bim package symmetry boundary condition at reactor outlet

[bim7]

Hi to all, 

I am using the latest bim-package to model a species transport problem in
cylindrical coordinates. 
It is a classical tube reactor, with an inlet, an impermeabel wall, a
symmetry line in the center and an outlet. 

Applying a Dirichlet boundary conditions for the inlet works quite fine and
a Neumann boundary with no flux at all through the wall works as well. 

I am having quite some trouble to get the right outlet boundary condition.
Naturally I would apply a symmetric boundary condition stating that there is
no concentration change in the axial direction over the boundary in normal
direction. 

I tried using a Neumann BC but this looks like setting the overall flux to
zero and not only the diffusive part of it. Counter-balancing that by the
redirected convective flux did not work for me so far.  
Robin does not work either, despite it seems to be the right choice.

Any ideas on how I could solve that issue? 

Best regards, 
Sebastian 


 



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