy Equations. 13. Similitudes, Dimensional Analysis and Modeling. 14. Viscous Flow in Pipes and Over Immersed Bodies. 15. Gettign Started in Heat Transfer: Heat Transfer Modes and Their Rate Equations. 16. Heat Transfer by Conduction. 17. Heat Transfer by Convection. 18. Heat Transfer by Radiation. Appendix. Tables, Figures, and Charts.

Three-dimensional Multiphase Mathematical Modeling of the Blast Furnace Based on the Multifluid Model

Abstract: The blast furnace process is a multi-phase chemical reactor whose main purpose is to reduce iron oxides producing hot metal. In the actual blast furnace operation several phases simultaneously interact with one another exchanging momentum, mass and energy. In this paper a three-dimensional multiphase mathematical model of the blast furnace is presented. This model treats the blast furnace process as a multiphase reactor in which all phases behave like fluids. Five phases are treated by this model, namely, gas, lump solids (iron ore, sinter, pellets and coke), pig iron, molten slag and pulverized coal. Conservation equations for mass, momentum, energy and chemical species for all phases are solved based on the finite volume method. In the discretized momentum equations, the covariant velocity projections are used, which is expected to give the best coupling between the velocity and pressure fields and improve the convergence of the calculations. This is a new feature of the present model regarding to the numerical procedures applied to the blast furnace modeling, which emphasizes its originality. In addition, gas and solid phases are treated as continuous phases possessing a pressure field and the SIMPLE algorithm is applied to extract the pressure field and ensure mass conservation. Hot metal, slag and pulverized coal are treated as discontinuous phases consisting of unconnected droplets. For such phases, momentum conservation is used to calculate the fields of velocity while the continuity equations are used to calculate the phase volume fractions. This model was applied to predict the three-dimensional blast furnace operation and predicted temperature distributions and operational parameters like productivity, coke rate and slag rate presented close agreement with the actual measured ones in the blast furnace process.

Microchannel flow with patchwise and periodic surface heterogeneity

Abstract: Surface heterogeneity is present in a variety of electrokinetic transport phenomena. It is desirable to understand the synergetic effects of the electrical double layer field and the surface heterogeneity on electrokinetic flow in microchannels. In this paper, a 3D, finite element based, numerical model for pressure-driven flow through microchannels with an arbitrary but periodic patchwise heterogeneous surface pattern has been developed. The model is based on a simultaneous solution to the Nernst−Planck, Poisson, and Navier−Stokes equations to determine the local ionic concentration, the double layer distribution, and the flow field. The presence of a heterogeneous patch is shown to induce flow in all three coordinate directions, including a circulation pattern perpendicular to the main flow axis. The strength of this circulation region is found to be proportional to Reynolds number and double layer thickness. While at low Reynolds number (i.e., Re < 1) the double layer distribution is diffusion dominate...

Numerical simulation of convective and diffusive transport effects on a high-pressure-induced inactivation process

Abstract: High-pressure-induced conversions, such as the inactivation of enzymes or of microorganisms, are dependent on the applied pressure and the temperature of the process. The former can be considered to be a spatially homogeneous quantity, while the latter, being a transport quantity, varies over space and time. Here we question whether the uniformity of a high-pressure conversion can be disturbed by convective and conductive heat and mass transport conditions. Enzyme inactivation is taken as a model process for a high-pressure conversion. To cover a broad range of parameters and to consider scale-up effects, the investigation is based on mathematical modeling and numerical simulation for different sizes of the pressure chamber and different solvent viscosities. Apart from viscosity, the physical properties of the enzyme solutions are assumed to be identical in all cases. Therefore, matrix effects other than that of viscosity are excluded. Moreover, the authors postulate that viscosity solely acts on the continuum mechanical scale of momentum exchange but not on the molecular scale on the inactivation kinetics. It has been found that nonuniform thermal conditions can strongly influence the result of a high-pressure process. A variation of the activity retention between 28% and 48% can be observed after 20 minutes for a 0.8-L high-pressure chamber and a matrix fluid with a viscosity comparable to that of edible oils. The same process carried out in a 6.3-L device leads to an activity retention that varies between 16% and 40%. From the analysis of the time scales for the inactivation and for hydrodynamic and thermal compensation, it can be deduced that a nonuniform activity retention has to be expected if the inactivation time scale is larger than the hydrodynamic time scale and smaller than the thermal compensation time scale.

Application of the discrete element modelling in air drying of particulate solids

Abstract: The Discrete Element Method (DEM) has been widely used as a mathematical tool for the study of flow characteristics involving particulate solids One distinct advantage of this fast developing technique is the ability to compute trajectories of discrete particles This provides the opportunity to evaluate the interactions between particle, fluid and boundary at the microscopic level using local gas parameters and properties, which is difficult to achieve using a continuum model To date, most of these applications focus on the flow behaviour This paper provides an overview of the application of DEM in gas–solids flow systems and discusses further development of this technique in the application of drying particulate solids A number of sub-models, including momentum, energy and mass transfer, have been evaluated to describe the various transport phenomena A numerical model has been developed to calculate the heat transfer in a gas–solids pneumatic transport line This implementation has shown a

The physics of radiation transport in dense plasmas

Abstract: Radiation transport redistributes energy within a medium through the emission and reabsorption of photons. These processes also have a pronounced effect on the spectrum of radiation that escapes the medium. As the deliverable energies of plasma drivers such as lasers and pulsed-power generators steadily increase, denser and/or more massive plasmas can be created. Such plasmas are more absorptive to their own emitted radiation, with portions of the line spectrum frequently being highly opaque. Thus, radiation transport becomes more important, along with the need to consider its impact on the design of experiments and their diagnosis. This tutorial paper covers the basic theory and equations describing radiation transport, its physical effects, experimental examples of transport phenomena, and current challenges and issues. Among the specific topics discussed are requirements for local thermodynamic equilibrium (LTE), conditions for diffusion and the use of the diffusion approximation, the formation of emission and absorption lines, the approach of an emitted spectrum to the Planck limit, and diagnostic applications of transport effects.

