The establishment of experimental method to observe bubble formation in high temperature molten salt system and estimation of generated bubble size

Recently, molten salt has attracted attention as an attractive liquid phase for gas-liquid processes at high temperature. Then a method of estimating the gas–liquid interfacial area in molten salt is required. However, the experimental and theoretical methods to estimate the gas bubble size in molten salt systems have not yet established. Especially, there is a lack of experimental techniques to directly observe the bubble formation in a high temperature system. The experimental observation of growth, stretch and detachment of bubbles is also necessary for modeling and numerical simulation. Thus, at first, this study tried to construct a reliable experimental method for observing the bubble formation in a high-temperature liquid. Clear images of the progress of bubble formation could be recorded by the experimental setups proposed in this study. The bubble formed in sodium nitrate was clear, spherically shaped and the shape was maintained longer than in aqueous system. Secondly, using this images, the bubble size generated in molten sodium nitrate was measured. The measured value was compared with the estimated values of the empirical formula established in the aqueous system. At dynamic conditions, where the difference of the physical properties has no effect on the bubble size, the same equation was applicable to those two quite different solutions. However, at static conditions, the measured bubble size was larger than the estimated value. It suggested that the empirical formula established for the aqueous system could not sufficiently estimate the effect of the surface tension in a molten salt solution.