Experimental study of droplet sizes across a spacer grid location under various reflood conditions

Droplet size and distribution across a spacer grid in a heated rod bundle during reflood stage of a postulated design basis accident loss of coolant accident (LOCA) are studied experimentally using the Rod Bundle Heat Transfer (RBHT) test facility. Effects of spacer grid conditions, quench front location during reflood, and inlet water subcooling on droplet field are investigated. Experimental results show that the droplet sizes decrease when they pass through a dry spacer grid due to cutting and shattering effects at the lower edges of the spacer grid. On the other hand, for a wet grid, the droplet sizes downstream of the grid could be larger than those at the upstream locations, as the droplet generation mechanism, for a wet grid, is due to the formation of liquid ligaments at the trailing edge of the spacer grid and the subsequent aerodynamic breakup of these ligaments into relatively larger droplets induced by instabilities. It is found that, as the quench front propagates upward, sizes of incoming droplets increase correspondingly as a result of the droplet-vapor thermal-hydraulic interactions along the flow channels. The experimental results also indicate that larger droplet diameters occur when the inlet water subcooling is higher. Results of the present study can be utilized to develop models for droplet field behavior under accident scenarios. These models can be incorporated into nuclear reactor thermal-hydraulic safety analysis codes such as COBRA-TF and TRACE.