Three-dimensional behaviour of quasi-detonations

Abstract

Building on previous experiments conducted in an obstructed narrow rectangular channel, new details of the three-dimensional propagation behaviour of supersonic combustion waves have been revealed. In this study, a square channel equipped with 50% blockage ratio obstacles was used. Average velocity measurements coupled with high-speed schlieren photography and sooted glass sheets were used to simultaneously capture wave propagation and triple-point trajectories from multiple fields-of-view. Experiments were carried out in mixtures of stoichiometric hydrogen-oxygen at initial pressures between 9 kPa and 60 kPa in a 3.66 m long, by 7.62 cm square cross-section channel with optical access. Results show that the increased channel width results in a lower maximum pressure for which fast-flame propagation occurs. At higher initial pressures, detonation kernels were initiated at the obstacle face-sidewall interface in either a symmetrical (both sides) or an asymmetrical (single side) formation across the channel width. Wall reflection generated detonations evolve to form transverse detonations propagating diagonally across the channel width in the shock-compressed region following the obstacle. The single wall ignition was found to lead to a stable single-head “zig-zag” detonation (diagonal propagation driven by sidewall reflection) at initial pressures from 17 kPa to 24 kPa where transverse detonation reflection leads to the generation of a reactive Mach stem that survives diffraction at the next obstacle pair. Soot foils displayed a unique narrow vertical band of cells where the transverse wave collides with the channel sidewall in this propagation mode, which is the only mode to not involve obstacle reflection re-initiation. The channel width w, being larger than the obstacle opening d, makes it possible for the transverse modes seen in an obstacle-free channel to lock in, like the single-head detonation propagation observed. Continuous detonation propagation through the channel core was seen at high CJ velocity deficits beginning at d/λ = 6.3, where λ is the detonation cell width, with higher initial pressures having cellular structure reach the channel walls between obstacles. Thus, continuous detonation propagation is governed by the diffraction process around the obstacles and d is the governing length scale.