Experimental study on the rewetting velocity on dry out surface due to stepwise boundary condition changes

Abstract

Post-boiling transition (post-BT) heat transfer is essential for analyzing the duration of surface dryout and peak cladding temperature during abnormal transients and accidents in light water reactors. The rewetting phenomenon, in which the liquid film front propagates to the dryout surface and becomes wet again due to a decrease in heat flux or an increase in flow rate, is very important for evaluating the dryout duration. Although rewetting velocity models have been proposed mainly for reflooding processes, mechanical rewetting velocity models applicable to much higher flow and heat flux conditions than reflooding processes have been developed in recent years. However, due to the lack of an experimental database on rewetting velocities under such a wide range of thermal–hydraulic conditions, sufficient data for model development and validation do not exist. Therefore, a database on rewetting velocities caused by stepwise boundary condition changes under a wide range and multiple combination of thermal–hydraulic conditions was obtained using a single-tube experimental apparatus. Based on this database and the characteristics of rewetting velocities obtained, an empirical correlation for rewetting velocity was proposed. This correlation predicts the rewetting velocity accurately by taking the change in the mass flux of the liquid or gas phase with stepwise transients as a parameter. This suggested that the change in the mass flux of the gas or liquid phase near the liquid film front has a strong influence on the rewetting under extremely high mass flux conditions compared to the reflooding process.