Numerical Simulation Research On Optimization Of Oxidation Ditch Section And Guide Wall

With the rapid development of society,environmental issues have become increasingly prominent.In many environmental problems,the problem of water pollution has become the most serious problem.Among numerous processes for urban sewage treatment,the oxidation ditch process is favored by most sewage treatment plants because of its advantages such as simple process,good effluent water quality,and strong anti-load capacity.The hydraulic characteristics of mixed liquid in the oxidation ditch play an important role in efficiency of the sewage treatment and operating conditions.CFD is an indispensable means to study the flow characteristics in the oxidation ditch.First of all,this paper uses the previous test data to verify the mathematical model chosen in this study to ensure the rationality and feasibility of the model.Then,based on the traditional rectangular section oxidation ditch,three new section forms are proposed,and the RNG k-E turbulence model and the VOF method are used to numerically simulate these four-section oxidation ditch,respectively.The flow field structure,velocity distribution on sections,velocity along the vertical distribution,and the average velocity of the liquid layer in the oxidation ditch were analyzed and compared to obtain the optimal cross-section oxidation ditch.Finally,the same method was used to study the influence of the radius of guide wall and eccentricity of half-width guide wall on the hydraulic characteristics in the oxidation ditch,and the most suitable radius of guide wall and eccentricity were obtained.The main conclusions of the study are as follows:(1)The simulated values of the measured point velocity and the average velocity of the section are in good agreement with the experimental values,this shows that the selected mathematical model can well simulate the hydraulic characteristics in the oxidation ditch.Since the aeration brush is installed near the water surface,the velocity near the water surface is significantly greater than the middle and lower velocity;the sections farther from aerated brush,the more uniform the velocity distribution along the vertical line near the water surface.(2)The simulation results of four types of section oxidation ditch indicate that the hydrodynamic characteristics of the three kinds of new section form,such as the flow field structure and velocity,have been improved.The flow field structure and velocity in the "U"section form are the best.Area of the recirculation zone on the horizontal sections is obviously reduced,and the multi-swirl structure is shown on the cross-section,which improves aeration and oxygenation capacity of the disk aerator,this conducive to operation of the oxidation ditch.The velocity along the lines,average velocity of each liquid layer and the average velocity of the cross-section have greatly improved,especially improvement of velocity at the bottom of the oxidation ditch,increase of the bottom velocity can effectively reduce the sludge deposition in the oxidation ditch.This research conclusion has a certain reference value and reference for design of the shape of oxidation ditch in practical engineering.(3)The addition of a guide wall can effectively improve the flow pattern of water on the straight channels and bends in the oxidation ditch,and enhance the velocity at the bottom.But the radius of guide wall should not be too large nor too small.Too large or too small will make the velocity distribution inhomogeneous on the bend,and there is a low velocity zone.When the radius of guide wall is equal to 2m,the flow field structure and velocity distribution in the oxidation ditch are all better than lm or 3m,which is more conducive to the operation of the oxidation ditch.(4)When the eccentricity of half-width guide wall is equal to 0.2m,the flow field structure and overall velocity distribution is improved.When the eccentricity reaches 0.4m,it has the opposite effect on the flow field structure and velocity distribution in the oxidation ditch.It is suggested that the eccentricity of half-width guide wall is set to about 0.2m.