Two-Phase Bubble Flow Modelling for Restoration of Eutrophic Lakes

For the remediation of eutrophic, anoxic lakes, this thesis considers artificial circulation as a technique by introducing air bubbles into the lake water, which has the advantage that no chemical substances will be added to the lake water. This study aims to improve the understanding of the hydrodynamic behaviour of water elements and air bubbles in a water body subject to aeration, and to investigate optimal aeration schemes for prevention of sedentary conditions and improvement of the anoxic conditions in eutrophic lakes. A computational fluid dynamics (CFD) analysis of two-phase flow was performed using two different types of model domains: a cylindrical bubble column; and lakes. Predictions of the flow field are obtained from numerically solving the Reynolds-averaged continuity and momentum equations, using the Eulerian-Eulerian multiphase method. The CFD results are validated through a comparison of the predictions with available experimental measurements of the quantities made from a laboratory water tank. Subsequently, the response of water and bubble motions to a selection of bubble size, air flow rate, and spatial configurations of injection was investigated. Using the results from these simulations, the beneficial effects of aeration on the enhancement of oxygen concentrations in the water column are analysed. The results show that a proper solution for bubble columns is crucially dependent on the correct modelling of interphase forces and turbulence models. The consideration of the effect of interfacial forces has improved the results, especially at larger distances from the centreline of the model domain. Oxygen is shown to transfer to water across bubble interfaces as the bubbles rise to the water surface. Oxygen transfer also occurs across the air-water interface at the free surface as a result of turbulence induced by bubble motions and water circulation. Many independent variables have influence on the bubble flow field. The first set of variables was related to air injection, including bubble size, initial velocity, and air flow rate. The second set of variables is diffuser variables, including the number of ports, port diameter or diffusion area, spacing between adjacent ports, port angle with the horizontal, and elevation of ports above the bottom. It has been demonstrated that a properly installed aeration system in a lake can possibly halt and eventually reverse anoxic condition. De-oxygenated bottom water is exchanged with highly oxygenated surface water. The opening of ports should be mounted at a certain height from the bottom. Specifically, the optimal height is shown to be 0.3 m for shallow lakes with a maximum depth of 2 m. Port spacing should be approximately equal to the maximum depth. This installation prevents re-suspension of bottom sediments while it creates full circulations around the injector. It reduces dead zones between two adjacent injectors and produces stronger downward flows. The installation induces the dispersion of air and increases oxygen transfer rate in water. The oxygen concentration is continuously increased with time and reaches a steady state. Thus, an aeration system can possibly halt and eventually reverse anoxic condition. This thesis has demonstrated that computer modelling of aeration has the potential to improve our understanding of complex bubbly flow processes through systematic simulations