Investigation into cavitation damage potentiality using pressure pulsation phenomena in a low head Francis turbine for small hydropower schemes

Abstract

This study investigated unsteady pressure pulsations in a low-head Francis turbine under the effects of strong cavitation for different plant discharge factors and submergence levels of the draft tube at normal water temperature. The effect of the submergence water level of the draft tube at a dominating frequency was found to be negligible underrated-load and upper-part-load plant-operating conditions. However, the amplitude increased as the submergence of the head of the draft tube increased. At a high plant discharge coefficient, i.e., under overload conditions, the maximum amplitude was found at the blade-passing frequency. Additionally, a cavitation region was observed in the vicinity of the blades of the Francis runner, indicating the start of cavitation damage. Moreover, the results revealed the characteristics of low-frequency high amplitudes, which were predominant in the draft tube for the lower-part-load condition. According to the relevant literature, such amplitudes can cause instability in the hydraulic system of a powerplant, potentially resulting in fatigue-related damage to the turbine during operation. This study's predicted numerical results indicated the potentiality of cavitation damage in low-head Francis runners for different plant-operating conditions.