so I had re-run the simulation for much longer and here are what I got.
From the vector plots, you can see that air is flowing into the fluid domain from mostly the top part and exiting through the bottom part which I think, mimics the reality quite accurately. However, although the mass imbalance has basically flat out, my residual plot has problem converging. Incompressible ideal gas is not suitable for my simulation as there is also heat transfer involved, with fluid temperature ranging from 80 C to 450 C.
In this case, can I still take this result as being accurate, and the problem with converging is basically caused by the reverse flow which doesn't really interfere with my actual fluid flow?
(Just for better understanding, here is what the model will looks like when installed, https://sermeta.com/wp-content/uploads/2018/08/Condensation-ENG3.png)