The use of boundary conditions for multiphase flow is very similar to that for single-phase flow. For details, see Boundary Condition Modeling. The main differences are:
Boundary conditions need to be specified for all fluids for all variables except the shared pressure field.
Volume fractions of all phases must be specified on inlet and opening boundary conditions. These must sum to unity.
A degassing boundary condition is available for multiphase flow containing a dispersed phase. For details, see Degassing Condition (Multiphase only).