Thanks for your comment,nstatic head is accounted using operating density set to zero.nthe initial temperature of the gas is at saturated temperature and the inlet liquid is 5K subcooled.nIn fact, I used polynomial density of gas via temperature, and apparently as the liquid comes in, the density is constant and so pressure goes up since gas was like incompressible liquid!nI am using Peng-Robinson density for gas now, and mass flow outlet BC instead of pressure outlet. I had a strange pressure changes at the very small times steps at the beginning but it becomes physical now using larger times steps (0.01 s).nThe simulation is running to see if I will have a physical volume fraction and temperature or not. I will update it here the possible problems of the model. Like now I can see that the liquid temperature is developing a strange colder column from outlet into the fluid that seems strange.nI highly appreciate if the topic is followed after the update.n