Propagation mechanism of quasi-detonations

Abstract

The propagation mechanism of quasi-detonations in very rough tubes is studied using high speed schlieren photography. Stoichiometric mixtures of H2, C2H4 and C3H8 in oxygen at an initial pressure range 10≤po≤160 torr are investigated in a 61.8×61.8 mm by 1.5 m long channel with two-dimensional obstacles with a height of 25.4 mm and for various obstacle spacings. The results indicate that shock reflections (transition from regular to Mach reflections) from the walls lead to re-initiation. The obstacle spacing is found to represent an effective reaction zone length (or cell length) of the quasi-detonation. At the critical condition of transition from the choking to the quasi-detonation regime, this effective reaction zone length is found to be about twice the normal cell length of the mixture in accordance with Shchelkin's stability criterion for a perturbed wave. The minimum open dimension of the channel is found to be of the order of a cell size λ for transition to the quasi-detonation regime in agreement with the previous results of Peraldi17 and Gu8 for rough tubes. Photographic observations of the propagation mechanism in the choking regime reveal the absence of ignition via shock reflection. The placement of wire screens to damp the shock reflections at the channel walls suppresses the transition to quasi-detonations indicating the essential role of shock reflections. It is not clear whether the adiabatic heating or the turbulent vortex mixing associated with the shear layer wall jet by the Mach stem near the wall is the responsible mechanism for re-initiation.