Mass Transfer & VOF

R Liew

Dear all,

I'm trying to solve a condensation problem. To do this, I use the VOF model and have written a complex condensation model. There are several problems:

- 1) The Absolute Pressure drops BELOW zero.

This should not be possible ofcourse. My operating pressure is 4 bar abs.

- 2) The mass transfer works not like it should:

I've tested this with a very simple condensation model (UDF):

from species = water.

to species = steam.

DEFINE_MASS_TRANSFER(Condensation,cell,mix,from_in dex, from_species_index, to_index, to_species_index)

{

Thread *gas = THREAD_SUB_THREAD(mix, from_index);

Thread *liq = THREAD_SUB_THREAD(mix, to_index);

real T_sat = 406;

real m_lg;

if (C_T(cell,mix)>T_sat)

m_lg = 5000;

else

m_lg=-5000;

return m_lg;

}

This means that if the cell temperature exceeds the saturisation temperature, evaporation should occur. Otherwhise evaporation occurs.

But if I solve the problem (unsteady) this is not the case. In my simulation, at one place the mass transfer is 5000 kg/(m3-s) from water to steam. This is true indeed if I look to the temperatures, but if I look to the phase, the opposite occurs (condensation instead of evaporation).

I use the explicit VOF sceme, no turbulence models, energy equation. Unsteady with dt=1e-6 sec.

Solution controls:

p-v coupling: coupled

pressure: Presto

momentum: Quick

VOF : modified HIRC

energy: Quick.

Please, can anyone help me??

Thanks in advance!

R Liew, Technical University of Eindhoven