Could you post a picture of the domain so that I can offer a suggestion?

I have attached a picture.

The circular extrusion will work as heat source and the rest is fluid.

I want that if the temperature of fluid rises above a certain limit the temperature of heat source should be reduced.

I think, that you do not need UDF for this purpose. The "new" Fluent expression language should be entirely sufficient. You can formulate one or several expressions for evaluating the fluid temperature at desired locations or as a volume/surface integral (e.g. at the outlet cross section). Then you can formulate another expression for the heat source, which you make functionally dependent on the first (set of) expressions. if-then-else control structures are available as well in the expression language.

Regards,

Dr. Th. Frank.