The Spontaneous Triggering of Small-Scale Vapour Explosions

Abstract

This thesis presents the results of experimental work on the spontaneous triggering of vapour explosions, a phenomenon which can sometimes occur when two liquids at very different temperatures are brought into contact. The energy for the explosion is derived from the thermal energy in the hot liquid which is transferred very rapidly to the cold liquid by a fast fragmentation and intermixing process. The latter is vaporised so rapidly that a pressure pulse propagates from the explosion source. The results of some 2000 small scale spontaneously triggered vapour explosion experiments are described, using four different liquid pairs. Spontaneous triggering is a term reserved here for those vapour explosions or interactions in the dropping mode of contact which occur without any external influence. In the case of molten metals it is generally associated with the collapse of an insulating vapour film at the liquid-liquid interface: hence a knowledge of the stability, heat transfer and breakdown characteristics of vapour films and bubbles is important to an understanding of spontaneous explosions. Small-scale experiments are described to investigate such characteristics by measuring the heat flux, (conventionally presented in the form of boiling curves), from metal spheres heated and rapidly quenched in water, and also the growth and collapse characteristics of spark generated vapour bubbles. A new technique is described for constructing minute surface thermocouples for surface temperature measurements on metal spheres. The most important single result is the identification of regions in two-temperature space, called “temperature interaction zones" (TIZ) inside which it is possible for explosions to occur but outside of which they do not occur spontaneously. Some of the factors which affect the position of the zone boundaries have been identified and investigated and the variation of violence and dwell times across the zone have been related to the measurements of heat transfer and vapour stability in the quench and bubble experiments. A simple turbulent flow model for heat transfer from hot spheres has been developed. Particle size distribution in the finely fragmented portion of interacted metal debris, important to the modelling of heat transfer, has been shown to be predictable and essentially independent of the metal and water temperatures, mass and trigger mode over the ranges studied.