Transformation of premixed flame behavior with hydrodynamic and body-force instabilities

Flame dynamics such as the formation of wrinkles is caused by instabilities such as thermal expansion and buoyancy. We have studied the combined thermal and buoyancy effects on the propagation of a premixed flame by using a front-tracking method. It was found that though wrinkled surface could be stabilized by a strong upward buoyancy force (negative gravity) in a downward propagating flame, wrinkles with short wavelengths remained intact, while those with long wavelengths were suppressed as the magnitude of gravity became sufficiently small. As such, compared to the zero-gravity condition, reduced ranges of cellular scales result in weaker interactions between multiscale wrinkles; consequently, the chaotic motion characterized by incessant merging and dividing of cells is also weakened. With downward buoyancy (positive gravity) as well as flame propagation, a steady flame structure is established. This is in contrast to the general realization of gravitational destabilization in that while a flame is readily deformed to a large wrinkle at the incipient stage of Darrieus-Landau instability, ensuing stability due to the coupling of Darrieus-Landau and Rayleigh-Taylor instabilities prevents further excitation of secondary Darrieus-Landau instability and hence associated unsteady evolution. However, if the positive gravity is sufficiently large, the ordered pattern degenerates and the highly irregular geometry of flame surface is formed without a specific cell structure. This is the typical manifestation of Rayleigh-Taylor instability caused by the buoyancy force exerted on an interface with a density jump. (C) 2008 American Institute of Physics.