The Mechanism Of Vortices Effect On Coagulation Collision In The Flow Field Around A Circular Cylinder

Coagulation process,as a classic liquid-solid two-phase separation technology,has been widely used in the water treatment industry.The coagulation effect is mainly influenced by two factors: the dosage of chemicals and hydrodynamic conditions.In order to explore the coagulation behavior of fine particles in vortex at the mesoscale,this study used calcium chloride as a coagulant to investigate the effects of hydraulic conditions on particle coagulation at a dosage of 333 mg/L.Firstly,Computational Fluid Dynamics(CFD)was used to simulate and calculate the flow field,and vorticity method and Q criterion were used to identify and obtain information about the vortex.The distribution of the flow velocity,pressure,and vortex structure around the cylinder in the wake flow field was investigated by changing the inlet velocity and cylinder diameter.Secondly,CFD coupled with Discrete Element Method(DEM)was used to simulate the coagulation of polystyrene particles in the wake vortex and to explore the effects of particle size,cylinder diameter,and inlet velocity on the coagulation effect.Finally,a verification experiment was conducted to design the coagulation of polystyrene particles to confirm the accuracy of the simulation model.The simulation results of the flow field showed that a vortex street could be formed in the cylinder wake flow under suitable flow velocity,and the velocity,pressure and vortex structure of the vortex street exhibited periodic changes.After the vortices detached from the back surface of the cylinder,the vortex size represented by the average radius of the vortex inertial centrifugal region showed a trend of rapid growth followed by slow and stable growth in the wake field.The vortex strength represented by the average positive circulation decreased overall in the wake field,and gradually weakened and dissipated when the energy dissipation reached a certain level.By changing the flow velocity at the inlet,the vortex strength could be enhanced,and the maximum vorticity of the first vortex increased approximately linearly with the increasing inlet flow velocity,with a fitting equation of y=2423.59x-59.01.The vortex size increased with the cylinder diameter and approximately followed a linear equation of y=0.013x+0.046.CFD-DEM simulation was used to obtain the motion trajectory and agglomeration process of polystyrene particles in a circular cylinder flow field.Based on the characteristics of the flow field,the number of particle adhesions and adhesion efficiency were statistically analyzed in 9 small zones of the flow field.The results showed that the change trend of particle adhesion efficiency in the flow field was similar to an S-shaped growth curve,which effectively reflected the growth process of agglomerates.The interaction between particles and the fluid promoted particle collision and adhesion,and their agglomeration behavior mainly occurred in the vortex shearing zone.From the perspective of material particle size,the larger the particle size,the more significant its destructive effect on the vortex,the higher the turbulence level and shear strength of the wake,and the better the particle agglomeration effects.The adhesion number and adhesion efficiency of particles show a trend of first increasing and then decreasing with the increase of cylinder diameter.When the cylinder diameter was 3 mm,the agglomeration effect of particles was the best,and the adhesion efficiency was the highest.With the increase of inlet flow rate,the adhesion number and adhesion efficiency of particles also increased first and then decrease,reaching a peak when the inlet flow rate was 0.08 m/s.When the vortex scale was about 10 times the particle size,it was most favorable for particles to collide and adhere.Verification experiments were carried out using a flocculation device with the same design and simulation conditions.When the flow rate was 0.08 m/s,the boundary layer thickness between the cylinder wake flow field and the two flat plates was 8 mm,and the flow field always showed a laminar state.Increasing the distance between the two flat plates to 10 mm eliminated the influence of wall effects.Under the same flow rate,the cylinder wake flow exhibited turbulent flow,and its fluid flow morphology was the same as that simulated.The experimental phenomenon showed that the particles rotated along with the vortex structure and exhibited similar behavior to the simulated results.The coalescence effect on the backside of the cylinder was good,and the agreement between the experimental parameters and the simulation parameters was high.The experimental process could reasonably verify the simulation process.There are 50 figures,13 tables and 97 references in this thesis.