outlet&outflow conditions

Hi,
First of all , I have internal 3D Laminar flow through duct . My question is how does the COMSOL handle the zero outlet pressure condition? It seems that COMSOL assumes linear pressure drop when you put zero pressure drop at the inlet.

The second , I have the same domain but it is now force convection heat transfer using conjugate heat transfer model in COMSOL. I need How does the COMSOL handle the outflow condition in terms of solving for the pressure along the domain?.

Thanks