Study On Mechanism Of The Development Of The Cavitation Vortex Rope In The Francis Turbin

Under the severe test of global warming and environmental pollution,exploring and developing clean energy has become the consensus of all mankind.The hydropower,a traditional clean energy,has the advantages of low cost,high efficiency,no pollution,and renewability.At the same time,the frequency modulation and peak load regulation capacity of pumped storage units have received increasing attention with the development of nuclear energy,wind energy and other clean energy.Nowadays,with the further research and development of high-head and ultra-high-head turbines,the operational stability of hydropower units has been paid more attention.Pressure fluctuation is the main reason for the instability of hydropower units,and its causes and control methods have always been one of the focuses of research.The generation and development mechanism,influencing factors and control methods of the draft tube vortex rope,which can directly cause low-frequency pressure fluctuation in the draft tube,have received extensive attention and research from a large number of scholars.Based on Francis turbine,this paper uses the research method that uses the model test as the basis and the numerical simulation as an extended.Two typical operation conditions of partial load and full load are selected for numerical simulation.The generation and development mechanism of cavitation vortex rope in the draft tube,the influence of cavitation coefficient and swirling number on the appearance,internal flow field,stability and entropy production loss,as well as the influence of the punching and slotting treatment on the runner cone.This paper provides reference and guidance for the design of the Francis turbines for practical engineering applications,and lays the foundation for the subsequent research on the cavitation vortex rope.The main research contents of this article are as follows:(1)Based on the results of the model test,the flow regime image and the pressure data of the pressure sensors in the draft tube has been processed.And the flow regime and cavitation vortex rope in the draft tube under various operation conditions of the model test has been analyzed in detail.The Fourier change of the data was used to obtain the frequency domain signal of pressure fluctuation,and then the time-frequency analysis was carried out.The results showed that the flow rate and the rotating speed of the runner have a synergistic effect on the shape of the cavitation vortex rope in the draft tube.Punching and slotting has a certain influence on the shape of the beginning of the spiral cavitation vortex rope and pressure fluctuation.(2)Based on the typical operation condition in the rotating vortex area,4 types of runner cones were selected for 4 sets of unsteady numerical simulation,to explore the effect of punching and slotting treatment on the pressure fluctuation in the draft tube.The results show that punching on the drain cone has no effect on the frequency of the pressure pulsation of the draft tube,but it will increase the corresponding pressure fluctuation amplitude,while the slotting will reduce the amplitude.(3)Based on the typical operation condition in the column vortex area,the SS runner cone was selected for single-phase and multi-phase numerical simulation,and the similarities and differences of two are compared and analyzed on the aspects of external characteristics,vortex rope shape and internal flow field.Further analysis of the multiphase results reveals the mechanism of generation and development of the cavitation vortex rope,and the stability analysis at different positions in the draft tube shows that continuous forced disturbance is required to control the shape of the cavitation vortex rope.(4)Based on the operation conditions in the rotating vortex zone and the column vortex zone,the multi-phase numerical simulation has been carried out with different cavitation coefficient and the swirling number to explore the influence of them on the aspects of cavitation vortex belt shape,internal flow field and entropy production loss.The results show that the cavitation coefficient and the swirling number have a great influence on the shape of the cavitation vortex zone,the entropy production distribution in the draft tube and the total entropy production.