Novel weak detonation initiation from normal shock reflection in square cross-section shock tubes

Normal shock wave reflection driven detonation initiation was investigated in a 7.63-cm square cross-section shock tube using novel stereo visualization. High-speed schlieren video captured shock reflection and detonation onset through the sidewall of the shock tube, and simultaneously, self-luminous imaging through the end wall enabled the determination of the location of detonation onset in three-dimensional space. Tests were carried out with nitrogen- diluted stoichiometric ethylene-oxygen. Weak and strong detonation initiation modes were observed at the low and high end of the reflected shock temperature range tested, respectively. A novel reflected shock bifurcation driven weak detonation initiation mode was observed at intermediate temperatures where a flame, ignited at one, or multiple, reflecting wall corners, accelerated along the channel wall corner reaching the bifurcated reflected shock wave. The flame rapidly spread through the bifurcation stagnation bubble transitioning to detonation. The reflected shock temperature range that this new weak initiation mode was observed depends on the reflected shock pressure. For low reflected shock pressures, typical of historic detonation initiation shock reflection studies, this weak bifurcation mode was not observed. Nonreactive three-dimensional Navier-Stokes simulations showed that the reflected shock bifurcation generates flow conditions along the channel wall corners that can explain the observed flame propagation from the end wall to the stagnation bubble, and the subsequent rapid spreading in the stagnation bubble.