How do I choose between polytropic efficiency and isentropic efficiency?
There are two ways to determine whether a compression or expansion process is isentropic or not. Either you use the isentropic efficiency, which is the ratio of actual work to theoretical work for an expansion process and the opposite for a compression process. Or you use a polytropic exponent k such that pvk = cste. You can find the value of k from the inlets / outlets of the pevek = psvsk conversion determined using the isentropic efficiency approach above.
