Possible to model multi-phase (more than 2) flow?
 

For 2-phases, the answer is definitely yes.
But how about more than 2 phases? For example,
oil and water mixing in a tank filled with air. Or even four phases, let us say two types of oil and water mixing in a tank filled with air?

Yes, there are several ways to model more than two fluids/phases.

1. water & oil mixing in tank w/ air: use 1-fluid, free surface, with density evaluation physics and specify RHOF and MU1 as density and viscosity of water, RHOFS and MUS as density and viscosity of oil (specify in prepin file under namelist PROPS). Use initial fluid regions to specify initial locations of oil and water, and designate them as such via the initial fluid region density (RHOF to RHOFS sets the mixture ratio). The air will have uniform pressure, although it can apply a wind shear to the surface if it is blowing (specify 'Wind' physics in v10, see User Manual for how-to in earlier versions).

2. Water, air, and water vapor: search on the Flow Science website for 'non-condensible gas' for a tech note and example.

3. Water and any dispersed phase (sediment, air bubbles, etc.): use the drift-flux model with density evaluation. Search the website for tech notes.

4. Two types of oil and water and air (four phases): not possible without code customization.

Thanks, JBurnham.

Can I use two fluid and density evaluation? That is one fluid for gas (defined by flow rate, because the volume and pressure are both increasing, thus cannot be treated as void). Another fluid for the other two phases: oil and water, differentiated by density evaluation?

Tried but to find it is not possible. Thinking of an alternative way.