mass balance problem in transient simulation
 

Hi, I am Nana, a new one of CFX online, I have a problem when running the transient simulation in bubble column. The problem is when the first seconds (before bubble reach the liquid surface), there only bubbles in, but no bubbles out, then the gas phase mass is imbalance, and if the liquid phase should go out from the top? How to set the outlet boundary condition? Degassing or opening? The software I am using is CFX 12.0.

The difference between these boundaries is explained in the documentation. Which one is suitable for your application will depend on what you are trying to model.

mass balance problem in transient simulation
 

Thank you for reply.
I want to know if there is a notice "A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 99.8% of the faces, 99.8% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: OUTLET. |
| The fluid name is: Air. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead." in the solver, then if I should change the degassing b.c. to opening b.c?

And I have another problem: using either degassing or opening b.c. in the three-phase flow transient simulation, the residual curve will be show a jump at the same timestep. Is the drag force improper or what is happening?

Thank you!