Dynamic Characteristic Study Of Francis Hydro Turbine Blades In Fluid-Structure Interaction

The strong vibrations of Francis hydro turbine unit occurr frequently both at home and abroad due to excitation by flowing fluid, and the problems of blade cracks of a runner with middle and high specific speed are commonly observed. The vibrations are possibly endangering the safety operation of the unit. However, an effective way to solve the problems has been not obtained so far.The interaction between hydraulic turbine blades and viscous flow field is the causes of the vibrations of the blades, and the strong vibrations can lead to the occurrence of fatigue fracture and dynamic loss stability of the blades. Therefore, the study on the dynamic characteristics of the blades is of great significance to understand the flow-induced vibration mechanism, to prevent resonance of the blades, and to ensure safty and stability of the unit.The main works of this thesis are as follows:1. Focusing the problem on transformation of the data of the blade mood-mould drawings of the Francis hydro turbine into a numerical model for a compuer, this paper transformed successfully the data of the traditional wood-mould drawings into a numerical model for auto-generation of the finit element mesh. This data format are convenient for the runner blades geometrc modeling. With data points given on some control profiles, the airfoil of the turbine blade is compeletly produced by the double cubic uniform B-spline surface, and the model are obtained to be applicable to surface fitting of turbine blade. The three-dimensional finit element model of the whole blades are established to take into account the fluid-structure interaction.2. Francis hydro turbine blades and the incompressible fluid are descretized by using Galerkin method and displacement-velocity potential scheme with fluid-structure interaction. The governing equations of the vibration of the coupling system are developed to take into account the influences of the velocity field on the dynamic characteristics of the blades. The computational methods including the added mass were introduced, and the influences of the added mass on the dynamic characteristics were detailly discussed.3. According to the vibrating features of the blades, Francis hydro turbineblades and compressible fluid are descretized by using Galerkin method with fluid-structure interaction. The governing equations of the vibration of the coupling system are derived by the displacement-pressure scheme. The natural frequencies and mode shapes of the whole blades and single blade both in air and in water were calculated, and the comparative analyses were done. Moreover, the effects of the centrifugal force, the hydraulic pressure and the changes of fluid density on the dynamic characteristics of the blades were discussed in detail. The results show that the influence of centrifugal force and of hydrostatic pressure on the dynamic characteristics of the blades are small, and may be neglected, but of the fluid compressibity and of the dynamical pressure must be considered.4. Based on the generalized variational principle, the viscosity of fluid, the boundary conditions of the system and the coupling conditions at the interfaces of the fluid and the structure are introduced into the governing equations of the system by appling the lagrangian multiplier method. A functional which is used to describe the mechanism of the interaction between the runner blades and 3D viscous flow field is proposed. On the base of these works, a mathematical model was successfully established and used to simulate the interaction between the Francis hydro turbine blades and 3D viscous flow field.