Interaction of Bubbles with Vortex Ring Launched into Bubble Plume ()
1. Introduction
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.
2. Experimental Set-Up and Method
2.1. Experimental Set-Up
Figure 1 outlines the experimental set-up. The experiment is conducted in a water tank made of transparent acrylic resin. The width and depth of the tank are 300 mm, while the height is 1000 mm. The top of the tank is open to the atmosphere. A vortex ring launcher, composed of a cylinder and a piston, is mounted at the bottom of the tank. The cylinder centerline is parallel to the vertical direction. A cathode is wound around the cylinder outlet, and an anode is placed on the tank wall. Applying a DC voltage between the electrodes, small hydrogen bubbles are released from the cathode. The bubbles rise by the buoyant force and induce the water flow around them, resulting in an annular bubble plume.
A vortex ring is launched vertically upward into the bubble plume by discharging the water in the cylinder with the piston. The push of the piston is performed with a slider connected to an AC servomotor. The velocity and stroke are controlled with a personal computer.
The vortex ring and the bubbles are visualized by a laser light sheet, of which power, wavelength and thickness are 100 mW, 532 nm and 1 mm, respectively. Their images in the vertical plane passing through the cylinder centerline are captured by a video camera. The spatial resolution, the frame rate and the shutter speed are 640 × 480 pixels, 200 fps and 1/200 s, respectively.
The water and bubble velocities are measured by using a PIV system. Nylon particles (mean diameter: 80 μm, specific weight: 1.02) are laden as the tracer for the water flow. Therefore, the bubbles should be removed from the visualized images when measuring the water velocity. In this experiment, the remove was properly performed by
setting the camera aperture at an appropriate level, be cause the brightness of the bubble differs greatly from that of the tracer particle.
2.2. Launch of Vortex Ring
Figure 2 shows the close up for the vortex ring launcher mounted at the bottom of the water tank. The water depth is 300 mm. The piston stroke L0 is 100 mm, and the top dead center is 46 mm below the cylinder outlet. The inner diameter of cylinder D0 is 42.5 mm, while the outer one is 57.8 mm. The height of cylinder outlet from the tank bottom is 45 mm. The origin of the vertical (z) and radial (r) axes is set at the center of cylinder outlet.