Information on the mass flow rate outlet boundary condition for single-phase flow is available in Mass Flow Rate (Bulk Mass Flow Rate for Multiphase). For multiphase flow, mass flow outlets can be specified either for a particular phase or for the bulk flow. In all cases, the volume fraction and pressure profiles at the outlet are an implicit part of the calculation. The option to specify a pressure profile together with the mass flow rate for single-phase flows is not available for multiphase simulations.

If the mass flow rate for a particular phase is specified, then the mass flow distribution is a function of the mass flow and volume fraction profiles just upstream of the outlet boundary, adjusted to enforce the specified mass flow rate of that phase.

Boundary conditions for the other phases are also required. These may also be phasic mass flow conditions or any of the other available velocity conditions. Two particular cases that may be appropriate are a zero velocity specification, in which case the boundary is closed to the phase, and a no slip velocity condition, which is possible using a CEL expression. The phasic mass flow outlet condition is not available for homogeneous multiphase simulations.

If a bulk mass flow rate outlet condition is applied, then the bulk mass flow distribution is again a function of the mass flow and volume fraction profiles just upstream of the boundary, adjusted to enforce the specified bulk mass flow rate.

The bulk mass flow outlet condition is available for both homogeneous and inhomogeneous multiphase simulations.