Hi tried a demo problem ( AcuSolve Training - Tut 2 - CHT - heating of fluid flowing through a heated pipe - outer pipe srf is at higher temperature) and the results says the exact opposite :
1) Heat transfer into the fluid domain (as well as solid) is negative!
Elaborated as follows:
A solid and a fluid always have two interfaces in between - coincident and exactly same overlapping surface meshes - one interface is attached to the fluid volume ( call this fluid_int) the other to solid volume (solid_int) - so heat is transferred from solid to fluid volume
so If I plot the "heat flux" in acuProbe on "solid_int" its +ve and on "fluid_int" is negative
Hence the (1) above!