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hi

I am trying to model the mixing of carbon powder and oxygen in a simple combustion chamber attached using a multiphase model. I modelled the chamber using gambit so I have two volumes a box (volume 1) with inlet and outlet and the volume 2 (the semispher) that represnt the carbon powder. the problem I am facing is how to define the boundary conditions for volume 2. Do I need to define the face between the two volumes as interface, interior or internal?. which model I have to use in such a case?

thanks a lot for your help.