Study On Aeration Bubble Dynamics Based On The Level Set Method

In the aeration tank, the contact between each phase such as microorganisms (mud), organic matter (water) and oxygen (gas) is a prerequisite for biochemical reactions. Such phase contact is mainly dominated by mesoscale fluid dynamics (i.e. bubble-scale). Therefore, it is significantly important to carry out the investigation of mesoscale fluid dynamics and to gain a deeper insight on the interaction of gas-liquid two-phase in the tank for the development of aeration technology with high efficiency and low energy consumption and for the improvement of the efficiency of aerobic biological wasterwater treatment.Firstly, after literature reviews on the current situation of gas-liquid two-phase flow in aeration tanks and of the numerical simulation technology of gas-liquid two-phase interfacial flow, a numerical model of mesoscale gas-liquid two-phase flow based on the Level Set method has been developed in this thesis, which overcomes the disadvantage of the conventional two-fluid model that can only provide the global hydrodynamics like flow pattern and gas holdup in the aeration tank. The numerical model improves the conservation of the Level Set method by adding a constraint condition on the re-initialized equation of the Level Set. Combining the nature of the Level Set function with respective motion equation of gas and liquid phases and boundary conditions at the gas-liquid interface, one-field governing equation of gas-liquid two-phase flow is then derived in this model.Secondly, the developed model has been validated through comparisons with experimental observations using high-speed photography and with published results in the literature. The mesh size and computational domain have been then optimized through the analysis of grid independence and boundary effects for the following simulation.Subsequently, the rising processes of a single bubble and bubble pairs in the aeration tank have been simulated numerically by applying the developed numerical model, respectively. On the one hand, the impact of initial conditions of bubble on the deformation and velocity of the single bubble rising in the aeration tank has been investigated deeply. The regime phase map of the bubble with the high Eo number changing from the spherical-cap into the toroidal bubble is established and the Mo number and Eo number determining the shape change are also obtained. The impact mechanism has been then explained in detail. Based on the mechanism, the impact of Eo number and Re number on the aeration bubble behavior is studied. On the other hand, the bubble behaviors such as distortion, attraction, and repulsion are investigated, too. Simulation results show that the behaviors of the rising two bubbles depend on the wake of the two bubbles and their interaction. Since the trailing bubble rises fast under the affect of the wake induced by the leading bubble, the two bubbles collide and coalescence takes place and their rise velocity first decreases and then increases during the coalescence process.Finally, the numerical simulations have been carried out on bubble formation in aeration tanks to characterize the bubble formation through the single orifice and the multiple orifices, with the focus on the impact of the cross-flow on the dynamics of aeration bubbles. The results show that the behavior of bubble formation from single orifice is different from that from the single orifice in the water. The bubble formation is nonsynchronous at multiple orifices. And the bubble sizes and bubble distribution are not uniform in a multi-orifice system because bubbles coalesce and break up more frequently due to bubble and bubble wake interaction. The simulation results also show that the water cross-flow in aeration tank has a strong impact on the bubble growth process. Because of the water cross-flow, the bubble inclines and grows to the downstream, and the time of bubble formation decreases with the increase of the liquid cross-flow velocity, resulting in the reduction of the size of generated bubbles. Such results will cause the enhancement of gas-liquid interfacial area and improve mass transfer ability in the aeration tank.