Steam Explosions of Single Drops of Molten Silicon-Rich Alloys
Commercial quantities of ferrosilicon alloys and silicon metal in the form of granules are available. They are produced in water granulation, a process involving the risk of large-scale steam explosions. To learn about these steam explosions, we have investigated single drops of molten alloys when released into water. When single drops of molten ferrosilicon alloy with 75 wt.% Si (FeSi75) or silicon metal at its liquidus or melting temperature were released into cold, room temperature or hot water, spontaneous explosions never occurred. We could, however, initiate steam explosions by exposing the drops to upwarddirected pressure transients generated in the water with an encapsulated impactor. For water at room temperature, we determined (a) the strength of the pressure transients needed to initiate the steam explosion of a single drop, and (b) the range of fall depths over which the drop can be triggered to explode with this transient. When varying the water temperature we learned that the triggered explosions became less vigorous as the temperature increased. The explosions became more vigorous also when the Al and/or Ca content in the molten drop decreased. Photographic images of the bubbles generated during the steam explosions indicate that the maximum energies released correspond to about 1 % to 2% (dependent on drop size) of the total enthalpy of the molten ferrosilicon at its liquidus temperature. During the explosions, tiny bubbles of a permanent gas, presumably hydrogen, were generated. Video images show that ferrosilicon drops with diameters between 3 mm and 11 mm fall through water at about 0.5 m/s. Shutter wheel photographs confirm these fall rates and also indicate the onset of solidification of the drops and show that steam explosions can occur in partially solidified drops.
Formation of microbeads during vapor explosions of Field's metal in water.
We use high-speed video imaging to investigate vapor explosions during the impact of a molten Field's metal drop onto a pool of water. These explosions occur for temperatures above the Leidenfrost temperature and are observed to occur in up to three stages as the metal temperature is increased, with each explosion being more powerful that the preceding one. The Field's metal drop breaks up into numerous microbeads with an exponential size distribution, in contrast to tin droplets where the vapor explosion deforms the metal to form porous solid structures. We compare the characteristic bead size to the wavelength of the fastest growing mode of the Rayleigh-Taylor instability.
Reactivity of Al-2.5 Pct Li alloy with water as studied by the exploding wire technique
The reaction of molten Al-2.5 pct Li alloy or pure aluminum with water was studied by the exploding wire technique incorporating a crowbar switch current interruptor and an inductive energy store. The extent of chemical reaction was measured by the amount of hydrogen gas product vs the electrical energy input to the alloy or Al wire placed in water. The minimum electrical energy required to trigger the chemical reaction of the molten alloy or Al with water was about 2 kJ per gram of the metal. A further increase in the energy input to the wire revealed a large difference in the degree of chemical conversion between the two metals; the alloy wire needed only 4 kJ/g of the energy to achieve more than 90 pct of chemical conversion, whereas pure aluminum wire to achieve the same degree of the conversion needed 10 kJ/g to vaporize most of the wire mass. The higher reactivity of the alloy is interpreted as a result of the role of lithium in the alloy in altering the structure and strength of the surface oxide layer and increasing the oxide formation rate whereby the mixing of the liquids is facilitated.