Abstract
In this study, the novel method was used to visualize the detonation initiation process by the high-speed spherical projectile using the soap bubble filled with the combustible mixture. The experiments were conducted using the stoichiometric hydrogen-oxygen mixtures with three argon dilution ratios of 40%, 50%, and 70% (2H2 + O2 + 2Ar, 2H2 + O2 + 3Ar, and 2H2 + O2 + 7Ar) in order to investigate the effect of the effective activation energy of the mixture on the criteria of detonation initiation by the projectile. The combustion phenomena around the projectile were observed using the shadowgraph optical system and the high-speed camera. The observation using the soap bubble indicated that the detonation initiation process included the multiple detonation initiations around the projectile that eventually developed into the spherical self-sustained detonation, when the Mach number of the projectile was lower than the Chapman-Jouguet (C-J) detonation Mach number. The initiation criteria based on the detonation cell width was consistent in the 2H2 + O2 + 2Ar and 2H2 + O2 + 3Ar mixtures, however, the detonation initiation was not observed even if the condition well above the criticality of other two mixtures in the 2H2 + O2 + 7Ar mixture, which indicated that the reduction of effective activation energy due to the high argon dilution ratio had the effect to reduce the capability of detonation initiation by the high-speed projectile. Two dimensionless parameters, which characterized the rapidness of the exothermic reaction and the induction length behind the shock wave, were applied to separate the conditions between the shock-induced combustion and the detonation initiation regardless of the argon dilution ratio, and the dimensionless parameters was able to separate the conditions for the detonation initiation, which could not be separated by the cell width.
