UDF help in liquid-vapor interface modelling
 

Hi community,
I am trying to simulate a vapor chamber in FLUENT 2020 R2 as shown in figure. Initially iam planning to model liquid and vapor flows seperately and give appropriate source terms at the interface. For example, I will have a negative mass source term in liquid side and the same positive mass source term in vapor side of the interface. However, I am struck with the problem of identifying the correct face threads to use.

I am not sure what type to keep the liquid-vapor interface as. I have a number of options like:
1) keeping them as walls (say x,y) and give their ID's as the thread ID's for which i give the source terms.
2) convert them to interface type and creat coupled interface (say named 'interf'). This leads to creating three things:
a) Two interface type instances of interf on both the walls with new interface ID's.
b) Two new walls (x-non overlapping, y-non-overlapping) with their own wall ID's.
c) Another two new walls with arbitrary names (z-1-1-1, z-1-1-1- shadow) with their own wall ID's.

This is where I am lost, what ID's should i use when i want to identify the face threads of my interface like:

thread_loop_f(t, domain)
{
if (THREAD_ID(t) == Interface_ID)
{
begin_f_loop(f, t)
{
...
...
}
end_f_loop(f,t)
}
}

What should be my Interface_ID?

as I see your problem you should keeping them as walls so you know ID's from fluent GUI

this condition, probably, will not give you link between faces
as numbering of faces on each walls could be random (should be tested)

so, my vision:
to find the link between face you need:
loop over faces of wall 1,
check coordinate of face1 (face on wall 1)
loop over faces of wall 2,
check coordinate of face2 (face on wall 2)
compare coordinates

if you find other way, please share