Numerical Simulations On The Control Of Algae Migration Under The Role Of Selective Withdrawal And Development Of Enhanced Selective Withdraw Technology

Most large and medium-sized cities in our country have built or are buildingreservoirs as source of water supply or emergency. In recent years, eutrophication ofreservoir source result from point, non-point and internal pollution is obvious. In orderto supply safe water and reduce purification cost, improvement of raw water quality isone of the major issues in water protection field.Considering the characteristics of temperature stratification and surface algaegrowth properties, it’s an effective way to use selective withdraw technology to avoidhigh algae-laden raw water and reduce algae concentration in water plant. TheComputational Fluid Dynamics （CFD） on the basis of classical Fluid Dynamics andnumerical calculation can simulate fluid field under the control of basic fluidequations.This paper use CFD commercial software FLUENT to simulate the flow fieldnear water intake in each typical withdraw condition in summer and autumn bychoosing a deep water supply reservoir of a city in northwest China as prototype. Thevelocity of surface water （less than）2.75x10^-4m/s is a threshold of withdraw avoidingsurface algae suggested in literatures. So we can draw the following conclusions:（1） The velocity of surface water changed along with water level changed, that isto say withdraw avoiding surface algae range changed. Choose the range of horizontal60m away from water inlet and vertical3m away from water level as the comparativeresearch range. The simulation confirm that withdraw in up layer each percent ofwithdraw avoiding surface algae range is17.6%，31.8%，49%and63.5%whenwithdr aws is6.64m³/s and the water level are5，10，15and20m.In the same condition, the withdraw range presented a symmetrical distribution when withdraw in the middlelayer. It can be concluded that the distance between water level and water intake ismore than22m, we can consider withdraws from none of surface water. So thewithdraw influence on surface water weakened along with water level elevated, that isto say the withdraw avoiding surface algae range increased.（2） Selective withdraw influence on surface water strengthen along with thewater withdrawals increased and avoiding surface algae range reduced. The simulationresults conform that the velocity of surface water increase when20m way from waterintake to reservoir water level and6.64，10，15.6and28m³/s water withdraws. Theavoiding algae areas ratios are63.5%,32.7%,18%and9.6%.（3） Inflow of reservoir can influence selective withdraw avoiding surface algae.Considering actual conditions like surface flow in summer and autumn, temperaturestratification, gradient before intake tower, we can use VOF（Volume Of Fluid） modelto simulate selective withdraw. It can be confirmed that the area of avoiding surfacealgae is dynamical and the potency of drawing water in middle-level is obvious in thecondition of10m³/s water withdraws and11.4,15,19.6and28m³/s surface flow.（4） Installment of baffle plate that is perpendicular to intake tower can changesurface water velocity distribution obviously. The longer baffle length, the largeravoiding surface algae withdraw area. In the condition of10m³/s water withdraws andbaffle plate is20m away from reservoir water level, the simulation confirm the ratio ofavoiding surface algae grow to be68%,73.2%and88.6%after installing0.5,1and2mbaffle as broad as water intake. It brings new ideas for avoiding surface algae andenhanced selective withdraw. According to the simulation results, we can develop theenhanced selective withdraw baffle plate device.