### 7.5.1. The Inhomogeneous (Interfluid Transfer) Model

Each fluid possesses its own flow field and the fluids interact via interphase transfer terms. In the inhomogeneous multiphase model, there is one solution field for each separate phase. Transported quantities interact via interphase transfer terms. For example, two phases may have separate velocity and temperature fields, but there will be a tendency for these to come to equilibrium through interphase drag and heat transfer terms.

Three different sub-models that differ in the way they model the interfacial area density and the interphase transfer terms are available. A model is selected for each fluid pair on the Fluid Pairs tab. The available options depend on the morphology of each phase of the pair (for example, continuous, dispersed, and so on), and on settings found on the Fluid Models tab (homogeneous options, free surface model option).

Descriptions of the three models follow.

#### 7.5.1.1. The Particle Model

This model is available when one of the phases is continuous and the other is dispersed or polydispersed. The dispersed phase particles or droplets are assumed to be spherical. It is suitable for modeling simple dispersed multiphase flow problems, for example, the dispersion of:

- Gas bubbles in a liquid.

- Liquid droplets in a gas or in immiscible liquid.

- Solid particles in a gas or in a liquid.

The MUSIG model is a variation of the particle model specialized for polydispersed phases. For details, see Polydispersed, Multiple Size Group (MUSIG) Model.

#### 7.5.1.2. The Mixture Model

This is a very simple model that treats both phases symmetrically. It may be appropriate as a first approximation or as the basis of user supplied interfacial transfer models for a calculation of non-disperse liquid-liquid or gas-liquid two phase flow. It requires both phases to be continuous. It can be used to model more complex multiphase flow problems, for example, Churn flow.

An interfacial length scale must be specified on the tab in CFX-Pre to relate the phasic volume fractions to the interfacial contact area. For details, see:

- Fluid Pair Models Tab in the CFX-Pre User's Guide.

- The Mixture Model in the CFX-Solver Theory Guide.

#### 7.5.1.3. The Free Surface Model

This model is applicable to free surface flows. The particle model or mixture model may also be used for these flows if there is entrainment of one phase inside another.

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