Knowledge of the isentropic efficiency etas provides  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic. To know this law, we must give ourselves additional hypotheses.


One of the most common leads to the  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic, which can cover slightly different definitions depending on the authors.

The hypothesis given here is to consider that the  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic, which amounts to supposing that, during any infinitely small step of the transformation, the  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic, 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  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic.

Modeling a  elementary efficiency of a stageirreversibilities are uniformly distributed throughout the expansionisentropic efficiency keeps a constant valuemultistage turbine using a polytropic approachno indication of the law followed by the fluid during irreversible expansionwidely used notion of polytropic is therefore more realistic than assuming that its isentropic efficiency is constant.