Rob
Ansys Employee

I suspect it'll be a maths definition somewhere, and may be defined in more detail in a reference given at the top of the section   https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v222/en/flu_th/flu_th_sec_mod_p1.html%23flu_th_radiation_p1_particles  

The heat sink (source) to the fluid volume is given at the end of the above.