Numerical Simulation Research On Optimization Of Oxidation Ditch Section And The Propeller's Submerged Depth

Oxidation ditch process is an improved form of activated sludge process in sewage treatment.It has the characteristics of long hydraulic retention time and low organic load.It is widely used in urban sewage treatment in the world.The section form of the oxidation ditch has an important influence on the hydraulic characteristics and mixing performance of the mixed liquid of ditch.As an energy input device in oxidation ditch,the propeller is very important to improve the flow of the ditch.The submerged position of the propeller determines the flow pattern and vertical velocity distribution of the oxidation ditch,and the treatment effect.After analyzing th,e previous articles,the paper adopts numerical simulation method to verify the experiment,which proves the feasibility of the selected model.Then,based on the traditional rectangular section of oxidation ditch,the new trapezoidal compound section of oxidation ditch was proposed,and the submergence depth of underwater propeller.By using RNG k-e turbulence model and VUF multiphase flow model,the above schemes are simulated and obtained the following conclusions:(1)In the model validation,the simulated and measured values are basically in agreement.The flow velocity near the wall is higher than that in the middle of the ditch,and the surface velocity is higher than that in the middle and lower parts of the water body.The flow around the inverted umbrella-shaped aerator has a sharp water jump.The air content in the curved channel is higher than that in the straight channel,and that in the upstream channel is higher than that in the downstream channel.The model can predict the velocity distribution of oxidation ditch with aeration booster equipment.(2)The low velocity zone in oxidation ditch is mainly located at the start and the end of the partition wall,the outer edge of the entrance of the bend,and the high velocity zone is mainly located at the outer side of the exit of the bend and near the outer wall.Compared with rectangular section,the horizontal eddy size of compound section oxidation ditch is smaller,there are more vertical eddies and the average flow velocity is larger.The bottom flow velocity of ditch without an aeration disk is larger.In the ditch with an aeration disk,The flow velocity of the middle lower portion is higher than that of upper ditch in the upstream of aeration disk,the high flow velocity zone moves upward and the velocity at bottom decreases.The flow velocity of compound section oxidation ditch is more sensitive to the setting of aeration disks.(3)The hydraulic radius of the oxidation ditch with compound section is larger,the zone which velocity is lower than 0.15m/s in the ditch decreases 7.94%,the zone that velocity is higher than 0.5m/s increases 10.95%,the water velocity and the flow capacity is increased.The same energy input can provide higher mixed liquid velocity in the oxidation ditch with compound section.The maximum velocity along the flow of the ditch is similar to the cosine curve,the peak value appears in the bend,and the valley value appears at the end of the straight channel.(4)In the four cases of submergence ratio of propeller,the hydraulic performance are better when the ratio is 0.709 and 0.535.The fitting curve shows that when the submergence ratio is 0.53,the water of which velocity higher than 0.3m/s is the largest.when the ratio is 0.51,the average velocity of the liquid is the highest.Considering other factors,it is suggested that the submergence ratio of the propeller could increase appropriately.If the mediumis is water,the ratio is suggest as 0.55,If the sewage consistence is high,the ratio can be further increased to eliminate the low velocity region at the bottom of ditches.(5)With the increase of the propeller's submergence ratio,the swirling intensity of water surface increases,and the water-air interface increases.The gas volume fraction has obvious regular pattern in vertical direction.The higher the propeller is,the large bubbles play a major role in the gas content.The lower the runner is,the smaller bubbles play a maj or role in the gas content.When the runner is installed on the water surface,the gas volume fraction is the largest.In the simulation of propeller's submergence ratio,the order of liquid gas content from large to small is case4,ease1,case 2 and case 3.