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Reaction in porous media - very slow convergence

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Old   November 27, 2015, 10:14
Default Reaction in porous media - very slow convergence
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Laurien Vandewalle
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Hi there!

I am trying to simulate a honeycomb catalytic reactor by using the porous media model in Fluent. For the moment, I am using a small axisymmetric 2D grid and my reaction network only consists of 6 reactions (adsorption/desorption of O2 and CO, surface reaction to CO2, desorption of CO2). I use the steady-state pressure-based solver.

First, I run the case without reactions. It converges almost immediately.
Then I set the surface-to-volume ratio to 91000 for the porous zone and I run it again. However, it looks like the reaction does only proceed at the inlet of the porous zone, and the reaction front moves VERY SLOWLY through the reactor. I monitor the mole fraction of CO2 at the outlet and this increases slowly during the entire simulation.
The case reaches convergence after almost half a million iterations, which is way too much since my grid is only 45x30 cells. All underrelaxation factors are set to 1 so that is not the problem. There are no instabilities in the residuals.

Does anyone know this problem or has an idea how to solve it? Thanks in advance!

Kind regards,
Laurien
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