Negative absolute pressure in heat pipe

    • fjulio


      I am trying to simulate a heat pipe. In a previous thread I mentioned the simulation diverging and I was able to identify the root cause being the absolute pressure becoming negative whenever I activate a UDF for saturation pressure.

      My question would be, can the UDF, which is just an implementation of the Antoine equation, cause the absolute pressure to be negative? Here is the UDF code:

      #include "udf.h"



      real sat_t;

      real vapor_p;

      real static_p = C_P(c,t);

      real operating_p = RP_Get_Real ("operating-pressure");

      vapor_p = static_p + operating_p;

      sat_t = (1723.6425/(8.05573-((double)log10((double)vapor_p/133.322)))-233.08)+273.15;

      return sat_t;


      And this is a snapshot of my solution methods. I am using VOF multiphase flow with liquid and vapor water as the two phases. The controls tab is unchanged (relaxation factors, etc.)

    • DrAmine
      Ansys Employee
      Is it possible to store that in a cell memory and check the values at the moment when the absolute pressure get negative? I cannot imagine how the absolute pressure will be negative as it is actually clipped to minimum based on the solution limits.
      Moreover I do not understand why you are casting the variable again which you have already defined. Remove all this double appearing in your expression and verify if the Antoine expression is correct.
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