Numerical investigation of blade tip cavitation on large discharge condition of diagonal turbine

A series of numerical simulations was carried out to estimate the cavitation characteristics for a diagonal flow turbine. The interaction between the tip leakage flow and the cavitation bubbles was investigated using the obtained numerical data. The evaporation and condensation process for cavitation phenomenon was described by using a simplified Rayleigh-Plesset equation. A two-phase homogeneous model was adopted to calculate the mixture of gas and liquid phases. Three types of cavitation with different origins were obtained around the blade tip region. The incipient cavitation was well-known tip clearance cavitation. In the present study, the occurrence of tip clearance cavitation was limited in the position where the clearance between blade tip and discharge ring spread, irrespective of Thoma number. Tip clearance cavitation was found out not to contribute to the decrease of the turbine efficiency. On the contrary, the turbine efficiency increased with the growth of tip clearance cavitation. When Thoma number decreased, the other type cavitation arose on the discharge ring surface. The development of the discharge ring cavitation led to the decrease of the turbine efficiency. In addition, so-called tip vortex cavitation obtained in the lower Thoma number condition than the appearance of discharge ring cavitation. These three types of cavitation were also observed in the experimental approach. The correlation between the decrease of the turbine efficiency and the development of discharge ring cavitation was also verified in the model test.