| In this paper,the FLUENT software is used to simulate the flow field in the grid flocculation tank.Compared the simulation analysis of different mesh size,the setting of the side wall block,water inflow and the height of the water hole between the vertical shafts.The turbulence kinetic energy,dissipation rate of turbulent kinetic energy and vortex velocity gradient are used as the evaluation index of coagulation effect.Intercept different directions,different locations,different heights of the characteristic line to analyze the velocity change on every characteristic line.Meanwhile,the numerical simulation is connected with the coagulation experiment to analyze how characteristics of flow field in the grid flocculating tank under different hydraulic conditions affect coagulation.The following conclusions can be drawn:(1)Through the numerical simulation of the whole flow field in the flocculation tank,coagulation effect would be comparatively good when the mesh size is 6×6mm and would be comparatively poor when the mesh size is 15×15mm.When the inlet and outlet shafts apply different mesh sizes,it is concluded that coagulation is effective when the inflow shaft uses dense mesh and outflow shaft uses sparse mesh.This finding could respond to the real producing practice which need dense mesh at front,sparse mesh in the middle and no mesh at the end of coagulation process.Comparison is made between flocculating tank side wall with block and those without block.The former has a relatively high energy utilization rate and a large turbulent range,helping to improve coagulation effect.So in real programs,there can be no need of setting up blocks on wall.Compared with the change of mesh size,influence of varying inflow in the flocculating tank is not evident.So in practice,design of mesh size should still be the research focus of optimizing grid flocculating tank.In different stages of the process,the height of the water hole should also be set up differently.Its height should be gradually increased with the coagulation process.(2)Analysis of changes in characteristic line velocity of every direction of partial flow field behind the grid shows that at a same location,velocity varying on x direction of the characteristic line is basically the same as that on y direction.Because block location on side wall is impacted by the wall surface,its velocity on the characteristic line is smaller than block location on non-side wall and has a relatively vehement fluctuation.With increasing water inflow,the fluctuation range of the characteristic line velocity value increases correspondingly.But the varying trend of velocity on every characteristic line is fundamentally invariant.Comparing overall velocity of different mesh size in the same condition can find that the velocity of 6×6mm is the largest,4×4mm the second largest and 15×15mm third largest.Due to vortex zone under the block,location of block on non-side wall with different mesh size has a characteristic line velocity which would first decrease and then increase with the growth of distance to the grid.Meanwhile,characteristic line velocity about the location of block on side wall and the mesh will increase first and then decrease and stabilize last.Analysis of velocity in x direction,y direction and z direction of characteristic lines can find that velocity in y direction is the largest and has a similar varying trend with resultant velocity.(3)Analyzing the data of coagulation experiment shows that when the mesh size is 6×6mm,the turbidity removal rate is the highest and the coagulation effect is the best.And when the mesh size is 15×15mm,outflow water quality is the worst.This is the same as numerical simulation analysis results.Increase of PAC dosage would enhance the outflow water quality correspondingly.When the inflow is in a relatively small scale,the agent can be mixed intensively with the raw water and the coagulation effect is relatively good.When the inflow is in a larger scale,the agent has aggressive shearing action on flocs and reaction time is too limited to have poor coagulation effect. |
