The changes in the flow field and combustion behavior of a scramjet combustor with a cavity flameholder under boundary layer bleeding condition were investigated by direct imaging, PIV measurement and numerical calculations, at the mainstream conditions of Mach number of 2.5 with total temperature of 580--760K and total pressure of 0.6MPa. Hydrogen gas was used as fuel, and the ignition was forced by precombustion gas injection. From PIV measurement and numerical calculations, it was found that the boundary layer bleeding decreased the static pressure, the density and the static temperature of the mainstream and increased the maximum velocity and the velocity of the boundary layer. Direct imaging of the combustion experiments and numerical calculations showed that the boundary layer bleeding increased the momentum flux of the mainstream and changed the interaction between the precombustion gas jet and the mainstreams, which in turn changed the static temperature and the local equivalence ratio distribution inside the cavity flameholder, resulting in a change in the combustion behavior more favorable to flameholding. It was concluded that the increase in momentum of the mainstream could improve the flameholding performance.
