Numerical Simulation And Optimization Of Flocculation Process Based On CFD

Flocculation is one of the important unit operations of water treatment.Under the background of increasing urban water consumption and high water quality requirements,the traditional flocculation process urgently needs to be optimized and upgraded.The research objects were based on the grid flocculation tank,the folded plate flocculation tank and the stirred flocculation reactor,which are widely used in engineering application.Combined with the theory of flocculation kinetics,computational fluid dynamics（CFD）numerical simulation software was used to simulate the main structural parameters and operating parameters of the flocculation process.From the perspectives of flow regime and energy distribution,the relationship between the flocculation effect and the hydraulic conditions and the structural characteristics of reactor was explained,which provides theoretical support for the optimization of the flocculation process.Based on CFD simulation,the standard k-εmodel and RNG k-εmodel as the numerical simulation method of turbulence were adopted in this thesis.The grid flocculation tank and the folded plate flocculation tank were simulated in two-dimensional,and three-dimensional simulation of flocculation stirred reactor was carried out.In order to analyze the fluid motion state and explore the best design parameters,the vortex velocity gradient,turbulent kinetic energy,turbulent energy dissipation rate,velocity distribution uniformity and vortex scale were selected as dynamic evaluation indicators.The results show that:1.Variations of parameters in the grid flocculation tank will directly affect the velocity distribution and the turbulence of water,and the equivalent size of the grid has the most significant effect on flow pattern.When the width-depth ratio of the tank is 0.37,the inlet velocity is 0.12m/s,the smaller grid size is 40mm,the grid plate spacing is 400mm,and the inlet height is 600mm,the flow field has better hydraulic conditions due to the turbulent synergy of water.The mixing degree is more intense,which is more suitable for initial flocculation stage.By establishing the relationship model between the mesh equivalent size and the vortex scale.It is found that when the mesh size is 20～100mm,the micro vortices which scale are similar to the floc size can be generated within 100mm below the grid plate.The order of the size range is 10^-4,According to this relationship model,the flocculation process can be controlled.2.The diffusion of the flow jet zone formed at the crest of the folded plate channel is the main producer of the water turbulence.Sixteen groups of models were established to carry out single-factor simulation of different folding angle,linear plate length and space between the two adjacent folding plates.Finally,the optimum combination of folding plate was determined as follows:the folded plate angle is90°,the linear length is 500mm and the adjacent distance is 300mm.On that basis,two structural optimization methods were proposed,and Adding spoilers into the center of each folding plate unit and using a corrugated plate with sharp corners removed can obviously increase the number of vortex in the two wave troughs.Therfore,it is strongly recommended that the folded plate unit with 120°diamond spoiler should be used in the front section of flocculation,and the corrugated plate with vertical plate should be used in the middle section of flocculation process.3.The impeller speed of the stirred flocculation reactor has little influence on the flow rate partition of the flow field,but has a greater influence on the energy distribution.The simulation results show that the overall mixing degree of pitched blade flow field is more balanced than the straight blade,and the work efficiency is much higher.Through the stirring flocculation verification test under different water quality conditions,and the comparison with the CFD simulation results,it is proved that the reactor with 15°angle four-blade impeller has the best treatment effect and is more suitable for treating low-temperature and low-turbidity water.