Study On Instability And Improvement Strategy Of Vortex Rope In Francis Turbine

Under the background of energy transformation in the world and the long march of science and technology in China,the energy industry is facing the transformation from extensive resource-intensive operation mode to efficient technology-intensive operation mode,which means that relevant upstream industries and related research should enhance innovation and ensure reliability.Hydropower,as a mature renewable energy,should ensure its reliability when it is integrated into the power grid.Therefore,the vibration of the turbine unit should be avoided and the stability of the whole unit should be maintained.Pressure fluctuation is one of the important reasons for the instability of the unit,especially the low frequency pressure fluctuation in draft tube is one of the important reasons for the severe vibration of the unit.When the turbine runs in off-design condition,the main source of pressure fluctuation is the tailrace vortex rope.In some cases,the pressure fluctuation caused by the tailrace vortex rope may even destroy the whole unit or even the factory building.Under different working conditions,the shape and development law of vortex band in draft tube show different characteristics,which becomes an important factor restricting the stable operation of hydraulic turbine.Therefore,a large number of scholars have done in-depth research on the formation mechanism,development characteristics and improvement measures of the tailrace vortex band,trying to find a reasonable and feasible way to intervene in the vortex band,reducing the pressure fluctuation and strong vibration caused by the vortex band.In this paper,by means of theoretical analysis,model experiment and numerical simulation,the characteristics of internal flow field in each part of draft tube are analyzed in detail.The formation mechanism of draft tube vortex band and the influence of runner cone modification on vortex band characteristics,instability and pressure fluctuation are studied.An efficient and accurate method is trying to be sought to improve the vortex band and weaken the disadverse effects of vortex rope's rotation,promoting the development of hydropower industry.(1)Combining with the videos obtained from the model test of hydraulic turbines carried out on the hydraulic test bench,the different forms of vortices and their development laws produced by the operation of hydraulic turbines under different operating conditions are analyzed.The operating characteristic curve of the turbine is divided into four parts: non-vorticity area,fragmentation area,rotating vorticity area and column vorticity area.Under the opening of 16 mm,the rotating vortex rope has a typical helical structure.The experimental data are processed by Fourier transform.It is found that the shape of vortex band can be changed and the pressure fluctuation of draft tube can be affected by the modification of runner cone.(2)Use the steady-state numerical simulation method to explore the formation mechanism of the vortex band and analyze its instability.In the rotating vortex region,because of the velocity circulation and pressure gradient at the runner outlet,the spiral visible vortex band can be formed,and the flow state at the draft tube inlet is relatively unstable.(3)By comparing the influence of different structure types of runner cone,it is found that the special-shaped runner cone is beneficial to the axial flow of the fluid,which makes the vortex band stretching and straightening,while the long straight runner cone will reduce the axial direction to a certain extent.(4)The unsteady calculation under 16 mm guide vane opening condition can confirm that the low-frequency pressure fluctuation of draft tube is from the vortex band,and the amplitude of the fluctuation is positively correlated with the rotating radius of the spiral vortex band.The possible high frequency pulsation at the draft tube inlet is from static and dynamic interference,but it will gradually disappear after entering the draft tube.The modification of the runner cone has a slight effect on the main frequency of fluctuation.The dynamic and static interference can be greatly weakened by the foundation runner cone and the special-shaped runner cone,and the effect of the long straight runner cone is better in reducing the amplitude of pressure fluctuation.