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58.0 kW 285.14) kJ/kg] Discussion Notice that in determining the power input to the compressor, we
used h2a instead of h2s since h2a is the actual enthalpy of the air as it exits the
compressor. The quantity h2s is a hypothetical enthalpy value that the air would
have if the process were isentropic. Isentropic Efficiency of Nozzles
Nozzles are essentially adiabatic devices and are used to accelerate a fluid.
Therefore, the isentropic process serves as a suitable model for nozzles. The
isentropic efficiency of a nozzle is defined as the ratio of the actual kinetic
energy of the fluid at the nozzle exit to the kinetic energy value at the exit of an
isentropic nozzle for the same inlet state and exit pressure. That is,
hN Actual KE at nozzle exit
Isentropic KE at nozzle exit 2
2s (7–66) Note that the exit pressure is the same for both the actual and isentropic
processes, but the exit state is different.
Nozzles involve no work interactions, and the fluid experiences little or no
change in its potential energy as it flows through the device. If, in addition, the
inlet velocity of the fluid is small relative to the exit velocity, the energy balance for this steady-flow device reduces to
h1 h2a 2
2a h 2
2a Then the isentropic efficiency of the nozzle can be expressed in terms of
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This document was uploaded on 11/28/2012.
- Spring '09