The process of solving a multiphase system is inherently difficult and you may encounter some stability or convergence problems.

For a steady solution: It is recommended that you use the Multiphase Coupled solver, described in detail in Coupled Solution for Eulerian Multiphase Flows in the Fluent User's Guide. The iterative nature of this solver requires a good starting patched field. If difficulties are encountered due to higher order schemes, or due to the complexities of the problem, you may need to reduce the Courant number. The default Courant number is 200 but it can be reduced to as low as 4. This can later be increased if the iteration process runs smoothly. In addition, there are explicit under-relaxation factors for velocities and pressure. All other under-relaxation factors are implicit. Lower under-relaxation factors for the volume fraction equation may delay the solution dramatically with the Coupled solver (any value 0.5 or above is adequate); on the contrary, PC SIMPLE would normally need a low under-relaxation for the volume fraction equation.

For a time-dependent solution: A proper initial field is required to avoid instabilities, which usually arise from poor initial fields. If the CPU time is a concern for transient problems, then the best option is to use PC SIMPLE. When body forces are significant, or if the solution requires higher order numerical schemes, it is recommended that you start with a small time step, which can be increased after performing a few time steps to get a better approximation of the pressure field.

When computing unsteady flows using Non-Iterative Time Advancement (NITA), good initial conditions are important. Stability problems may arise in models with poor meshes or in the presence of large body forces.

If you are using the MRF model for steady-state or quasi-steady analysis and are experiencing convergence problems, you can switch to the unsteady solver and attempt to converge to a steady solution. When using NITA with the MRF model, you should be aware of NITA robustness problems due to poor mesh quality or large source terms in the momentum equations at the MRF boundaries. The Iterative Time Advancement (ITA) is preferable for MRF simulations as it gives you more control over the number of iterations per time step.

In addition, Ansys Fluent offers a Full Multiphase Coupled solver where all velocities, pressure correction and volume fraction correction are solved simultaneously, which currently is not as robust as the others.

Furthermore, Ansys Fluent has an option to solve stratified immiscible fluids within the Eulerian multiphase formulation. This feature is similar to the single fluid VOF solution, but in the context of multiple velocities.