Structure of the Shock-Flame Complex in Free-Standing Supersonic Hydrogen-Air Combustion

A free-standing hydrogen-air shock-flame complex has been stabilized in a supersonic swirl flow. Scalar field images obtained through laser Rayleigh scattering showed features analogous to triple points observed transiently during deflagration-to-detonation transition in planar systems. The scalar fields were used to quantify statistics of the shock position, shock-flame distance, shock curvature and number of compression wave intersections observed conditioned on the spatial position of the shock-flame system. The largest likelihood of observing a triple point was about half a nozzle diameter downstream of the nozzle exit irrespective of the shock leading point location. However, when the shock leading point was upstream of its mean location, the separation between the axial locations of the intersection point and the leading point were likely to be greater than when the leading point was downstream of its mean position. The shock-flame distance increased as the system moved downstream while the curvature decreased. The combined trends indicate that local explosions due to hot-spots might be responsible for strengthening shock-flame coupling and driving portions of the shock-flame complex upstream. As this region travels upstream, the increasing leading point curvature weakens the system and decouples it. The transient coupling/de-coupling of the shock and local explosions might be the governing mechanisms for flame stabilization in this system.