Modes of chocked flame instability defined by the peculiarities of combustion kinetics at rising pressure

Abstract

The aim of the paper was to analyze the structure and the stability of the chocked flames to understand the origins of different possible combustion modes, including quasi-stable supersonic flames and deflagration-to-detonation transition. By means of numerical analysis, it is shown that the chocked flame structure and its stability are defined by two basic mechanisms: compression of the fresh mixture ahead of the flame front and compression of the reacting mixture inside it. The first mechanism provides burning velocity increase; the second one can either accelerate or decelerate reaction depending on the pressure-dependent reaction behavior in the observed pressure range and depending on the rate of compression. In case of reaction intensification with rising pressure, a detonation forms on the leading edge of the flame front. Otherwise, the flame propagates in a quasi-stable supersonic regime consisting of consequential stages of deceleration and re-acceleration of the flame. On the deceleration stage, the compressed fresh mixture priorly chocked by the supersonic flow near the flame tip flows downstream generating the compression wave ahead. The new contact surface between this packet of compressed mixture and the fresh mixture ahead of the flame front can become the kernel of the exothermal reaction, evolving in a new deflagration wave or even detonation.