Study On Low-Re Number Turbulence Controlling Method And Its Application In Flocculation

As an important part of removing impurities in water treatment process,the effect of flocculation directly affects the subsequent precipitation and filtration,thus affecting the final effluent water quality.At present,most water plants in our country adopt hydraulic flocculation tank,which has the advantages of simple structure,convenient management and maintenance.However,the hydraulic condition can not be adjusted according to the actual operating conditions,there are still some defects in dealing with sudden water pollution events,low temperature and low turbidity water sources,night load water and so on.Meanwhile the ineffective energy consumption accounted for the proportion of larger,leading to the long flocculation time,large volume of pool and high cost of infrastructure.Therefore,this paper proposes a flocculation equipment that can strengthen the flocculation and flexibly adjust on the flocculation process according to the operation situations and parameters of raw water quality,water load,medicament type and dosage.The hydraulic conditions were intervened by adding the adjustable vortex generators in the traditional flocculation gallerys,so as to improve the flocculation efficiency and final effluent water quality.In this paper,the wake characteristics and vortex control method of the adjustable vortex generator are analyzed by the large eddy simulation method.The effects of vortex scale,intensity,shedding frequency and Strouhal number on aggregation and growth of floc particles at different stages are investigated.It was applied to the traditional reciprocating flocculation tank gallerys,exploring the flocculation effect and energy efficiency under different blade forms by the flocculation experiment of PAC and kaolin,and the optimization experiments were carried out under different temperature,suspended solids concentration and flow rate,the details are as follows:(1)Through the analysis to colloid in the natural water and structure of impurities,these point out that the reasons of colloid can be stably dispersed.Then,the mechanism of flocculation was studied from the perspective of destroying its stability and four representative forms of action are summarized.Meanwhile the driving force of colloid collision aggregation is expounded and it points out the application scope and effect of them.The dynamic model is obtained by studying the force of floc particles in vortex motion.Finally,the main influencing factors of flocculation process were discussed.(2)Large eddy simulation is used to study the blade vortex generator that used in the adjustable flocculation tank at diverse blade forms and Reynolds numbers,the change rules of vortex scale,intensity,shedding frequency and Strouhal number were explored calculated at different velocities and attack angles.The initial vortex size increases with the increase of absolute value of the blade angle and irrelevant to velocity,while the vortex intensity increases at larger attack angle and velocity.The vortex shedding frequency increases at bigger velocity and smaller angle.The Strouhal number can be approximated as constant that is similar to other issues of flow around bluff.Finally,the aggregation,growth and crush of floc particles in flocculation process were analyzed comprehensively.(3)The traditional reciprocating flocculation tank model was designed by Froude criterion number and flocculation indexes value of G and Gt were verified.Based on that,an adjustable vortex flocculation tank experimental platform was built by adding adjustable vortex generator in gallery.Before the experiment,the optimum dosage ratio of flocculant was confirmed(22.5mg/L)by the change of turbidity and zeta potential in jar stirring test,and the mixing uniformity of flocculant and raw water that through the SK mixer was verified by using large eddy simulation.(4)The turbidity removal and energy consumption at different working conditions were investigated by changing the attack angle of vortex generator in the partial and overall flocculation tank galleries.It is more appropriate when the attack angle of blade vortex generator is-45°.At the same energy consumption,the flocculation effect can achieve better when blade angle decreases continuously along the flow direction.Meanwhile,the similar experiments were carried out in different temperature,flux and suspended matter concentration,the results show that the improvement of flocculation effect is more obvious with the addition of vortex generator that compared with conventional flocculation tank at low flow,turbidity and temperature,and the turbidity removal efficiency increased with temperature,flux and turbidity of raw water.Thus the optimization control of flocculation process was realized.