Real-Gas Aerothermodynamic Phenomena

Real-gas aerothermodynamic phenomena in the context of this book are the so-called real-gas effects and flow phenomena related to hypersonic flight. Hypersonic flight usually is defined as flight at Mach numbers M ^ 5.[40] Here appreciable real-gas effects begin to appear. In this chapter we discuss the important real-gas phenomena with the goal to understand them and their implications in vehicle design.[41]

The basic distinction regarding real-gas phenomena is that between ther­mally and calorically perfect or imperfect gases. The thermally perfect gas obeys the equation of state p = pRT. A calorically perfect gas has constant specific heats cp and cv. We speak about a “perfect” or “ideal gas”, if both is given.

A gas can be thermally perfect, but calorically imperfect. An example is air as we treat it usually in aerothermodynamics. If a gas is thermally imperfect, it will also be calorically imperfect, and hence is a “real gas”. We note, however, that in the aerothermodynamic literature, and also in this book, the term “real gas” is used in a broader way to describe gases, which are thermally perfect and calorically imperfect. We will see in the following that real-gas effects in aerothermodynamics usually are high-temperature real-gas effects.

We first have a look at the classical “real gas”, the van der Waals gas. After that the high-temperature real-gas effects are treated which are of ma­jor interest in aerothermodynamics. Essentially we treat air due to the flight speed/altitude domain considered in this book as a mixture of the “thermally perfect gases” N2, N, O2, O, NO, implying that these are calorically imper­fect. Rate effects are explained, and also catalytic surface recombination. Finally computation models are considered.

We give only a few illustrating examples of real-gas effects, because real – gas phenomena, like flow phenomena in general, usually cannot be treated in an isolated way when dealing with aerothermodynamic problems of high­speed flight.