Onboard Measurements of Pressure and Strain Fluctuations in a Model of low Head Francis Turbine. Part 2 : Measurements and Preliminary Analysis Results

In this second part of a 2 papers series, we present the analysis of “onboard” measurements on the runner of a Low Head Francis model and comparison with the same type of measurements on a similar prototype, as well as a comparison with numerical calculations. The details of the experimental procedures for onboard measurement of pressure and strain fluctuation in the model runner are presented in the first part (Ref. 1). Load fluctuations on the blade of a Francis runner depend largely on the different hydrodynamic phenomena that may be present. For instance, the partial load rope vortex, blade trailing edge Karman vortices and any other cavitating occurrences at off-design operating points. Almost all of these phenomena can lead to fatigue failures, which translates into the interest of better understanding their behavior and interaction with the structures. The present paper shows how onboard measurements can be a valuable source of information on these phenomena and how it brings solutions to the evaluation of the fatigue life of turbine components. The good comparisons between the prototype and model results illustrate that the phenomena can be observed at both scales, although the transposition of dynamic component of pressure or stresses, is not yet feasible. The also very good correlation between measurements and numerical calculations of the flow and of the structural response of the runner to the static loading, enables us to progress to the next step, that is, dynamic analysis and fatigue life prediction with reliable data. Finally, we will show also that such measurements can be very useful in the development of numerical calculation tools used for performance optimization.