what is the utility is to be used to get the nusselt number for example F_V(f,ft) will give the velocity in y direction similarly what is the utility is to be used to get the nusselt number from fluent

define a custom field function for h*Dh/k where h is surf heat transfer co-eff from standard Fluent post

Nu_fluent=BOUNDARY_HEAT_FLUX(f,ft)*x[0]/(k*NV_MAG(A)*(F_T(f,ft))-Tbulk));

is the equation which i got the inf to use but here what does boundary heat flux mean and what does it give

my model is a shell and tube heat exchanger so i need inner nusselt number and outer nusselt number values i need to extract from fluent and need to store these values in a file so what is the utility i should use to get nusselt number value

pls let me know the how to get the utility for nusselt number asc wellas for heat transfer coefficient if any doubts reg my ques pls let me know