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Old   November 20, 2012, 07:03
Default thermal conductivity expression
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Thomas Hoffmann
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hi all.
I would like to add an expression for thermal conductivity
k=-0.000837*T^2-0.030641*T+0.147212
But I get the following error:
'
Bad expression value '-0.000837*T[K^2]^2-0.030641*T[K]+0.147212' detected in parameter 'Thermal Conductivity' in object '/LIBRARY/MATERIAL:Ice cream/PROPERTIES/THERMAL CONDUCTIVITY'.
CEL error:
Error at position 12. Units ([K^2]) are associated with a term that is not dimensionless.
'
I also tried to write it like this:
k=-0.000837*T[K^2]^2-0.030641*T[K]+0.147212

Does anybody know how to use the expressions in cfx?
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Old   November 20, 2012, 07:19
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Talita Possamai
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Hello Thomas,

You must indicate the units of each constant. So it should be:

k=-0.000837 [W m^-1 K^-3]*T^2-0.030641 [W m^-1 K^-2]*T+0.147212 [W m^-1 K^-1]

This will give you thermal conductivity in [W m^-1 K^-1]

Regards,
Possa
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Old   November 20, 2012, 07:27
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easy
thanks!
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Old   December 6, 2012, 04:34
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Thomas Hoffmann
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Hi Possa.
It worked for me to apply the material with this expression for thermal conductivity, but the solver has problem working with it.
I tried to change all the parameters to water one by one, and found out that this expression is the only problem. Do you know if cfx nows that T is temperature? Do I have to define it somewhere?
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Old   December 6, 2012, 05:18
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Glenn Horrocks
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The variables and their names is listed in the CFX reference manual, under variables. You should consult it to make sure you use the correct name for variables.
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Old   December 6, 2012, 06:47
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Talita Possamai
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Hello Thomas,

Yes, you're right. You must change T for Temperature for this to work.

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Possa
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Old   December 7, 2012, 08:34
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I had a look in the reference manual, and it says that the short variable name for temperature is 'T'. But it doesn't work for me.
Do you know where I have to define T in cfx-pre if I want to make my thermal conductivity temperature dependent?
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Old   December 7, 2012, 08:53
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Talita Possamai
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Hi,

Try putting like this in the expression definition tab:

-0.000837 [W m^-1 K^-3]*Temperature^2-0.030641 [W m^-1 K^-2]*Temperature+0.147212 [W m^-1 K^-1]

It is Caps sensible (it will recognize Temperature but not temperature).

It should work like that.

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Old   December 7, 2012, 09:00
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Great. This worked for me. Thanks!!
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