The role of cavities in the initiation and growth of explosion in liquids

Abstract

A study has been made of the growth of explosion in thin films and also three-dimensional charges of liquid explosives including nitroglycerine, nitromethane‒nitric acid mixtures, hydrogen peroxide‒ethanol mixtures and diethyleneglycol dinitrate (DEGDN). Observation was by high-speed photography at microsecond framing rates. In the thin film experiments burning was initiated at the centre of the film by the rapid discharge of a condenser across a spark gap. The transition from deflagration to much faster reaction (low velocity detonation) was found to depend on the production and collapse of cavities ahead of the reaction front. Experiments with inert liquids showed that the cavitation generated in the thin film arrange­ment was produced by a surface wave propagating on the liquid/solid interface. In other ex­periments air bubbles of chosen size were deliberately inserted into the thin films. Cavities and bubbles were observed to cause transition to fast reaction in nitroglycerine and then help sustain it by various processes, namely: (i) adiabatic collapse of cavities by pressure waves from the deflagration front; (ii) presentation, of a larger burning surface to the deflagration front as it entered the cavitated liquid, (iii) jetting during the collapse which dispersed liquid droplets in the heated cavity or produced high impact pressures. The delay before transition in nitroglycerine, and the dominant process causing it, depended on the acceleration of the deflagration front and the cavity size when the front and cavities inter­acted. Transitions did not take place in the other liquid explosives with the confinements and for the propagation distances considered. In these cases the deflagration front never accelerated sufficiently to reach cavitated liquid but always propagated into homogeneous liquid in which cavities had been ‘sealed’ by pressure waves from the front. In three-dimensional charges it was possible to propagate a deflagration front into homo­geneous liquid; the reaction then progressed throughout as a deflagration. When cavities were introduced by pre-shocking, transitions to faster reaction occurred with velocities which depended on cavity size.