Gas bubbles released into a liquid induce the liquid flow as they rise due to the buoyant force. Such bubble-driven flow or bubble plume can attain the active contact and mixing between the gas and liquid phases. Therefore, it is utilized in engineering devices handling chemical reaction, bio-process, and coal liquefaction. It is also employed in environmental applications for ice prevention in lakes and purification of water. A number of researches have thus far been performed on bubble plumes. Various methods to predict the entrained liquid flow rate [1,2] and the plume characteristics [3,4] have been proposed. The relation between the meandering motion of rising bubble and the bubble flow rate has also been investigated [5]. The authors [6] carried out a numerical simulation for a plane bubble plume, and they analyzed the large-scale vortical structures as well as the bubble meandering motion. To heighten the performance and efficiency of the devices utilizing bubble plumes, it is effective to increase the interfacial area concentration of the plume. But there are few attempts at such increment. 

In single-phase mixing layer, it is well known that the mixing and momentum transport of the fluid is dominated by the large-scale organized eddies. Rightley and Lasheras [7] performed an experimental study on a water mixing layer laden with small air bubbles, and elucidated that the bubble distribution is chiefly governed by the large-scale eddies. Druzhinin and Elghobashi [8] and Yang et al. [9] conducted numerical investigations on bubble-laden mixing layers, and reported that the bubbles concentrate preferentially around the center of largescale eddy. In single-phase jet, there are also organized large-scale eddies near the nozzle outlet. Milenkovic et al. [10,11] carried out experiments on a bubble-laden jet to study the bubble entrainment into the large-scale eddies generated by disturbances near the nozzle.

Vortex ring can transport mass and momentum through its convection with the self-induced velocity. The transport ability is so high that it has received much attention, and some attempts at applying a vortex ring to transport particles are reported. Domon et al. [12] performed an experimental study on the transport of solid particles in water. A vortex ring was loaded with small resin particles at the launch into still water, and the behavior of the particles convected with the vortex ring was observed. Yanagida et al. [13] proposed an olfactory display method for a virtual reality of the next generation. Scented particles, charged into a vortex ring, were efficiently delivered to a specific user’s nose by the convection of the vortex ring through the air. Yagami and Uchiyama [14] numerically simulated the convection of glass particles by a vortex ring in the air, and demonstrated that the particles are successfully transported when their Stokes number is small. Uchiyama [15] conducted a numerical simulation of a water jet laden with small air bubbles, and made clear that a vortex ring induced near the nozzle outlet by an axisymmetric disturbance involves the bubbles and convects with the bubbles. The abovementioned results suggest that a vortex ring is usefully employed for the control of bubble motion. Therefore, a vortex ring launched into a bubble plume may increase the interfacial area concentration. As a vortex ring convects with inducing the liquid flow, it may also enhance the mixing between the two phases. But there are few researches on a vortex ring laden with bubbles. Sridhar and Katz [16] investigated experimentally the motion of a bubble entrained into a vortex ring, and showed that the motion calculated by the measured lift and drag forces agrees well with the experimentally visualized result. Sridhar and Katz [17] also studied the effect of entrained bubbles on the strength and vortical structure of a vortex ring, and made clear that the deformation of the vortex ring occurs even when a few bubbles are entrained. These experimental studies did not examine the changes of dimension and convection velocity of vortex ring caused by the bubble. The number of bubbles was so small, and the bubble volume fraction within the vortex ring was not measured. Consequently, the interactions between a vortex ring and the entrained bubbles are not fully clarified. 

The objective of this study is to explore experimentally the interactions of bubbles with a vortex ring launched vertically upward into an annular 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 vortex ring convecting vertically upward in the plume. In this study, a laminar vortex ring is launched into a bubble plume. The mean bubble diameter is 0.2 mm, and the bubble flow rate is 4.1 mm³/s. The behavior of the vortex ring and the bubble motion are investigated in a region, of which vertical height from the cylinder outlet is six times the inner diameter of the cylinder.