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Heat loss through wall to surroundings, wall Heat flux

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Old   December 20, 2017, 11:02
Default Heat loss through wall to surroundings, wall Heat flux
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I need to calculate the Heat loss to the surroundings by an Insulation wall. The insulation is solid domain. one side is in contact with the heat source and the other side(ie wall) is exposed to ambient conditions.

I use the following two options in CFX-Pre. Why are they both give different values? additionaly I see 'Wall heat Flow' variable in cfx - pre variables, what is that?

1. Heat loss = (areaAve(T)@walls-ambtemp)*htc*surface area of the exposed wall
2. Heat loss = areaInt(Wall Heat Flux)@walls

@walls - this refers to the area of Insulation exposed to atmosphere.

I want to observe the heat loss during solver run as I need this data for every time step.
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Old   December 20, 2017, 17:30
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Be careful with the heat transfer coefficient. By default it does not use your ambient conditions as the reference condition, but your equation references it against ambient. You need to set an expert parameter for the HTC to be referenced to a defined temperature.
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Old   December 20, 2017, 18:29
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Quote:
Originally Posted by ghorrocks View Post
Be careful with the heat transfer coefficient. By default it does not use your ambient conditions as the reference condition, but your equation references it against ambient. You need to set an expert parameter for the HTC to be referenced to a defined temperature.
Thank you for the response.
In the above equation: amptemp= 25 degC. htc = 10 W m-2 K-1. I am not taking system variables. I defined above two as values using expressions. In this case why am I getting difference values for the above two formulas? and also any info. on Wall Heat flow available in CFX Pre variables?
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Old   December 20, 2017, 18:43
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Oh, I see. You are defining the HTC as the external wall of a solid domain, not as an internal fluid boundary.

How big is the difference between the two heat flows?

Note that areaAve(T) will give you the area averaged temperature of the control volumes, but areaInt(Wall Heat Flux) integrates over the integration points which is a finer resolution. For a fine mesh the difference should be very small.
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Old   December 20, 2017, 19:36
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Quote:
Originally Posted by ghorrocks View Post
Oh, I see. You are defining the HTC as the external wall of a solid domain, not as an internal fluid boundary.

How big is the difference between the two heat flows?

Note that areaAve(T) will give you the area averaged temperature of the control volumes, but areaInt(Wall Heat Flux) integrates over the integration points which is a finer resolution. For a fine mesh the difference should be very small.
12832 [J], -21167[J] for 1st formula and 2nd formula. "-" sign for the second formula is because of Heat flux term, like heat is going out. There is significant difference. regarding areAve(T), I use this areaAve(T)@exposed wall. So, will not take the average temperature of the wall/surface area exposed instead of the whole domain?

Which formula gives less error or values close to real?
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heat flux., heat transfer cooling, insulation, wall heat flux, wall heat transfer

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