Adiabatic Processes in the Liquid–Vapor Two-Phase Region. 1. Pure Fluids
2014
Adiabatic processes in the liquid–vapor two-phase region were studied with several equations of state. The comparison of the resulting isentropes, particularly their patterns in quality (fraction of vapor phase) vs temperature diagrams, indicates that there are two different classes of fluids: One class shows a simple pattern where isentropes entering the two-phase region never leave it again; the other shows a more complicated pattern with reentrant isentropes, which may either cross the entire two-phase region or exhibit a retrograde behavior. The existence of these two classes can be related to the shapes of entropy–volume or temperature–entropy curves, and these in turn to the temperature dependence of ideal-gas heat capacities. The two-phase isentrope that runs to the critical point approaches it with an infinite slope in the quality–temperature diagram. The slope is positive for reference equations of state, but negative for all other equations of state used in this work.
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