Motion of Small Bubbles near a Grid Spacer in a Two by Three Rod Bundle

Motion of small bubbles upstream of a grid spacer in a two by three rod bundle is experimentally investigated. Trajectories of single bubbles upstream of the grid spacer are recorded by using a high-speed video camera. An image processing method based on the combination of the Sobel filter and the Hough transform is developed to measure void distributions in dilute bubbly flows in the rod bundle. Mean liquid velocities upstream of the spacer are calculated based on 10,000 instantaneous velocity distributions measured by a Particle Image Velocimetry (PIV). Effects of the grid spacer on bubble motion are discussed based on the difference between the measured bubble motion and liquid velocity distribution. The main conclusions obtained are as follows: (1) just upstream of the grid spacer, bubbles are apt to migrate toward the rods and accumulate in the vicinity of the rod surface, and (2) lateral displacement of bubbles toward the rod wall is larger than that of streamline due to the centrifugal force induced by curved streamlines just upstream of the grid spacer.