Some thermodynamic remarks on non-equilibrium fluid streams

The non-reversible heat transfer between two fluid streams is a complex problem requiring many data and becomes more complicated if the two streams involved in the process include two-phase and two-component fluids.This paper is presented to make some thermodynamic remarks and, in particular, to show that along a heat exchanger, in whatever section normal to the flow rate, every non-equilibrium fluid state can be represented by its corresponding equilibrium state and a nonequilibrium–equilibrium deviation measured by the corresponding entropy difference or essergy difference. Within this general statement, somewhat different results are obtained in the cases of single-phase fluids, two-phase one-component fluids, two-phase two-component fluids, and mixtures of a single-phase fluid and a two-phase fluid. It is necessary to point out that the concepts of “maximum obtainable work” and of “distance from equilibrium” have been often associated, directly or implicitly, to the concept of exergy, also in good books, that have considered exergy as the basic argument. The analysis developed by Evans and by others showed that not always the two concepts can be represented by a unique parameter. In the presence of non-equilibrium states in the system, the hypothesis of a reversible way cannot be assumed, not even as a limit. Thus, it was suitable the definition of essergy as a potential which never increases in the system time evolution and which represents the distance of the system state from the environment state. In addition, it is to be remarked that, if one determine the essergy ε for a system and F is a whatever strictly increasing function, also F∘ε is an essergy parameter with the same properties of the parameter ε.