Effect Of Temperature-control Curtain On The Outflow Temperature And Its Thrust Force In A Deep Stratified Reservoir

Thermal stratification often occurs in deep reservoirs,which makes the outflow different from natural inflow in spring and summer and brings ecological and environmental problems to the downstream areas.Temperature-control curtain（TCC）is an effective facility of selective withdrawal,and both the withdrawal effect and structure safety are of great concern,which have been less studied in previous research.Taking the deep Sanbanxi Reservoir in China as a case study,through numerical simulations and hydraulic model tests,the mechanism of stratified flow and source of outflow are investigated,the effect of TCC on the outflow temperature and thermal stratification of reservoir area are obtained,and the thrust force caused by water flow are analyzed and estimated.The research result shows the following:1)The mechanisms of stratified flow before the dam is investigated,and the source of outflow is tracked.Outflow temperature is closely related to the vertical temperature distribution,negatively related to the water level,and positively related to the flow rate.Withdrawal zone is located within 5±30 m above the intake,which contributes approximately 82%to the outflow temperature.Among all water layers,the most effective layer which contributes the most to the outflow temperature is located 7.5 m above the top of intake.Contribution rates of surface and hypolimnetic layers are small.A forecasting formula of outflow temperature with five parameters is proposed and verified,including elevation of most effective layer(h_main=h_422.5),up-characteristic elevation(h_up=h₄₃₀),low-characteristic elevation(h_down=h₄₁₀),maximum(Q_full=870 m³/s)and actual（q）flow rate of intake.This formula systematically considers the withdrawal zone,most effective layer and discharged tracer proportion,and provides a rapid and accurate method predicting outflow temperature.2)Focusing on the bottom-TCC,three layout schemes of TCCs are proposed and elaborated including suspended TCC,floating TCC and hybrid TCC.Their structural compositions,construction methods and operation schemes under varing water levels,advantages and disadvantages,scopes of application are discussed.In engineering practice,the suspended TCC is recommended under small thrust forces;the floating TCC is recommended in small reservoir or river channel whose cross section are close to rectangle;and the hybrid TCC is recommended when the thrust force is notably large,such as the Sanbanxi Reservoir.3)The flow and temperature fields of whole reservoir under TCC are simulated,and the change rules of outflow temperature and thermal stability are obtained.When the water-retaining proportion（P_r）of bottom-TCC increases,the temperature difference between inflow and outflow monotonously decreases,while the thermal stability first increases and later decreases.The maximum thermal stability exists at P_r=62.5%,it goes against water quality improvement and should be avoided in practice.A bottom-TCC with P_r>80%is practical for deep reservoirs such as Sanbanxi Reservoir to decrease the temperature difference between inflow and outflow without the increase of thermal stability.In terms of top-TCC,as P_r increases,the temperature difference between inflow and outflow monotonously increases and thermal stability decreases.The top-TCCs are recommended when a smaller thermal stability is more preferentially considered than outflow temperature,or a cool outflow in the summer is required for downstream coldwater fishes.In addition,the TCC cannot decrease or increase the outflow temperature all of the time throughout the whole year,and it primarily changes the phase and variation range of outflow temperature.4)The variation laws of pressure difference and thrust force of different TCCs are analyzed.Under the uniform inflow condition,the distribution of pressure difference on the TCC is relatively even,and the maximum value is located at the upper part of curtain.When the water is unstratified,the hydrodynamic characteristic and thrust force of bottom-TCC are basically in line with those of top-TCC.When arc length-height ratio increases,the location of maximum pressure difference gets lower.The variation law of thrust force on the TCC conforms to the classical resistance equation.The drag coefficient is found to fit a power function of the water-retaining ratio,a secondary polynomial function of arc length-height ratio,and linear function of inclination ratio.The results also yield a simplified forecasting formula of thrust force on the TCC which is proposed and verified using flume simulations and a real reservoir model test.The newly developed formula systematically considers the water-retaining height,arc length and inclination degree,provides a rapid and accurate algorithm to predict the thrust force.5)The effect of thermal stratification on the thrust force is further considered and estimated.For the bottom-TCC in a stratified reservoir,because of the temperature difference between the two sides of the TCC,the thrust force on the TCC is divided to hydrostatic and hydrodynamic pressure differences,and both are in the downstream direction.Between the two parts,the proportion of the hydrodynamic pressure difference is much larger.When flow height decreases,the thrust force on the TCC rapidly increases.Both hydrostatic and hydrodynamic pressure differences increase,and the latter changes more rapidly.When thermal stratification becomes stronger,the hydrostatic pressure difference increases and the hydrodynamic pressure difference decreases,the thrust force on the TCC decreases.For a top-TCC,the hydrostatic pressure difference points upstream direction.However,because of the giant hydrodynamic pressure difference,the thrust force on the top-TCC is much larger than that on the bottom-TCC.The result in this paper provides a theoretical basis for the application of TCC,and offers a powerful support for temperature management.With more deep reservoirs being built and attention being paid to water environment management,the research and application of TCC will continue to grow.