22.2.1 The Volume Fraction Equation

The tracking of the interface(s) between the phases is accomplished by the solution of a continuity equation for the volume fraction of one (or more) of the phases. For the $q$th phase, this equation has the following form:

$$\frac{\partial \alpha_q}{\partial t} + {\vec v} \cdot \nabla \alpha_q = \frac{S_{\alpha_q}}{\rho_q}$$ (22.2-1)

By default, the source term on the right-hand side of Equation  22.2-1 is zero, but you can specify a constant or user-defined mass source for each phase. See Section  22.5 for more information on the modeling of mass transfer in FLUENT's general multiphase models.

The volume fraction equation will not be solved for the primary phase; the primary-phase volume fraction will be computed based on the following constraint:

$$\sum_{q=1}^n \alpha_q = 1$$ (22.2-2)