Study On Bubble Plume Flow And Destratification In Step Stratified Water

Due to the per capita shortage and uneven distribution of water resources in China,the role of impounded reservoirs is increasing.However,the density stratification of water in the reservoir will inhibit the exchange of nutrients and oxygen and other substances between the upper and lower layers of water,which will affect the water quality of the reservoir.Therefore,the bubble plume is often used to destroy water stratification.What's more,the study on the flow morphology and de-layering efficiency of bubble plumes in stratified water bodies is not perfect.Therefore,the following studies have been conducted in this paper:?1?By studying the bubble plumes under different experimental conditions of water density and aeration,we found that they can be divided into two flow types:the plume-jet pattern with large gas volume and weak stratification,the fountain pattern with strong stratification and small aeration.Also,the dimensionless number,P_NE,is defined by the ratio between the liquid potential energy and the effective bubble kinetic energy in the step stratified water,and is used to predict the transition of the bubble plume flow pattern.In addition,whenP_NE?130,bubble plume flow pattern is called plume-jet,and when P_NE>130,bubble plume flow pattern fountain is called fountain.?2?The relationship between bubble plume flow pattern and destratification efficiency in step stratified water was studied through experiments.It was found that the fountain flow pattern has higher destratification efficiency than plume-jet.Therefore,the amount of aeration should be satisfiedQ_B<N_E³ H⁴/?130gS_f?in engineering applications.That is,the flow pattern of the bubble plume is fountain.?3?The Euler-Eulerian two-phase flow model and the RNG k-eturbulence model were used,and the salinity transport equations for the density stratification of liquid water were introduced.Numerical simulations were performed and the results were compared with the experiments,and two flow type de-layering mechanism were found.In addition,the plume expansion rates of the plume-jet and fountain flow patterns were respectively 0.055 and 0.041,respectively,which were smaller than those of the bubble plumes in the uniform water.The bubble diffusivities are 0.12 and 0.19,respectively,and the kinetic energy of the gas flow in the fountain flow pattern is more transmitted to the liquid phase than plume-jet,in further confirming that the fountain flow pattern has a higher destratification efficiency.?4?Finally,the study found that for the fountain flow pattern,with the increase of the aeration time,it will gradually turn into a plume-jet flow pattern,and its de-stratification efficiency will gradually decrease.Therefore,it is proposed to further reduce the amount of aeration by adopting a method of gradually reducing the amount of aeration to improve its de-layering efficiency.