Confused about Surface Nusselt number evaluation in Fluent


I am really confused about Nusselt prediction in Fluent. I aware that there is a method to predict it from the surface integral>wall fluxes>surface Nusselt number. I tried to evaluate Nusselt on another method based on the temperature gradient in which :

q'' = -k (dT/dx) 

then h= q''/(T-Tref) = -k(dT/dx)/(T-Tref)

finally Nu=hL/k = -(dT/dx) * L/(T-Tref)

I found the Nusselt predicted from the two methods is not the same why? knowing that I set Tref at the reference values





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