Hi Rob! As you said, I decreased the growth rate and changed the mesh to polyhedral. In the beginning, everything was fine and I thought that it solved the problem. However, then I lengthened the simulation time and the error with gas phases stocking before the porous zone came back. The growth rate is only 1.05 and the aspect ratio is 8,42.

Furthermore, I am getting the upward flow near the walls and downward in the centrum of geometry, while from experimental data it should be opposite. Before the porous zone, the flow is correct.  

Do you think that it's still the mesh? If I decrease the growth rate even further then I'm getting mesh over a million. Now, I'm using 250k.