Flame Propagation with Hydrodynamic and Body-Force Instabilities

The hydrodynamic (Darrieus-Landau) instability is an intrinsic mechanism that wrinkles the flame surface. In the nonlinear stage, propagation of flame wrinkles can evolve to a quasi-stable state characterized by a solitary wave or chaotic form with corrugated front. The underlying structures, i.e. incessant merging of near wrinkles and creation of new cells, have been studied numerically. It reveals the significance of asymmetric perturbation in breaking the symmetry. The effect of gravity was also investigated. It was found that, while wrinkled flames can be stabilized by negative gravity of moderate magnitude, the wrinkles at short wavelength, \textit{$\lambda $}, remains intact if the magnitude is small while those at long \textit{$\lambda $ }are suppressed. As such, compared to the zero-gravity state, diminishing multiplicity of cellular scales and subsequently decreasing interactions among the multi-scale wrinkles mollify the chaotic complication. When slight positive gravity is introduced, the unsteady evolution is suppressed. The somehow stabilizing effect, while in contrast to the destabilization at linear stage, is due to the coupling of D-L instability and Rayleigh-Taylor instability that prevents excitation of secondary D-L instability. If the magnitude is strong enough, however, the ordered pattern degenerates and highly irregular flame surface is formed without specific cell structure. This is a typical appearance of R-T instability caused by buoyancy.