CFD Online Discussion Forums

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   FLUENT (https://www.cfd-online.com/Forums/fluent/)
-   -   Equilibrium Reaction Rates/Gas Chemistry (https://www.cfd-online.com/Forums/fluent/30311-equilibrium-reaction-rates-gas-chemistry.html)

Greg Perkins September 17, 2002 20:33

Equilibrium Reaction Rates/Gas Chemistry
 
Has anybody worked out a way to have Fluent calculate equilibrium gas chemistry and/or equilibrium reaction rates when using the finite-rate/eddy-dissipation type reaction models???? Or other models??

Is this not possible through some simple user code etc.?

Greg

Duraivelan.D September 30, 2002 02:35

Re: Equilibrium Reaction Rates/Gas Chemistry
 
Ya is it possible with user codes. I have worked with both interpretated or Complied.. But I have one problem..I am astonished to see the fluent gives decreased species mass fraction even if there is no reaction.. This behaviour i saw when i have a velocity field..But i patch up the whole area with same mass frction but then too I get results like this.. If you are working in species mass fraction Please do inform me abt the procedures you follow.. I can tell you how to write the codes for our own mechanisms or rates.. Regards Duraivelan.D

Greg Perkins September 30, 2002 02:54

Re: Equilibrium Reaction Rates/Gas Chemistry
 
I'm not sure of your problem but for species you must add the source terms for the individual species and a source term for the sum of the species sources to be added to the continuity equation (this is because the last species is not solved, but its mass fraction is determined from the difference of the total continuity and all others).

For the reaction rates I'm interested to know what you did. Did you write a volume reaction rate udf??

I wrote one, but using this method is it actually equivalent to an equilibrium model?? I wanted equilibrium for water-gas-shift reaction, and used the volume reaction udf - I did however find the numerical solver convergence was sensitive to the forward rate and where I implemented the equation:

R = kf C1 C2 - kf/Kp C3 C4

where Kp is equilibrium constant. At equilibrium R = 0 and kf is immaterial, but within Fluent these need values. I tried to select kf as high as possible, but found I couldn't have it very high at all without convergence difficulty. The output results were a little sensitive to the value - I think a factor of 10 difference in kf changed the output mole percentage of some components by 2% - which seemed to indicate to me that complete equilibirum wasn't achieved.

Perhaps you tackled it differently, I'd be interested to know how. If you can send me your code that would be appreciated. If you want I can send you my vol. reaction rate udf so you can see my approach.

Thanks

Greg


Duraivelan.D September 30, 2002 03:05

Re: Equilibrium Reaction Rates/Gas Chemistry
 
Hi, I feel if you give me the Udf i can see what i can do..Because i worte My Udf to be dependent on mass fractions..So this seemed to be solvable..since you have the rate to be function of concentrations.. But I might see what i can do for you..What kind of UDF did you use..is it inter or Compiled.. Ok..Coming to my problem where do i enter the source terms for individual as well as for sum..Please clarify that for me.. Regards Duraivelan.D

Greg Perkins September 30, 2002 04:02

Re: Equilibrium Reaction Rates/Gas Chemistry
 
Your should have a DEFINE_SOURCE for each species being solved (N-1 in system) and a DEFINE_SOURCE that represents the sum of these source terms.

These are added into the boundary conditions panel for the fluid thread beign solved - click on source terms and select the udfs.

I'll send you my udf.

Greg

ashishpande April 27, 2009 15:05

Quote:

Originally Posted by Greg Perkins
;102656
Has anybody worked out a way to have Fluent calculate equilibrium gas chemistry and/or equilibrium reaction rates when using the finite-rate/eddy-dissipation type reaction models???? Or other models??

Is this not possible through some simple user code etc.?

Greg

Hi,
I have the same problem but for a volumetric liquid phase equilibrium reaction. My species balance seems to be sensitive to the value of forward reaction rate constant. Do you still remember the solution to this problem?
THanks,
Ashish


All times are GMT -4. The time now is 15:50.