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Hi,

I am conducting a 2D simulation of water-vapor condensation upon a pipe wall. My simulation runs fine when using the default Lee's model in fluent.

However I'm trying to implement a mass transfer phase change model from a paper which will provide more accurate results. I'm implementing this through the UDF macro DEFINE_MASS_TRANSFER.

Although when I run the simulation with my mass transfer udf, it runs for approx 100 iterations and then the simulation starts to diverge. My UDF is listed here:
#include "udf.h"

DEFINE_MASS_TRANSFER(liq_gas_source, cell, thread, from_index,from_species_index, to_index, to_species_index)
{
real m_lg;
real T_SAT = 373.15;
Thread *gas = THREAD_SUB_THREAD(thread, from_index);
Thread *liq = THREAD_SUB_THREAD(thread, to_index);
real delta_t;
delta_t = CURRENT_TIMESTEP;
real lc = C_H(cell,gas)-C_H(cell,liq)

m_lg = 0.;
if (C_T(cell, liq) >= T_SAT)
{
m_lg = (1./lc)*C_VOF(cell,liq)*(C_R(cell,liq)*C_CP(cell,liq)* fabs(T_SAT-C_T(cell,liq))/delta_t);
}
else if ((m_lg == 0. ) && (C_T(cell,gas) <= T_SAT))
{
m_lg = (1./lc)*C_VOF(cell,gas)*(C_R(cell,gas)*C_CP(cell,gas)* fabs(T_SAT-C_T(cell,gas))/delta_t);
}

return (m_lg);
}

Any ideas of what could be causing this?

Many thanks!