Evaluation of structural effect of BWR spacers on droplet flow dynamics

Highlights: • An experimental system was established to visualize a droplet passing through a spacer. • A ferrule and grid type spacers were simulated to evaluate an effect on downstream droplets. • There was a relationship between a diameter and velocity of droplet specific to each spacer. • A CFD analysis for gas phase flow to drive the droplets showed characteristics of each spacer. • Good correlations between a diameter and velocity of droplet were found by data fitting. We have established an experimental system to visualize a droplet flow in a simulated BWR fuel sub-channel optically and measure the diameter and velocity of droplet after passing through a spacer. For representative spacers of ferrule and grid type, an effect of them on downstream droplets was evaluated with the experimental system. When a ferrule type spacer was simulated and implemented in both the center and side sub-channel, the vertical velocity of droplets got faster especially in the range of small diameter compared to the case of no spacer. When a grid type spacer was simulated and implemented in the center sub-channel especially, a large dispersion of vertical velocity of droplets occurred especially in the range of small diameter compared to the case of no spacer. By a computational fluid dynamics analysis for gas phase flow to drive the droplets in the sub-channel, it was confirmed qualitatively that the characteristics of droplet behavior observed in this experiment were dependent on the structure and geometry of spacer and sub-channel. Furthermore, it was revealed that a relation between a droplet diameter and velocity can be organized with a non-dimensional function derived from a momentum equation of particle in driving fluid and its drag coefficient has linear correlation with a gas Froude number.