Dynamic stress assessment in high head Francis runners

The deregulation of the energy market in the recent years has lead to an increased willingness to operate hydro turbines over a larger operational range and with increased number of start and stops. This means that the turbine will be subject to a more frequent and larger load variation. Especially for high head turbines, the impact of dynamic forces has been of considerable interest. High head turbine runners are subjected to dynamic forces originating from various sources. One significant source is the interaction between the runner and guide vanes, also called rotor - stator interaction (RSI). RSI creates periodic pressure fluctuations in the runner. Combined with stress concentration and material defects, the induced pressure fluctuations can give an increased risk of fatigue cracks. For Francis runners, the areas near the trailing edge of the runner blade towards the band and/or crown has been identified as critical areas for fatigue, and for high head runners there has been several examples where fatigue failure have been initiated near the trailing edge toward the band. The thesis presents an estimation of dynamic stresses in a high head Francis runner. The stresses were estimated by Finite Element analysis based on pressure variations obtained via unsteady CFD analysis. The dynamic stresses calculated by FEA were compared with dynamic stresses measured by strain gauge measurements carried out on a prototype runner. A CFD result for one operational condition was calculated. Since the pressure variations on the blades are periodic, only a pressure load for one whole cycle is needed for the FEA. Symmetry between the number of runner blades and guide vanes made it possible to reduce the computational model and thereby reduce computational time. The Finite Element model contained a portion of the crown and band and two runner blade channels. The FEA was carried out for time steps corresponding to a full periodic cycle for one operational condition, i.e one guide vane opening angles. The results were compared with site measurements.Validerat; 20101217 (root