Hi I see, thanks for the advice. I will make the changes and simulate it again.
Regarding the back flow, the fluid is coming from outside the fluid domain, as shown in the following pathline diagrams, particle released from outlet.
And here is the mass flow rate report. Around 40% of mass has gone missing.
So, is this result acceptable?
Will changing the BC from pressure outlet to pressure farfield improve the situation? As I am simulating compressible flow, outflow BC is not appropriate, according to user guide.
The reason backflow is not really favourable/realistic is because the closed cavity of condensing heat exchanger actually has a slightly higher pressure (~5 mbar) than to say, the open space, where the fluid is supposed to be exhausted. So there is almost no way for fluid to reenter the closed cavity from open space and into the outlet pipe in my model. In my case, I had set the gauge pressure at pressure outlet to be 0 Pa just so that the fluid can flow properly. Operating pressure is at 1 atm.