Fluent用户手册.doc

TheFLUENTUsers Guide tells you what you need to know to useFLUENT. At the end of the Users Guide, you will find a Reference Guide, a nomenclature list, a bibliography, and an index.!Under U.S. and international copyright law, Fluent is unable to distribute copies of the papers listed in the bibliography, other than those published internally by Fluent. Please use your library or a document delivery service to obtain copies of copyrighted papers.A brief description of whats in each chapter follows: Chapter1, Getting Started, describes the capabilities ofFLUENTand the way in which it interacts with other Fluent Inc. and third-party programs. It also advises you on how to choose the appropriate solver formulation for your application, gives an overview of the problem setup steps, and presents a sample session that you can work through at your own pace. Finally, this chapter provides information about accessing theFLUENTmanuals on CD-ROM or in the installation area. Chapter2, User Interface, describes the mechanics of using the graphical user interface, the text interface, and the on-line help. It also provides instructions for remote and batch execution. (See the separateText Command Listfor information about specific text interface commands.) Chapter3, Reading and Writing Files, contains information about the files thatFLUENTcan read and write, including hardcopy files. Chapter4, Unit Systems, describes how to use the standard and custom unit systems available inFLUENT. Chapter5, Reading and Manipulating Grids, describes the various sources of computational grids and explains how to obtain diagnostic information about the grid and how to modify it by scaling, translating, and other methods. This chapter also contains information about the use of non-conformal grids. Chapter6, Boundary Conditions, explains the different types of boundary conditions available inFLUENT, when to use them, how to define them, and how to define boundary profiles and volumetric sources and fix the value of a variable in a particular region. It also contains information about porous media and lumped parameter models. Chapter7, Physical Properties, explains how to define the physical properties of materials and the equations thatFLUENTuses to compute the properties from the information that you input. Chapter8, Modeling Basic Fluid Flow, describes the governing equations and physical models used byFLUENTto compute fluid flow (including periodic flow, swirling and rotating flows, compressible flows, and inviscid flows), as well as the inputs you need to provide to use these models. Chapter9, Modeling Flows in Moving Zones, describes the use of single rotating reference frames, multiple moving reference frames, mixing planes, and sliding meshes inFLUENT. Chapter10, Modeling Turbulence, describesFLUENTs models for turbulent flow and when and how to use them. Chapter11, Modeling Heat Transfer, describes the physical models used byFLUENTto compute heat transfer (including convective and conductive heat transfer, natural convection, radiative heat transfer, and periodic heat transfer), as well as the inputs you need to provide to use these models. Chapter12, Introduction to Modeling Species Transport and Reacting Flows, provides an overview of the models available inFLUENTfor species transport and reactions, as well as guidelines for selecting an appropriate model for your application. Chapter13, Modeling Species Transport and Finite-Rate Chemistry, describes the finite-rate chemistry models inFLUENTand how to use them. This chapter also provides information about modeling species transport in non-reacting flows. Chapter14, Modeling Non-Premixed Combustion, describes the non-premixed combustion model and how to use it. This chapter includes details about usingprePDF. Chapter15, Modeling Premixed Combustion, describes the premixed combustion model and how to use it. Chapter16, Modeling Partially Premixed Combustion, describes the partially premixed combustion model and how to use it. Chapter17, Modeling Pollutant Formation, describes the models for the formation of NOx and soot and how to use them. Chapter18, Introduction to Modeling Multiphase Flows, provides an overview of the models for multiphase flow (including the discrete phase, VOF, mixture, and Eulerian models), as well as guidelines for selecting an appropriate model for your application. Chapter19, Discrete Phase Models, describes the discrete phase models available inFLUENTand how to use them. Chapter20, General Multiphase Models, describes the general multiphase models available inFLUENT(VOF, mixture, and Eulerian) and how to use them. Chapter21, Modeling Solidification and Melting, describesFLUENTs model for solidification and melting and how to use it. Chapter22, Using the Solver, describes theFLUENTsolvers and how to use them. Chapter23, Grid Adaption, explains the solution-adaptive mesh refinement feature inFLUENTand how to use it. Chapter24, Creating Surfaces for Displaying and Reporting Data, explains how to create surfaces in the domain on which you can examineFLUENTsolution data. Chapter25, Graphics and Visualization, describes the graphics tools that you can use to examine yourFLUENTsolution. Chapter26, Alphanumeric Reporting, describes how to obtain reports of fluxes, forces, surface integrals, and other solution data. Chapter27, Field Function Definitions, defines the flow variables that appear in the variable selection drop-down lists inFLUENTpanels, and tells you how to create your own custom field functions. Chapter28, Parallel Processing, explains the parallel processing features inFLUENTand how to use them. This chapter also provides information about partitioning your grid for parallel processing.18. Introduction to Modeling Multiphase FlowsA large number