Generation Mechanism of the Impulsive Pressure Rise during Start-up on Blade at High Head Low Specific Speed Francis Turbine

Abstract

Extraordinary stress on runner blade during start-up at the low specific speed Francis turbine cause severe damage to the blades. To make clear the mechanism of the extraordinary stress, CFD analysis in addition to the measurement of the model and the prototype turbine was carried out. By the analysis of the model and the prototype turbine, it became clear that the phenomenon is commonly arisen as the transient hydraulic behavior, and the impact load causing extraordinary stress on runner blades is related to the impulsive pressure rise. From a comparison of CFD and model test result, it is concluded that the impulsive pressure rise was caused by the result of chain-reaction of the cavitation cloud in the runner passage.