多相流数值模拟一

CHMLTECH501Computational Fluid Dynamics Introduction to Modeling Multiphase Flows

A large number offlows encountered in chemical engineering are a mixture of phases.Phys-ical 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 multiphaseflow,a phase can be defined as an identifiable class of material that has a particular inertial response to and interaction with theflow and the potentialfield in which it is immersed.For example,different-sized solid particle 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 theflowfield. Multiphase Flow Regime

Multiphaseflow can be classified by the following regimes,grouped into four categories:•gas-liquid or liquid-liquidflows

–bubblyflow:discrete gaseous orfluid bubbles in a continuousfluid.

–dropletflow:discretefluid droplets in a continuous gas.

–slugflow:large bubbles in a continuousfluid.

–stratified/free-surfaceflow:immisciblefluids separated by a clearly-defined in-terface.

•gas-solidflows

–particle-ladenflow: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 duneflow,slug

flow,packed beds,and homogeneousflow.

–fluidized beds:consists 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 gasflow rate,bubbles appear and rise through the

bed,intensifying the mixing within the bed.

•liquid-solidflows

–slurryflow:transport of particles in liquids.the fundamental behavior of liquid-solidflows varies with the properties of the solid particles relative to those of

the liquid.In slurryflows,the Stokes number(St=τd/t s)is normally less than

1.When the Stokes number is larger than1,the characteristic of theflow is

liquid-solidfluidization.

–hydrotransport:densely-distributed solid particles in a continuous liquid.

–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-phaseflows,e.g.gas-liquid-solidflows.

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Each of theseflow regimes is illustrated in Figure(1).

Figure1:Multiphase Flow Regimes.

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