Premixed flame stability and transition to detonation in a supersonic combustor

Simulations of a supersonic, reacting, premixed flow in a channel were performed to investigate the effect of flow speed on ignition, flame stability, and transition to detonation. The configuration studied was a rectangular channel with a supersonic inflow of stoichiometric ethylene-oxygen, a transmissive outflow boundary, and no-slip adiabatic walls. The compressible reactive Navier-Stokes equations were solved by a high-order numerical algorithm on an adapting mesh for inflow Mach numbers, M∞, of 3 to 10. For M∞= 3, the fuel-oxidizer mixture does not reach a sufficient temperature for autoignition. Boundary layers that form on the top and bottom walls deflect the incoming flow, resulting in the formation of an oblique shock train. For M∞ ≥  5, the fuel-oxidizer mixture ignites in the boundary layers and the flame front expands into the channel. The flame front becomes unstable and turbulent with time due to a Rayleigh-Taylor (RT) instability at the interface between the low-density burned gas and high-density unburned gas. Detonation is initiated in several locations at the flame front and in the unburned gas through an energy-focusing mechanism. As M∞ increases, the time scales for growth of the RT instability at the flame front and eventual detonation increase significantly. Despite the difference in time scales, the flame evolution process is qualitatively independent of M∞: ignition in the boundary layer, laminar flame expansion, growth of an RT instability at the flame front, turbulent flame expansion, and deflagration-to-detonation transition.