I am trying to model a two-phase simulation with evaporation and condensation mass transfer process, To start the simulation I am trying to model a simple 2-d model. The geometry is like a bottle with a pressure outlet on the top, filled half with liquid and vapour phase of water. Heat flux is provided through the bottom of the bottle. The geometry is attached below.
I am running a transient simulation. My solution is diverging, but if I put the wall boundary condition instead of the outlet then my simulation works fine. So, according to my understanding, the pressure outlet is causing the divergence in the problem. I am unable to understand that when I specify my pressure outlet open to atmospheric pressure (0 Gauge pressure) it should work fine in principle but why it is unable to find the solution. Is there a theoretical mistake in describing the problem?