Mechanism of the onset of detonation in blast initiation

The problem of blast initiation of gaseous detonation has been studied by focusing on the onset of detonation, i.e. the development of a pressure pulse during the quasi-steady period which leads to an abrupt acceleration to form a self-sustained detonation. This study has been carried out by numerical simulation of the one-dimensional Euler equations in a planar geometry. For the chemical kinetics model, a single-step Arrhenius law was assumed. It was found that for the critical energy required to initiate a detonation, the onset starts with the development of a pressure pulse between the reaction front and the shock front. The formation of the pressure pulse was attributed to the rapid energy release in the long induction length during the quasi-steady period. It was observed that within the framework of the present analytical model of a single-step Arrhenius rate law without losses, it is difficult to define a precise value for the critical initiation energy. However, the abrupt increase in the run up distance when the initiation energy reaches some critical range can be used to define the critical initiation energy. The present results show that initiation process has the same mechanism for both stable and unstable detonations. However, for unstable detonations when the activation energy is very high, no unique value can be defined for the critical initiation energy. It was found that analytical models based on the Zeldovich criterion cannot predict the critical initiation energy over the full range of activation energies considered in this study. This is because the Zeldovich criterion does not consider any dynamic effects during the quasi-steady period. Comparing previous research on initiation which used other initial conditions and the "blast initiation" which was studied in the present work, it was concluded that the onset of detonation during the quasi-steady period has the same mechanism for "deflagration to detonation transition" and "direct initiation". The role of hot spots in detonation onset was also studied. A single temperature perturbation was used to generate a hot spot. It was observed that the effect of the hot spot is mainly a gasdynamics effect. Hot spots promote onset through a multi-step shock merging mechanism. An optimum perturbation amplitude which can facilitate the initiation faster has been identified.