Concept of polytropic efficiency

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Knowledge of the isentropic efficiency etas provides . To know this law, we must give ourselves additional hypotheses.


One of the most common leads to the , which can cover slightly different definitions depending on the authors.

The hypothesis given here is to consider that the , which amounts to supposing that, during any infinitely small step of the transformation, the , equal by definition to the polytropic efficiency etap which thus appears to be an infinitesimal isentropic efficiency.

We can then show that, for an ideal gas, the law of evolution followed by the fluid is of the type Pv k = Cste, but it is not so simple for a vapor.

For similarly constructed, high-stage multistage machines, such as a steam turbine, this efficiency has a clear physical meaning: it is sort of the .

Modeling a is therefore more realistic than assuming that its isentropic efficiency is constant.