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 Thomashoffmann November 20, 2012 07:03

thermal conductivity expression

1 Attachment(s)
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?

 Possa November 20, 2012 07:19

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

 Thomashoffmann November 20, 2012 07:27

easy :)
thanks!

 Thomashoffmann December 6, 2012 04:34

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?

 ghorrocks December 6, 2012 05:18

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.

 Possa December 6, 2012 06:47

Hello Thomas,

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

Regards,
Possa

 Thomashoffmann December 7, 2012 08:34

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?

 Possa December 7, 2012 08:53

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.

Regards

 Thomashoffmann December 7, 2012 09:00

Great. This worked for me. Thanks!!

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