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Old   March 31, 2008, 02:35
Default meaningless solution please help
  #1
alp
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I simulate an octane steam reforming in a tubular reactor. reaction type finite rate and just forward reaction occurs not include backward rxn. when I run the fluent, programme converged successfully. there is no backflow.. but when I look at the results, there is an increase in molecular concentration not only for products but also for reactants (steam and i-octane)... how can it be?

please help it's urgent

thanks in advance...
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Old   April 7, 2008, 08:25
Default Re: meaningless solution please help
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Aris
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Steady or unsteady? What convergence criteria do you have? For species i believe you should have 10^-6. Moreover which is the last species you defined? This species (for example steam) will be not solve. It's mass fraction would be calculated as (1- sum of mass fraction of other species). This means that the error accumulates in steam. You should define as last the species with highest concentration in your field
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Old   April 7, 2008, 08:49
Default Re: meaningless solution please help
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alp
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steady yes for species 10^-6 last species is one of the products yeah, I know that we enter n-1 species to the fluent

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Old   April 7, 2008, 08:53
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Aris
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If it's not so time consuming could you check unsteady?
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Old   April 7, 2008, 14:59
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alp
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I also tried unsteady but nothing changed and defined the steam as last species
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Old   April 8, 2008, 04:07
Default Re: meaningless solution please help
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Aris Nikolopoulos
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I see.

I don't have your case but let me try to suggest some things:

-Backward reaction. I think that with finite rate backward reaction is ON. Check user guide! -Try eddy dissipation model! - Check your species properties carefully. You may have a wrong Enthalpy or Entropy! - Finite rate model's contants. Were did you find them?

I hope i helped
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Old   April 8, 2008, 04:37
Default Re: meaningless solution please help
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alp
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I tried backwardrxn on but I encountered an error: amg solver divergence or something like that. the constants were taken from a journal paper so the problem is not related with them. may be enthalpy or entropy of iso-octane is wrong. others' are ok..

is the problem about meshing??
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Old   April 8, 2008, 07:11
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Aris
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What about eddy disipation?
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Old   April 9, 2008, 04:13
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alp
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when I turn on eddy disipation, I encounter reverse flow and temperature limitation to 1 K at iteration level
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Old   April 10, 2008, 04:31
Default Re: meaningless solution please help
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Aris Nikolopoulos
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Reverse flow. It's not a problem if after some iterations the solution doesn't have reverse flow! The same about temperature. But temperature 1K makes me a little suspecious. You should very carefully check your enthalpies. Check out also the heat release from your reactions. Are they endothermic or exothermic?
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Old   April 11, 2008, 07:14
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alp
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reverse flow continues for whole. I checked species enthalpies and entropies from literature and they are matching with each other..reaction is endo I checked it also... do you have any other suggestion?

I am stuck with this problem....

thanks in advance
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Old   April 14, 2008, 04:45
Default Re: meaningless solution please help
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Rami
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I cannot comment specifically for the problem at hand, but rather on the reversed flow in general. As posted above, reversed flow may be only an intermediate iteration result (although it may also be physical in some cases). If this is the case, and very unrealistic values are prescribed at the outlet (e.g., if the default temperature is 0K), then this can lead to convergence problems or even divergence (as seems to be your case). The posssible remedy is straightforward - prescribe some reasonable values for all variables at all openings, so that when reversed flow occurs, it will not spoil the solution.

I hope this helps.
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Old   April 14, 2008, 05:49
Default Re: meaningless solution please help
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HP
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Yes! As Rami says, check out the reverse flow temperature you have chosen. It should have a realistic value
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