Dynamic Analysis of Francis Runners - Experiment and Numerical Simulation

Abstract

The present paper shows the results of numerical and experimental modal analyses of Francis runners, which were executed in air and in still water. In its first part this paper is focused on the numerical prediction of the model parameters by means of FEM and the validation of the FEM method. Influences of different geometries on modal parameters and frequency reduction ratio (FRR), which is the ratio of the natural frequencies in water and the corresponding natural frequencies in air, are investigated for two different runners, one prototype and one model runner. The results of the analyses indicate very good agreement between experiment and simulation. Particularly the frequency reduction ratios derived from simulation are found to agree very well with the values derived from experiment. In order to identify sensitivity of the structural properties several parameters such as material properties, different model scale and different hub geometries are numerically investigated. In its second part, a harmonic response analysis is shown for a Francis runner by applying the time dependent pressure distribution resulting from an unsteady CFD simulation to the mechanical structure. Thus, the data gained by modern CFD simulation are being fully utilized for the structural design based on life time analysis. With this new approach a more precise prediction of turbine loading and its effect on turbine life cycle is possible allowing better turbine designs to be developed.

Failure investigation of a large pump-turbine runner

“…The main excitation forces generated while the machine was in operation were applied to the runner. This is known as dynamic behavior and it allows the structural response of the runner to be calculated [1,2]. The main dynamic forces that act on the runner are described below.…”

Analysis of the dynamic response of pump-turbine impellers. Influence of the rotor

“…Other publications have presented detailed experimental and numerical analysis considering the effect of surrounding mass of water [5][6][7] but in a reduced scale model and without taking into account the boundary conditions of the casing. Other publications about dynamic behavior in Francis turbines [8] do not give details about the impeller response.…”

Selection of Optimal Number of Francis Runner Blades for a Sediment Laden Micro Hydropower Plant in Nepal

“…Current technology using Computational Fluid Dynamics (CFD) is capable of design optimization of the hydraulic turbines both statically and dynamically [5,6]. Fluid Structural Interaction (FSI) analysis is performed for the structural analysis of the runner after it is hydraulically designed [7,8].Wang et al [9] demonstrated that reducing comprehensively the number of runner blades can increase the turbine capacity and found 13 blades is the optimal choice for the case of a small hydropower plant.…”