thermal conductivity expression

Thomashoffmann · November 20, 2012, 06:03

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, 06:19

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, 06:27

easy

thanks!

Thomashoffmann · December 6, 2012, 03:34

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, 04:18

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, 05:47

Hello Thomas,

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

Regards,
Possa

Thomashoffmann · December 7, 2012, 07:34

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, 07:53

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, 08:00

Great. This worked for me. Thanks!!

December 6, 2012, 04:18		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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. Goenitz likes this.

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November 20, 2012, 06:19		#2
Possa New Member Talita Possamai Join Date: Sep 2012 Posts: 23 Rep Power: 13	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

November 20, 2012, 06:27		#3
Thomashoffmann Member Thomas Hoffmann Join Date: Oct 2012 Posts: 67 Rep Power: 13	easy thanks!

December 6, 2012, 03:34		#4
Thomashoffmann Member Thomas Hoffmann Join Date: Oct 2012 Posts: 67 Rep Power: 13	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?

December 6, 2012, 05:47		#6
Possa New Member Talita Possamai Join Date: Sep 2012 Posts: 23 Rep Power: 13	Hello Thomas, Yes, you're right. You must change T for Temperature for this to work. Regards, Possa

December 7, 2012, 07:34		#7
Thomashoffmann Member Thomas Hoffmann Join Date: Oct 2012 Posts: 67 Rep Power: 13	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?

December 7, 2012, 07:53		#8
Possa New Member Talita Possamai Join Date: Sep 2012 Posts: 23 Rep Power: 13	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

December 7, 2012, 08:00		#9
Thomashoffmann Member Thomas Hoffmann Join Date: Oct 2012 Posts: 67 Rep Power: 13	Great. This worked for me. Thanks!!