42 Modeling Dispersed Multiphase Flows

Dispersed multiphase flows occur in a number of industrially important reactors including stirred tank reactors, fluidized bed reactors, bubble column reactors, com-bustors and so on. The modeler is often confronted with a complex flow process, in which some fundamental problems are still unsolved. In the simplest case, the reactor engineer has to deal with a one-way problem: prediction of particle trajectories or distribution in the known single-phase flow field. However, dispersed phase particles are not passive contaminants and the presence of these particles may influence the flow of the continuous phase. The level of interaction becomes especially complex for a turbulent flow field. When the size of the dispersed phase particle is very small or the mass loading of the particles is small, the influence of dispersed phase particles on the flow field of the continuous phase may be neglected. This is called one-way coupling. When the dispersed phase volume fraction is increased, the presence of dispersed phase will significantly affect the continuous phase flow field. This is called two-way coupling. If the particle number density is sufficiently large to allow direct particle-particle interactions, the modeler is faced with four-way coupling: continuous phase-dispersed phase particles-dispersed phase particles-continuous phase.

It is essential to examine the extent of coupling between dispersed and continuous phase to select an appropriate modeling approach. This is especially important for

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