This study is concerned with an experimental exploration for the interactions of bubbles with a vortex ring launched vertically upward into a bubble plume. A vortex ring launcher, composed of a cylinder and a piston, is mounted at the bottom of a water tank. Small hydrogen bubbles are released into still water from a cathode, which is wound around the cylinder outlet, by the electrolysis of water. The bubbles rise by the buoyant force and induce a bubble plume. The water in the cylinder is discharged into the bubble plume by the piston, resulting in a laminar vortex ring convecting along the central axis of the plume. Just after the launch of the vortex ring, the bubbles are spirally entrained into the vortex ring with the roll up of the shear layer. The void fraction within the vortex ring increases with the convection of the vortex ring until a certain displacement of the vortex ring, where the reduction occurs. The vortex ring convects with a constant velocity higher than that in still water. The entrained bubbles reduce the strength of the vortex ring.

An experimental study searching for the possible generation and transport of a bubble cluster by a vortex ring in water is performed. A vortex ring is launched vertically upward into a water tank by discharging the water from a cylinder mounted at the bottom of the tank with a piston. Air bubbles are successively injected into the vortex ring from a needle attached to the cylinder outlet. The cylinder inner diameter D 0 and the piston stroke L 0 are 42.5 mm and 100 mm, respectively. The circulation of the vortex ring is less than 20000, and accordingly laminar vortex rings are launched. The mean diameter of the bubbles is 3.4 mm. The generation of bubble cluster and transport of the cluster by the vortex ring can be classified into four patterns according to the piston velocity (strength of the vortex ring) and the air volumetric flow rate. When the strength of the vortex ring is low, the bubbles are less affected by the vortex ring and instead rise with the buoyant force at a higher velocity than the vortex ring. With an increase in the strength of the vortex ring, the bubbles are entrained in the vortex core and form a cluster. The bubbles entrained in the vortex core circumferentially disperse around the vertical axis of the vortex ring, and they are successfully transported by the convection of the vortex ring. The convection velocity of the vortex ring is scarcely affected by the entrained bubbles, but the radius is enlarged slightly. The circulation of a vortex ring that entrains and transports air bubbles in this study is nearly accurately predicted by the formula of Milenkovic et al., which gives the circulation of a vortex ring entraining a single bubble in the vortex core. Index Terms-Bubble cluster, bubble entrainment, vortex ring, vorticity, visualization.

The entrainment and transport of air bubbles by a vortex ring are experimentally illves − tigated． A vortex 血 g launcher ， composed of a cylinder and a piston， is mounted on the bottom of a water ta 皿 k． Air bubbles are injected from a needle ， which is attached on the cyllnder surface ， into the vortex ring ． The bubble motions relative to the vortex ring are found to be classi 丘ed according to the strength of the vortex ring and the bubble flow rate ． When the s七 rength of the vortex ring is low， the bubbles are 且 ess affected by the vortex ring ． With increasing the strength ， も he bubbles are favorably ep 七 rained into the vortex core and 七 ransported by the convection of the vortex ring ， Key VVords： Vortex ring ， Bubble ， Entrainment aud transport ， Vorticity