interface condition in gas-solid flows
 

Hello
I am working on a computer code which simulates the hydrodynamics of gas-solid flow in a fluidized bed rectors with two types of particles by using the Eulerian-Eulerian method. This code determines solid volume fraction corrections instead of solid pressure corections to speed up the numerical solution. SIMPLE algorithm is used to link pressure and velocity fields.
When volume fraction of solid paticles approach zero on gas-solid interfaces, unexpected variations occure; especially in vertical velocities and solid volume fraction corrections. It seems I need to exert some conditions on these interfaces, but I don't know about them.

Please help me to complete the code.
Thankyou...