Study On Bi-directional Hydraulic Characteristics And Shape Optimization For The Vertical Pipe Intake-outlet

The pumped hydro energy storage(PHES)station is mainly composed of the upper reservoir,the lower reservoir,the water conveyance system and powerhouses.The station is equipped with reversible turbines that have functions of both the hydropower station and the pump station.According to the load of the power system,PHES stores electrical energy in the form of hydraulic potential energy by pumping water from a lower reservoir to an upper one during off-peak hours,and water is conversely released during peak hours to generate electrical energy.The energy conversion between electric energy and water potential energy is realized through this way,so PHES plays an important role in the current energy transformation process in China.As an important part of the water conveyance system of PHES,the water intake-outlet is an engineering facility built between the upper and the lower reservoir to control water flow.The water flow presents bi-directional characteristics of inflow and outflow when PHES operates under different working conditions.Therefore,shape design of the water intake-outlet is directly related to operation safety and economic benefits of PHES.As an important type of the intake-outlet,the vertical pipe intake-outlet has many unique advantages: the structure is relatively compact,work quantities during the excavation are small and the site location is relatively flexible.In recent years,its application in domestic and foreign projects has gradually increased.Tunnels of the intake-outlet are vertically connected with the reservoir.The flow pattern is very complex because water coming from the vertical pipe firstly passes through a diffuser and then changes the flow direction from vertical to horizontal in the outlet mode;therefore the hydraulic conditions are very difficult to control.At present,there have been some problems in the previous studies for the bi-directional hydraulic characteristics of the vertical pipe intake-outlet:(1)model experiment research is mainly limited to specific projects,lacking of a general summary of main problems;(2)Computational Fluid Dynamics(CFD)has been a main research method in the field of the intake-outlet shape optimization,but there is little literature involving systematic discussions about accuracy of the CFD calculation;(3)shape optimization research is mostly based on the single factor comparison method,and more precise investigation about the parameter influence is deficient,which can neither provide a reasonable shape parameter selection interval nor analyze the sensitivity and the interaction of different shape parameters;(4)The optimization method based on the surrogate model has been widely used in the field of shape simulation and optimization.However,there are few discussions based on the vertical pipe intake-outlet.Furthermore,how to reduce the uncertainty of the surrogate model and form a reliable optimization process for the vertical pipe intake-outlet still needs further discussion.Based on the above problems,this thesis investigates the bi-directional hydraulic characteristics and the shape optimization for the vertical pipe intake-outlet.The research content and main results are as follows:1.Taking the vertical pipe intake-outlet in the upper reservoir of Yimeng PHES station as the research object,hydraulic characteristics of the intake-outlet in pump and generation modes are studied through a model experiment.Summarizing experimental results of similar PHES projects that have been constructed in China,it is difficult to eliminate the reverse velocity in pump mode(outflow).Therefore,the main problem of the shape optimization at present is how to eliminate the reverse flow velocity of outflow and give consideration to the bi-directional hydraulic characteristics at the same time.In order to solve this problem,the optimization algorithm and the surrogate model are proposed in this thesis to seek the optimal shape that can eliminate the reverse velocity and take the bi-directional characteristic into account at the same time.2.To solve the problem of CFD calculation accuracy,a numerical model is built with the background of the vertical pipe intake-outlet.Considering the calculation efficiency and the accuracy,the Grid Convergence Index(GCI)is introduced.Based on the GCI,a selection method for the reasonable grid scale and turbulence model is proposed,and the matching scheme suitable for the subsequent parameter analysis and optimization research is given.When the orifice grid scale is 0.03 D,and the turbulence model is Realizable k-?,this scheme can well balance the efficiency and the accuracy of numerical simulations.3.Based on the single factor comparison method,effects of the height of the orifice,radius of the cover plate,short half axis of the diffusion section and long half axis of the diffusion section on the hydraulic characteristics are studied.Furthermore,second-order response surface models are constructed from the sample data,and its significance and accuracy are verified.Based on the response surface model,the sensitivity and interactions of the shape parameters to the hydraulic characteristics are analyzed.4.According to the bi-directional characteristic of the vertical pipe intake-outlet,reasonable shape parameters and optimization objectives are determined.Combining the optimization algorithm and multiple surrogate models,a fast and robust shape optimization process is discussed.In order to make the method fast and convenient,an integrated method is adopted in the sample data acquisition process,including geometric modeling,mesh generation and CFD calculation.For the purpose of meeting the requirement of robustness and reliability,the uncertainty of single surrogate model is discussed,and a series of approaches are adopted in the optimization process,including hyper-parameter optimization,multiple surrogates building and comparison among multiple optimization results.5.By comparing and analyzing the optimization results from combinations of the optimization objectives,the optimal shape that can eliminate the reverse velocity and take into account the bi-directional hydraulic characteristics is obtained,and a detailed CFD analysis is carried out.The results show that the hydraulic characteristics of the optimal shape have been greatly improved: the maximum coefficient of inflow velocity is reduced by 14.71%,the non-uniform coefficient of outflow velocity is reduced by 36.01%,and the total head loss coefficient is reduced by 7.47%.The further discussion shows that not only the reverse velocity at the trash rack section has been completely eliminated,but also the development degree of flow separation inside the intake-outlet has been greatly weakened,and the internal flow pattern has been well adjusted.