Cellular instabilities and self-acceleration of outwardly propagating spherical flames

Using a recently developed constant and high-pressure combustion chamber, an experimental study was conducted on several aspects of cellular instabilities of outwardly propagating spherical premixed flames. Propane/air and hydrogen/oxygen/nitrogen flames of different concentrations and under elevated pressures were used to systematically identify the influences of thermal expansion ratio, flame thickness, global activation energy, mixture Lewis number, and global stretch rate on the generation of hydrodynamic and diffusional-thermal cells over the flame surface. In particular, it was demonstrated that hydrodynamic instability is greatly enhanced with increasing pressure and hence decreasing flame thickness, although the influence can also be moderated by the progressively important three-body termination reactions as the pressure increases. The onset of cellular instability was examined in light of the theory of Bechtold and Matalon, and satisfactory qualitative and acceptable quantitative comparisons were observed. The cellular flames were found to be self-accelerating, including those that are diffusionally unstable, with fractal dimensions between 2.20 and 2.25.