Droplet deposition enhancement by ferrule and vaned grid spacers in BWR fuel assemblies

Spacers in a BWR fuel assembly have an effect of improving the critical heat flux (CHF) by enhancing droplet deposition on the fuel rod surface. In this study, the behavior of droplets in the downstream region for a ferrule-type spacer and a vaned grid-type spacer was observed with a test section simulating the center subchannel of a BWR fuel assembly. The effects of the spacer on droplets were defined as the “turbulent promotion effect” and the “run-off effect”. The former refers to an effect on droplets in the dispersed flow due to turbulence downstream caused by the spacer. The latter refers to an effect on droplets generated from the liquid film on the spacer structure. The turbulent promotion effect affected droplets in a diameter standardized by the hydraulic equivalent diameter of the sub-channel less than 0.008 (the ferrule-type spacer) and 0.02 (the vaned grid-type spacer), and enhanced their deposition on the fuel rod surface downstream of the spacer. The droplet deposition distribution downstream of the spacer had two peaks, one in the immediate vicinity of the spacer and the other farther downstream (over 10 times the hydraulic equivalent diameter of the sub-channel), and both frequencies depended on the geometry. The run-off effect affected droplets generated by the spacer varying their outflow angle widely, and the droplet deposition distribution had a steep peak in the immediate vicinity of the spacer. Based on the experimental results, a mathematical model of droplet deposition enhancement was developed for each spacer type.