Hydrodynamic Characteristics And Load Ratio Control Of Bidirectional Contra-Rotating Axial Flow Pump-Turbine

With the proposal of carbon peaking and carbon neutrality goals,China has accelerated the process of energy transformation.With relative stability and high energy density,tidal energy is one of the most promising clean energy technologies of the future.Conventional tidal power plants only have bidirectional turbine mode,but in order to improve the economic efficiency of tidal power generation by peak shifting and peak generation,bidirectional pump mode should be added.Contra-rotating machinery has the advantages of high efficiency,a wide high efficiency zone and a compact structure.In addition,it can be designed with a symmetrical two-stage impeller,which can have the same bidirectional hydraulic performance.However,the interaction between the two impellers will lead to the problem of high load ratio on the two impellers in actual operation,which is not conducive to the requirements of long-term stable operation of the unit.To address these issues,this paper adopts the streamline method for the design of bidirectional contra-rotating axial pumps and investigates the influence of impeller geometry and operating parameters on the external characteristics,internal flow characteristics and operational stability of the unit based on load ratio control.The relevant research results provide an important theoretical basis and technical support for the independent development of contra-rotating axial flow pump-turbines.The main research contents and the creative results achieved in this paper are as follows:(1)In order to verify the accuracy of the numerical simulation results of the counter-rotating unit,a contra-rotating axial flow pump-turbine(under pump mode)test bench was built,and the pressure collected at the position between the two impellers during the test was corrected by combining the numerical simulation calculation results,reducing the error caused by the unstable flow between the two impellers for similar conversion of the test results.The hydraulic design of the contra-rotating axial flow pump is carried out using the streamline method and the effect of different contra-rotating impeller arrangements on the performance of the unit is analyzed.The results show that the design parameters of the front impeller can be maintained similar to those of the single-stage axial flow pump,and the same hydraulic model with different rotational directions is used for the rear impeller.The final numerical simulation results are in good agreement with the experimental results.(2)In order to achieve the load ratio control of the contra-rotating unit,this paper adopts the contra-rotating unit scheme with unequal number of blades and the control of single-stage impeller speed.Numerical simulations are carried out for the contra-rotating units with equal and unequal number of blades in both pump and turbine conditions,and the effect of the change in the number of impeller blades on the external characteristics of the unit is analyzed.On this basis,variable speed is adopted as the load ratio control means of the unit and its effect on the unit performance is analyzed.The results show that,under pump mode,reducing the speed of the rear impeller can improve the unit efficiency and reduce the load ratio.Under turbine mode,increasing the rear impeller speed can achieve higher efficiency,and will lead to a decrease in the unit load ratio.Increasing the number of rear impeller blades requires a higher rear impeller speed than increasing the number of front impeller blades to reach the high efficiency point.(3)In order to obtain a more accurate flow field simulation,a refined grid with Y+ less than2.23 and a total grid node number of 23.9 million was used to continue the numerical simulation.Based on the entropy theory,the internal hydraulic loss distribution of the contra-rotating unit under different conditions was studied for different schemes,and the changes of entropy hydraulic loss inside the impeller at each stage before and after the load ratio control are compared and analyzed.The results show that the area of high entropy inside the impeller is mainly located at the back of the blade near the outlet side and the trailing area,as well as the area where the leakage vortex at the top of the blade.(4)In order to study the transient characteristics of the contra-rotating unit before and after load ratio control,an SST-DES model is used to carry out transient numerical simulations of the contra-rotating axial flow pump-turbine.The pressure pulsation analysis is carried out using twostage impellers with different blade numbers to reveal the mechanism of dynamic-dynamic interference between the two impellers of the contra-rotating unit.The analysis compares the pressure pulsation characteristics inside the contra-rotating unit with the fluid forces on the different stage impellers under different load ratios.The results show that increasing the number of rear impeller blades and decreasing the rear impeller speed can significantly improve the radial force vector pulsation of the unit under pump mode;under turbine mode,increasing the number of impeller blades leads to an increase in the radial force vector pulsation of the unit at every stages.Increasing the speed of the rear impeller reduces the radial and axial forces on the rear impeller,while the center of mass of the radial force trajectory will move towards the origin.(5)In order to construct an optimization strategy for the bidirectional contra-rotating axial flow pump-turbine based on load ratio control,this paper uses orthogonal tests and correlation analysis to study the influence of the main impeller design variables on the external characteristics of the unit.In turn,the parameters with a high correlation to the unit’s load ratio and efficiency are extracted to achieve dimensionality reduction of the impeller design variables of the contra-rotating unit.A neural network is used to establish an approximate function model between the geometric parameters and the target parameters of the optimization.A genetic algorithm is used to find the optimum.The results show that by selecting the impeller exit angle,inner streamline wrap angle and outer streamline wrap angle for optimization,the optimized model can reduce the shaft power of the rear impeller while ensuring high efficiency.In summary,the hydrodynamic characteristics and load ratio control of bidirectional contrarotating axial flow pump-turbine are studied in this paper.The flow line method is used to design the impeller of the pump-turbine,the flow mechanism and evolution law of the unit are analyzed,and the test bench of the unit under the pump mode is built,and the numerical simulation results of the same model are verified.This paper focuses on the influence of geometric parameters and operating parameters of impeller based on load ratio control on the external characteristics,internal flow characteristics and operating stability of the unit.An optimization strategy of the contrarotating axial flow pump-turbine impeller based on multi-parameter and multi-objective genetic algorithm is constructed,and a set of impeller hydraulic model with high performance is obtained.