An experimental investigation of the direct initiation of spherical detonations

Abstract

A study is described of the direct initiation, by solid explosives, of detonation waves in stoichiometric propane-oxygen diluted with nitrogen and contained in plastic balloons of between 1–2 m dia. The ignition source energies lie in the approximate range of 1–50 kJ. Quantitative information on the flow field behind the initiating blast wave, near the source, is obtained from pressure profiles given by pressure-bar gauges made of high tensile steel. Wavefront velocities are continuously monitored by a 3 cm microwave interferometer. Velocity profiles exhibit similar characteristics to those predicted by the “phenomenological” model of Lee et al. During the transition process from initiating blast wave to detonation large fluctuations are sometimes observed in the flame front velocity. This observation is not unlike the reported findings of Lee et al., Brossard and Struck, each of whom has observed, using spark initiation, a quasi-steady low velocity regime just prior to the establishment of a steady C-J wave. A simple criterion is postulated which relates the duration of the spherical blast wave flow, when its velocity is equal to the C-J velocity, and the duration of the subsonic flow behind the front in a self-sustaining C-J wave. This enables the critical weight of solid explosive to be predicted for various gaseous systems. The values predicted by the criterion are found to agree reasonably well for those determined experimentally for propane-oxygen and acetylene-oxygen mixtures.