Dynamics of curved flames in compressible flows

Abstract: Influence of flow compressibility on propagation and stability of premixed laminar flames in gaseous combustible mixtures has been investigated. A linear analysis of the intrinsic hydrodynamical instability of a planar flame front has shown that the instability growth rate is very sensitive to the compressibility effects and can increase significantly even for rather small Mach numbers of the flame generated flow.The nonlinear problem of propagation of a curved stationary flame in a compressible flow has been studied by means of 2D numerical simulations based on the complete set of flame dynamics equations. It was shown that curved shape of a flame in a compressible fuel mixture leads to stronger increase of the flame velocity than in the case of slow isobaric flames. It was found that acceleration of a flame due to the development of the curved shape generates a shock of noticeable intensity ahead of the flame front. The shock compresses the fresh fuel and, in turn, changes parameters of the flame. Additional amplification of the flame velocity was obtained due to the development of asymmetrical structures at the flame fronts in wide tubes.The interaction of curved flames with a weak shocks has been also examined. It was shown that the result of a head-on flame-shock collision depends on the shock intensity and on the heating coefficient of the burning matter. The critical intensity of a shock needed to invert the shape of a curved flame has been calculated. It was obtained that for sufficiently large intensities of the shock the flame-shock interaction leads to considerable increase of the flame curvature and velocity. Special attention was paid to the flame-shock collisions for the case of shock intensities close to the intensities of shocks generated by a flame in a closed burning chamber. It was shown that most of the laboratory flames in closed burning chambers are destabilized by weak shocks.