Spontaneous Triggering of Vapor Explosion: Results of Experimental Studies

In the paper, the results of experimentally studying of spontaneous vapor explosion triggering are presented. They were obtained under model conditions when heated solid hemispherical samples were used and under laboratory conditions during the interaction of molten NaCl droplets with a distilled water pool at 15-70 degrees C. It is shown that the development of violent, disordered 3-D perturbations (oscillations) of the vapor film surface precedes spontaneous vapor explosion triggering. Such oscillations are accompanied by vapor film destruction and the creation of local short-term contacts of hot and cold fractions and intense, explosion-like formation of a vapor phase of the volatile fraction. The experiments also showed that, under laboratory conditions, spontaneous vapor explosion at the molten droplets of usual metals and alloys (lead, tin, POS-61 solder, Wood's and Rose's alloys, etc.) occurred very seldom and researchers are forced to apply an external triggering to provide a vapor explosion. At the same time, during the interaction of NaCl melt with water spontaneous vapor explosion triggering was practically attained with a probability of 100%. It was also shown that, during the simultaneous interaction of several NaCl melt droplets with water, the spontaneous vapor explosion triggering that has been originated at one of the droplets after a time lag of some tens of microseconds provokes explosion-like water evaporation at the other droplets also. Spontaneous triggering is accompanied by a typical sharp pressure pulse in the working medium and luminescent flashes at the molten droplet surfaces. The thesis was postulated that there is no common mechanism of fine hot fraction fragmentation, which is true for any situations and regime conditions.