Diffusion flames and supersonic combustion 2

Abstract

Some problems related to the fluid dynamical and chemical phenomena appearing near the injector exit of an idealized supersonic combustion burner, are theoretically investigated. When hydrogen is injected into a coflowing supersonic stream of air,a wake-like configuration appears in most cases,improving the mixing process. It is only natural to suggest that whenever the ignition delay length, computed assuming isobaric mixing and reaction, is not much larger than the recirculation zone length, the wake will influence the ignition zone. In order to calculate the ignition delay length, the usual assumption is made that fuel and oxidizer mix without appreciable concentration change and heat release resulting from chemical reactions, although radicals, mainly atomic hydrogen, are produced. The chemical kinetics scheme is reduced to one overall chemical reaction; and the presence of radicals introduced in the mixing zone from outside is taken into account. A two-dimensional mixing zone has been considered. Although many different mechanisms will be responsible for the fortuitous or provoked production of these radicals either in real flight or in ground testing facilities, dissociation at the injector outer boundary layer is considered in this paper as the main radicals producing mechanism. As it is shown, the temperature of the injector outer wall and, to a less extent, pressure, injector length and the conditions outside of the boundary layer, control the amount of radicals introduced in the mixing layer, and hence the ignition delay length. Finally, it is shown that ignition delay lengths are, in most cases of interest, comparable to the near-wake length. The paper ends with a discussion of the main weaknesses of the work presented and with some suggestions for further work