| Summary: | Direct contact condensation affects the performance of pressure suppression system of nuclear reactor. Since the heat and mass transfer varies with the condensation regimes as well as steam-water interface shapes, the identification and classification of the condensation regimes are important. The experiment of steam direct contact condensation was carried out and the corresponding condensation regimes were studied. Specifically, the experiment was studied by injecting steam vertically into a wet well with subcooled water. The physical process of steam condensation phenomenon with varying undercooling degree of wet well was recorded by high-speed camera. According to the location of the steam-water interface and the shape of the steam bubble, the four condensation regimes were studied, namely internal chugging, external necking, spheroidal upward bubble and T-shaped upward bubble. Besides, it is found that thermal stratification occurs in the wet well at low steam mass flux, the influence of each condensation regime on the thermal stratification of wet well was studied. The experimental results show that the internal chugging and external necking cause a large downward circulation of subcooled water, which enhance the flow mixing around the discharge tube and the bottom of the wet well. Consequently, the thermal stratification phenomenon does not happen in those condensation regimes. However, the regimes like spheroidal upward bubble and T-shaped upward bubble cause the upward circulation of subcooled water in wet well, which cause the thermal stratification. The critical Richardson number (Ri) determines the condensation regimes. Furthermore, when the Ri is smaller than 1, the condensation regime is external necking. Besides, when the Ri is greater than 1, the condensation regimes are spheroidal upward bubble and T-shaped upward bubble.
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