The problem is about gas-solid flow. gas and solid enter the riser from bottom, and leave out of the top outlet which is set as pressure boundary. In CFX, the default value of pressure at outlet is zero. During my simulation, the pressure decreases from inlet to outlet gradually. But near the outlet, the pressure donot decrease to zero, it is about 10^2. The simulation results indicate the large solid concentration at outlet. This is defintely not right. I guess the reason is outlet pressure. So what should deal with the outlet condition or do something else? Thanks in advance.

Hi Neil

My guess would be that the outlet/press bdry are too close to the action. BCs work best when they are far enough away from regions where stuff is happening that they are not affected by whats going on in the domain.

Is the high solds conc. caused by reverse flow into the domain, or is it caused by solids convected up from the bottom. If they come from the bottom, and you think they shouldn't then definately try moving the BC up higher.

I hope you can solve your problem