Hello drAmine, seems like I've found a strange problem.
This is the area ave of pressure at the inlet.
I stopped the simulation at that time, since I know the pressure should be constant (with constant velocity inlet), am I correct?
Also, the ave area of inlet density looks like this. It should be constant, right?

(EDIT: I have just found out that the static pressure read in the plot is actually gage pressure).
And I have noticed, that the trendline of the RH is very similar to the trendline of density. So I'm thinking that the density is may be the one that causes the problem?
As for the remainder of DrAmine's reply, I will try to monitor the plot vs iterations. And I would like to ask, so this whole time my results have never converged? I have set the convergence conditions to be timestep convergence.
I also have noticed below that the pressure takes quite amount of time steps to reach a constant value, does this mean the initial timesteps does not give the correct value and should be "sacrificed" ?