Dynamics and Statistics of Cellular Instability in Laminar and Turbulent Expanding Flames

Abstract

This dissertation consists of a series of studies on the dynamics and statistics of the cellular instability in both laminar and turbulent expanding flames. First of all, local quantities including the flame-induced velocity, curvature and strain rates of the cellularly-unstable laminar expanding flames have been measured for the first time to our knowledge. The statistics of these quantities and their relations with the global flame propagation have been investigated. Then, the flow field induced by the cellularly-unstable laminar expanding flames has been further processed to address the hypothesis of self-turbulization. The flow field near the unstable flamefront exhibits turbulence-like characteristics, and a power-law scaling relation in the energy spectra of tangential velocities on the flamefront have been discovered. It is demonstrated theoretically that such scaling relation is caused by the fractal nature of the wrinkled flame surface. Next, in light of the knowledge of cellular instability in laminar expanding flames, the effects of cellular instability in turbulent expanding flames have been studied thereafter. A scaling analysis has been conducted to show the interaction between Darrieus-Landau instability and turbulence in different regimes of the turbulent flame regime diagram. The turbulent flame speeds have also been measured in these regimes to validate the scaling analysis. Furthermore, the effect of the diffusional-thermal instability has also been investigated experimentally. It is shown that Darrieus-Landau instability controls the flame acceleration, while diffusional-thermal instability affects the magnitude of the flame speed. Next, the local statistics of the turbulent cellularly-unstable expanding flames in the corrugated flamelet regime have been studied, and the interaction between cellular instability and turbulence are assessed based on the statistics of the strain rates, the curvature and the surface spectra.