Study On Flow Characteristics And Outflow Temperature Through Selective Withdrawal From Thermally Stratified Reservoir

The thermal stratification causes low water temperature in the hypolimnion,which exerts to a negative impact on the downstream aquatic ecosystem.The selective withdrawal is highly recommended and applied in the reservoirs for improving outflow temperature.There exists several common structural forms of selective withdrawal,such as multilevel intake,stop-log gate intake,floating intake and control curtain.The main stream region presents near the intake owing to the influence of thermal stratification.The water body in the mainstream area is the main source of the outflow water body,and its temperature determines the final outflow temperature.It is of great significance to study the flow characteristics to explore the flow formation mechanism,and to reveal the cause and regulation effect of the outflow temperature.In addition,the difference in the flow characteristics,induced by the different structure of intaks,can lead to the differnce of outflow temperature.Thus,comparison and analysis of the flow characteristics and outflow temperature of different intakes can provide theoretical basis for the selection of intake.Considering the influence factors of temperature distribution,submergence depth and outflow rate,the experiment and numerical simulation method are used to study the flow characteristics and outflow temperature of multilevel intake,floating intake and control curtain.And the differences between different intakes are compared.The main research contents and results are as follows:(1)The variation law of flow characteristics is studied through the experiment and numerical simulation,and the differences between intakes are compared.Under the condition of no stratification,the withdrawal region is the whole water depth and the velocity distribution is uniform.The withdrawal layer is formed near intake level under the combined action of traction of intake and buoyancy produced by density gradient,and the withdrawal layer thickness is inversely proportional to the density gradient.The velocity distribution presents bell shaped when the water temperature is stratification.The change of density gradient induced by adjusting the submergence depth causes the variation of withdrawal layer thickness.With the increase of outflow rate,the withdrawal layer thickness and maximum velocity increase,but the nonuniform coefficient decreases.Under the same condition,the flow characteristics of three intakes modes are basically same when the temperature is not stratified.When the water temperature is stratification,the withdrawal layer thickness of multilevel intake is the largest,and that of control curtain is the smallest.And the maximum velocity and nonuniform coefficient for multilevel intake is the smallest,and that of control curtain is the largest.(2)The formation of outflow temperature and its response to the influence factors are revealed through numerical simulation,the prediction formula of outflow temperature for floating intake is established,and the outflow temperature of different intakes is compared.The outflow water comes from withdrawal layer,and the outflow temperature is determined by the temperature distribution corresponding to the withdrawal layer.With the decrease of submerged depth or increase in outflow rate,the outflow temperature increases.The variation amplitude caused by submerged depth depends on the temperature gradient between the two submerged depths.According to the internal relationship between the outflow temperature and the withdrawal layer,the formula of outflow temperature with high prediction accuracy is proposed based on the dimensional analysis and energy conservation.Under the same conditions,the floating intake corresponds to the highest outflow temperature,while that of control curtain is the lowest.(3)Based on numerical study of velocity distribution upstream of multilevel intake,the velocity formula is put forward.A vertical velocity distribution formula suitable for water temperature stratification is proposed by studying the velocity distribution along the vertical direction.The velocity decay tendency along the centerline of multilevel intake orifice is independent of three influence factors,which indicates that three influence factors do not affect the longitudinal flow.The temperature stratification has little effect on the velocity near the multilevel intake orifice.And the potential flow theory is used to derive the velocity formula near the orifice,and the calculated results are in good agreement with simulation results.(4)The influence of different factors on the resistance coefficient of the control curtain is studied based on theory of flow,the minimum submergence depth of control curtain is determined by sensitivity analysis,and the weight of influence factors is calculated by entropy weight method.The resistance coefficient increases with increasing density gradient below intake level and decreasing submergence depth or outflow rate.And the relationship between resistance coefficient and influence factors is obtained by dimensional analysis.The minimum submerged depth H/h=0.11 corresponds to the best improvement of outflow temperature.The influence degree of three factors on the outflow temperature in descending order is temperature distribution,submergence depth and outflow rate.