Morphology and propagation of cellularly unstable flames

We report herein our recent experimental observation and mechanistic interpretation of the evolution and propagation of constant-pressure, spherically expanding H₂/O₂/N₂ flames, subjected to hydrodynamic and diffusional-thermal instabilities over a wide range of pressure, equivalence ratio and thermal expansion ratio. Analysis of high speed Schlieren images of these expanding flames show the existence of three distinct stages of flame propagation affected by the development of the instability cells, namely smooth expansion, transition, and saturated stages. The onset of the cellular structure is mainly controlled by the diffusional-thermal instability, while characteristics of the subsequent transition to and maintenance of the saturated state is controlled by the hydrodynamic instability. The study also systematically investigates the effect of pressure, thermal expansion ratio, equivalence ratio or Lewis number on these self-accelerating flames. The acceleration exponent for the fully developed saturated instability is found to be around 1.2-1.4, which is smaller than 1.5, the suggested value for self-turbulization.