Effects of unequal blockage ratio and obstacle spacing on wave speed and overpressure during flame propagation in stoichiometric H2/O2

Experiments on flame propagation and detonation onset behind two solid obstructions were carried out in premixed stoichiometric hydrogen-oxygen mixtures at 20 kPa in a closed-ended tube. Obstacles with three different blockage ratios (25%, 40%, and 80%) were used, and the arrangement between the obstacles was changed in terms of blockage distribution (increasing, decreasing, and equivalent); obstacle distance (38, 76, and 114 mm); and opening geometry. Changes in the obstacle pair characteristics resulted in shocks with distinct intensities and averaged Mach numbers, (M) over bar (s), that propagated into the undisturbed mixture; (M) over bar (s) varied from 1.3 to 4.5 for the range of obstacle pairs tested. Four distinct deflagration-to-detonation transition (DDT) timescale groups were identified with average values varying between 0.7 and 12 ms. Obstacle pairs with increasing blockage ratio (25-80% and 40-80%) resulted in jet ignition downstream of the second obstacle, reducing the DDT length and timescale significantly when compared to the other obstruction combinations investigated. Soot foil records showed that detonation onset was via ignition from one or more hot spots near the flame front for all obstacle pairs that resulted in jet ignition. The results from this work suggest that channels with irregular obstacle patterns can experience faster detonation onset depending on the position of the obstructions relative to the ignition point.