# Writing a UDF for heat loss from a composite wall

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 January 11, 2012, 08:34 Writing a UDF for heat loss from a composite wall #1 Member   Join Date: Jun 2011 Posts: 83 Rep Power: 8 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. Thank you.

January 14, 2012, 21:36
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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 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. Thank you.

January 15, 2012, 06:52
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Quote:
 Originally Posted by ComputerGuy 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.

January 15, 2012, 09:21
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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 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.

January 15, 2012, 09:43
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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 Thanks, I think I have got the solution here: http://cape-forum.com/index.php/topic,3.0.html

January 15, 2012, 10:10
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Quote:
 Originally Posted by ComputerGuy 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.

January 15, 2012, 10:27
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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 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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