ABSTRACT
So far, we have restricted ourselves to the study of the flows of two-parameter, or simple gases, whose state is completely determined by a pair of the basic thermodynamic parameters, say, the pressure p or the density ρ and any energy-type variable, such as the temperature T, the enthalpy h, the internal energy e, or the entropy s. Earlier (see Sections 1.1 to 1.4) we noted that only gases in equilibrium state or in equilibrium process, which is understood to be a sequence of equilibrium states superseding one another at an infinitely slow rate, could be assigned to the two-parameter gases (in what follows this fact will be rigorously proven). However, actual rates of these processes are always finite, so that in the general case the state of a gas is nonequilibrium and is determined by the set of kinetic variables λi as well. These can be the mass (or volume, etc.) concentrations of the species that form the gas mixture, or the degrees of excitation of their internal degrees of freedom, that is, the vibrational (for molecules) or electronic levels. These parameters are determined by the differential equations of physicochemical kinet-
ics governing the course of physical processes or chemical reactions and having the general form of Equation 1.2.8. The problem consists in the specification of their right-hand sides i. At equilibrium the parameters λi = λie(p,T) are single-valued functions of the pressure and the temperature, so thatwe deal againwith a two-parameter gas (we refer the subscript e to the equilibrium state parameters). Naturally, this equilibrium solution must be a part of the totality of the relaxation equation solutions, that is, equilibrium gas dynamics outlined previously represents a particular case of the general nonequilibrium, or relaxation, gas dynamics. The subjectmatter of nonequilibriumgas dynamics can be described under two headings
(correspondingly, two chapters in this book). The first one contains the physicochemical model of thenonequilibriummixture ofmoderately dense (in the sense of Sections 1.1 through 1.4) reacting or relaxing gases, that is, the description of their equations of state, the generating functions i, and so on, for given flow conditions. These questions are treated in this chapter. The model follows from theoretical and experimental studies within the framework of different divisions of physics and chemistry, such as quantum and statistical physics, kinetic theory of gases, theory of chemical reactions, and so on. For this reason, our presentation has only descriptive, or phenomenological, character, sincewe simply could not do otherwise within the scope of a book on gas dynamics.∗
Real Gas Flows with
The second part of nonequilibrium gas dynamics is related with some specific effects of a gas dynamic nature caused by relaxation processes. They will be treated in the next chapter. In both chapterswewill deal with inviscid flows only. Nonequilibriumdissipative effects will be considered in Chapter 13.
