I would suggest you to look at the Tutorial 15: Multiphase Flow in Mixing Vessel. The problem is similar to your case except that this is a batch reactor, whereas your case is a continuous flow type

 

This will give you the concepts involved in solving multiphase flows.

 

I would also suggest you to go through the following chapter in the online manual:

 

ANSYS CFX-Solver, Release 10.0: Modelling | Multiphase Flow Modelling | Modelling Advice for Multiphase Flow

 

This will give all the details of how to setup cases for multiphase flows.

