Experimental research of bubble number density and bubble size in narrow rectangular channel under rolling motion

Bubble number density and bubble size are two important parameters in the research of subcooled flow boiling and the channel power of water-cooled nuclear reactor are greatly affected by the two parameters through the neutron feedback caused by the variation of local void fraction. By using the high speed camera in combination with the digital image processing technology, the bubble number density and bubble mean diameter in an up-flow narrow rectangular channel under rolling motion are experimentally researched. Experimental results indicate that the bubble number density and the bubble mean diameter under rolling motion periodically fluctuated with the same period as that of the rolling motion. The camera captured bubbles consisted of sliding bubbles coming from the upstream flow and nucleation bubbles emerging in the scope of the observing window. Variations of the bubble mean diameters under rolling motion are analyzed by the distribution of the bubble diameters. Both the diameters of sliding bubbles and nucleation bubbles periodically changed under the effect of the rolling motion, and the two kinds of bubble share the same variation trend. The fluctuation amplitude of the bubble number density and the bubble mean diameter is determined by the rolling period and amplitude. The fluctuation amplitudes of the above two parameters are intensified by the rolling amplitude, whereas the effect of the rolling period is weak. At the same time, local pressure, wall temperature and local fluid velocity oscillated periodically in the rolling motion. Effect of the fluid velocity oscillation can be neglected due to its tiny fluctuation amplitude in this research. Variation of the effective wall superheat induced by the change of wall temperature and local pressure is the main reason accounting for the fluctuation of bubble parameters under rolling motion.