# Confused about Surface Nusselt number evaluation in Fluent

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• Last Post 17 January 2020
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khalidb77 posted this 12 January 2020

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

abenhadj posted this 13 January 2020

It depends in the reference values in for Tref and Lref.

Best regards,

Amine

khalidb77 posted this 13 January 2020

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

abenhadj posted this 13 January 2020

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.

Best regards,

Amine

khalidb77 posted this 13 January 2020

So do you mean that they are not the same?

Khalid

abenhadj posted this 13 January 2020

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

Best regards,

Amine

khalidb77 posted this 13 January 2020

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

abenhadj posted this 13 January 2020

Surface Heat Transfer Coefficient

Best regards,

Amine

khalidb77 posted this 13 January 2020

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?

abenhadj posted this 13 January 2020

Yeah as I expected. That is the method which I know moreover heat transfer is not only due to conduction but to convection to.

Best regards,

Amine

khalidb77 posted this 14 January 2020

which method do you mean that heat transfer due to convection and conduction?

Khalid

abenhadj posted this 14 January 2020

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.

Best regards,

Amine

khalidb77 posted this 14 January 2020

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

abenhadj posted this 14 January 2020

right

Best regards,

Amine

khalidb77 posted this 14 January 2020

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

rwoolhou posted this 15 January 2020

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

khalidb77 posted this 17 January 2020

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