HELP - Wall Heat Flux CFX

Karkoura · March 25, 2011, 12:29

Hi,

I have one question.
How does CFX calculate the wall heat flux? I understand the wall heat transfer coefficient is obtained from
htc = qw / (Tw-Tbulk)

But I don't see where qw comes from when a fixed temperature is specified at the wall.

Thank you for you help.

singer1812 · March 25, 2011, 15:03

qw = (rho)(cp)(u*)(Tw-Tf)/T+

u* and T+ are based on scalable wall functions or automatic wall treatment.

Best you read the help manual to determine what is appropriate to your case.

Karkoura · March 28, 2011, 18:09

Thank you for your reply.

I've been reading the documentation. And I have one question. If I understand correctly, the wall heat flux, when calculated based on the scalable wall functions or automatic wall treatment, is dependent on the thermal conductivity of the fluid. So I need to make sure that particular parameter, the fluid thermal conductivity, is correct in my simulations.

The thermal conductivity of the solid wall does not come into play unless a solid subdomain is created and volumetric heat transfer is to be analyzed.

Is that correct?

Again, thank you for your help.

ghorrocks · March 28, 2011, 18:24

The thermal conductivity of the wall is included in the heat transfer coefficient. The whole idea of the heat transfer coefficient is you draw a line in the system and say that everything beyond that line I will just simplify down to a coefficient.

Karkoura · March 28, 2011, 18:29

Hi Glenn and thanks for your quick reply.

So the thermal conductivity of the wall is included in the heat transfer coefficient. This means I need to make sure that CFX uses the correct material for the solid blade I am analyzing if I want to match experimental data.

However I did not create a solid subdomain in CFX because I do not want to analyze volumetric heat transfer. And I am not able to specify the material type (for ex. ceramic) for the blade. Which in my CFD domain is defined as a no-slip wall boundary condition with a specified fixed temperature.

Would you know how to specify material type for a wall boundary?

singer1812 · March 29, 2011, 12:44

You are using fixed wall temperature for your wall. There is no need to include the solid domain if this correctly models what you want it to model.

If you generalize results for analysis, you will have to state that your htc is based on a constant wall temperature, which is different than actually having the solid body in there.

If you do include the solid body, you would no longer use a fixed wall temperature. The solid will equilibriate based on the solid body BCs and the fluid interface.

March 25, 2011, 12:29	HELP - Wall Heat Flux CFX	#1
Karkoura New Member Join Date: Jan 2010 Posts: 13 Rep Power: 16	Hi, I have one question. How does CFX calculate the wall heat flux? I understand the wall heat transfer coefficient is obtained from htc = qw / (Tw-Tbulk) But I don't see where qw comes from when a fixed temperature is specified at the wall. Thank you for you help.

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March 25, 2011, 15:03		#2
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	qw = (rho)(cp)(u)(Tw-Tf)/T+ u and T+ are based on scalable wall functions or automatic wall treatment. Best you read the help manual to determine what is appropriate to your case.

March 28, 2011, 18:09		#3
Karkoura New Member Join Date: Jan 2010 Posts: 13 Rep Power: 16	Thank you for your reply. I've been reading the documentation. And I have one question. If I understand correctly, the wall heat flux, when calculated based on the scalable wall functions or automatic wall treatment, is dependent on the thermal conductivity of the fluid. So I need to make sure that particular parameter, the fluid thermal conductivity, is correct in my simulations. The thermal conductivity of the solid wall does not come into play unless a solid subdomain is created and volumetric heat transfer is to be analyzed. Is that correct? Again, thank you for your help.

March 28, 2011, 18:24		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,694 Rep Power: 143	The thermal conductivity of the wall is included in the heat transfer coefficient. The whole idea of the heat transfer coefficient is you draw a line in the system and say that everything beyond that line I will just simplify down to a coefficient.

March 28, 2011, 18:29		#5
Karkoura New Member Join Date: Jan 2010 Posts: 13 Rep Power: 16	Hi Glenn and thanks for your quick reply. So the thermal conductivity of the wall is included in the heat transfer coefficient. This means I need to make sure that CFX uses the correct material for the solid blade I am analyzing if I want to match experimental data. However I did not create a solid subdomain in CFX because I do not want to analyze volumetric heat transfer. And I am not able to specify the material type (for ex. ceramic) for the blade. Which in my CFD domain is defined as a no-slip wall boundary condition with a specified fixed temperature. Would you know how to specify material type for a wall boundary?

March 29, 2011, 12:44		#6
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	You are using fixed wall temperature for your wall. There is no need to include the solid domain if this correctly models what you want it to model. If you generalize results for analysis, you will have to state that your htc is based on a constant wall temperature, which is different than actually having the solid body in there. If you do include the solid body, you would no longer use a fixed wall temperature. The solid will equilibriate based on the solid body BCs and the fluid interface.