| As is well known,the current situation of water resources in China is facing various problems such as resource shortage,uneven distribution,and severe pollution.The per capita share of water resources is only one fourth of the average population share of countries around the world.In this context,research on water environment protection and sewage purification and recycling is particularly important.Aeration tank,as an improved activated sludge treatment process,is characterized by its longer hydraulic retention time and sludge age.Compared with the traditional activated sludge process,the structure of some aeration tanks is more simplified,and even the secondary sedimentation tank can be omitted,which makes it have unique hydraulics characteris tics and working characteristics.In this paper,the mathematical model selected in this study was verified by previous experimental data to ensure the rationality and feasibility of the model.Then,for the same cross-section aeration tank,the RNG k-ε model is used to simulate the turbulent motion in the aeration tank,and the VOF model is used to track the free water surface.By analyzing and comparing the hydraulic characteristics of the aeration tank under various operating conditions,such as flow field structure,cross-sectional flow velocity distribution,and cross-sectional gas-phase volume distribution,by changing the inlet velocity of the aeration hole,initial water filling height,and pressure difference of the pusher,the optimal initial parameter setting combination of the aeration tank under operating conditions can be obtained.Complete the optimal setting of the aeration tank to improve efficiency and reduce loss.The main research contents include:(1)Referring to previous experiments,the experimental device of the aeration tank was modeled and numerically simulated.The numerical simulation results were compared and analyzed with the experimental reference results.The results showed that the numerical simulation effect was good,and the hydraulic characteristics and phenomena in the simulated model were consistent with the actual gas-liquid two-phase flow characteristics in the aeration tank.The numerical analysis results of the model were basically consistent with the experimental data in various parameter comparisons,Prove that the selected mathematical model is reasonable for numerical simulation of gas-liquid mixing process in an aeration tank.(2)Four sets of working conditions are set to ensure that the shape of the aeration tank,the location,size and number of the aeration holes are the same.The numerical simulation of the tower aeration tank model is carried out by changing the inlet velocity of the bottom aeration hole and the initial water filling height of the internal space of the aeration tank.The results show that when h/w=2.25,the transverse mixing of the upper streamline of the aeration tank is weak,and the combination effect of oxygen and sewage is poor.After increasing the gas velocity,it is improved,and the ability to absorb air from the upper space is strengthened.The aeration effect is strengthened,the oxygen transfer efficiency is improved,and the internal flow field structure is relatively better.In the flow field,the velocity gradient change in the high and low velocity regions is slowed down,the velocity difference in different regions is reduced,and the velocity distribution tends to be uniform.The low inlet inlet velocity has a great influence on the gas content in the middle and upper part of the aeration tank,and the effect of high inlet velocity on the gas content in the lower part of the aeration tank is more obvious.When h/w=4.5,the velocity distribution trend of the three lines in condition(b)is the closest among the four conditions,and the difference between the uniformity of velocity distribution and the maximum velocity in the whole flow field is the smallest.The gas volume fraction of the liquid surface and the bottom of the tank has the smallest change,which proves that the gas content is greatly affected by the aerator and the aeration effect is remarkable.(3)Five sets of working conditions were set up to ensure that the shape of the oxidation ditch,the location,size,number and inlet velocity of the aeration hole were the same,and the pressure difference of the thruster was changed to numerically simulate the circulating mixed aeration tank model.The flow field structure,velocity distribution and gas volume fraction obtained by simulation are compared and analyzed.The results show that the streamlines of the flow field inside the oxidation ditch tend to be more uniform and smooth with the increase of the pressure difference in the working conditions(a)-(e).After the pressure difference increases to 600 Pa,the improvement effect of the pressure difference on the cross-section streamline is significantly reduced.In condition(d),the flow velocity in most areas of the ditch meets the basic non-silting condition of the aeration tank;as the pressure difference rises,the gas content of each layer in the aeration tank gradually increases.At 600 Pa,the gas content is the highest and the high and low gas content regions are excessively gentle,and the oxygen is most evenly distributed in the three planes.The optimal condition is still condition(d). |
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