The transition from CJ-deflagration to detonation in a square channel

Experiments were performed in a 7.6 cm square channel to study the transition of a Chapman-Jouguet (CJ) deflagration to detonation in stoichiometric propane-oxygen, hydrogen-oxygen, and acetylene-oxygen with and without argon dilution. The subcritical transmission of a CJ detonation wave through a perforated plate produced a CJ deflagration. High-speed side-view schlieren, end view chemiluminescence visualization, and soot foils were used to identify the location and nature of the deflagration-to-detonation transition (DDT) process. Detonation initiation was categorized as prompt or delayed depending on the effect of the compressible-turbulent flow immediately downstream of the perforated plate. For prompt initiation the transverse waves generated by the decoupled detonation were directly responsible for detonation initiation. The mixture detonation cell structure irregularity played no role in prompt initiation but played a significant role for delayed initiation. For undiluted propane-oxygen progressive strengthening of the transverse waves ultimately led to detonation initiation at the channel walls because of lateral transverse wave collisions. For the argon diluted mixtures that have regular detonation cell structures, detonation initiation typically occurred in the corners, most likely due to flame boundary layer interaction due to the absence of transverse wave collisions at the DDT location. The mechanism for transverse wave amplification was not present for the regular cell structure mixture.