BUBBLE BEHAVIOR AND MEAN DIAMETER IN SUBCOOLED FLOW BOILING

Bubble behavior and mean bubble diameter in subcooled upward flow boil ing in a vertical annular channel were investigated under low pressure and mass flux conditions. A high-speed video system was used to visua lize the subcooled flow boiling phenomenon. The high-speed photographi c results indicated that, contrary to the common understanding, bubble s tend to detach from the heating surface upstream of the net vapor ge neration point. Digital image processing technique was used to measure the mean bubble diameter along the subcooled flow boiling region. Dat a on the axial area-averaged void fraction distributions were also obt ained using a single-beam gamma densitometer. Effects of the liquid su bcooling, applied heat flux, and mass flux on the mean bubble size wer e investigated A correlation for the mean bubble diameter as a functio n of the local subcooling, heat flux, and mass flux was obtained.