Effects of mitigation and suppression of vapor explosion have been investigated by adding a surfactant, polymer, and neutral salt into the water droplet impinging onto a molten alloy pool surface. This configuration was selected to attain good reproducibility and visibility, because premixing events prior to the triggering restrained. In addition, an adequate amount of a surfactant can be adsorbed at the vapor/liquid interface compared to that in the case of conventional configurations such as a molten alloy droplet injecting into a water pool. Dilute anionic and nonionic surfactant aqueoues solutions did not affect the triggering conditions or the pressure pulse generated by vapor explosion, even if the surfactant was added up to a density 25 times higher than the critical micelle concentration. Polymeric additives, which increase the viscosity of a fluid, have little suppression effect on vapor explosion. Spontaneous vapor explosion was, however, suppressed by a 200 wppm polyethylene glycol (molecular weight of 4 x 106) solution, since deposition of the solute due to cloudy-point phenomenon may stabilize the vapor film and prevent the solution from mixing finely. In order to exert this effect, molecular weight should be heavier so that a cloudy-point temperature is below the boiling point at the tested system pressure. Additionally, this threshold concentration became denser as the impingement velocity increased. Thus, a denser concentration and heavier molecular weight should be used to suppress vapor explosion when the vapor film may be subjected to large inertia and/or external force. When a neutral salt was added, the initial molten alloy temperature range where vapor explosion occurred shifted to a higher temperature and became wider. Vapor explosion was observed on the molten zinc surface, which does not explode spontaneously for water.
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