• The transfer of mass across a system boundary, in addition to
work and energy, characterizes open systems.
• Any mass that enters or exits the system carries a certain
amount of energy into or out of the system, respectively.
• Therefore, we need to account for the change in the energy of
the system due to mass flow.
• The work associated with flow sometimes is referred to as
flow work.
• Flow work is calculated by determining the work involved in
pushing a certain mass through the system boundary.
• Consider a differential element of fluid of uniform properties
entering an open system ( see Fig. ).
• If the cross-sectional area of the element is A and the pressure of
the fluid is P, then the force required to push this element through
the system boundary is
This general equation is for a system that may have p inlet
streams or q exit streams for a fluid to flow in or out,
respectively. If there is only one inlet (location 1) and exit
(location 2) for a system, then
where Qm and Wm are the transfer of heat and work per unit mass,
respectively. The specific volume V is replaced with 1/ρ in
Equation (1.95).