Fig. 4.9 Schematic of an open system that does no expansion work. However, the unit may perform or accept shaft work, as do the pump and turbine in Fig. 4.3. It may also exchange heat with the surroundings, which is the function of the boiler and condenser in the simple steam power cycle.
The principal difference between the system of Fig. 4.9 and the closed systems that were analyzed in Chap. 3 is the presence of inlet and outlet flow ports in the open system. The boundary of the open system in Fig. 4.9 is composed of the inner surface of the physical device plus the imaginary meshes on the inlet and outlet ports through which fluid enters and leaves the unit. The mass flow rate through the device, expressed in kg/s, is assumed to be constant in time. The properties of the fluid at the entrance and exit are fixed by any two thermodynamic properties at these locations. Pressure and temperature are indicated in Fig. 4.9, although any two properties suffice for a single-phase substance and any one property fixes all others if the fluid is a vapor-liquid mixture. In addition, the velocities of the fluid entering and leaving are important because they enter into the First law applied to the open system. High entrance and exit velocities arise in cases of high mass flow rates through inlet and outlet ports having small cross sectional areas.
The volumetric flow rate is the product of the area of the port, A, and the velocity of the fluid, V:
volumetric flow rate = AV
The volumetric flow rate can be converted to the mass flow rate m by dividing by the fluid's specific volume, giving:
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