Deflagration-to-detonation transition in laser-ignited explosive gas contained in a smooth-wall tube

Abstract

The effect of laser ignition on the deflagration-to-detonation transition (DDT) was experimentally investigated. Explosive gas, which was 0.87[(1/4)C2H4+(3/4)O2]+0.13N2 contained in a smooth-wall tube at 100 kPa and approximately 20 °C, was ignited by a 1064-nm 12-ns laser at either 8 or 88.8 mm from the closed tube end connected to the gas-feeding pipe, where the incident laser energy was either 40, 80, or 120 mJ. When the gas was ignited at 8 mm from the closed tube end, although laser ignition promoted DDT, the run-up distance to DDT was shortened by approximately 10% only. This was because the DDT behavior was essentially the same as the one observed typically in ordinary spark-plug ignition, in which a flame is accelerated approximately to be a choked flame before the detonation onset. When the gas was ignited at 88.8 mm from the closed tube end, two DDT scenarios were observed. The first scenario was the typical one mentioned above. In the second scenario, the detonation onset was induced by the collision between the flame and a shock wave following the flame before the flame became the choked flame. Significant DDT promotion was realized in the second scenario by the use of laser ignition with a larger energy. The shock wave inducing the detonation onset was likely created by the end-gas autoignition near the tube end or inside the gas-feeding pipe. As a result of the analysis on the shock wave, the mixing of the unburned and burned gasses induced by the vortex generation at the flame front by the baroclinic torque (Richtmyer-Meshkov instability) is the most plausible mechanism for initiating the micro-explosions that trigger the detonation onset.