Fluids

Fluids

Some questions about user defined specific heat

    • Sky0101
      Subscriber

      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
      Ansys Employee
      Can you reformulate your question?
    • Sky0101
      Subscriber

      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
      Ansys Employee

      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
      Ansys Employee
      Fluent requires to know how to get the sensible enthalpy out of the user defined heat capacity.
    • atulsbhat
      Subscriber

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


      Where Can I set Tref for UDF?


    • DrAmine
      Ansys Employee
      Is hard coded.
    • atulsbhat
      Subscriber

      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
      Subscriber
      How to Add Tref as 600K? , For Cp UDF..
    • rachels1001
      Subscriber


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


       

    • DrAmine
      Ansys Employee
      No that is wrong. Only for multiphase there is an option because of latent heat.
    • atulsbhat
      Subscriber
      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
      Ansys Employee

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

    • ahmad91
      Subscriber

      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
      Ansys Employee

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

    • atulsbhat
      Subscriber

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


      Regards

    • DrAmine
      Ansys Employee
      Which function for Cp you want to incorporate?
    • atulsbhat
      Subscriber

      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
      Ansys Employee
      I still not understand: rational or not rational use the inbuilt function in Fluent.
    • atulsbhat
      Subscriber



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


       

    • DrAmine
      Ansys Employee
      You might need to limit temperature bounds at the beginning. Try that and report
    • atulsbhat
      Subscriber

      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
      Ansys Employee
      What are you using now? Udf or inbuilt?
    • DrAmine
      Ansys Employee
      As you created another thread I am lo king this one.
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