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a_cucen July 12, 2012 11:46

Latern Heat failure
I m trying to change Latern Heat with Temperature. I imported a table in CFX. But It gives this failure. How can I set Latern Heat to change with temperature?

Parameter 'Latent Heat' in object '/FLOW:Flow Analysis C3BC/DOMAIN:ICEngine/FLUID PAIR:Gas Mixture | C7H16liq/COMPONENT PAIR:C7H16 | C7H16l/LATENT HEAT' is not allowed to be assigned an expression value that depends on variables. It must be assigned a numeric value, or an expression that resolves to a constant value.

Danial Q July 12, 2012 18:08

Could you post your Expression here because you have included some parameter in expression which is not CFX built-in parameter and being calculated as a solution, which causes problem like this normally.


a_cucen July 12, 2012 19:26

I have imported a table that shows enthalpy changes with temperature. I have located this table to Latent heat value in Liquid Evaporation Model as a function of temperature.

Danial Q July 12, 2012 23:44

You have to solve it (latent heat) to dimensionless value. I mean latent heat value should resolve to a constant value (without units of latent heat). I hope you know how to make some value dimsensionless.

flotus1 July 13, 2012 00:42

Very interesting issue. What kind of process requires to change the latent heat as a function of temperature?

Isn't latent heat by definition the amount of heat stored or released by a material while the temperature remains constant?

a_cucen July 13, 2012 03:35

I don t know if latent heat is enthalpy of formation? But Enthalpy values change with temperature. If latent heat represents enthalpy of formation, how can I import enthalpy values to my case because vapourization entalpies must change according to temperature.

Danial Q July 13, 2012 03:58

Well, There are two different things , i guess you are just confusing them;
1 > Enthalp of system
2> Latent Heat

So, Enthalpy of system = sensible heat + latent heat (for phase change). And yes Enthapy of system changes with temp but latent heat is added/removed at constt temp.So,
heat of formation will be "that heat added/removed to make some new compound" so its is infcat enthalpy change. IN your case, YES, heat of formation is latent heat as per my understanding. And how can you do that, go through some tutorials, i guess you have to make (.rgp) table and import it in cfx pre. Otherwise, i told you just make a CEL expression.


a_cucen July 13, 2012 04:33

Thanks for your help Danial. But I have imported the table. But I don't know where to put these values. I could not find any enthalpy terms apart from Latent heat. How can I represent changes of enthalpy difference with temperature?

Here is an example of my table. I have imported this table to cfx and modelled like Latentheat(T). It takes suitable enthalpy values with temperature values.

Temperature (K) Enthalpy (kJ/kg)
186,72 -400,41
188,81 -396,30
190,90 -392,19
192,99 -388,08
195,07 -383,96

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