How to tackle the reverse flow . what is the cause of reverse flow. the outlet is already set to pressure outlet so what else to be checked out. this is regarding simulation of two phase gas solid flow.

Try to extend your boundary and see if flow still reverses. For one-phase flow, sometimes one uses a convective outflow boundary condition to avoid errors at boundary. You may want to check that (Ref: CFD by Ferziger and Peric)