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Yang Cao
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8540, Japan

Zensaku Kawara
Department of Nuclear Engineering, Kyoto University, Kyoto-Daigaku katsura, Nishikyo-ku, Kyoto, 615-8540, Japan

Takehiko Yokomine
Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga-kouen 6-1, Kasuga, Fukuoka 816-8580, JAPAN; Department of Nuclear Engineering, Kyoto University, Kyoto-Daigaku Kastura, Nishikyo-Ku, Kyoto, 615-8540 Japan

Tomoaki Kunugi
Department of Nuclear Engineering, Kyoto University, Kyoto Daigaku-Katsura, Nishikyo-ku, Kyoto, Japan

DOI: 10.1615/TFESC1.mph.012773
pages 1983-1990

KEY WORDS: Experiment, Subcooled flow boiling, Bubble coalescence, High tempo-spatial observation, Upward flow, Annular channel


Experimental study has been conducted on the bubble behaviors of forced convective subcooled boiling in an upwards annular channel. The images of single bubble behavior and multi-bubbles coalescence behaviors have been observed with both high spatial and temporal resolutions by using a high speed video camera and a Cassegrain microscopic lens. The experiments were carried out in relatively low degrees of subcooling less than 10 K in this study. It was found that when a certain nucleate bubble reaches its maximum size, some new bubbles were often generated in the downstream separation region of that bubble and surprisingly even in the upstream of it. The new nucleated bubbles grow rapidly and reach its maximum size in a short time period before the pre-existing bubble departs from the wall. The reason is because the vortex flow generated in the up- or down-streams of the preexisting large bubble can enhance the thermal mixing of the fluid, the temperature becomes higher compared to the saturation temperature and eventually new bubbles are generated there. There is a high probability that the new nucleated bubbles will coalesce. The coalescence of bubbles will cause the wave propagation on the bubble surface and make the bubble deform strongly. This interface deformation can enhance the heat transfer because of the surface enlargement, so the coalescence bubble collapses rapidly in a short time period because the condensation rate of coalescence bubbles is much faster than that of a single bubble. The bubble coalescing phenomenon will form a long plug-shaped vapor clot near the heating wall, which may be considered as one of the precursor phenomena of the Departure from Nucleate Boiling (DNB) and indicates the start of transition from nucleate boiling to film boiling.

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