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Old   September 19, 2001, 03:13
Default reaction rate expression
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tang
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Hi all i am simulating the combustion of CO and dry air using the finite-rate-model. In FLUENT the reaction rate expression is used with water steam as follows:

k=2.239E+12exp(-1.7E+8/T)(CO)(O2)^0.25(H2O)^0.5

but d like to get the expression WITHOUT WATER STEAM. please give me a hand.

thanks in advance.
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Old   September 20, 2001, 14:00
Default Re: reaction rate expression
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sivakumar
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use the DEFINE_VR_RATE MACRO to program your reaction rate into each cell in the domain.
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Old   September 21, 2001, 02:22
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tang
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thanks sivakumar,id like to know the reaction rate expression without water stream, not how to implemente the expression in the code.
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Old   September 21, 2001, 04:39
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Greg Perkins
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This is the standard global form for this equation. The dependence on steam is due to reaction of radicals such as OH if a multi-step scheme is used (See Westbrook & Dryer, Comb Sci Tech., vol 27, 1981).

If your system doesn't have steam you might try leaving off the steam dependence. I haven't seen a form like this in the literature though. Perhaps others can comment on this.

Physically though CO and O2 react almost instantaneously at high temps (1500-3000K). Depending upon your setup, you might find Fluent has difficulty with these reactions using only the finite rate model. I've had to use some numerical limiting using a udf to get solutions and then test the sensitivity of these limiters with respect to the solution, for laminar diffusion flames, since Fluent can't handle the stiff kinetics. I believe other software (CFD-ACE?) can handle this better than Fluent.

Have you managed to get any converged solution using the rates above. What solver and settings have you tried?

Greg

According to Westbrook & Dryer
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Old   September 21, 2001, 05:12
Default Re: reaction rate expression
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tang
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Hi Greg Perkins, thank you for your comments. Yes, only using the finite reate model in Fluent is very difficulty to get a reasonalbe solution. I hve used the eddy-dissipation/finite-rate model with segrated solver. The converged solution i can get. The simulated Temperature, CO2 and O2 content are very good agreement with the experimental results. But the calculated CO content is 5 to 10 times higher or lower than the experiment data.
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Old   September 22, 2001, 03:21
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Greg Perkins
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Is your problem turbulent or laminar?

Greg
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Old   September 24, 2001, 04:38
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tang
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Hi,the gas flow in my system is turbulent.
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