Abstract :
For flame propagation in tubes or channels filled with obstacles, it was observed that deflagration to detonation transition (DDT) occurred mostly in the vicinity of an obstacle. Furthermore, onset of detonation originated near the upstream side of the obstacle. This observation suggests that the interaction between the leading shock front of an accelerated flame with the obstacles along its path plays an important role in the transition process. To understand this interaction, initiation of detonation resulting from a collision of a shock wave with obstacles (repeated baffles and wedge-baffle combination) was studied in a 9 × 9 cm channel containing hydrogen-oxygen mixtures. Stroboscopic schlieren photographs showed that shock-focusing, from shock diffraction and reflection, can create local hot spots capable of causing a strong ignition develops into a detonation wave. For collision of a shock wave with a wedge-baffle combination in a stoichiometric H2---O2 mixture at 7.9 kPa (60 torr) initial pressure, the critical incident shock Mach number was measured to be about 2.25. This critical value depends on the wedge angle as well as the size of the baffle obstacle. Even though the effects of turbulence in DDT have been well documented, present results suggest that for highly accelerated flames (with flame speed approaching that of a CJ deflagration), local heating by shock-focusing, resulting from a collision of the leading shock with obstacles, is sufficient to cause transition to detonation.
