[cp703]

Hi everyone,

I'm trying to write a UDF that describes phase change (just working on getting evaporation right now). I've been trying out my UDF on a "1D" Stefan problem (see attached PDF). My biggest problem right now is with an if statement in the mass source macro (DEFINE_SOURCE) that is hooked to my primary phase, the vapor. Here's the macro in question:

DEFINE_SOURCE(gas, cell, thread, dS, eqn)
{
    
    real source = 0.0; /* Initialize the variable "source"*/

real a_c = 0.04; 
    real a_c_coeff=(2.0*a_c)/(2.0-a_c); 
    real M = 0.018015; /* molar mass [kg/mol] */
    real R = 8.314; /* universal gas constant [J/molK] */
    real Tsat = 373.1; /* saturation temperature [K] */
    real Psat = 101325.0; /* saturation pressure [Pa] */
    real DeltaH = 2256560.0; /*vaporization enthalpy [J/kg] */
    real rho_L = 958.4; /* liquid density [kg/m3] */
    real rho_V = 0.597; /* vapor density [kg/m3] */

    /* Initialize and define thread pointer/pointer array  */
    Thread *tm = THREAD_SUPER_THREAD(thread); /* *tm is a pointer to the mixture-level thread */
    Thread **pt = THREAD_SUB_THREADS(tm); /* **pt is a pointer ARRAY, whose elements contain pointers to the phase-level threads */
    
   

    /* If the temperature of the cell (located in a primary phase cell thread) is greater than Tsat */
    /* AND the volume fraction of the primary phase is less than one (i.e. there is some liquid in the cell), then... */
    
        if (C_T(cell, thread) > 373.1 && C_VOF(cell,thread) < 1.0)
        {

        source = C_UDMI(cell,tm,0)*a_c_coeff*sqrt(M/(2.0*M_PI*R))*((C_P(cell,thread)/sqrt(C_T(cell,thread)))-(Pvaporavg/sqrt(Tvaporavg)));
      
        }
        else
        {
            source = 0.0;
        }
    

    /* Set the user-defined memory location, labeled 1, as the mass source term for the vapor phase*/
    C_UDMI(cell, tm, 1) = source;
  
    /* Set the user-defined memory location, labeled 2, as the energy source term, which is just the mass source term multiplied */
    /* by the latent heat of vaporization. */
    C_UDMI(cell, tm, 2) = -1.*source*DeltaH;

    dS[eqn] = 0;
  
    return source;
}

FYI: My operating pressure is set to be 0 Pa.

My specific issue is that the solver doesn't always enter into the if statement when it should. When the interface temperature gets above 373.1K, I would expect a mass source to appear along the entire length interface. However, I don't see that at all. Instead, it's just one localized circular region where a mass source appears.

Does anyone have any experience with if statements inside DEFINE_SOURCE macros for a mass source in a multiphase simulation? Have I formatted it correctly? I've been trying to figure out if it's something to do with the data structures in multiphase situations, i.e. phase-level threads versus mixture-level threads, or a data type problem, or a problem with the if statement syntax. I haven't been able to find anything helpful in the UDF manual or elsewhere online. I've submitted my question to ANSYS but am waiting to hear back.

Thank you!!