# Confused about Surface Nusselt number evaluation in Fluent

Hello

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

Thanks

Khalid

• edited January 13
It depends in the reference values in for Tref and Lref.
• I was set Tref & Lref correctly into reference values. My question if I set the local Nusselt number on a custom field function as below :

Nu = -dT/dx*(Lref/(T-Tref))

why it gives different Nusselt from that obtained by wall fluxes > surface Nusselt number? knowing that I was estimate both numbers on the same line?

Khalid

• edited January 13

Fluent uses the wall heat transfer coefficient in the definition of Nusselt number. And the latter is build up with the conservative wall heat flux.

• So do you mean that they are not the same?

Khalid

• edited January 13

What does alpha(HTC)*Lref/heat conductivity provide if put in CFF?

• Excuse me what do you mean with the term alpha(HTC)?

• edited January 13

Surface Heat Transfer Coefficient

• What does alpha(HTC)*Lref/heat conductivity provide if put in CFF?

With this relation it gives the same value as the method of wall fluxes but not the same as the method whuch define Nu in terms of derivative dT/dx?

• edited January 13
Yeah as I expected. That is the method which I know moreover heat transfer is not only due to conduction but to convection to.
• which method do you mean that heat transfer due to convection and conduction?

Khalid

• edited January 14

Fluent calculates his Surface Nusselt Number as already mentioned and the way it is documented in the User's Guide.

If you want to have your own definition of Nusselt Number do not compare with the one of Fluent. Your definition does only take into account for thermal conduction and not for convection.

• You mean Surface Nusselt Number in Fluent based on conduction and convection right?

• edited January 14

right

• Thanks Mr. Abenhadj for the response. May you tell me please the relation that Fluent uses in evaluating the Total Surface Heat Flux because I failed to find it on the User's Guide?

Khalid

• edited January 15

Have a look in the field variables part of Field Function Definitions.

• Have a look in the field variables part of Field Function Definitions.

I found the relation at last that Fluent uses to predict the surface heat flux

for horizantal  wall   q=-k dT/dy

for vertical wall  q=-k dT/dx

I remarked this post as solved now