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Old   April 18, 2010, 09:47
Default bulk Temperature - htc
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Hi all,

i am doing calculations to determine the heat transfer coefficient. I am interested in values averaged over an area.

Everything works fine. But if i am setting a bulk temperature in the experts parameters to compare the values to literature, cfx gives me averaged values of zero. what is going wrong?

Thanks in advance!
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Old   April 18, 2010, 13:23
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Hello!

Quote: "if i am setting a bulk temperature in the experts parameters to compare the values to literature, cfx gives me averaged values of zero"

Is it close to zero or exactly zero? Which tbulk do you specify?
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Old   April 20, 2010, 10:15
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Hi,

thanks for reply!

This is an Air-Flow around a body. For bulk temperature, i use the inlet temperature of 293K. But everytime i specify an arbitary bulk temperature, the htc is exactly zero.

For evaluating the htc, i just use the function calculator. Is this correct? Or do i have to use some kind of expression?


Regards!
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Old   April 20, 2010, 11:30
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Quote: "For evaluating the htc, i just use the function calculator. Is this correct?"

It's all about bulk temperature - its choice depends on what the flow is doing. If you are sure of the bulk temperature, then it's correct.

If htc is exactly zero then there's no heat flux. Are you sure you have thermal (total) energy enabled? Or may be your wall of interest is adiabatic?

Anyway, you can calculate htc yourself in cfx-Post: WallHeatFlux/(Twall-Tair)
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Old   April 20, 2010, 17:01
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Hi again!

The problem seems to be the function calculator. If i choose the location

1) Domain Interface Fluid

areaAve Wall Heat Flux: 0 Wm^-2
areaAve Wall Adjacent Temp: 300 K (bulk Temp. is this case...)
areaAve Temperature: 590 K
--> areaAve Wall Heat Transfer Coefficient: 0

2) Domain Interface Solid

areaAve Wall Heat Flux: 4012 Wm^-2
areaAve Wall Adjacent Temp: undefined
areaAve Temperature: 590 K
--> areaAve Wall Heat Transfer Coefficient: undefined

3) Solid Region behind Domain Interface

areaAve Wall Heat Flux: 4012 Wm^-2
areaAve Wall Adjacent Temp: undefined
areaAve Temperature: 590 K
--> areaAve Wall Heat Transfer Coefficient: undefined

So i think i could easily calculate htc by hand. But the strange thing is, this only happens, when a bulk temperature value is set. If not, i could get a htc-value by the function calculator.


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Old   April 21, 2010, 09:12
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What kind of wall BCs do you use?
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Old   April 21, 2010, 11:36
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I attached a .ccl -File of this problem. It's a very simple model for testing. The solid is just a simple plate in air domain with a hole in the middle where som heat flux is applied. Domain Interface 1 is plate front, 2 is plate back, 3 are the plate edges. The .gtm -File is quiet large, so i could not attach this.

Thanks and Regards!
Attached Files
File Type: zip Unnamed.zip (3.7 KB, 20 views)
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Old   April 22, 2010, 00:23
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Could you please provide some screenshots from cfx-Pre (mesh with BCs marked) and cfx-Post (plane temperature distribution)?

Anyway, your interfaces are OK, but you should set lower RMS and use conservation target for CHT. It's also important to monitor some temperatures in both fluid and solid domains while solving to make sure they don't change significantly within final iterations. That would ensure that fluid/solid timescales are OK.
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Old   April 22, 2010, 10:44
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Thanks for the hints!

Unfourtunaly, i could not access cfx the next days. But i don't think the problem is the model setup. I also tried the heating coil example provided with the installation (Ansys Version 12.1) and the same behaviour occurs there: you could not get an areaAve htc value with the function calculator when a bulk temperature is set ...

I am also nut sure, how the htc value is calculated, when no bulk temp is set. When i do the calculation by hand using areaAve Wall Temperature, Heat Flux and WallAdjacentTemperature, the result does not match the value the function calculator gives me!

So what is the htc value via functioncalculator and no bulk temperature worth? It seems to be meaningless because it is not clear, witch reference temperature is used...

Regards!
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Old   April 22, 2010, 11:38
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Default reference temperature for htc is the temperature in the first node off the wall. Yes, it seems meaningless that way.
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