Deflagration to Detonation Transition by Forced Ignition near the Wall behind an Incident Shock Wave

Abstract

For deflagration to detonation transition (DDT), several explanations on initiation have been given. Nevertheless, the knowledge on DDT is still insufficient for predicting where and when detonation occurs. In order to improve the reproducibility, an ethylene oxygen mixture was ignited forcibly by spark discharge behind an incident shock wave near the wall. The process of flame development was visualized by Schlieren imaging and analyzed by drawing several wave element trajectories on the shock waves ahead of the flame. As a result of varying the timing of spark discharge, detonation initiation was promoted as the boundary layer Reynolds number Re_ign increases. For Re_ign of more than 5.0 × 10⁶, DDT was caused at 45 ± 10 μs. The processes of flame development were classified as Mode 1 and 2, which denote Re_ign of less than transition Reynolds number and more than it, respectively. Although the times for detonation initiation were markedly different in Mode 1 and 2, it was found that the both flame developments were similar. The accelerated flame near the wall propagates in upstream direction along the wall, resulting in approaching the shock wave front. This makes the shock stronger by coalescing of numerous compression waves. As the strengthened shock compresses the unburned gas, the flame was more accelerated, so that at the position where the flame front reached the shock front detonation initiation occurred. The difference of flame development between Mode 1 and 2 was observed in the initial stage in particular in the early 20 μs. Detonation initiation was caused at the position where following three conditions were satisfied: (1) A local Mach number reaches 2.4. (2) The flame front approaches and reaches the shock front ahead of it. (3) A concavity is generated on the flame/shock front, compressing the unburned gas coming into the point.