Define heat flux on specific faces of mesh
 

Hi! I need to add a heat flux on specific faces of my mesh (not at boundaries). I've looked up the related functions that can be used on UDF but none of them is applicable. (DEFINE_HEAT_FLUX defines the heat flux as functions of the heat transfer coefficients, whereas I need to define just the value of the heat flux, and DEFINE_PROFILE is defined only on the boundaries).
So, my question is whether is possible to define and call other functions than the ones that are already available at the UDF manual of FLUENT. Also, how is this possible? I can’t find any instructions/examples in the manual about that. Please let me know if someone has any insight about this. Thank you!