In Fluent, reactions can be set up via an incoming mechanism, or by UDF. The surface reaction UDF will override any existing surface reactions defined via the Species Model Reactions area, so if you are setting up surface reactions via UDF, all your surface reactions should be defined via UDF. The same is true for volumetric (gas phase) reactions.
You can take a look at the Fluent Customization manual DEFINE_SR_RATE UDF template to see an example of a surface reaction rate implementation. Similarly, DEFINE_VR _RATE can be used to specify your gas phase reactions.
If you are reading in a surface or volume reaction mechanism and don't plan to use the REACTIONS in it, you can delete all the surface or volume reactions between REACTIONS and END of a particular mechanism to enter a dummy reactions that will allow Fuent to read in the other mechanism data without error.
Best Regards,
Judy
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