The following limitations apply to the mixture model in Ansys Fluent:
You must use the pressure-based solver. The mixture model is not available with the density-based solver.
When the mixture model is used, do not model streamwise periodic flow with specified mass flow rate.
Do not model solidification and melting in conjunction with the mixture model.
Do not use the relative formulation in combination with the MRF and mixture model (see Limitations in the User’s Guide).
The mixture model does not allow for inviscid flows.
When tracking particles with the DPM model in combination with the Mixture model, the Shared Memory method cannot be selected (Parallel Processing for the Discrete Phase Model). (Note that using the Message Passing or Hybrid method enables the compatibility of all multiphase flow models with the DPM model.)
The mixture model, like the VOF model, uses a single-fluid approach. It differs from the VOF model in two respects:
The mixture model allows the phases to be interpenetrating. The volume fractions
and 
for
a control volume can therefore be equal to any value between 0 and
1, depending on the space occupied by phase 
and phase 
.The mixture model allows the phases to move at different velocities, using the concept of slip velocities. (Note that the phases can also be assumed to move at the same velocity, and the mixture model is then reduced to a homogeneous multiphase model.)
The mixture model is not compatible with non-premixed, partially premixed, and premixed combustion models.
The mixture model solves the continuity equation for the mixture, the momentum equation for the mixture, the energy equation for the mixture, and the volume fraction equation for the secondary phases, as well as algebraic expressions for the relative velocities (if the phases are moving at different velocities).