CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Multiple species and UDS source terms

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 4, 2002, 07:05
Default 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
  Reply With Quote

Old   September 8, 2002, 22:54
Default 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
  Reply With Quote

Old   September 9, 2002, 04:45
Default 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
  Reply With Quote

Reply


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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Questions for a species transport problems (snapshots attached) aleisia FLUENT 2 October 9, 2011 05:40
Species transport eqn using UDS Vishal FLUENT 0 March 22, 2006 00:00
UDF Scalar Code: HT 1 Greg Perkins FLUENT 8 October 20, 2000 13:40
UDFs for Scalar Eqn - Fluid/Solid HT Greg Perkins FLUENT 0 October 14, 2000 00:03
UDFs for Scalar Eqn - Fluid/Solid HT Greg Perkins FLUENT 0 October 11, 2000 04:43


All times are GMT -4. The time now is 08:39.