DrAmine
Ansys Employee
Is the flow turbulent or laminar? Can you do a rough estimate of the Reynolds Number? 
You wrote you OQ is more than 0.01: okay but that is a warranty for smooth run. I recommend to have a OQ not smaller than 0.1 or 0.2 and Cell Volume Change should not exceed 1.2. Aspect Ration should be smaller than 100. 

I do not understand your "mass flow" BC and how you are flowing in to or flowing out of the domain: is your BC a sort of pump sucking in and out? You can if the density is constant use instead of velocity inlet with dedicated fow direction but changing the direction of the inflow (outwards now)