[sajaalrifai]

hello,
I've tried to model heat and mass transfer in the cross flow staggered heat exchanger using the multiphase flow, where the condensation take place at the outer surface of the inner tube where the tube material is porous material.
I model the multiphase model with the species transport. I've tested both models available in fluent (Eulerian and VOF), and I faced the same problem:


1. When I turn on the mass transfer, using the evaporation-condensation model, the outlet temperature of the both fluid looks wired, the outlet temperature of the hot fluid change only from 355 K to 342 K, and for the cold fluid the outlet temperature changes from 293.5 to 288. this is should be wrong based on the conservation of energy.

2. When I turn off the mass transfer, and run the simulation, the outlet temperature reasonable for both fluid, the hot fluid from 355 to 311 K, the cold fluid from 293 to 298 K.

3. I wrote a simple UDF (without turning on the mass transfer) to model the condensation/evaporation and calculate the mass transfer from the water vapor to the water liquid. The condensation rate was reasonable, but the tempearture as in case 1.

Anyone can explain the results in case of using the evaporation-condensation mass transfer in case 1 and 2.

I used the same geometry and boundary condition to simulate the single phase flow and I validate the results against the experimental data. everything was okay.

thank you
saja