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 mali28 January 11, 2012 08:34

Writing a UDF for heat loss from a composite wall

Hello,

I want to write a UDF file to calculate heat loss from the system to the atmosphere. The wall is insulated. The outside temperature is ambient.

The heat loss equation would be:

q = U(overall) * (Twall - Tambient);

I know the ambient temperature and overall heat transfer coefficient. q (heat flux through the wall) and Twall will be calculated by Fluent I presume.

:confused:

Thank you.

 ComputerGuy January 14, 2012 21:36

Hi! Can you describe the system a bit more? It's not really clear, if the wall is thermally insulated (heat flux=0=adiabatic), why you are specifying a heat transfer coefficient.

Do you mean you have a composite wall with a very low thermal conductivity? If so, Fluent has a built-in functionality to calculate the fluxes through surfaces(find it in Plots).

ComputerGuy

Quote:
 Originally Posted by mali28 (Post 338795) Hello, I want to write a UDF file to calculate heat loss from the system to the atmosphere. The wall is insulated. The outside temperature is ambient. The heat loss equation would be: q = U(overall) * (Twall - Tambient); I know the ambient temperature and overall heat transfer coefficient. q (heat flux through the wall) and Twall will be calculated by Fluent I presume. :confused: Thank you.

 mali28 January 15, 2012 06:52

Quote:
 Originally Posted by ComputerGuy (Post 339342) Hi! Can you describe the system a bit more? It's not really clear, if the wall is thermally insulated (heat flux=0=adiabatic), why you are specifying a heat transfer coefficient. Do you mean you have a composite wall with a very low thermal conductivity? If so, Fluent has a built-in functionality to calculate the fluxes through surfaces(find it in Plots). ComputerGuy

The wall is thermally insulated but there are some heat losses through the wall. Fluent does not have the functionality to add composite wall. I first have a layer of deposit at the inside of the geometry, then I have the metal wall, then I have a thick layer of insulation. The heat will e lost from inside to outside (ambient). I can only specify one wall with one set of thermal properties and thickness.
So I have to write a UDF, through which I calculate the heat flux.

Thank you.

 ComputerGuy January 15, 2012 09:21

Unless I understand your problem incorrectly, a 3 material wall is absolutely possible in Fluent. Simply construct your mesh such that the layers of the walls share nodes/faces at their interfaces, and Fluent will calculate heat conduction through them. You'll need to name each wall volume individually, but I guarantee multi-layered walls are perfectly acceptable and easily solved in Fluent.

ComputerGuy

Quote:
 Originally Posted by mali28 (Post 339362) The wall is thermally insulated but there are some heat losses through the wall. Fluent does not have the functionality to add composite wall. I first have a layer of deposit at the inside of the geometry, then I have the metal wall, then I have a thick layer of insulation. The heat will e lost from inside to outside (ambient). I can only specify one wall with one set of thermal properties and thickness. So I have to write a UDF, through which I calculate the heat flux. Thank you.

 mali28 January 15, 2012 09:40

Quote:
 Originally Posted by ComputerGuy (Post 339342) Hi! Can you describe the system a bit more? It's not really clear, if the wall is thermally insulated (heat flux=0=adiabatic), why you are specifying a heat transfer coefficient. Do you mean you have a composite wall with a very low thermal conductivity? If so, Fluent has a built-in functionality to calculate the fluxes through surfaces(find it in Plots). ComputerGuy

Thanks, I think I have got the solution here:
http://cape-forum.com/index.php/topic,3.0.html

 ComputerGuy January 15, 2012 09:43

OK. Just wanted to make sure you understand that I think Fluent can do what you want without having to write a single line of UDF code.

Quote:
 Originally Posted by mali28 (Post 339381) Thanks, I think I have got the solution here: http://cape-forum.com/index.php/topic,3.0.html

 mali28 January 15, 2012 10:10

Quote:
 Originally Posted by ComputerGuy (Post 339382) OK. Just wanted to make sure you understand that I think Fluent can do what you want without having to write a single line of UDF code.
Fluent only calculates heat loss for a single wall, not an insulated wall. Because in the wall properties, I can only specify properties of one wall.

 ComputerGuy January 15, 2012 10:27

If you can only specify "one wall," you haven't constructed the wall in such a way that Fluent understands that there are three distinct layers. Ensure you have created a named selected for each of the three wall volumes (Fluent 12+).

You might want to revisit some of the geometry creation and meshing tutorials for more help. If you think you've solved your problem, however, great.

ComputerGuy

Quote:
 Originally Posted by mali28 (Post 339388) Fluent only calculates heat loss for a single wall, not an insulated wall. Because in the wall properties, I can only specify properties of one wall.

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