| The grid flocculation tank has the advantages of short flocculation time and good treatment effect.The grid flocculation tank is used by various scale waterworks at present,but there are two problems in its application.Firstly,the parameters of grid flocculation tank design specification are derived from experience and have a wide range.Designers need to rely on their own experience in the design of a large subjective difference,resulting in the design of the grid flocculation pool many did not achieve the desired effect.Secondly,there are still some problems in the reaction principle of the grid flocculation pool.For example,within a certain range from the grid plate,the flow field can be considered as isotropic turbulence with zero velocity gradient.According to the traditional velocity gradient theory,the reaction efficiency of the grid flocculation pool should be the worst,but the actual situation is that the flocculation effect of the grid flocculation is better than that of other flocculation equipment.Aiming at the existing problem of grid flocculation pool this paper uses the Fluent software to numerical simulation of grid flocculation field,combined flocculation index(turbulent kinetic energy,turbulent kinetic energy dissipation rate,discrete coefficient,etc.),analysis and evaluation of different grid plate geometry influence on grid flocculation reaction,and then puts forward the better and more reasonable grid flocculation pool design parameters.The main conclusions of this paper are as follows:1.The results of numerical simulation and PIV measurement show that the flow pattern,velocity and vorticity are consistent.This shows that the numerical simulation results are in line with the expected standard and the simulation results are accurate.2.The mesh size gradually increases to affect the flow field:(1)The maximum turbulent kinetic energy decreases in the flow field,which makes floc less likely to be broken;(2)The uniformity of water flow mixing increases first and then decreases,among which the flow field uniformity with mesh size of 10 mm is the best;(3)The dissipation rate of the average turbulent kinetic energy decreases in the flow field,which leads to the decrease of the flow field's promoting effect on flocculation;(4)The effective distance of grid plate is reduced.3.The influence of increasing the width of lattice block on the flow field:(1)The maximum turbulent kinetic energy in the flow field increases significantly,which may cause floc breakage and affect the flocculation effect;(2)When the grid width increases to more than 12 mm,the flow field inhomogeneity increases significantly,so it is suggested that the grid width should be less than 12mm;(3)The dissipation rate of the average turbulent kinetic energy increases and the effect of the flow field on the flocculation reaction is enhanced;(4)The effective distance of grid plate rises.4.If the mesh size and the width of the grid are increased,the effective distance of the grid plate will increase,but the effect on improving the flocculation effect is small.5.The influence of mesh size increasing and lattice width decreasing on the flow field:(1)The maximum turbulent kinetic energy in the flow field decreases greatly;(2)The uniformity of the flow field first increases and then decreases;(3)In the flow field,the dissipation rate of the average turbulent kinetic energy decreases greatly,resulting in a significant decrease in flocculation promotion;(4)The effective distance of grid plate decreases greatly.6.The influence of plate thickness on flow field:(1)The maximum turbulent kinetic energy of the flow field tends to decrease;(2)The uniformity of the flow field varies little;(3)The dissipation rate of the average turbulent kinetic energy decreases;(4)The effective distance of grid plate is little affected.7.The influence of grid plate mounting spacing on flow field:(1)The maximum turbulent kinetic energy decreases slightly in the flow field;(2)The inhomogeneity of the flow field increases;(3)The dissipation rate of the average turbulent kinetic energy decreases in the flow field;(4)The grid plate installation spacing is less or greater than the effective distance of the grid plate is not conducive to flocculation.8.According to the evaluation indexes of the flow field,the best scheme of the grid plate in the front section of the grid flocculation pool is the mesh size of 8mm and the grid width of 12 mm.The best plan of grid plate in the middle section of the grid flocculation tank is the mesh size of 10 mm and the grid width of 10 mm.The thickness of mesh plate is 5mm preferably.Grid plate installation spacing equal to the effective distance of the grid plate is the best. |
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