Setting Initial Site Coverage
Good day, dear all!
I study catalytic conversion of CO, NO & C3H6 on Pt/Rh, using the mechanism from detchem.com.
There's no convergence with C3H6-reactions on Pt after 100 iterations, N2 conc. falls to zero. Only reduced mech. seems to be converging.
But, there's another question. In mixture properties -> reaction mechanism I specify two surface sites: Pt and Rh with site density 2.04e-08 and 0.68e-08 respectively. I must specify Initial Site Coverage (Define.. button). When I set them to Pt/1 on Pt and Rh/1 on Rh, I have quick convergent solution - if no other species are mentioned in these dialog box. Solution is symmetric in a cylindric pipe. When I specify them, for example, with all surface species with equal (or unequal) fractions, I have asimmetric solution in a pipe, that converges slowly, and has questionable reliability.
How should I set Initial Site Coverage? Is there any guide rules?
And does it mean I turn off all surface species production, if no species except Pt and Rh were choosen in Initial Site Coverage dialog box?
1) Initial Coverages are set by the known data - for ex., if I know, that catalyst is mostly coated with CO - then set CO fraction the largest. It's only initial guess. Real equilibrium (if it is) coverages are rapidly calculated by FLUENT.
It's only initial guess for solution of the problem of finding a steady-state for surface coverage. If it's only one steady-state present, the problem will converge to that steady-state from different initial guesses.
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