Analysis Of The Effects Of Bottom Oxygenation Of Small And Medium-sized Reservoir On The Temperature Stratification And Sediment Disturbance Based On Numerical Simulation

China's small and medium-sized reservoirs are the main ways of water supply in cities and towns.Most of them have the characteristics of water temperature stratification in summer and autumn,but more than 60% of the reservoirs are currently seriously polluted.Even if the external pollution is cut off to the greatest extent,when the bottom of the water body is deficient in oxygen,the nutrients and heavy metals in the sediment are easily released in excess and cause water environmental problems.However,the existing oxygenation measures at the bottom of the water body can easily damage the natural water temperature stratification state or disturb the sediment,which may bring secondary pollution to the water body.Therefore,for small and medium-sized reservoirs with stratified water temperature in summer and autumn,it is necessary to improve the water quality while maintaining the natural state of water temperature stratification and surface sediment while oxygenating at the bottom.Through a large number of literature investigations,it has been found that the air bubble plume diffusion system and the jetting enriched-oxygen water system oxygenation mode have a significant effect on increasing the dissolved oxygen concentration of the water body,and are widely used in practical projects.However,aeration flow rate,aerator length,bubble size,bubble diffusion area,jet flow rate,jet angle,can affect the mixing strength and sediment resuspension of the water body,and it is impossible to compare these factors through physical model test or field monitoring test.With the rapid improvement of computer performance,it is a major method in scientific research to construct a mathematical model,use mechanism test data to rate and verify its parameters,and carry out preset calculation of scenario conditions under various combinations.In view of this,this paper uses Open FOAM,an open source software of fine fluid dynamics,to construct a flow field-temperature field-dissolved oxygen concentration field model for bottom oxygenation of small and medium-sized reservoirs,and conducts the following simulation analysis:(1)The effects of different aeration flow rates,aerator lengths and bubble diameters on the water body were simulated inair bubble plume diffusion.The comprehensive analysis of oxygenation rate,sediment resuspension and water temperature stratification were carried out.(2)The effects of jet flow rates,injection tube lengths,jet angles,and dissolved oxygen concentration in jetting enriched-oxygen water system were simulated.The comprehensive analysis of oxygenation rate,sediment resuspension and water temperature stratification were carried out.(3)The advantages and disadvantages of the two methods were compared and analyzed.The following conclusions are obtained:Air bubble plume diffusion can mix the upper and lower water body,supply the dissolved oxygen in the water body without disturbing the sediment,and reduce the dissolved oxygen concentration difference and temperature difference between the upper and lower water body,destroying the original dissolved oxygen stratification and water temperature stratification.Moreover,the thermal stability of the water body is reduced,and the water body is promoted to change from a layered state to a mixed state.(1)Increasing the aeration flow rate and reducing bubble diameter can increase the oxygenation rate,and the degree of influence is: bubble diameter> aeration flow rate> aerator length.(2)Reducing the aeration flow rate and aerator length can reduce the buoyancy frequency change rate,and the degree of influence is: aeration flow rate> aerator length> bubble diameter.(3)Reducing the bubble diameter can increase the oxygenation rate to the greatest extent,and cause less damage to the water temperature stratification.(4)The calculation formulas of oxygenation rate,buoyancy frequency change rate and delamination efficiency are obtained,which can provide the basis for parameter setting for the operation of bubble plume diffusion oxygenation system and optimize its application in the reservoir.Jetting enriched-oxygen water system can maintain the original water temperature stratification without disturbing the sediment,but it supply the dissolved oxygen to the bottom water body by convective diffusion,and has little effect on dissolved oxygen stratification.Increasing jet flow rate and dissolved oxygen concentration can improve the oxygenation rate,and the degree of influence is: the dissolved oxygen concentration> jet flow rate.The formula of oxygenation rate of jetting enriched-oxygen water system is obtained,which can providethe basis of parameter setting for the operation of jetting enriched-oxygen water system and optimize its application in the reservoir.Comparing the two oxygenation modes by jetting enriched-oxygen water system and air bubble plume diffusion,it is concluded that air bubble plume diffusion can greatly oxygenate the water body and destroy the dissolved oxygen stratification,while jetting enriched-oxygen water system can maintain the original water temperature stratification of the water body.Therefore,these two aspects should be considered when choosing oxygenation method.The research results of this paper will provide a good reference for promotion and application of air bubble plume diffusion and jetting enriched-oxygen water system in the actual bottom oxygenation of small and medium-sized reservoirs.