Analysis Of Flow Field And Unidirectional Fluid Structure Coupling In Small Opening Of Guide Vane During Start-up Of Francis Turbine

The start-up process of hydraulic turbine often causes vibration of the unit and damage of runner blades,which also leads to poor performance and even threatens the safe and stable operation of the unit.In this paper,the Francis turbine is taken as the research object,and the numerical analysis is carried out for the small opening operating point of the guide vane during the start-up process.The influence of different guide vane opening on the performance of hydraulic turbine is studied.A total of 10 operating points with guide vane opening of 1.8°,2.2°,2.7°,3.2°,3.7°,4.2°,4.7°,5.2°,5.7 ° and 6.1 ° are selected to analyze the variation law of internal flow and runner stress.In order to study the influence of the start-up process of Francis turbine on the hydraulic performance and structural characteristics of the unit,the geometric model of the whole flow channel of Francis turbine was established.Based on SST Turbulence model,sequential coupling method and prestressed modal analysis,the numerical simulation and one-way fluid structure coupling calculation of Francis turbine were carried out.During the start-up process,with the increase of guide vane opening,the high pressure area in the volute and guide vane gradually extends to the runner area,the maximum pressure of the runner blade first decreases and then increases,the eddy viscosity in the runner increases,and the areas with large eddy viscosity are concentrated in the discharge cone and runner outlet,the maximum velocity in the vaneless area first decreases and then increases,and the obvious flow separation phenomenon appears in the draft tube.The results of fluid calculation are loaded into the solid domain for fluid solid coupling.It is found that the maximum equivalent stress of runner is mainly distributed in the connection between blade and ring and upper crown,and the maximum deformation of runner is mainly located in the part of blade near upper crown.With the increase of guide vane opening and unit flow,runner stress increases gradually.The unidirectional fluid structure coupling calculation of Francis turbine was also carried out at three operating points with large opening of guide vanes of 19.7°,23.5 ° and 31.5°respectively.The stress-strain characteristics of runner structure with small opening of guide vanes were compared with the former.It is found that the maximum equivalent stress of runner with small opening of guide vane during start-up is smaller,while the maximum deformation is larger,which will have adverse effects on the structural characteristics of turbine blade during start-up.When the prestressed modal analysis of runner with different guide vane opening is carried out,it is found that the mode of vibration of runner with different guide vane opening are almost the same.The research content of this paper can provide a certain reference for the further study of the characteristics of Francis turbine during start-up.