Investigation of steam explosion features on metal droplets and solid hemispherical surfaces

The results of an experimental study of the explosive interaction of high-temperature molten tin droplets and solid metallic hemispheres with cooling water are presented. The experiments were performed to investigate the initial triggering stage of small-scale steam explosions at atmospheric pressure, with initial melt and cooling-water temperatures of 600–800 °C and 18 °C, respectively. High-speed videography, piezoelectric recording of pressure pulses, and conductometric measurements of the contact area on a solid metallic surface were employed as the primary diagnostic techniques. It was established that explosive fragmentation of a liquid droplet is preceded by direct contact between the coolant and the superheated metal surface at several local regions, with the number of such contact events reaching 5–6. In the pressure traces, these events are manifested as a sequence of pressure peaks with progressively increasing amplitude. Wavelet analysis of the pressure signals revealed several simultaneously occurring processes in the frequency range of approximately 2–80 kHz, associated with deformation of the vapor volume and ejection of fine liquid-metal droplets into the water. A qualitative similarity was established between the triggering mechanisms and subsequent explosive interaction with water for liquid droplets and solid metallic samples of hemispherical geometry. However, substantial quantitative differences were observed: for molten tin droplets, the amplitudes of the pressure pulses and the duration of the explosive interaction were several times greater than the corresponding values for the solid samples, whereas the calculated values of the released acoustic energy were several orders of magnitude higher. It is shown that solid samples can reproduce the qualitative features of the initial triggering stage of a small-scale steam explosion but do not fully reproduce the energetics and dynamics of the explosive interaction between a molten droplet and water.