Lean blowoff behavior of cavity-stabilized flames in a supersonic combustor

Abstract

Experiments are performed to investigate the near blowoff characteristics of cavity-stabilized flames in an ethylene-fueled scramjet combustor. Heated air enters the combustor with conditions of Ma = 2.52, stagnation pressure p=01.6 Mpa and stagnation temperature T0= 1400 K. The lean blowoff and ignition limits are obtained for different-sized cavity flameholders with upstream flush-wall fuel injection, and the flame structures and dynamics near blowoff are captured with high-speed chemiluminescence imaging. Though the flow residence time is believed to increase with increasing cavity size, it is observed that the lean ignition and blowoff limits do not vary monotonically with the cavity size. Under the same fuel injection conditions, the flame intensity in larger cavities is weaker than that in smaller cavities though the residual flame can survive for an obviously longer period in larger cavities. This may be attributed to the leaner mixture within larger cavities when the lean blowoff limits are approached. Thus, there exists an optimal cavity size which is the most suitable for the ignition and flame stabilization near lean blowoff conditions. This optimal cavity size is determined by the competition between the flow residence time and the local equivalence ratio that greatly depends on the interactions between the fuel jet and the cavity.