| The flocculation process is the basic process of water purification plant,whose effect on flocculation is not only affected by the nature of flocculant,but also has strict requirements on the hydraulic condition of equipment.Due to its superior flocculation effect and simple structure,the reciprocating baffle flocculation process has been widely used in large and medium-sized water plants in China.Many problems have been exposed during the long-term operation:the large ineffective energy dissipation in the tank owing to its poor hydraulic conditions;large tank volume but low flocculation efficiency.In view of the flow field movement characteristics of the reciprocating baffle flocculation tank,the dynamic cause of flow field restricting the flocculation effect are explored and effective optimization schemes are put forward,which is of great significance to give full play to the efficient flocculation effect of the traditional flocculation process.The research progress of flocculation kinetics was reviewed to clarify the important role of turbulent eddy motion in flocculation process.CFD flow field numerical simulation and model test were carried out to improve the proportion of micro-eddy in the flow field as the main optimization measure.CFD numerical simulation results show that the flocculation effect of traditional reciprocating baffle flocculation process mainly depends on the head loss at inlet and corner of corridor.However,in the long straight corridor of flocculation tank,the turbulent kinetic energy and dissipation rate of turbulent kinetic energy in the flow field are low,which results in inadequate particles collision during flowing with fluid.On the other hand,the energy consumption caused by near-wall turbulence has little effect on flocculation.The above explicates the main reason for the ineffective energy consumption of this process and the unsatisfactory flocculation effect.The flow velocity,turbulent kinetic energy and dissipation rate of turbulent kinetic energy in the flow field increase gradually with the increase of simulated inlet water quantity,but the low effective energy consumption and poor hydraulic condition in the flow section of the corridor are still not improved.Therefore,it can be considered that increasing the flow rate cannot significantly improve the flocculation effect.In order to improve the proportion of free turbulent flow in the flow field,two types of obstacles,grid and orifice plate,were added into the inlet corridor of flow field respectively.The simulation results indicate that water flows around the grid and forms a vortex region behind the grid,which reflects the vortex phenomenon caused by boundary layer separation.Through the simulation calculation of the flow field of three kinds of grid(right square prism grid,cylindrical grid and triangular prism grid)respectively,it is found that the flow field turbulence effect of the triangular prism grids is the best.In addition,the turbulent kinetic energy and turbulence eddy dissipation in the flow field with the edge of the triangular prism facing water are higher than those with the cylinder facing water.The smaller gap between grids results in increasing of flow velocity,which contributes to higher turbulent kinetic energy and effective energy consumption.The local flow field velocity and energy consumption caused by the grid at different positions are basically the same when the grid is changed.Therefore,the adding position of obstacles can be reasonably selected according to the distribution of energy consumption in the flow field.The flow through the orifice plate is consistent with the characteristics of the orifice jet.By changing the opening and closing angles of orifice plate in turn,the bending orifice plate promoted the water flowing through the orifice to merge into one stream,so that the energy consumption curve in the flow field presented different distributions on the characteristic lines in the z and x directions,but the flow field turbulence was the highest when the orifice plate angle is 180°.The smaller the aperture of orifice plate added in the flow field,the larger the flow velocity,the higher the turbulence of the flow field,and more available energy consumption for flocculation.In the model test,PAC was used as flocculant.With the increase of water intake,turbidity removal rate increased first and then decreased,and increasing water intake could not effectively improve the flocculation effect.Two orifice plates were added into the inlet corridor of the reciprocating baffle flocculation reaction tank,and the tests were carried out under the conditions of pore sizes of 4mm,6mm,8mm and 10mm successively.Due to the significant interaction between inlet water and turbulence,the local turbulent kinetic energy and effective energy consumption in the flow field will be significantly increased with the increase of flow rate.When the inlet flow rate is small,the turbidity removal rate increases with the decrease of the pore size of the orifice plate.However,as the inlet flow rate increases to a certain extent,the small pore size will cause the fragmentation of flocs,leading to the increase of turbidity of the effluent.Changing the orifice angle in the experiment did not cause marked improvement in turbidity removal rate,for the change of orifice plate angle has no significant improvement effect on hydraulic conditions.With the increase of water inflow,the eddy scale decreases gradually.The power function equations are obtained by fitting the water intake and the eddy scale under each test conditions.The function equation is=12(v/Q)~23~12 under the condition that no orifice plate is added into the reaction tank model,and the correlation coefficient is 0.98198.The fitting curve functions of two 10mm plates,two 8mm plates,two 6mm plates and two 4mm plates in the reaction tank model are=11.544(v/Q)~23~12?=10.365(v/Q)~23~12?=8.386(v/Q)~23~12 and=5.8(v/Q)~23~12respectively,The correlation coefficients were 0.98326,0.993,0.99917 and 0.9994respectively.The fitting curve functions of two orifice plates with holes of 8mm and bending angles of 150°,120°and 90°are=10.332(v/Q)~23~12?=10.228(v/Q)~23~12 and=10.082(v/Q)~23~12respectively.The corresponding correlation coefficients were 0.99404,0.99516 and 0.996,respectively.Through numerical simulation,the characteristics of the flow field of the reciprocating baffle flocculation process were deeply understood.Combined with the hydraulic evaluation indexes,schemes for flow field optimization were provided,and the optimization effect was verified by model test.Based on the numerical simulation and experimental study,the??Q equations under various working conditions were obtained,which explained the root cause of the improvement of flocculation effect from the perspective of vortex.This study is of great reference value for the optimization of flow field of the reciprocating baffle flocculation process in waterworks. |
PDF Full Text Request