Study on the modal characteristics of runner with cracked blade for Francis turbine under the effect of fluid

Abstract The cracks in hydro-turbine runner blades impact turbine unit reliability and stability. The vibration monitoring based on natural frequency variation of hydro-turbine is an accurate and potential online monitoring method. In this paper, the modal characteristics of runner blades with different cracks under flow-induced vibration are studied numerically in different modes. The results indicate that the presence of cracks in the blade leads to a decrease in the natural frequency compared to healthy blades. Furthermore, the natural frequency of the wet mode experiences a significant decrease compared to the dry mode and the prestressed mode. The analysis reveals that cracked blade lead to large frequency difference ( f d ) of the natural frequencies of the runner mainly at the 4th–12th orders in the prestressed mode. For the wet mode, cracks cause large f d of the natural frequencies of the runner mainly at the 3rd–20th orders, but the most significant ones are at the 3rd–10th orders. The frequency-doubling ( f fd ) values of runner blade with crack under the 4th–12th orders in the prestressed mode or under the 3rd–10th orders in the wet mode are concentrate on 0.3–1.0. The results of modal analysis can provide a valuable reference for the crack detection of turbine runner.