S0802-2-2 強制対流サブクール沸騰中のボイド率発展に関する一考察(原子力システムおよび要素技術(2))

Abstract

The mechanism of net vapor generation in subcooled flow boiling was investigated experimentally. The test section was a vertical rectangular channel and one side face of the test channel was heated uniformly. The test fluid was water and the system pressure was close to the atmospheric pressure. The inlet mass flux and the wall heat flux were kept constant while the inlet subcooling was changed parametrically to control the thermal equilibrium quality at the measuring section. The void fraction distribution was measured using an optical void probe and bubble behavior was observed using a high speed camera. In the present experiments, bubbles were usually lifted off the heated surface within a short period of time after the nucleation. After the lift-off, the bubbles disappeared quickly due to condensation upstream of the point of net vapor generation (PNVG). However, when the bubble coalescence took place, drastic change was observed. Since the coalesced bubbles frequently repeated the coalescence with other bubbles, typical bubble size increased discontinuously. Since the increase of the bubble size led to the decrease of the interfacial area concentration, the bubble life-time and the cross-sectional area-averaged void fraction increased significantly if the bubble coalescence took place. It was concluded that the bubble coalescence is a key phenomenon in determining the PNVG at least in the experimental condition tested in the present work.