[yacarioca]

Hey! I'm working on the axisymmetric problem of a hollow fiber permeator for membrane distillation. I am trying to implement a UDF for computating the heat flux through the membrane due to water evaporation (water evaporates at the feed side and condenses at the permeate side). I have very little experience with UDFs and C language. I have come up with the following sketch, although I know there are MANY mistakes.

UDF for the feed side:

#include "udf.h"
#include "math.h"
#include "mem.h"
#define ID 4 /* id number of the permeate zone */
DEFINE_PROFILE(membrane_feed, t, i)
{
real x[ND_ND];
real y;
face_t f;
face_tp fp;
Thread *tp = Lookup_Thread(domain,ID);
begin_f_loop(f, t)
{
real ri = F_CENTROID(x, fp, tp);
real ro = F_CENTROID(x, f, t);
real ti = F_T(ri, tp);
real to = F_T(ro, t);
pi = exp(23.238 - ((3841 / (ti - 45))));
po = exp(23.238 - ((3841 / (to - 45))));
q = 0.0000002*(po - pi) * 2375000;
F_PROFILE(f,t,i) = -q;
}
end_f_loop(f, t)
}



UDF for the permeate side:

#include "udf.h"
#include "math.h"
#include "mem.h"
#define ID 5 /* id number of the feed zone */
DEFINE_PROFILE(membrane_permeate, t, i)
{
real x[ND_ND];
face_t f;
face_tf ff;
Thread *tf = Lookup_Thread(domain,ID);
begin_f_loop(f, t)
{
real ri = F_CENTROID(x, f, t);
real ro = F_CENTROID(x, ff, tf);
real ti = F_T(ri, t);
real to = F_T(ro, tf);
pi = exp(23.238 - ((3841 / (ti - 45))));
po = exp(23.238 - ((3841 / (to - 45))));
q = 0.0000002*(po - pi) * 2375000;
F_PROFILE(f,t,i) = q*0.45/0.725;
}
end_f_loop(f,t)
}


Basically I need a UDF that accounts for the vapor pressure difference across the membrane and I intend to do it through two boundary conditions (one at the membrane-feed face and the other at the membrane-permeate face). Could someone with more experience please help me?

My geometry is in the picture. The rotation axis is at the bottom. The lower layer stands for the permeate, the intermediate one, the membrane and the upper layer stands for the feed.

Geometry: https://ibb.co/hLPKtnP