Detonation initiation from a deflagration wave interacting with fence-type obstacles

Experiments were performed in an 80 mm square channel to study the transition of a unique shock flame complex to detonation in stoichiometric hydrogen-oxygen mixture. The shock flame complex structures were generated at the end of the first 2.48 m smooth-walled channel, and deflagration-to-detonation transition (DDT) occurred in the following 0.63-m long test section with fence-type obstacles on bottom wall. High-speed schlieren, pressure sensors, and soot foils were used to identify the flame structure formation and DDT process. The shock flame complex was categorized as fast flame or strange wave, depending on the coupling between the leading shock and the trailing flame. An advancing flame collides head-on with the reflected shock wave (FSI) from the end wall of the channel twice, and the second FSI produces a funnel-shaped flame with a large potential for acceleration. For a funnel-shaped flame, the pre-FSI flame state, which governs the post-FSI flame acceleration, is the dominant factor determining whether coupling occurs. For fast flame the mixing of the hot products with the unburned mixture at an obstacle corner initiates a localized explosion that transitions to a detonation wave, with no involvement of the leading shock. For strange wave detonation initiation can be prompt or delayed, governed by the strength of the precursor shock. The critical Mach number for the transition of a strange wave to detonation was experimentally found to be 2.24.