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Chp18 Document Transcript

  • 1. Chapter 18. Introduction to ModelingMultiphase Flows A large number of flows encountered in nature and technology are a mix- ture of phases. Physical phases of matter are gas, liquid, and solid, but the concept of phase in 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 dif- ferent 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 in FLUENT, and Chapters 19 and 20 provide details about the multiphase models mentioned here. Chapter 21 provides information about melting and solidification. Information in this chapter is presented in the following sections: • Section 18.1: Multiphase Flow Regimes • Section 18.2: Examples of Multiphase Systems • Section 18.3: Approaches to Multiphase Modeling • Section 18.4: Choosing a Multiphase Model c Fluent Inc. November 28, 2001 18-1
  • 2. Introduction to Modeling Multiphase Flows18.1 Multiphase Flow Regimes Multiphase flow can be classified by the following regimes, grouped into four categories: • gas-liquid or liquid-liquid flows – bubbly flow: discrete gaseous or fluid bubbles in a continuous fluid – droplet flow: discrete fluid droplets in a continuous gas – slug flow: large bubbles in a continuous fluid – stratified/free-surface flow: immiscible fluids separated by a clearly-defined interface • gas-solid flows – particle-laden flow: discrete solid particles in a continuous gas – pneumatic 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 parti- cles 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-solid flows – slurry 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 Equation 18.4-4) 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 con- tinuous liquid18-2 c Fluent Inc. November 28, 2001
  • 3. 18.2 Examples of Multiphase Systems – sedimentation: 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 Figure 18.1.1.18.2 Examples of Multiphase Systems Specific examples of each regime described in Section 18.1 are listed below: • Bubbly flow examples: absorbers, aeration, air lift pumps, cavita- tion, evaporators, flotation, scrubbers • Droplet flow examples: absorbers, atomizers, combustors, cryo- genic pumping, dryers, evaporation, gas cooling, scrubbers • Slug flow examples: large bubble motion in pipes or tanks • Stratified/free-surface flow examples: sloshing in offshore separator devices, boiling and condensation in nuclear reactors • Particle-laden flow examples: cyclone separators, air classifiers, dust collectors, and dust-laden environmental flows • Pneumatic transport examples: transport of cement, grains, and metal powders • Fluidized bed examples: fluidized bed reactors, circulating flu- idized beds • Slurry flow examples: slurry transport, mineral processing • Hydrotransport examples: mineral processing, biomedical and phys- iochemical fluid systems • Sedimentation examples: mineral processing c Fluent Inc. November 28, 2001 18-3
  • 4. Introduction to Modeling Multiphase Flows slug flow bubbly, droplet, or particle-laden flow stratified/free-surface flow pneumatic transport, hydrotransport, or slurry flow sedimentation fluidized bed Figure 18.1.1: Multiphase Flow Regimes18-4 c Fluent Inc. November 28, 2001
  • 5. 18.3 Approaches to Multiphase Modeling18.3 Approaches to Multiphase Modeling Advances 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 Approach The Lagrangian discrete phase model in FLUENT (described in Chap- ter 19) 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 parti- cles, 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 sec- ond phase occupies a low volume fraction, even though high mass loading (mparticles ≥ mfluid ) is acceptable. The particle or droplet trajectories are ˙ ˙ computed individually at specified intervals during the fluid phase cal- culation. 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 Approach In the Euler-Euler approach, the different phases are treated mathemat- ically as interpenetrating continua. Since the volume of a phase cannot be occupied by the other phases, the concept of phasic volume fraction is introduced. These volume fractions are assumed to be continuous functions of space and time and their sum is equal to one. Conserva- tion 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 in- formation, or, in the case of granular flows, by application of kinetic theory. c Fluent Inc. November 28, 2001 18-5
  • 6. Introduction to Modeling Multiphase Flows In FLUENT, three different Euler-Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model.The VOF Model The VOF model (described in Section 20.2) is a surface-tracking tech- nique 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, slosh- ing, 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 Model The mixture model (described in Section 20.3) 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 Model The Eulerian model (described in Section 20.4) is the most complex of the multiphase models in FLUENT. It solves a set of n momentum 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 of18-6 c Fluent Inc. November 28, 2001
  • 7. 18.4 Choosing a Multiphase Model kinetic theory. Momentum exchange between the phases is also depen- dent upon the type of mixture being modeled. FLUENT’s user-defined functions allow you to customize the calculation of the momentum ex- change. Applications of the Eulerian multiphase model include bubble columns, risers, particle suspension, and fluidized beds.18.4 Choosing a Multiphase Model The first step in solving any multiphase problem is to determine which of the regimes described in Section 18.1 best represents your flow. Sec- tion 18.4.1 provides some broad guidelines for determining appropriate models for each regime, and Section 18.4.2 provides 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 Guidelines In 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 Section 18.4.2. • For bubbly, droplet, and particle-laden flows in which the dispersed- phase volume fractions are less than or equal to 10%, use the dis- crete phase model. See Chapter 19 for more information about the discrete phase model. • For bubbly, droplet, and particle-laden flows in which the phases mix and/or dispersed-phase volume fractions exceed 10%, use ei- ther the mixture model (described in Section 20.3) or the Eulerian model (described in Section 20.4). See Sections 18.4.2 and 20.1 for details about how to determine which is more appropriate for your case. • For slug flows, use the VOF model. See Section 20.2 for more information about the VOF model. c Fluent Inc. November 28, 2001 18-7
