Thermal Efficiency and Isentropic Efficiency

In Chapters 4 and 5 we defined the thermal efficiency of a cycle as:
A flow diagram of an isentropic turbine.
  • We want to define a new efficiency that can be used to evaluate the performance of a single process such as a pump, compressor, turbine, or even a nozzle.
  • The thermal efficiency is based on the comparison of the performance of a real cycle to the performance of a hypothetical cycle which could convert heat completely into work.
  • This hypothetical cycle violates the 2nd Law.
In Chapter 6, we learned that a Carnot Cycle has the maximum thermal efficiency and we showed that for any Carnot Cycle:
  • In order to develop an efficiency for individual processes, we will compare the performance of real, irreversible processes to the performance of isentropic processes (adiabatic and internally reversible ).
  • We will call this new type of efficiency the

    isentropic efficiency

    and it will be represented by ηs
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Ch 8, Lesson C, Page 1 - Thermal Efficiency and Isentropic Efficiency