Spatiotemporal visualization of instantaneous flame structure in a hydrogen-fueled axisymmetric supersonic combustor

Spatiotemporal visualization of instantaneous flame structures in a hydrogen-fueled axisymmetric supersonic combustor was investigated using multiview planar laser-induced fluorescence of the hydroxyl radical, coupled with high-speed photography and pressure measurement. The axisymmetric cavity generates a loop-shaped recirculation flow and shear layer that sustains the flame. An irregular and wrinkled flame loop with a central hole is formed near the loop-shaped region. Due to turbulent disturbances, multiple small-scale holes and fragmented flames are randomly distributed in the flame loop or near the wrinkled flame front. The combustion near the cavity shear layer is more likely to be stronger and sustained. As the thickness of the cavity shear layer increases along the axial direction, the flame loop is expanded toward the core flow and the cavity. The flame base anchors near the cavity leading edge with a low global equivalence ratio (GER). The increased GER expands the flame loop to compress the high-speed core flow dramatically, promoting the flame base to propagate upstream along the hydrogen jet wake. The flame base is unable to anchor near the thin boundary layer. Consequently, it propagates reciprocally to enhance the combustion oscillation that disturbs the flame structure dramatically. The flame structure becomes more complex and tendentially fragmented, which increases the fractal dimension, especially near the middle part of the combustor. In comparison, the flame structure near the ramp is more resistant to disturbances due to the dramatic expansion of local flame loop, extending the favorable combustion environment. Despite the instantaneous flame structure being severely wrinkled and even tendentially fragmented, it is primarily sustained within a relatively regular loop region near the cavity recirculation flow and the cavity shear layer.