| This study is carried out under the financial support from National Key Research & Development Program of China(2016YFB0200901).Inverse umbrella aerator is widely used in the wastewater treatment process because of its simple structure,large action area,high power efficiency and so on.However,the flow of an inverse umbrella aerator consists of water jump,entrainment and wave breaking,while its aeration is significantly affected by the speed,immersion depth,blade number and so on.All of these indicate that the internal flow analysis and the aeration performance prediction in the inverse umbrella aerator are difficult to be solved.In order to reveal the flow mechanism and establish the aeration performance prediction method,the operation characteristics and the aeration performance of the inverse umbrella aerator are researched deeply in this study by the method of combining theoretical analysis,experimental tests and numerical simulation.The main work and research results are as follows:1.A high-speed photography experiment rig was set up to capture the liquid surface of the aeration tank driven by an inverse umbrella aerator.It is found that:(1)the liquid rising from the water jump mainly comes from the upper side of the plate.The liquid flows from the front passage to the next passage and then is ejected by the pressure side.(2)When the impeller rotates,a low pressure area will be formed and a large quantity of air triangular in shape will be entrained at the suction side.2.With the high-speed photography result,the VOF method was used to simulate the three dimension unsteady motion of the inverse umbrella aerator.The results show that:(1)the surface deformation appears near the impeller zone firstly and then spreads to the wall until the whole surface fluctuates.The surface velocity has the same motion trail.The motion of the circulation vortex is in coincident with the surface deformation.(3)The stirring action to the shallow layer is stronger than the bottom layer.The fluid in the bottom layer is mainly driven by the liftingcapacity of the inverse umbrella aerator,which leads to the smooth flow and low turbulent kinetic energy.3.The aeration performance test was carried out to analyze the influence to the inverse umbrella aerator with different speed,liquid level,blade number and immersion depth.The results show that:(1)when the liquid level is constant and the impeller has the same height with the liquid surface,the standard oxygen mass transfer coefficient and the standard oxygen transfer rate are proportional to the speed.But the standard aeration efficiency has no clear relation with the speed.(2)The immersion depth together with the speed has an effect on the aeration performance.The immersion depth has a slight effect on the aeration performance in the low speed.The aeration performance increases with the increase of the immersion depth in the high speed.(3)When the speed keeps constant,the standard oxygen mass transfer coefficient decreases with the increase of the liquid level,but the standard oxygen transfer rate and the standard aeration efficiency have the opposite rule.(4)The blade number has a slight affect on the aeration performance.4.The prediction model of aeration performance was established based on dimensional analysis method to resolve the problem that the scaled prototype can’t predict the aeration performance of the prototype.The results show that:(1)the advanced absorption number and the aeration efficiency number of the inverse umbrella aerator are only related to the Froude number.(2)The predictive value of the aeration performance of the larger inverse umbrella aerator has little difference with the experimental value.The maximum relative error of the standard oxygen transfer rate is 2.56% and the average relative error of the standard oxygen transfer rate is 1.39%.The maximum relative error of the standard aeration efficiency is6.14% and the average relative error of the standard aeration efficiency is 4.05%.It shows that the prediction model has high accuracy. |
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