Mechanism of detonation transition from accelerating flames in a channel

Abstract

In order to clarify the mechanisms associated with a local explosion leading to deflagration to detonation transition (DDT) in a rectangular channel, detonation initiation behind the incident shock wave with a weak spark discharge was caused. In this approach the incident shock wave plays a role of the precursor shock wave, while the propagating flame is replaced with a flame kernel generated by the spark discharge. This method was successful in controlling the time and location of detonation initiation with good repeatability. The DDT process was visualized in two perpendicular directions to understand the three-dimensional behavior of the flame and the shock wave generated from flame acceleration. The following conclusions on the DDT process are deduced. (1) A shock wave coupled with a propagating flame eventually reflects off the wall as Mach reflection, behind which an accelerating flame and a shock wave ahead of it are caused. (2) The shock wave interacts with the other shock wave (shock–shock interaction) with more than a Mach number of 3. (3) This shock–shock interaction increases the temperature and pressure of the mixture locally, leading to the generation of a sufficiently strong shock wave with a larger MCJ. (4) The induced chemical energy release sustains the shock wave that decays to MCJ, resulting in propagation as detonation. Because the emergence of the accelerating flame and the shock–shock interaction process are completely three-dimensional phenomena, an understanding of three-dimensional flame and shock behavior is essential to reveal the DDT mechanism.