The development and structure of flame instabilities and cellularity at low Markstein numbers in explosions

Flame instabilities and the formation of cellular structures during spheric al gaseous explosions have been studied experimentally using natural light and schlieren high-speed cine photography, as well as single-shot planar la ser-induced fluorescence (PLIF) from the OH radical. High-pressure, rich-hy drocarbon and lean-hydrogen flames at low Markstein numbers were employed. Ranges of unstable wavelengths have been identified as a function of Markst ein and Peclet numbers. The cine photography enables the dynamics of cell g rowth and fissioning to be studied and qualitatively interpreted, in terms of flame stretch rates and thermodiffusion. The PLIF technique enabled unst able wavelengths to be measured and flame fracture at negatively stretched cracks to be observed. A cascade of unstable wavelengths terminates in a ce llular structure. This structure appears at a second critical Peclet number . The smaller cells are continually destabilizing and restabilizing. As the y increase in size, the localized stretch rate on the cell surface decrease s and the cell becomes unstable. It restabilizes by fissioning into smaller cells with higher localized stretch rates. The cells are bounded by cracks in regions of negative curvature. At sufficiently small Markstein numbers the cracks are fractured. The results are interpreted within the theoretica l framework of the stability analysis of Bechtold and Matalon. (C) 2000 by The Combustion Institute.