Bubble collapse and the initiation of explosion

Experiments were conducted to investigate the initiation of an emulsion explosive containing cavities. Cylindrical cavities were created in thin sheets of either gelatine or an ammonium nitrate/sodium nitrate emulsion confined between transparent blocks. Shocks were launched into the sheets with either a flier-plate or an explosive plane-wave generator so as to collapse the cavities asymmetrically. The closure of the cavities and subsequent reaction in the explosive was photographed by using high- speed framing cameras. The collapse of the cavity proceeded in several stages. First, a high-speed jet was formed which crossed the cavity and hit the downstream wall sending out a shock wave into the surrounding material. Secondly, gas within the cavity was heated by rapid compression achieving temperatures sufficient to lead to gas luminescence. Finally, the jet penetrated the downstream wall to form a pair of vortices which travelled downstream with the flow. When such a cavity collapsed in an explosive, a reaction was observed to start in the vapour contained within the cavity and in the material around the heated gas. The ignition of material at the point at which the jet hit was found to be the principal ignition mechanism.

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