Mass Fluxes Does not Match on Boundary, and Different Results from Different Solvers

Hi all,

I'm currently modeling a mass diffusion problem, using Transport of Dilute Species and Laminar Flow modules. On the boundaries of two adjacent diffusion domains, I use the Stiff-Spring method as introduced in the dialysis example.

flux=M*(c2-k*c1), where k=1.

The first solver I chose was segregated solver. I've tried both iterative and direct methods. They converge to the same numerical value. However, the nominal total mass flux on the two sides of the boundaries are not the same. It means the mass balance is break.

In the attached excel, I copied the probe results from the segregated solver. Column D and E, Column F and G should have the same absolute value. But they differ a lot.

Then I tried fully-coupled solver. This time, the fluxes on the boundaries satisfy the mass balance. But the results it converges to is very different with that of segregated solver.

So is segregated solver not suitable in this problem? How to determine which solver to use?

Thank you!

Xiao

(Due to size limit of attachments, I deleted the results in the model attached.)