Numerical Simulations Of Effects Of Thermal Stratification And Water Depth On Flow Around Water-lifting Aerator

The Ware-lifting Aerator, aiming at solving water quality problems of eutrophication and pollutants'releasing from sediments in deep reservoirs, is a new water quality improvement device. By mixing water or oxygenating water directly, the aerator can improve and change the anaerobic state in the lower waters, which can control the pollutants'(such as nitrogen, phosphorus, iron, manganese, organic materials) releasing from sediment into the water body. By aerating, the waters in the upper-layer and lower-layer can be mixed, the thermal stratification can be destructed, and algae can then be brought down to the lower-layer where algae would die finally because of no light.With the help of the discretization method and the computer, the continuous equation, momentum equation and energy equation can be solved by Computational Fluid Dynamics (CFD) simulation, and complex flow motion can be numerically simulated and analysed. Using CFD simulation, the basic physical parameters (such as temperature, velocity, pressure, concentration) can be easily predicted at different locations and times. As a powerful computing package, FLUENT was employed to simulate the complex motions of air and water.Through the Fluent simulation, the effects of thermal stratification and water depth on the flow field around Water-lifting Aerator were investigated and the main conclusions were as follows:(1) Whether in the summer or winter, thermal stratification had little effect on the area without algae when the aerator worked stably, and the proportion of area without algae retained about 75%. If the temperature difference between the surface and bottom of the reservoir was noticeable, the time of developing the eddy zone was long. If the temperatures on the surface and bottom of the reservoir were the same, the deeper the thermocline layer, the nearer the eddy zone to the reservoir bottom.(2)Under the same conditions of affecting radius, gas shell period and outflow velocity, the time for steady flow development was long when the Water-lifting Aerator worked under the deep water condition. However, the proportions of area without area retained same.(3)Using Fluent's function of tracing flow, the algae motion in the reservoir can be analyzed when the water-lifting aerator worked stably. Gong with the water flow, algae stayed in the decay area longer than in the production area. After several times of circulation, algae continued to consume their own organic materials and died ultimately.(4)If the water depth increases, the time of algae's flowing through the decay area will increase, algae removal efficiency will increase. However, the stirring effect of outlet flow will be weakened accordingly, larger flow resistance will be overcome to develop gas shells, more energy will be required and the Water-lifting Aerator's running costs will be higher. Therefore, comprehensive consideration of algae removal efficiency and operating costs should be given to select the optimum layout of the Water-lifting Aerator in the reservoir.