Transition to detonation of an expanding flame ring in a sub-millimeter gap

Deflagration-to-detonation transition of a flame ring circularly expanding in a 260 μm gap filled with stoichiometric ethylene/oxygen mixtures initially at atmospheric pressure and temperature has been experimentally visualized. The results show that DDT can occur under the influence of wall confinement even for an expanding flame. DDT could be observed at a distance as short as ∼70 mm from the ignition spot, which corresponds to ∼130 μs after the ignition spark voltage breakdown. Velocity overshoot of reaction front velocity exceeding Chapman–Jouguet velocity was characterized. Cell structures were observed on the reaction fronts after DDT occurred. The visualizations also showed that smooth circular flame developed right after ignition quickly evolved into wrinkled flame as the flame ring propagated outwards. Flame propagating velocity was accelerated from ∼600 m/s to ∼1000 m/s during the wrinkled flame stage. A series of local explosion on the flame ring was observed during the DDT process, and resulted in an abrupt surge on reaction front propagation velocity.