Pressure in Flow Work (u + Pv)

[QQB]

Why is the pressure used in flow work the pressure of the fluid? If the fluid was say at 10kPa and the external pressure was at 0Pa, why don't we use the 0Pa in our calculations for flow work, since it's the external pressure that's the resistance? Or is it not possible to have this difference at all in fluid and external pressure?

 

[Chestermiller]

The two pressures have to match at the boundary of the control volume. Pressure is continuous.

In a closed system, the external pressure always matches the gas pressure at the piston face. It is just that, for an irreversible deformation, the pressure in the cylinder is not uniform and the average pressure in the cylinder is not equal to the pressure at the piston face.