Species Outlet: A fundamental Problem
 

Dear friends of the CFD Forums,

I've found myself several times in the situation where I have a geometry that, in some sense, acts like a reactor. This is, the concentration of species in the outlet has been reduced in respect to the inlet, where the concentrations are knonw.

This can be either an actual tubular reactor or other systems like particles deposition on walls, specially when the particle phase is simulated as an User Defined Scalar. In other words; every time an UDS is used to simulate a species which is destroyed or created inside the geometry, the same problem arises: What to write in the outflow condition?

Fluent allows a specification for UDS concentration or UDS flux in outlet boundaries, but those values might not be known in several applications. Which alternatives exist? How would implement another condition, like zero derivatives? Is there any way of making Fluent calculate the output by itself?

All suggestions/comments will be very wellcome.

Thanks,

Josep