  • 8. Introduction to Modeling Multiphase Flows • For stratified/free-surface flows, use the VOF model. See Sec- tion 20.2 for more information about the VOF model. • For pneumatic transport, use the mixture model for homogeneous flow (described in Section 20.3) or the Eulerian model for granular flow (described in Section 20.4). See Sections 18.4.2 and 20.1 for details about how to determine which is more appropriate for your case. • For fluidized beds, use the Eulerian model for granular flow. See Section 20.4 for more information about the Eulerian model. • For slurry flows and hydrotransport, use the mixture or Eulerian model (described, respectively, in Sections 20.3 and 20.4). See Sections 18.4.2 and 20.1 for details about how to determine which is more appropriate for your case. • For sedimentation, use the Eulerian model. See Section 20.4 for 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 Guidelines For stratified and slug flows, the choice of the VOF model, as indicated in Section 18.4.1, 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, β, and the Stokes number, St. (Note that the word “particle” is used in this discussion to refer to a particle, droplet, or bubble.)18-8 c Fluent Inc. November 28, 2001
  • 9. 18.4 Choosing a Multiphase ModelThe Effect of Particulate Loading Particulate loading has a major impact on phase interactions. The par- ticulate loading is defined as the mass density ratio of the dispersed phase (d) to that of the carrier phase (c): αd ρd β= (18.4-1) αc ρc The material density ratio ρd γ= (18.4-2) ρc is 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]: 1/3 L π1+κ = (18.4-3) dd 6 κ where κ = β . Information about these parameters is important for γ determining how the dispersed phase should be treated. For example, for a gas-particle flow with a particulate loading of 1, the interparticle L space dd is about 8; the particle can therefore be treated as isolated (i.e., very low particulate loading). Depending on the particulate loading, the degree of interaction between the phases can be divided into three categories: • For very low loading, the coupling between the phases is one-way; i.e., the fluid carrier influences the particles via drag and turbu- lence, but the particles have no influence on the fluid carrier. The discrete phase, mixture, and Eulerian models can all handle this type of problem correctly. Since the Eulerian model is the most expensive, the discrete phase or mixture model is recommended. c Fluent Inc. November 28, 2001 18-9
  • 10. Introduction to Modeling Multiphase Flows • For intermediate loading, the coupling is two-way; i.e., the fluid carrier influences the particulate phase via drag and turbulence, but the particles in turn influence the carrier fluid via reduction in mean momentum and turbulence. The discrete phase, mixture, and Eulerian models are all applicable in this case, but you need to take into account other factors in order to decide which model is more appropriate. See below for information about using the Stokes number as a guide. • For high loading, there is two-way coupling plus particle pressure and viscous stresses due to particles (four-way coupling). Only the Eulerian model will handle this type of problem correctly.The Significance of the Stokes Number For systems with intermediate particulate loading, estimating the value of the Stokes number can help you select the most appropriate model. The Stokes number can be defined as the relation between the particle response time and the system response time: τd St = (18.4-4) ts ρ d2 d where τd = 18µd and ts is based on the characteristic length (Ls ) and the c characteristic velocity (Vs ) of the system under investigation: ts = Ls . V s For St 1.0, the particle will follow the flow closely and any of the three models (discrete phase, mixture, or Eulerian) is applicable; you can therefore choose the least expensive (the mixture model, in most cases), or the most appropriate considering other factors. For St > 1.0, the particles will move independently of the flow and either the discrete phase model or the Eulerian model is applicable. For St ≈ 1.0, again any of the three models is applicable; you can choose the least expensive or the most appropriate considering other factors.18-10 c Fluent Inc. November 28, 2001
  • 11. 18.4 Choosing a Multiphase ModelExamples For a coal classifier with a characteristic length of 1 m and a characteristic velocity of 10 m/s, the Stokes number is 0.04 for particles with a diameter of 30 microns, but 4.0 for particles with a diameter of 300 microns. Clearly the mixture model will not be applicable to the latter case. For the case of mineral processing, in a system with a characteristic length of 0.2 m and a characteristic velocity of 2 m/s, the Stokes number is 0.005 for particles with a diameter of 300 microns. In this case, you can choose between the mixture and Eulerian models. (The volume fractions are too high for the discrete phase model, as noted below.)Other Considerations Keep in mind that the use of the discrete phase model is limited to low volume fractions. Also, the discrete phase model is the only multiphase model that allows you to specify the particle distribution or include com- bustion modeling in your simulation. c Fluent Inc. November 28, 2001 18-11
  • 12. Introduction to Modeling Multiphase Flows18-12 c Fluent Inc. November 28, 2001