[koad]

I am having a problem using the washcoat factor for surface reactions. The washcoat factor increases the surface area for reactions, but is not taken into account in the mass flow calculations. This results in unphysical results. I believe this is an error in Fluent.

The setup of my problem is simple: a straight, cylindrical tube with incompressible ideal gas flow in steady state. The simulation is 2D, axisymmetric, with multi-component diffusion of 4 species. Let's call them AB, A, B, and C. C is the carrier gas.

Species AB and C flow into the tube from 1 end. On the inside surface of the tube AB reacts with the surface to create A and B:

AB(g) -> A(s) + B(g)

Gas species AB is consumed, solid species A is deposited on the inside surface of the tube, and gas species B is created. This is the only chemical reaction in the simulation. Gas species C, B, and the remnants of A flow out of the other end of the tube.

If the washcoat factor is 1 on the inside surface of the tube, then the simulation results make sense. The total mass flow is balanced:

Mass flow in = mass flow to wall + mass flow out

And the species mass flow is balanced:

mass flow to wall = rate of mass deposition of species A on wall
mass flow of B out = mass flow of AB in - mass flow of AB out - rate of mass deposition of species A on wall

However, if I set washcoat factor to 2, the mass flow of B doubles but the total mass flow to the wall and the rate of mass deposition of species A do not change. This is not physically possible. Fluent maintains mass balance by changing the mass flow of the carrier gas, C, but that is unphysical because C is neither created nor consumed in the system.

Has anyone else encountered this problem? Is there a work around? (Other than always setting the washcoat factor to 1, that is.)

[koad]

Talked to Fluent tech support. They confirmed that this is a bug in Fluent. They are working on a solution.