Experimental study of the possibility of occurrence of a vapor explosion at the initial stage of a melt jet breakup


Аuthors

Vasil’ev N. V.*, Vavilov S. N., Lidzhiev E. A.

,

*e-mail: nikvikvas@mail.ru

Abstract

When a melt with a high temperature gets into cold water, under certain conditions, processes of its explosive boiling are possible, accompanied by fragmentation of the melt, leading to an increase in the contact area of the hot and cold substances. This phenomenon, called a vapor explosion, can cause destructive consequences at facilities of the nuclear, metallurgical, pulp and paper industries. At the first stage of the vapor explosion process, a hydrodynamic disintegration of a jet of molten substance covered with a vapor film into individual drops and their mixing in the volume of cold liquid (premixing) occurs. The next stage is called triggering and occurs, apparently, as a result of fluctuations in the thickness of the vapor film up to the possibility of local direct contact of the hot and cold substances. Triggering provokes a fine fragmentation of one of the drops and local sharp vaporization due to an increase in the contact area of water and the melt. Rapid vaporization on one of the droplets provokes a pressure wave propagating in the liquid, which leads to a repetition of similar processes on neighboring droplets and a rapid increase in the volume of the vapor phase.
At the same time, there are regimes of jet breakup, in which disintegration into large fragments is accompanied by the formation of so-called satellite droplets with sizes an order of magnitude smaller than the diameter of the jet. These satellite droplets are potentially more likely to spontaneously trigger a vapor explosion, which will provoke further spread of the process to neighboring large fragments of the jet breakup. The work is devoted to an experimental study using high-speed video filming of the vapor explosion process on a fragmenting jet of molten salt NaCl in water. The modes of jet breakup into large parts, accompanied by the separation of small satellite drops with sizes an order of magnitude smaller than the jet diameter, are studied. The possibility of a vapor explosion due to spontaneous triggering of the process on the satellite droplet at the initial stage of the first stage of rough fragmentation and mixing of the melt jet and its spread to large fragments of jet breakup is demonstrated. It is shown that the probability of events developing according to this scenario decreases with increasing water temperature.

Keywords:

vapor explosion, subcooled water, jet breakup, satellite drop, molten salt, spontaneous triggering, high-speed video filming

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