Détection vibratoire de l'érosion de cavitation des turbines Francis

Cavitation erosion of Francis turbines continues to impose costly repairs and loss of revenues. This erosion is primarily caused by leading edge cavitation. A revue of the literature reveals the non-existence of an absolutely validated detection technique of this cavitation in operating Francis turbines. Considering the impulsive character of the implosion of a cavitation structure, the assimilation of cavitation erosion to a process of mechanical fatigue of materials and the vibratory response at high frequencies of a linear structure to multiple random excitations whose responses add on a power basis, we have proposed that the vibratory response of a Francis turbine at high frequencies is tightly related to its cavitation erosion rate. The absolute measurement of cavitation aggressiveness on the blades can be made by the normalization of lower guide bearing vibration measurements by an average Transmissibility function measured by reciprocity with the runner underwater. The proposed method has been validated and it is now possible to state that the MSV of inferred forces at the cavitation impact rate of hydrodynamic origin is proportional to the erosion rate. The approach has been validated in excellent manner in laboratory set-ups with little or no flows by comparison to absolute cavitation aggressiveness assessment methods such as the DECER electrochemical technique and the pit counting technique on polished material samples. The validation was then pursued with laboratory homologous flows on a NACA 009 profile in the LMH high-speed cavitation tunnel with blade mounted DECER sensors as reference. Once again, impact frequencies of hydrodynamic origin, following here a Strouhal law, were observed and the MSV of the profile acceleration at high frequencies was proportional to the average erosion rate on the profile. Notwithstanding the difficulty of the measurements, the results were excellent and fared well with the erosive power of the flow in V3L2 (V= flow velocity, L= length of the attached vapour cavity). Preliminary measurements on a Francis model with blade mounted DECER sensors agreed well with the MSV of lower guide bearing acceleration at high frequencies. Later measurements on the Rapide Blanc model confirmed the damage model observed on the NACA profile. Hydrodynamic impact frequencies were however not governed by the tunnel Strouhal law but were found to be determined by the blade passage frequency. Stationary measurements at the lower guide bearing were equivalent to runner on board measurements. Reciprocity, typical to linear structures, was verfied. As on the profile, maximum aggressiveness on the runner blade occurs in the leading edge cavity closure area. This was confirmed by the application of "Prescale" pressure sensitive film on the blades for operating conditions generating maximum vibratory response at the cavitation hydrodynamic frequencies. High-speed video recordings shot at 2000 frames/s allowed detailed observation o