Isentropic Efficiency of SteadyFlow Devices

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Isentropic Efficiency of Steady-Flow
Devices Isentropic efficiency is a measure of
the deviation of actual processes from the
corresponding isentropic (idealized)ones.
Isentropic Efficiency of Turbines The
isentropic efficiency of a turbine is defined as
the ratio of the actual work output of the
turbine to the work output that would be achieved
if the process between the inlet state and the
exit pressure were isentropic.
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Isentropic Efficiency of 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.
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ENTROPY BALANCE The second law of thermodynamics
states that entropy can be created but it cannot
be destroyed. The entropy change of a system
during a process is equal to the net entropy
transfer through the system boundary and the
entropy generated within the system.
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Entropy Generation, Sgen Entropy generation is a
measure of the entropy created by irreversibility
such as friction, mixing, chemical reactions,
heat transfer through a finite temperature
difference, etc. For a reversible process (a
process that involves no irreversibility), the
entropy generation is zero and thus the entropy
change of a system is equal to the entropy
transfer. The entropy transfer by heat Q/T is
zero for adiabatic systems, and the entropy
transfer by mass ms is zero for systems that
involve no mass flow across their boundary (i.e.,
closed systems).
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• Entropy balance for any system undergoing any
  process can be expressed more explicitly as

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Closed Systems The entropy change of a closed
system can be expressed as

• The entropy balance for a closed system and its
  surroundings can be written as

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Control Volumes General entropy balance
relations for control volumes can be expressed as
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The entropy balance relation for a general
steady-flow process can be written as
Note Equation (6-86) indicates that the specific
entropy of the fluid must increase as it flows
through an adiabatic device since Sgen gt 0.
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