Using the Second Principle
Turbomachinery - Thermodynamics of energy conversion

Let's consider the real evolution of a fluid with only internal irreversibilities, which in principle occurs when the system in question does not exchange heat with the outside environment.

Let's compare the actual transformation with a reversible transformation, marked by the index rev, which takes the fluid through the same physical and kinetic states. The First Principle allows us to write between two successive states that are infinitely close: