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-   -   Multiple species and UDS source terms (http://www.cfd-online.com/Forums/fluent/30267-multiple-species-uds-source-terms.html)

 Ale September 4, 2002 06:05

Multiple species and UDS source terms

Hi to all,

I wanted to check the right way to write a source term in fluent, so I solved a simple problem of laminar non-premixed combustion, using the Multiple Species method and ticking off only the option Volumetric Reaction.

In the same simulation, I attempted to calculate the profiles of the species by UDF.

The source term I have written for methane is the following:

DEFINE_SOURCE(p_source_ch4, c, t, dS, eqn) {

double source_ch4;

source_ch4=-16/C_R(c,t)*(2.119e11*exp(-2.027e8/8314/C_T(c,t))*pow((C_UDSI(c,t,0)*C_R(c,t)/16),0.2)*pow((C_UDSI(c,t,1)*C_R(c,t)/32),1.3));

return source_ch4; }

C_UDSI(c,t,0) is methane mass fraction; C_UDSI(c,t,1) is oxygen mass fraction. Other source terms for other species are similar. The kinetic expression is the one used by Fluent for the methane-air mixture.

The problem is that contours of UDS are different from the corresponding mass fraction contours calculated by Fluent.

I used for diffusivity the constant-dilute-appx (2.88e-5) and I set to the same value for UDS Diffusivity.

Has anyone idea of the reason of this discrepancy? Where's my mistake?

Thank you very much!

Ale

 Greg Perkins September 8, 2002 21:54

Re: Multiple species and UDS source terms

If you're trying to replicate a chemical reaction you need to have sources for all the species participating in the reaction - some will be positive (products) and some negative (reactants).

Thus for ch4 + 2O2 -> co2 + 2h2o

you'll need sources for each of the four species to get the right mass fractions!

Ps: Its possible to solve this yourself - but for laminar systems where chemical kinetics generate a stiff matrix, you may find solutions diverge unless you are careful with the implementation. Fluent's added a stiff chemistry solver in version 6

Greg

 Ale September 9, 2002 03:45

Re: Multiple species and UDS source terms

Thank you for reply.

I have written of course a source term for each of the four species; I reported only the one of methane for the sake of brevity.

Anyway, I noticed such discrepancy in the results of converged simulations (I didn't have problems with convergence).

I made these simulations just to check the right way to write a source term in fluent (I chose a laminar sistem just to avoid mixing effects).

Anyway, thank you very much for your response.

Ale

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