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Isentropic Efficiencies of Steady-flow Devices

Isentropic Efficiencies of Steady-flow Devices. A standard for efficiencies. We have the Carnot cycle for cyclic devices. Need something for discreet devices. Most designed to work adiabatically. Best is always reversible. Therefore, isentropic efficiencies are used.

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Isentropic Efficiencies of Steady-flow Devices

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  1. Isentropic Efficiencies of Steady-flow Devices

  2. A standard for efficiencies • We have the Carnot cycle for cyclic devices. • Need something for discreet devices. • Most designed to work adiabatically. • Best is always reversible. • Therefore, isentropic efficiencies are used. • Under isentropic conditions for the same inlet state and same outlet pressure.

  3. Isentropic efficiency of a turbine • Defined as: Actual turbine work/Isentropic turbine work. • both at same inlet state and exit pressure. • Usually ignore kinetic and potential energy changes. • So work is just the change in enthalpy. • That is if it is adiabatic.

  4. Adiabatic turbine

  5. Adiabatic compressor Isentropic efficiency is inverted If the compressor is cooled, use isothermal efficiency: For a pump, the isentropic efficiency is:

  6. Adiabatic nozzles – Actual KE at exit / Isentropic KE at exit If KE’s in are small and no work or Q: h1-h2 = KE2 Therefore: Which is the same as that of a turbine.

  7. Remember that the 2s state is always hypothetical and is at the same pressure as the 2a state for all three devices.

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