| Energy storage technology is a technology that stores electric energy in an appropriate form when the load of power grid is low and releases it when the load of power grid is high.The advantages of peak cutting and valley filling,load smoothing and load fluctuation mitigation can improve the power supply stability and reliability of power system.Pumped storage is widely used in the field of energy storage all over the world because of its advantages of large capacity,high efficiency and mature technology.Small-scale pumped storage can overcome the constraints of long construction period,large initial investment and high demand for natural conditions of conventional pumped storage power stations,and can complement distributed energy sources.At present,its application is developing rapidly.Pump-turbine is the key component of two-unit reversible pumped storage unit.Its performance directly affects the operation effect of the whole pumped storage unit.According to the fundamental principle of turbomachinery,the hydraulic design and analysis of pump-turbines suitable for small-scaled pumped storage systems were carried out by using the design software of turbomachinery and the numerical calculation method.The design included impeller,front and back pump chambers,volute and draft tube.The loss analysis of pump-turbines under different conditions was carried out by using the numerical simulation method.The influence of sealing structures and different clearance widths on the performance and internal flow of pump-turbine was studied.Afterwards the related optimization of impeller on performance and internal flow were studied.The main research contents were as follows:1.Research on hydraulic design method and internal flow loss analysis of pump-turbine.Aiming at the unit requirements of a small pumped storage system,the design of a pump-turbine was studied.Based on the binary design theory,hydraulic designs of the impeller,volute,pump chamber and draft tube were carried out.The flow of the design results under different conditions were numerically simulated.Based on the simulation results,the total pressure loss of each component of pump turbine was analyzed,and the components with concentrated total pressure loss were determined.The total pressure loss at pump mode mainly concentrates in the front pump chamber and impeller,while at turbine mode,the total pressure loss mainly concentrates on the impeller.2.Leakage flow analysis of impeller sealing ring.The flow characteristics of leakage under different conditions were studied.The influence of leakage flow on the performance of pump and turbine modes under different sealing structures and clearance widths was compared.It was found that under the same sealing structure and clearance width,the influence of leakage flow on the performance of turbine mode was greater than that of pump mode.The two kinds of sealing structures' total pressure loss of each component and the relationship between leakage and impeller flow at pump and turbine mode were studied.The influence and reason of the mixing of leakage flow and mainstream flow at the impeller inlet at pump mode were analyzed in detail.When the width of clearance was 0.2 mm,the maximum entropy of the two sealing structures happened in the front pump chamber.The leakage was small and the sealing was good.The increase of circumferential absolute velocity at mixing position proved that mixing causes circumferential secondary flow.Friction loss and impact loss increased with the decrease of liquid flow angle and relative liquid flow angle respectively.When the width of clearance was 0.2 mm,the impact loss at the inlet of impeller caused by leakage flow was smaller.3.Research on optimization design of key hydraulic components of pump-turbine.Orthogonal experimental design method was used to optimize the design of impeller with concentrated total pressure loss.Efficiency of pump design mode,the rate of head offset and maximum efficiency of turbine mode were selected as test objectives.Impeller outlet diameter,impeller outlet width,blade outlet angle and blade wrap angle were selected as test factors,and three test levels were selected for each factor.Through numerical simulation,range analysis and variance analysis,the analysis and optimization were carried out.Finally,comparing the numerical results with the prototype,it was found that the efficiency at pump design mode was 86.01%and the head was 17.44 m after optimization,which was 0.89%higher than that of original pump-turbine,and the flow in impeller was more uniform.The maximum efficiency of turbine mode was 1.19%higher than that before optimization,and the flow condition of impeller inlet was improved.The performance curves of the two modes shifted to the small flow conditions due to the increase of blade wrap angle after optimization. |
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