| In the nuclear power station and metallurgic plant,the steam explosion accident caused by accidental contact between cooling water and high-temperature molten metal possibly may occur,which leads to the heavy casualties and economic losses.This interaction is a complex transient process involving phase transition,heat transfer and chemical changes.Hence,it is difficult to explore the internal mechanism of the interaction.To date,there is no comprehensive and in-depth understanding of this phenomenon.It is meaningful to study on the interaction between cooling water and high-temperature molten metal for the analysis of relevant safety accidents.In this paper,a series of experiments is performed on the interaction between the water droplet and high-temperature molten aluminum using the visualized experimental device.The experimental apparatuses mainly compose of water injection device,high-speed camera image acquisition system and intermediate frequency furnace.The specific research work of this article is as follows:The dynamic and thermodynamic characteristics of a single water droplet impacting on the molten aluminum surface are studied under different falling heights of water droplet and initial temperatures of molten aluminum.Three typical experimental regimes are observed: the jet regime,the jet-bounce regime and the bounce regime.In the jet regime,a water jet is formed after the water droplet impacting on the molten aluminum surface,while the water droplet remains on the surface during the whole process.As the falling height of water droplet increases,the jet-bounce regime is observed in which a water jet is formed and then bounced off the molten aluminum surface after the water droplet impacting on the surface.Under the experimental condition that the falling height of water droplet and initial temperature of molten aluminum are both relatively large,the water droplet is directly bounced off the molten aluminum surface.This is the bounce regime.Furthermore,the maximum jet height is inversely proportional to the falling height of water droplet and the initial temperature of molten aluminum,while the maximum bounce height of water droplet is proportional to the falling height of water droplet and inversely proportional to the initial temperature of the molten aluminum.The evaporation-condensation model of the vapor film between water droplet and molten aluminum surface is proposed.As the initial temperature of molten aluminum increases,the complete evaporation time of water droplet decreases significantly owing to the dramatically increased radiative heat transfer between the droplet and molten aluminum.Combined with the additional experiments of a water droplet imapcting on the molten tin surface,it is found that the local collapse of the vapor film is caused by Rayleigh-Taylor instability and Kelvin-Helmholtz instability.After that,the lower-limit temperature to trigger direct contact interface of water droplet is lower than 208.67℃.In addition,the intensity of steam explosion increases with an increase in the initial temperature of molten metal.As the initial temperature of molten metal exceeds 500℃,the steam explosion of water droplet is unable to occur during the interaction.It is inferred that the vapor film is relatively thick and stable as the initial temperature of molten metal is higher.Therefore,the vapor film is unable to collapse locally under the action of its instability.The direct contact heat transfer between water droplet and molten metal is inhibited.As a result,the steam explosion can not be triggered as the temperature of water droplet is unable to reach its spontaneous nucleation temperature.The physical-chemical coupling characteristics of the interaction between water droplet and molten aluminum are studied.Three typical experimental regimes are observed in the interaction of continuous water droplets with molten aluminum under different falling heights and initial temperatures of molten aluminum: the water droplet polymerization regime,the water droplet breakup regime and the transition regime.The distribution of three regimes depends on the falling height of the continuous droplets,but has no obvious relationship with the initial temperature of molten aluminum.In addition,the complete evaporation time of cooling water decreases linearly with the increase of molten aluminum temperature when the molten aluminum temperature is relatively low(700-900℃).However,as the molten aluminum temperature exceeds 900℃,the complete evaporation time of cooling water decreases violently because of the greatly increased radiative heat transfer.The volume of hydrogen generated in the interaction between cooling water and molten aluminum is collected under different initial volumes of cooling water and initial temperatures of molten aluminum.The amount of hydrogen in the interaction increases with the initial temperature of molten aluminum,whereas it increases firstly and then gradually stabilizes as the initial volume of cooling water increases.The schematic illustration of hydrogen generation during interaction is established,and the physicochemical mechanism is analyzed in detail.It is proposed that the hydrogen and vapor film play a role of “insulated blanket”,which sharply reduces the intensity of the heat transfer between the cooling water and molten aluminum.The vapor film is unable to collapse locally because the instability(thickness)of the vapor film is relatively large.Consequently,no steam explosion is observed in the whole process of the interaction. |
