| When a detonation propagates in an explosive that partially fills a channel, a shock wave can be generated in the air gap between the explosive and its confinement. This is typically referred to as the channel effect. In the air gap, the detonation products behave as a piston, which drives the precursor air shock wave ahead of the detonation. Since the shock wave runs ahead of the detonation, it preconditions the explosive and the detonation propagates into shocked explosive. This can affect the detonation in various ways, depending on the nature of the explosive. Properties such as heterogeneity, porosity and sensitivity of the explosive will determine how the precursor shock will affect the detonation propagation. Four different coupling mechanisms have been identified and are discussed in this thesis. They have been called: precompression, detonation initiation, surface ignition and dead pressing.; In the present study, three cases are investigated experimentally: precompression, detonation initiation and the case where no coupling occurs. The goal is to elucidate the respective propagation mechanisms. It is found that boundary layers on the channel walls significantly affect the precursor shock wave propagation. This effect is modeled and the results are compared to experiments.; When the explosive is PETN powder, the detonation is found to accelerate to 1.5 times the Chapman-Jouguet velocity. Experiments performed indicate that this is due to precompression of the PETN. Again this effect is modeled and compared with experiments.; It is also demonstrated that in the present experiments, coupling via initiation does not occur. However, experiments were performed to determine why the explosive is not initiated by the precursor shock wave. It is found that the initiation delay for the strength of precursor shock generated is simply too long for any coupling to occur. |
