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-   -   UDF changing the rate exponent of a reaction (http://www.cfd-online.com/Forums/fluent-udf/68364-udf-changing-rate-exponent-reaction.html)

Stefan H September 16, 2009 13:20

UDF changing the rate exponent of a reaction
 
Hi everyone I am new here!



I have a problem that I can not solve... Perhaps there is someone who can help me out?



I am using fluent 6.3 and I am simulating the combustion of a turbulent propane flame using the RSM in combination with the EDC.


There are 5 reactions in the combustion mechanism that I am using. One of these reactions is the reverse oxidation of hydrogen where the following is valid.

Reaction: H2O -> H2 + 0.5 O2
(A=xxx, Beta=xxx, Ea=xxx)
Reaction order: [H2]^-0.75[O2][H2O]

All reactions data are implemented by the standard GUI reaction panel.

What I would like to do in the UDF is to change the rate exponent for H2 to 1 instead of -0.75, if the concentration of H2 is lower than a specified value, say 10^-5. This is to prevent the reaction rate from becoming extremely large at low concentrations of H2, which leads to convergence problems.


I have been thinking about using the macro DEFINE_VR_RATE but maybe it is not possible to use with the EDC model? Is it possible to use DEFINE_NET_REACTION_RATE instead? One other possible way could perhaps be to add the modified reaction as a 6th reaction by means of the GUI panel and then have the udf to select the "right" reaction...


Could you please give me some advise?


Kind regards,
Stefan


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