The mixing of liquids by bubble plumes.
When bubbles are continuously released from a localised source at the bottom of a liquid layer, a bubble plume is produced. As the bubble plume rises due to its buoyancy, it entrains surrounding liquid, which is carried upward with the stream of bubbles. This dissertation investigates the effects of bubble size and density variation in the liquid on bubble plume behaviour and mixing pattern produced in the surrounding environment.
The research was divided primarily into two areas. Firstly, the motion of a plume of low-Reynolds number bubbles in a stratified system consisting of two homogeneous layers of different densities was investigated. The liquid environment was of finite lateral extent. A theoretical model for the flow of the bubble plume and the surrounding liquid was developed. The theoretical predictions obtained were compared with new experimental results using plumes of small bubbles generated via electrolysis of an aqueous solution of sodium chloride and with previous experimental results (McDougall, 1978; Baines and Leitch, 1992).
Secondly, a comparative study on the behaviour of a plume of large-Reynolds number bubbles in a stratified environment was carried out. The effect of the bubble size on plume structure and flow pattern was investigated. It was found that the wake transport led to an increase in the density at higher levels in the tank. This was at variance with the mixing induced by a plume of low-Reynolds number bubbles, which did not possess wakes but were more efficient at destratifying the lower regions of the tank (Chen and Cardoso, 2000). Coupling the radial density gradients and the continuous exchange of liquid between the plume and the surrounding environment, a dispersive effect on the density profile in the environment was observed. The experimental results showed that the extents of advection and dispersion in the environment depend on the size of individual bubbles and the degree of stratification in the environment.
The theoretical model developed in this work was used to investigate the viability of using bubble plumes to induce mixing in reservoirs of liquefied petroleum gas.