[cfdEng_]

Hi,

I would have a naive question about energy conservation on Fluent.

Background: My domain is very simple: BL flow over a flat plate.

Problem: If I calculate the total heat transfer rate at the inlet (Report>Fluxes>Total heat transfer rate) and I compare it with the value I get from CFD-post: areaInt(Density*Velocity u*CP*T0)@inlet, the two results are completely different.

By checking the energy balance from Report>Fluxes>Total heat transfer rate, energy conservation is respected. However, it is not with CFD-post.

Am I doing something wrong?

Thank you in advance.

[MKuhn]

Check if you have the same reference state for calculating the enthalpy. Maybe in Fluent Total heat transfer rate at a flow boundary is only massflow by enthalpy, without the kinetic part. But I'm not sure.

[somia baali]

Hello every one
my geometry is 3D flat plate solar collector
i want to ask you about calculating Nusselt number in fluent
i try in fluent -report-surface integral-area weight average-wall flux- surface nusselt number but the result is incorrect
please help me how can i found correct result

[cfdEng_]

Originally Posted by MKuhn

Check if you have the same reference state for calculating the enthalpy. Maybe in Fluent Total heat transfer rate at a flow boundary is only massflow by enthalpy, without the kinetic part. But I'm not sure.

I think you are right. Apparently Fluent uses Tref=298.15K as the reference temperature in all enthalpy calculations.

Therefore, if I work out areaInt(Density*Velocity u*CP*(T0-298.15[K]))@in I get the same result that comes from the total heat transfer rate calculation. By adopting Tref, the energy balance holds on CFD-post as well.

What puzzles me is whether Tref=298.15K is also used to calculate the wall heat flux. Any idea?

Thank you in advance.

[MKuhn]

I would say, the value of the wall heat flux at a flow boundary is quite meaningless if you consider the single value. It is only usable for the caluclation of the energy conservation (sum up of all wall boundaries), because in this case you don't care about the reference state.

[cfdEng_]

For stationary flows, I get a pretty good approximation in terms of energy balance.

On the other hand, I am still experiencing a severe energy imbalance in the turbulent regime (implicit LES). Is there a quick way to assess the energy balance for transient flows with Fluent?

Thank you in advance.

[LoGaL]

Which one are you controlling for imbalance? Average one or istantaneous one?

[lei]

The Nu number and HTC calculation in Fluent depends on the ref temperature settings, please make sure you set a meaningful reference value in the Reference Values panel before calculating heat transfer coefficients.

https://www.afs.enea.it/project/nept...ug/node972.htm

Originally Posted by somia baali

Hello every one
my geometry is 3D flat plate solar collector
i want to ask you about calculating Nusselt number in fluent
i try in fluent -report-surface integral-area weight average-wall flux- surface nusselt number but the result is incorrect
please help me how can i found correct result

[somia baali]

Originally Posted by lei

The Nu number and HTC calculation in Fluent depends on the ref temperature settings, please make sure you set a meaningful reference value in the Reference Values panel before calculating heat transfer coefficients.

https://www.afs.enea.it/project/nept...ug/node972.htm

thank you for your reply, your answer is true, my problem now in how can i determine the reference values

[somia baali]

Originally Posted by lei

The Nu number and HTC calculation in Fluent depends on the ref temperature settings, please make sure you set a meaningful reference value in the Reference Values panel before calculating heat transfer coefficients.

https://www.afs.enea.it/project/nept...ug/node972.htm

thank you so much Lei, your reply is true, but my problem now in how can i determine the reference values exactly.