Learning Forum

[Yannik Danner]

Hey,

I have a question about the boundary condition for multiphase modeling. I have a Eulerian-Eulerian model for a water-air bubble column.

The column ist half filled  with water initially. For the outlet-BC I am using pressure outlet with water: backflow turbulent intensity = 0.001% and backflow hydraulic diameter = D_column and for air multiphase backflow volume fraction = 1. This works well for 2-phase-modeling. Now i want so use multiple air-phases (3) for PBM. With this I have to define new backflow volume fraction for the air-phases. But volume fraction is not constant any more. do i have to write UDF to define exact volume fraction for each phase or can i maybe chosse 0.33, 0.33 and 0.33?

Thanks for quick answering!

[Prashanth]

Hi 

You mentioned multiple air phases. If its just air, you can just keep it under one phase, ignore the top air gap and use degassing boundary condition.

[Yannik Danner]

Hey, thanks for the quick response.

There are severeal issues with this. First I cant keep it under one phase when using inhomogeneous PBM. All Air Phases represent different bubble size classes with different velocities. Therefore multiple Air Phases are needed. Secoundly  Degassing Boundary is just for one continous Phase and one disperse Phase. I wrote UDF fixing that that works well so I am able to use degassing for several disperse phases. But Air Volume Fraction is getting up to nearly 30% so if I would use degassing at initial water level I would ignore a enormous part of the column filled with water-air system.

[Rob]

If you have a high fraction of bubbles use degassing with some caution: read how it works. 

Also, for PBM, check which option you want to use. Some collect all the diameters into one phase. Otherwise, yes, you may need extra phases for each diameter. For outlet backflow I'd probably pick the largest diameter bubble phase as it shouldn't penetrate too far into the domain.