Experimental Research On The Influence Of Aeration Factors Affecting On Disstratification

Thermal stratification always occurs in deep lakes, and it will make the dissolved oxygen (DO)stratified. As the DO is stratified, the water on the upper layer of the lake will be oxygen-enriched,while the water on the under layer will be oxygen-poor, and it is harm for the ecosystem. Usingaeration to destroy the stratification is an economical and useful method. There are many theoreticaland experimental studies about this method. But most of those studies were focused on theinteraction of water and air, aeration equipment and so on, none of them studied the aerationparameters. To make up those insufficient studies, this paper tries to research the influence ofair-flow-rate and bubble size on destratification.First we took a field research in Qiandao Lake to investigate the depth and temperature.These data helped us know more about the Qiandao Lake’s thermal stratification. With the help ofinternational cooperator, we got the temperature and DO data of Vesij rvi Lake in Finland. Thatmakes it possible for us to using Classification and Regression tree model to find the relationshipbetween thermal stratification and DO stratification.To study the influence of aeration parameters on destratification, we designed a specialaeration experiment which can be conducted under laboratorial condition. This experiment includesseveral main parts, such as designing and setting up the experiment table, making the wateroxygen-poor, stratifying the DO in water, controlling the aeration parameters. The oxygen-poorwater was obtained by mixing the polluted river-water and clean water. Fostering this mixture forseveral days, the DO will reach to a very low level. After obtaining oxygen-poor water, we enrichedthe DO in the upper layer using aeration array and consumed the DO in the bottom using the ironsponge. Then we got the stratified water.After designing and testing the experiment table, we did several experiments. The alternatingquantities of these experiments were air flow rate and bubble size. In each experiment, we studiedthe DO variance in different layers, and then compared the DO decrease rate in Layer0m betweendifferent layers. Comparing these experiments’ results we found that: the DO increase from aerationhas a positive correlation with air flow rate and has a negative correlation with bubble size. Thatmeans the increasing air flow rate will lead to fast DO balance, while the increasing bubble size willslow down the balancing. On the other hand, we found that the aeration has little influence on thearea blow the aerator, that means the water convection induced by bubbles only appear above theaerator.The mathematical model will help people have a better understanding of the phenomenon. So we tried to study basic fluid dynamics equations of aeration model, such as mass conservationequation, momentum conservation equation and energy conservation equation. Besides, discretizingthe model was precondition of CFD, so we studied different types of grid and then meshed themodel with structured grid. A brief CFD processes introduction was made at the end of this section.At the end of this paper, we used Fluent to simulate the thermal destratification. That made ushas a visual recognition of the aeration processes. We tried to set different mass flow rate tocompare the differences of each simulation results. During the simulation, how to choose themulti-phase model, how to set thermal stratification as initial condition and how to create a monitorplane were the key points. We made a simple introduction of these questions in this section.