Abstract:

By using high speed photography technology combined with shadow method, the motion of a single rising bubble near a vertical wall in stationary water is experimentally studied. The effects of bubble size and the initial dimensionless distance between the nozzle and the wall ($S^{\ast})$ on the rising motion characteristics of bubbles were compared. The wall effect, bubble dynamic mechanism and energy variation rule before and after the collision between bubbles and the walls are analyzed. The results show that for the Reynolds number $Re \approx 580\sim 1100$, and the initial dimensionless distance between the nozzle and the wall $S^{\ast } < 2\sim 3$, the bubbles collide with the wall surface and the bubble trajectory changes from three-dimensional spiral under unconstrained conditions to two-dimensional zigzag periodic motion. However, when $S^{\ast } > 2\sim 3$, the wall effect weakens, and the movement characteristics of the bubble with wall constraint tends to be consistent with that without constraint. Before and after the bubble collides with the wall, the wall effect causes the peak value of transverse velocity to drop to 70% of the original peak value, and vertical velocity drop to 50%. Before the bubble collides with the wall, the vertical velocity variation rule can be predicted by the distance between the bubble center and the wall ($x/R)$ and the modified Stokes number correlation formula. In the process of collision between the rising bubble and the wall surface, the deformation energy of the bubble surface is transmitted to the transverse kinetic energy of the bubble in one direction, so that the deforming bubbles can maintain a relatively constant bouncing motion. The prediction model of the average resistance coefficient of bubbles in the repeated bouncing with the wall surface is proposed, which can describe the dimensionless parameters of Reynolds number, Weber number and Eo number reflected by the experimental data.

Key words: bubbles, vertical walls, periodic bounces, collisions