Discussion Forum

Hello everyone!

I bulit a 1D model to simulate transient diffusion-processes with multiple components, I used the "transport of concentrated species" model. At first I created a model with 3 components, H2/H2O/N2 and set N2 from mass constraint. At one side I have set a bounday condition with fixed mass fractions (the inflow) and on the other side I have an outward-flux of H2 and a complementary inward-flux of H2O (flux_x = (sin(2pi*f*t)/2F)*mass_x). What happens then is, when I plot the fluxes at the boundary (outflow), that I get an additional flux of N2 as well. I figured out, that it's because of the different molecule-weight of H2/H2O and Comsol balances the system like that. I worked around that problem by applying a convection term on the model which is directed away from the open boundary towards the "inflow". This makes sense, because in reality as well there would be a convection velocity because of the growing partial pressure of H2O. So far so good.

Then I extendet my model with 2 additional components, CO/CO2 and a reaction term for the water-gas-shift reaction. CO and H2O are consumed and H2 and CO2 are produced, but there is no flux of CO/CO2 over the boundary!
The problem now is, that I get fluxes of CO/CO2 and N2 over the open boundary, wether I set a "no flux" bc or not. If I disable the reaction term, the N2 flux disappears but the CO/CO2 stays...

Does anyone have an idea why Comsol reacts that way? I really would appreciate any kind of help, because I'm stuck right now with my assignement.

Thanks in advance - Helgo.