Transition to detonation in hydrogen-air mixtures due to shock focusing in a 3-wall 90-deg corner

Abstract

This work presents an experimental investigation on the transition to detonation due to shock-wave focusing in a 3-wall 90-deg corner in hydrogen-air mixtures (17.5 – 60% H2) initially at 298 K and 1 bara. The reflector was placed at one side of 2 – m long, 0.14 m internal diameter tube. The other side of the tube was equipped with an ignition source and a flame acceleration section. The shock wave approached the reflector with a velocity in the range of 580 – 970 m/s. The reflector was placed symmetrically to the tube axis. The research proved that transition to detonation due to shock focusing highly depends on the shock wave strength and can be triggered in a stoichiometric mixture at shock speed VS = 605 m/s which corresponds to M = 1.485 or 59.9% of the speed of sound in combustion products. The ignition delay time recorded for transition to detonation was at the order of few microseconds. The detonability range observed with the 3-wall 90-deg reflector proved to be 15 - 70 % H2 in air compared to 18 - 59% H2 observed in work performed for a 90-deg wedge reflector configuration at the same initial conditions. Additionally, the scaling factor between critical shock velocity between both types of reflectors is almost constant and equal to 0.84 throughout the investigated mixtures. The high focusing ability of the 3-wall corner has also proved the minimum transition pressure recorded in the corner tip at the level of 8.95 MPa in a stoichiometric mixture. This value corresponds to a nearly 17-fold increase in pressure compared to normal shock post-reflection conditions. Considering the shock focusing as the major trigger for detonation the presented results may explain the differences between the detonability limits observed in obstacle-laden ducts and large-scale experimental setups.