I tried to simplify the problem to see if I find what is causing the problem. My mesh is too large (56 Million cells), so that even making changes takes a lot of time.
Instead of my geometry I tried the Tutorial "Chapter 22: Using the Mixture and Eulerian Multiphase Models", which uses a 2D mesh for a tee junction.
It works if I have air bubbles in water, but it also diverges if I have water droplets in air.
I only changed the materials. Boundary conditions were kept the same.
Not sure what the problem could be:
Upward flow
Density ratio
Boundary condition for the water phase
Is there a tutorial for the Eulerian or the Mixture models, where the second phase is heavier than the first phase?