of flows encountered in nature and technology are a mixture of phases. Physical phases of matter are gas, liquid, and solid, but the concept of phasein a multiphase flow system is applied in a broader sense. In multiphase flow, a phase can be defined as an identifiable class of material that has a particular inertial response to and interaction with the flow and the potential field in which it is immersed. For example, different-sized solid particles of the same material can be treated as different phases because each collection of particles with the same size will have a similar dynamical response to the flow field.This chapter provides an overview of multiphase modeling inFLUENT, and Chapters19and20provide details about the multiphase models mentioned here. Chapter21provides information about melting and solidification.18.1 Multiphase Flow RegimesMultiphase flow can be classified by the following regimes, grouped into four categories:gas-liquidor liquid-liquidflowsbubbly flow: discrete gaseous or fluid bubbles in a continuous fluiddroplet flow: discrete fluid droplets in a continuous gasslug flow: large bubbles in a continuous fluidstratified/free-surface flow: immiscible fluids separated by a clearly-defined interfacegas-solidflowsparticle-laden flow: discrete solid particles in a continuous gaspneumatic transport: flow pattern depends on factors such as solid loading, Reynolds numbers, and particle properties. Typical patterns are dune flow, slug flow, packed beds, and homogeneous flow.fluidized beds: consist of a vertical cylinder containing particles where gas is introduced through a distributor. The gas rising through the bed suspends the particles. Depending on the gas flow rate, bubbles appear and rise through the bed, intensifying the mixing within the bed.liquid-solidflowsslurry flow: transport of particles in liquids. The fundamental behavior of liquid-solid flows varies with the properties of the solid particles relative to those of the liquid. In slurry flows, the Stokes number (see Equation18.4-4) is normally less than 1. When the Stokes number is larger than 1, the characteristic of the flow is liquid-solid fluidization.hydrotransport: densely-distributed solid particles in a continuous liquidsedimentation: a tall column initially containing a uniform dispersed mixture of particles. At the bottom, the particles will slow down and form a sludge layer. At the top, a clear interface will appear, and in the middle a constant settling zone will exist.three-phase flows (combinations of the others listed above)Each of these flow regimes is illustrated in Figure18.1.1.Figure 18.1.1:Multiphase Flow Regimes18.2 Examples of Multiphase SystemsSpecific examples of each regime described in Section18.1are listed below:Bubbly flowexamples: absorbers, aeration, air lift pumps, cavitation, evaporators, flotation, scrubbersDroplet flowexamples: absorbers, atomizers, combustors, cryogenic pumping, dryers, evaporation, gas cooling, scrubbersSlug flowexamples: large bubble motion in pipes or tanksStratified/free-surface flowexamples: sloshing in offshore separator devices, boiling and condensation in nuclear reactorsParticle-ladenflow examples: cyclone separators, air classifiers, dust collectors, and dust-laden environmental flowsPneumatic transportexamples: transport of cement, grains, and metal powdersFluidized bedexamples: fluidized bed reactors, circulating fluidized bedsSlurry flowexamples: slurry transport, mineral processingHydrotransportexamples: mineral processing, biomedical and physiochemical fluid systemsSedimentationexamples: mineral processing18.3 Approaches to Multiphase ModelingAdvances in computational fluid mechanics have provided the basis for further insight into the dynamics of multiphase flows. Currently there are two approaches for the numerical calculation of multiphase flows: the Euler-Lagrange approach and the Euler-Euler approach.18.3.1 The Euler-Lagrange ApproachThe Lagrangian discrete phase model inFLUENT(described in Chapter19) follows the Euler-Lagrange approach. The fluid phase is treated as a continuum by solving the time-averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a large number of particles, bubbles, or droplets through the calculated flow field. The dispersed phase can exchange momentum, mass, and energy with the fluid phase.A fundamental assumption made in this model is that the dispersed second phase occupies a low volume fraction, even though high mass loading () is acceptable. The particle or droplet trajectories are computed individually at specified intervals during the fluid phase calculation. This makes the model appropriate for the modeling of spray dryers, coal and liquid fuel combustion, and some particle-laden flows, but inappropriate for the modeling of liquid-liquid mixtures, fluidized beds, or any application where the volume fraction of the second phase is not negligible.18.3.2 The Euler-Euler ApproachIn the Euler-Euler approach, the different phases are treated mathematically as interpenetrating continua. Since the volume of a phase cannot be occupied by the other phases, the concept of phasic volume fractionis introduced. These volume fractions are assumed to be continuous functions of space and time and their sum is equal to one. Conservation equations for each phase are derived to obtain a set of equations, which have similar structure for all phases. These equations are closed by providing constitutive relations that are obtained from empirical information, or, in the case of granular flows, by application of kinetic theory.InFLUENT, three different Euler-Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model.The VOF ModelThe VOF model (described in Section20.2) is a surface-tracking technique applied to a fixed Eulerian mesh. It is designed for