Abstract

The seminal Navier-Stokes equations were stated even before the creation of the foundations of thermodynamics and its first and second laws. There is a widespread opinion in the literature on thermodynamic cycles that the Navier-Stokes equations cannot be taken as a thermodynamically correct model of a local "working fluid", which would be able to describe the conversion of "heating" into "working" (Carnot's type cycles) and vice versa (Afanasjeva's type cycles). Also, it is overall doubtful that "cycle work is converted into cycle heat" or vice versa. The underlying reason for this situation is that the Navier-Stokes equations come from a time when thermodynamic concepts such as "internal energy" were still poorly understood. Therefore, this paper presents a new exposition of thermodynamically consistent Navier-Stokes equations. Following that line of reasoning-and following Gyftopoulos and Beretta's exposition of thermodynamics-we introduce the basic concepts of thermodynamics such as "heating" and "working" fluxes. We also develop the Gyftopoulos and Beretta approach from 0D into 3D continuum thermodynamics. The central role within our approach is played by "internal energy" and "energy conversion by fluxes." Therefore, the main problem of exposition relates to the internal energy treated here as a form of "energy storage." Within that context, different forms of energy are discussed. In the end, the balance of energy is presented as a sum of internal, kinetic, potential, chemical, electrical, magnetic, and radiation energies in the system. These are compensated by total energy flux composed of working, heating, chemical, electrical, magnetic, and radiation fluxes at the system boundaries. Therefore, the law of energy conservation can be considered to be the most important and superior to any other law of nature. This article develops and presents in detail the neoclassical set of Navier-Stokes equations forming a thermodynamically consistent model. This is followed by a comparison with the definition of entropy (for equilibrium and non-equilibrium states) within the context of available energy as proposed in the Gyftopoulos and Beretta monograph. The article also discusses new possibilities emerging from this "continual" Gyftopoulos-Beretta exposition with special emphasis on those relating to extended irreversible thermodynamics or Van's "universal second law".

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