Study on the instabilities of the leading edge for the diffusion flames under strong crossflow conditions

The instability and burning behaviors at the leading edge of diffusion flames under strong crossflow were investigated. Experiments were conducted on the boundary layer diffusion flame with different fuel types using a scaled-down wind tunnel, to explore the effects of crossflow velocity and fuel flow rate on the pulsation frequency, stabilization distance of flame leading edge and soot free length. Due to the combined effects of the hydrodynamic and thermodiffusive instabilities, the instability mode at the flame leading edge depends on crossflow velocity. The pulsation amplitude of the flame leading edge increases as crossflow velocity increases, therefore the pulsation frequency decreases continuously. The stabilization distance increases with crossflow velocity but decreases with fuel flow rate. Compared to propane flames, methane flames have smaller critical blow-away velocities and larger stabilization distances. A unified model was proposed to predict the stabilization distance based on the Damköhler number and analysis of the effective velocity at the flame leading edge. The soot free length is related to the crossflow velocity and fuel flow rate. Through theoretical analysis on the oxygen supply rate, an empirical model was established to characterize the soot free length.