Heat flux to be supplied to the external wall as a boundary condition is available through the DEFINE*PROFILE macro on the fluid-side wall. The heat transfer coefficient can be defined with the DEFINE*HEAT*FLUX macro. This macro calculates the heat flux by the following equation. qid = cid[0] + cid[1]*C*T(c0,t0) - cid[2]*F_T(f,t) - cid[3]*pow(F*T(f,t),4) qir = cir[0] + cir[1]*C*T(c0,t0) - cir[2]*F_T(f,t) - cir[3]*pow(F*T(f,t),4) qid: convective heat flux qir: radiant heat flux To specify the heat transfer coefficient, one can be set the same value for the cid [1] and cid [2] within the UDF. For example, if you want to fix the heat transfer coefficient in a wall as 10 SI units, it can be realized as the follows. DEFINE*HEAT*FLUX(flux,f,t,c0,t0,cid,cir) { if(THREAD*ID(t) == wall*id){ /* specify boundary heat transfer coefficient */ cid[1] = 10.; cid[2] = 10.; } } wall*id is the zone ID of the wall you want to fix.