Visualization study on bubble dynamical behavior in subcooled flow boiling under various subcooling degree and flowrates

An experimental study was conducted to investigate the characteristics of bubble dynamical behavior in subcooled flow nucleate boiling in an upward annular channel. The bubble dynamical behavior under wide ranges of subcooling degrees (ΔTsub = 5-50 K) and flowrates (vm = 0.1-0.8 m/s) were observed and recorded with high spatial and temporal resolutions by using a high speed video camera and a Cassergrain microscopic lens. The bubble behaviors at different subcooling degrees and flowrates were found to be quite different. At a medium subcooling degree, the bubble deformation at upstream-side interface during its growing and departure period was observed and was attributed to the condensation caused by the cold liquid coming from the subcooled bulk flow. At high subcooling degrees and low flowrates, contracting deformation was found in both upstream and downstream bottom of the bubble, the phenomenon of two successive bubbles was also observed. At high subcooling degrees but high flowrates, bubble was strongly pushed by the incoming liquid flow and concave deformation in the middle of bubble upstream-side interface was observed. At lower subcooling degrees, bubble behaviors at different flowrates are quite similar. The bubble shape was very close to a sphere, only very weak deforming phenomena was observed.