Flow process

The region of space enclosed by open system boundaries is usually called a control volume.

It is convenient to define the shape of the control volume so that all flow of matter, in or out, occurs perpendicular to its surface.

[1] Then the inflowing matter performs work as if it were driving a piston of fluid into the system.

These two types of work are expressed in the equation: Substitution into the equation above for the control volume cv yields: The definition of enthalpy, H = U + PV, permits us to use this thermodynamic potential to account jointly for internal energy U and PV work in fluids for a flow process: During steady-state operation of a device (see turbine, pump, and engine), any system property within the control volume is independent of time.

Therefore, the internal energy of the system enclosed by the control volume remains constant, which implies that dUcv in the expression above may be set equal to zero.

During steady, continuous operation, an energy balance applied to an open system equates shaft work performed by the system to heat added plus net enthalpy added.