[Navsing]

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!

[RestlessBreeze]

Hi,I want to know what formula your m_lg is based on? Thanks

[m.r.soufivand]

Is your problem solved? I have a similar problem.

[cbooks77]

You should use the linearized mass transfer UDF like Ansys recommends. The solver generally expects linearized source terms to remain stable and avoid under/over shoots. You might get away with non linear mass transfer if its a very simple case. Also you should be running this in transient and find a time step that is stable. Also you will want to underrelax the pressure and momentum quite a bit (0.3)

[Navsing]

Yes I've solved this problem. I just needed to swap the phases over (the from and to index) as I defined the primary phase as the condensate and the vapour as the secondary.

[Navsing]

Originally Posted by RestlessBreeze

Hi,I want to know what formula your m_lg is based on? Thanks

The formula is based on the simple heat transfer energy equation which deduces the volumetric mass transfer rate between the liquid-vapour interface. The full explaination can be found from this reference:

A.S. Rattner, S. Garimella, Simple mechanistically consistent formulation for
volume-of-fluid based computations of condensing flows, J. Heat Transf. 136
(7) (2014), https://doi.org/10.1115/1.4026808.