Learning Forum

[Sky0101]

Hello, everyone. I have some questions about DEFINE_SPECIFIC_HEAT(my_user_cp, T, Tref, h, yi)

i get an example in udf manual, as follow:

In my case, all thermal properties,such as rho, lambda, mu, are variables about temperature.

i tested some cases, and found *h plays an key role in energy equation.

i want to know how does the pointer *h work in fluent.

thanks 

[DrAmine]

Can you reformulate your question?

[Sky0101]

When l compile this udf into my case, my case becomes unstable and difficult to converge . So i want to know what the code ,*h = cp*(T-Tref), means and how to get a reasonable Tref. Thanks 

 

[Rob]

Read the example code in the manual: the code calculates cp and sets "the sensible enthalpy to the referenced value". You've set *h=0 which may mess with some of the maths in Fluent: to understand which parts read the Theory guide (energy equation) and any references used in those sections. Re the approach to define cp, I don't understand what you're trying to do: if you wanted a piecewise profile use a table. 

[DrAmine]

Fluent requires to know how to get the sensible enthalpy out of the user defined heat capacity.

[atulsbhat]

I am Not Sure Where to Set Tref For Cp in the Materials Panel

Where Can I set Tref for UDF?

[DrAmine]

Is hard coded.

[atulsbhat]

Should I Define  it in My UDF ?

Like real Tref=600;

or its Not Possible to Alter it (by default its 298K) ?

In the Manual There is No Mention of how to add it..

[atulsbhat]

How to Add Tref as 600K? , For Cp UDF..

[rachels1001]

I believe that you should be able to set the reference temperature for materials within the materials dialog box.

 

[DrAmine]

No that is wrong. Only for multiphase there is an option because of latent heat.

[atulsbhat]

Yes You are Right, There is No Option for Single Phase Flow, So We Cannot Change Tref for Single Phase , Is That what you Mean? Or is There a Option to define it Within The Code?

[DrAmine]

Try to incorporate the changes of Tref in the way *h should be calculated.

[ahmad91]

Read the example code in the manual: the code calculates cp and sets "the sensible enthalpy to the referenced value". You've set *h=0 which may mess with some of the maths in Fluent: to understand which parts read the Theory guide (energy equation) and any references used in those sections. Re the approach to define cp, I don't understand what you're trying to do: if you wanted a piecewise profile use a table.

Hello rwoolhou,

how can I use a table to get piecewise profile?

[Rob]

If you have  cp V T data why not use a profile? Saves writing the UDF. 

[atulsbhat]

Sir, So I should Change My *h Expression in my UDF with all The Required Changes ?

Regards

[DrAmine]

Which function for Cp you want to incorporate?

[atulsbhat]

https://forum.ansys.com/forums/topic/how-to-write-a-correct-udf-fluent-for-specific-heat-as-function-of-temperature-for-polynomials/

Cp As A Function of Temperature, In Terms of Rational Polynomials, 

If (T<=450K)

{Cp= polynomial 1;}

Else if () 

....

So can I Change the expression for *h for Tref =600K (Default Tref 298K)

*h= cp (T-(Tref+302)); 

???

Will Tref effect my Cp or its only for sensible enthalpy calculation??

[DrAmine]

I still not understand: rational or not rational use the inbuilt function in Fluent.

[atulsbhat]

I have Checked My Polynomials, I had 4 polynomials for Cp, Exactly, they were Correct, But I got this Error

 

[DrAmine]

You might need to limit temperature bounds at the beginning. Try that and report

[atulsbhat]

I tried That in solution control limits, It Didnt work

 

https://forum.ansys.com/forums/topic/invalid-cp-error-after-running-program-with-udf/

[DrAmine]

What are you using now? Udf or inbuilt?

[DrAmine]

As you created another thread I am lo king this one.