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Setting Initial Site Coverage

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Old   April 4, 2012, 18:10
Smile Setting Initial Site Coverage
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John M.
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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?
-----
Thanks, Villager.

Last edited by villager; April 7, 2012 at 04:57.
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Old   April 16, 2012, 16:20
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John M.
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My thoughts
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.
2)
Quote:
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?
Yes, it does. I've checked it.
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Old   October 16, 2013, 11:01
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John M.
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Quote:
Originally Posted by villager View Post
2) Yes, it does. I've checked it.
No, certainly, it doesn't (the mechanism mentioned above works quite unstable in fluent, that was a case)
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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