On the correlation between runner blade dynamic stresses and pressure fluctuations in a prototype Francis turbine

Francis turbines operating in off-design conditions are subject to pressure fluctuations resulting from the development of hydrodynamic instabilities in the draft tube. Depending on the nature of the flow-induced pressure fluctuations (synchronous or convective), this may induce dynamic stresses on the runner blades, increasing fatigue and the risk of crack propagation. This paper proposes to identify the impact of draft tube flow instabilities on the dynamic stresses of Francis turbine runners. Measurements are conducted on a prototype Hydro-Québec Francis turbine from low-load to full-load, including pressure and strain measurements on the stationary and rotating components, respectively. It is first noted that the convective component of the part-load vortex is the main source of excitation for the runner blades. The amplitude of the corresponding dynamic stresses is however reduced at locations closer to the leading edge, for which the dominant fluctuations result from the propagation of synchronous pressure fluctuations. Finally, correlations between runner dynamic stresses and pressure fluctuations measured in water passages are tentatively established for flow instabilities observed at both deep part-load and part-load conditions. This aims to evaluate the feasibility of estimating runner blade dynamic stresses based on signals measured in the stationary components for further investigation.