Cavity-enhanced combustion stability in an axisymmetric scramjet model

Cavity-enhanced combustion in an optical axisymmetric scramjet with a diverging combustor is experimentally investigated in Mach 4.5 flows. The axisymmetric scramjet emphasizes generic combustion and flameholding dynamics without the corner boundary-layer effects that can distort flame propagation in rectangular geometries. Combustion dynamics is characterized by pressure distributions along the combustor wall as well as heat flux measurements at the combustor exit. Flame structures are resolved by instantaneous hydroxyl planar laser-induced fluorescence imaging. The cavity flameholder enhances combustion stability through improved fuel-air mixing and promotes the growth of the flame length scales in the diverging combustor. Direct heat addition into the core flow through mixing dominates the combustor performance and can lead to thermal choking. The combustor area relief needs to balance the combustion heat loading for optimal combustor performance such that sufficient and stable energy release can occur in the combustor. Excessive area relief or fast flow expansion can quench the reacting flow, leading to deterioration in performance.