# Multiple species and UDS source terms

 Register Blogs Members List Search Today's Posts Mark Forums Read

 September 4, 2002, 06:05 Multiple species and UDS source terms #1 Ale Guest   Posts: n/a 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

 September 8, 2002, 21:54 Re: Multiple species and UDS source terms #2 Greg Perkins Guest   Posts: n/a 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

 September 9, 2002, 03:45 Re: Multiple species and UDS source terms #3 Ale Guest   Posts: n/a 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

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post aleisia FLUENT 2 October 9, 2011 04:40 Vishal FLUENT 0 March 22, 2006 00:00 Greg Perkins FLUENT 8 October 20, 2000 12:40 Greg Perkins FLUENT 0 October 13, 2000 23:03 Greg Perkins FLUENT 0 October 11, 2000 03:43

All times are GMT -4. The time now is 02:09.