Liquid film behavior and heat-transfer mechanism near the rewetting front in a single rod air–water system. Issue 1 (2nd January 2020) - Wada, Yuki
ABSTRACT: The rewetting front propagation may occur when the fuel rod is cooled by the liquid film flow after it is dried out under accident conditions for boiling water reactor cores. Our previous study has revealed the importance of precursory cooling, defined as a rapid cooling just before the rewetting, which has a significant effect on the propagation velocity. To understand the mechanism of the precursory cooling, we conducted heat-transfer experiments using a single heater rod contained inside the transparent glass pipe to measure heat-transfer behavior with simultaneous observation using a high-speed camera. The results showed characteristic effects of the wall temperature on the liquid film flow and liquid droplets formation at the rewetting front, i.e. sputtering. Even when the liquid film flows in rivulets under adiabatic condition, horizontally uniformed rewetting front was observed with increasing wall temperature due to enhanced flow resistance by sputtering. This sputtering effect was also confirmed from observations of the liquid film thickness, which increased with approaching the rewetting front. Heat-transfer coefficients were predicted roughly well with a single-phase heat-transfer correlation with entrance effects, suggesting that the thinner thermal boundary layer downstream of the rewetting front may be one of the precursory cooling mechanisms.