UDF macros for surface reaction
 

Hello everybody,I am modeling a surface reaction in porous medium in fluent.I want to write an UDF for the reaction rate,in my problem gas reacts in the porous medium only on the surface of porous solid.

I don't know should I use the macro : "FLUID_THREAD_P(t)" (that is used in the udf manual of fluent for an example of volume reacion rate) or not?

It is written in the manual that this macro is used to determine if a cell thread is fluid rather than solid thread.,and for volume reactions,if it's false,rate=0.
Should this macro be true for my surface reaction or false?
I'd appreciate anyone help.