Experimental Investigation of the Interaction between Rising Bubbles and Swirling Water Flow

This study experimentally investigates the interaction between rising bubbles and swirling water flow imposed around the central (vertical) axis of a bubble plume in a cylindrical water tank. Small air bubbles are successively released from the bottom of the tank to generate a bubble plume, and a stirring disc at the bottom of the tank is rotated to impose a swirling water flow around the central axis of the bubble plume. The bubbles disperse further with the increasing rotational speed of the stirring disc. Some bubbles shift toward the central axis of the swirling flow when is high. The nondimensional swirling velocity of water reduces with increasing bubble flow rate when is lower than a certain value. However, it is less affected by the bubbles when is higher. The precessional amplitude for the upper end of the vortex core increases due to the presence of the bubbles. With increasing , the nondimensional precessional velocity decreases, and the bubble effect also reduces. 1. Introduction Gas bubbles successively released into a liquid induce liquid flow as they rise, due to the buoyancy force. Such bubble-driven flow (bubble plume) is utilized in various engineering devices concerning matter and heat transfer, mixing, and chemical reactions. A number of studies have previously been performed on bubble plumes, and methods to predict entrained liquid flow rate [1, 2] and plume characteristics [3, 4] have been proposed. The relation between the meandering motion of rising bubbles and bubble flow rate has also been investigated [5]. The authors [6, 7] developed simulation methods and carried out numerical simulations for bubble plumes. The simulations successfully analyzed bubble meandering motion and large-scale vortical structures induced by the bubbles. Bubble motion is one of the important and elementary phenomena governing the plume characteristics, and it is closely related to the performance of the device incorporating a bubble plume. Therefore, the control of bubble motion promises to optimize device performance. Investigations of the bubble behavior in a mixing layer [8] and in jets [9–11] showed that bubble motion in such shear flows is dominated by large-scale eddies. It is well known that vortex rings, which represent large-scale eddies, have a higher ability to transport matter through their convection process with a self-induced velocity. Therefore, the authors [12] experimentally explored the possibility of controlling bubble motion in a bubble plume with a vortex ring. A vortex ring launcher, composed of a cylinder and a piston, is