We now begin the study of premixed combustion. As we have learned from Chapter 6, a nonpremixed flame is supported by the stoichiometric, counterdiffusion of fuel and oxidizer. Thus, once ignited, a nonpremixed flame will situate itself somewhere between the fuel and oxidizer sources in order to satisfy this stoichiometry requirement. However, once ignition is achieved in a combustible fuel–oxidizer mixture, the resulting premixed flame tends to propagate into and consume the unburned mixture, if unrestrained through some aerodynamic means. Thus a premixed flame is a wave phenomenon.
In this chapter we shall study the simplest, idealized mode of wave propagation, namely the steady propagation of a one-dimensional, planar, adiabatic, wave relative to a stationary, combustible mixture in the doubly infinite domain. We shall call such a wave a standard wave or standard flame. In Section 7.1 we shall identify all such possible waves by constraining, through the conservation of mass, momentum, and energy, the states far upstream and downstream of the wave where the nonequilibrium processes of diffusion and reaction both vanish. Such an analysis yields the Rankine–Hugoniot relations, which show that two classes of waves can propagate in a combustible mixture, namely subsonic deflagration waves and supersonic, detonation waves. These waves have distinctively different properties.
Since the wave structure is not described at the level of the Rankine–Hugoniot analysis, the problem is not closed in that the crucial parameter of the wave response, namely the wave propagation speed, needs to be given.