Isentropic process

An isentropic process is one in which the entropy of the system does not change, ΔS = 0. For a closed system this is achieved by a process that is both adiabatic (no heat crosses the boundary, so dQ = 0) and reversible (no entropy is generated internally). Since dS = dQ_rev/T, both conditions together force dS = 0.

Reversible adiabatic compressions and expansions are the canonical examples, which is why adiabats are also called isentropes. For an ideal gas an isentropic process follows PV^γ = constant and TV^(γ−1) = constant, where γ is the ratio of heat capacities.

Isentropic legs are the vertical sides of the Carnot cycle in the temperature–entropy plane and serve as the ideal benchmark against which the efficiency of real compressors and turbines is measured.

The isentropic ideal grew out of the study of adiabatic processes in the nineteenth century and Carnot's reversible cycle; it became central to the engineering analysis of turbines and compressors in the twentieth.