The Study On PIV Measurement And Flocculation Experiment Of The Grid Flocculation Tank

Particle image velocity(PIV)was applied to measure flow field of grid flocculation tank,and obtain the information about the flow field of grid plates with different sizes in different inflows.Based on the result of PIV measurements,the velocity field is divided into different regions according to change rules of velocity.Combined with large eddy PIV method,distribution of turbulent kinetic energy k and turbulent dissipation rate ? in grid flocculation tank is explored,and the relationship between distribution and divided regions is found.And in the same conditions,flocculation experiment is conducted,to study the relationship between hydraulic characteristics and flocculation effect of grid flocculation tank flow field.Through PIV measurement of flow field and flocculation experiment research of grid flocculation tank,the following main conclusions can be drawn:1.Make comprehensive analysis of velocity of different grid plates,to know that the velocity change is mainly longitudinal velocity after the grid plate,and the ratio of longitudinal velocity and transverse velocity is bigger when the distance from the grid plate is shorter;the ratio of longitudinal velocity and transverse velocity is bigger when the distance from the mesh center is smaller;and the ratio of longitudinal velocity and transverse velocity is smaller when the distance from grid center is smaller.2.Make analysis of velocity on characteristic sections of the specific grid plates(6×6mm),to know that the velocity change includes three types after flow passes the grid plate: acceleration region,blending region and stabilization region;if hydraulic conditions are changed,the region division scope will have weak fluctuation.In the test,rules of velocity change of flow filed gained in No.1 grid plate(4×4mm)and No.2 grid plate(6×6mm)include three processes: acceleration region,blending region and stabilization region.Rules of velocity change of flow filed gained in No.3 grid plate(10×10mm)only experience acceleration region and blending area.3.The flow velocity is bigger when inflow is bigger,and the probability of impact of particles is bigger,so that turbulent kinetic energy increases and dissipation rate of turbulent kinetic energy increases.Utilize large eddy PIV method to analyze flow field.In a certain distance from the grid plate and in the same inflow,turbulent kinetic energy of different grid plates and fluctuation amplitude thereof are different;fluctuation amplitude of turbulent kinetic energy in acceleration region and blending region is bigger and smaller in stabilization region;fluctuation amplitude of dissipation rate of turbulent kinetic energy is bigger in acceleration region and smaller in blending region and stabilization region.4.It can be concluded according to the flocculation experiment that change of hydraulic condition has obvious influence on flocculation effect.When mesh size is too small,inflow is small and the flow velocity passing the grid plate can reach the scope of designed flow velocity,therefore,flocculation effect is good;inflow is too big and flow velocity passing the grid plate is too big to cause big turbulent fluctuation to flow field,which will damage the formed flocculation structure,to increase the number of small flocs gradually and make flocculation worse.When mesh size is too big,flow velocity passing the grid plate cannot reach the design scope as change of inflow and the probability of impact of floc particles is decreased,the flocculant can only have reaction in local part at high concentration and cannot fully touch more suspended solids to generate flocs and conduct sedimentation,which results in poor flocculation effect.Through comprehensive comparison,it is concluded that flocculation effect of No.2 grid plate(6×6mm)is better.