Transport phenomena in the porous cathode of a proton exchange membrane fuel cell

Abstract: Simultaneous heat and mass transfer in the porous cathode of a proton exchange membrane (PEM) fuel cell is generated by the exothermic chemical reaction at the catalyst layer. A two-dimensional steady state model in a cross section of the porous electrode taken normal to the gas flow in the channels is presented. We consider a multicomponent inlet feed composed of oxygen, nitrogen, and water vapor on the cathode side. The mathematical formulation reduces to a system of four nonlinear second-order elliptic partial differential equations subject to appropriate nonlinear boundary conditions. Numerical solutions are obtained using a finite-difference method. Results are presented for various operating conditions and design parameters in order to identify the important factors in the performance ofthe fuel cell. From the calculated values of the vapour pressure and temperature, the regions of vapor oversaturation are identified.

Transport and solidification phenomena in molten microdroplet pileup

Abstract: This article presents a predominantly numerical investigation of the transient transport phenomena occurring during the pileup (deposition one upon another) of molten, picoliter-size liquid metal droplets relevant to a host of novel micromanufacturing processes. The investigated phenomena last fractions of a millisecond in severely deforming domains of typical size of a small fraction of a millimeter. The prevailing physical mechanisms of the pileup process (occurring simultaneously) are identified and quantified numerically. These are the fluid mechanics of the bulk liquid, capillarity effects at the liquid–solid interface, heat transfer, solidification, and thermal contact resistance effects at all interfaces. In terms of values of the Reynolds, Weber, and Stefan number the following ranges are covered: Re=281–453, We=2.39–5.99, and Ste=0.187–0.895. This corresponds to molten solder droplets impinging at velocities ranging between 1.12 and 1.74 m/s having an average diameter of ≈78 μm. The initial subst...

Chemical kinetics with electrical and gas dynamics modelization for NOx removal in an air corona discharge

Abstract: A non-stationary reactive gas dynamics model in a mono-dimensional geometry, including radial mass diffusion, gas temperature variation and chemical kinetics, is developed in this paper. The aim is to analyse the spatio-temporal evolution of the main neutral species involved in a corona discharge used for NO pollution control in polluted air at atmospheric pressure and ambient temperature. The present reactive gas dynamics model takes into account 16 neutral chemical species (including certain metastable species) reacting following 110 selected chemical reactions. The initial concentration of each neutral species is obtained from a 1.5D electrical discharge model. The gas temperature variations are due to direct Joule heating during the discharge phase, and also result from the delayed heating due to the relaxation of the vibrational energy into a random thermal energy during the post-discharge phase. The simulation conditions are those of an existing experimental setup (anode voltage of 10 kV in the case of a point to plane geometry with an interelectrode distance of 10 mm). The obtained results show that the diffusion phenomena and the gas temperature rise affect quite well the gas reactivity and the neutral species evolution. This allows us to better understand the different reaction processes and transport phenomena affecting the NO concentration magnitude inside the discharge channel.

Macrodispersivity for transport in arbitrary nonuniform flow fields: Asymptotic and preasymptotic results

Abstract: [1] We use homogenization theory to investigate the asymptotic macrodispersion in arbitrary nonuniform velocity fields, which show small-scale fluctuations. In the first part of the paper, a multiple-scale expansion analysis is performed to study transport phenomena in the asymptotic limit e ≪ 1, where e represents the ratio between typical lengths of the small and large scale. In this limit the effects of small-scale velocity fluctuations on the transport behavior are described by a macrodispersive term, and our analysis provides an additional local equation that allows calculating the macrodispersive tensor. For Darcian flow fields we show that the macrodispersivity is a fourth-rank tensor. If dispersion/diffusion can be neglected, it depends only on the direction of the mean flow with respect to the principal axes of anisotropy of the medium. Hence the macrodispersivity represents a medium property. In the second part of the paper, we heuristically extend the theory to finite e effects. Our results differ from those obtained in the common probabilistic approach employing ensemble averages. This demonstrates that standard ensemble averaging does not consistently account for finite scale effects: it tends to overestimate the dispersion coefficient in the single realization.

Towards a mesh-free computation of transport phenomena

Abstract: This paper formulates a computational procedure based on the Trefftz method for the solution of nonlinear transport phenomena. This new unified approach is particularly important when solving coupled, nonlinear, inhomogenous, anisotropic, multiphase, and multifield heat and mass transfer problems. Physical system represents the general transport equation, standing for a broad spectra of mass, energy, momentum, and species transfer problems. This equation is cast into non-linear Poisson form and expanded with respect to the transport variable. Fully implicit time-discretization is used. The particular solution method is applied as a general solution framework. The solution of the inhomogenous part is based on the radial basis function global approximation, and the solution of the homogenous part is based on the Trefftz method Laplace equation fundamental solution global approximation. The discrete approximate method results in a global point-collocation based grid and hence eliminates the need for polygonization of the computational boundary and domain.