Experimental investigation of droplet transient behavior during reflood cooling process of single-rod channel

The reflood process after a loss of coolant accident (LOCA) in a nuclear reactor involves complex two-phase flow with low pressure and flow rate, as well as boiling phase transition. Accurately predicting the transient behavior of liquid droplets during the dispersed flow stage is crucial for evaluating reactor accidents, including peak cladding temperature (PCT). However, existing prediction models for liquid droplets in dispersed flows are typically based on quasi steady-state annular flow models and do not account for transient behavior, such as the propulsion characteristics of the quench front during strong transient flooding processes, and the formation mechanism of large droplets. In this study, we present the results of a visualization experiment of reflood cooling in a single-rod channel to investigate the development mechanisms of droplet formation and fragmentation. We observe the evolution behavior of the vapor-liquid interface during the reflood process and quantitatively analyze the transient size distribution characteristics of the droplets. Based on these findings, the transient characteristics of the quench front propagation during reflood, Kelvin-Helmholtz (K-H) instability, hot wall ejection mechanisms for droplet formation, and aerodynamic and capillary breakup mechanisms for droplet breakage, as well as entrainment and evaporation of droplets under high-speed steam are considered. A prediction model for droplet transient behavior during single-rod channel reflood cooling is established, and the results are compared with experimental data.