two or more immiscible fluids where the position of the interface between the fluids is of interest. In the VOF model, a single set of momentum equations is shared by the fluids, and the volume fraction of each of the fluids in each computational cell is tracked throughout the domain. Applications of the VOF model include stratified flows, free-surface flows, filling, sloshing, the motion of large bubbles in a liquid, the motion of liquid after a dam break, the prediction of jet breakup(surface tension), and the steady or transient tracking of any liquid-gas interface.The Mixture ModelThe mixture model (described in Section20.3) is designed for two or more phases (fluid or particulate). As in the Eulerian model, the phases are treated as interpenetrating continua. The mixture model solves for the mixture momentum equation and prescribes relative velocities to describe the dispersed phases. Applications of the mixture model include particle-laden flows with low loading, bubbly flows, sedimentation, and cyclone separators. The mixture model can also be used without relative velocities for the dispersed phases to model homogeneous multiphase flow.The Eulerian ModelThe Eulerian model (described in Section20.4) is the most complex of the multiphase models inFLUENT. It solves a set ofnmomentum and continuity equations for each phase. Coupling is achieved through the pressure and interphase exchange coefficients. The manner in which this coupling is handled depends upon the type of phases involved; granular (fluid-solid) flows are handled differently than non-granular (fluid-fluid) flows. For granular flows, the properties are obtained from application of kinetic theory. Momentum exchange between the phases is also dependent upon the type of mixture being modeled.FLUENTs user-defined functions allow you to customize the calculation of the momentum exchange.Applications of the Eulerian multiphase model include bubble columns, risers, particle suspension, and fluidized beds.18.4 Choosing a Multiphase ModelThe first step in solving any multiphase problem is to determine which of the regimes described in Section18.1best represents your flow. Section18.4.1provides some broad guidelines for determining appropriate models for each regime, and Section18.4.2provides details about how to determine the degree of interphase coupling for flows involving bubbles, droplets, or particles, and the appropriate model for different amounts of coupling.18.4.1 General GuidelinesIn general, once you have determined the flow regime that best represents your multiphase system, you can select the appropriate model based on the following guidelines. Additional details and guidelines for selecting the appropriate model for flows involving bubbles, droplets, or particles can be found in Section18.4.2.For bubbly, droplet, and particle-laden flowsin which the dispersed-phase volume fractions are less than or equal to 10%, use the discrete phase model. See Chapter19for more information about the discrete phase model.For bubbly, droplet, and particle-laden flowsin which the phases mix and/or dispersed-phase volume fractions exceed 10%, use either the mixture model (described in Section20.3) or the Eulerian model (described in Section20.4). See Sections18.4.2and20.1for details about how to determine which is more appropriate for your case.For slugflows, use the VOF model. See Section20.2for more information about the VOF model.For stratified/free-surfaceflows, use the VOF model. See Section20.2for more information about the VOF model.For pneumatictransport, use the mixture model for homogeneous flow (described in Section20.3) or the Eulerian model for granular flow (described in Section20.4). See Sections18.4.2and20.1for details about how to determine which is more appropriate for your case.For fluidizedbeds, use the Eulerian model for granular flow. See Section20.4for more information about the Eulerian model.For slurryflows and hydrotransport, use the mixture or Eulerian model (described, respectively, in Sections20.3and20.4). See Sections18.4.2and20.1for details about how to determine which is more appropriate for your case.For sedimentation,use the Eulerian model. See Section20.4for more information about the Eulerian model.For general, complex multiphase flows that involve multiple flow regimes, select the aspect of the flow that is of most interest, and choose the model that is most appropriate for that aspect of the flow. Note that the accuracy of results will not be as good as for flows that involve just one flow regime, since the model you use will be valid for only part of the flow you are modeling.18.4.2 Detailed GuidelinesFor stratified and slug flows, the choice of the VOF model, as indicated in Section18.4.1, is straightforward. Choosing a model for the other types of flows is less straightforward. As a general guide, there are some parameters that help to identify the appropriate multiphase model for these other flows: the particulate loading, and the Stokes number, St. (Note that the word particle is used in this discussion to refer to a particle, droplet, or bubble.)The Effect of Particulate LoadingParticulate loading has a major impact on phase interactions. The particulate loading is defined as the mass density ratio of the dispersed phase (d) to that of the carrier phase (c):The material density ratiois greater than 1000 for gas-solid flows, about 1 for liquid-solid flows, and less than 0.001 for gas-liquid flows.Using these parameters it is possible to estimate the average distance between the individual particles of the particulate phase. An estimate of this distance has been given by Crowe et al.42:where. Information about these parameters is important for determining how the dispersed phase should be treated. For example, for a gas-pa