The analysis of steady, one-dimensional, isentropic flow of nonideal compressible fluids is simplified greatly by the use of an approximate model for the PV or Pρ relation, this relation being called the isentrope equation. The simplest isentrope model is the perfect-gas or polytrope equation PVn = constant. During the past 80 years, five isentrope equations have evolved: the polytrope, Walker, van der Waals, Rayleigh, and Callendar models. The historical development and limitations of each model are discussed. Only the polytrope and Rayleigh model yield simple, closed-form, analytic solutions for isentropic flow properties. A comparison with calculated data for sonic flow properties reveals the superiority of the Rayleigh model for the prediction of isentropic flow.

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