Study on Condition for Cavitation Occurrence in Cross-Flow Water Turbine and Its Effect on Turbine Performance

抄録

Cross-flow water turbines are prone to cavitation owing to their short runner vane chord and high vane loading per unit area. In this study, experiments and computational fluid dynamics analysis (CFD) were conducted to clarify the effects of the effective head and net positive suction head available (NPSHa) on cross-flow turbine performance. Flow observations revealed that the critical conditions for cavitation in each flow area could be expressed by a linear equation of the NPSHa and effective heads. However, these lines all passed through a point of NPSHa =1.15m instead of passing through a point of NPSHa =0m when the effective head was zero. Furthermore, it was inferred that bubbles of cavitation grew using the released air bubbles as the nuclei. In the case of low NPSHa, air was suctioned into the region where cavitation had occurred without intake air, therefore cavitation was reduced with intake air. Under experimental conditions in this study, the ratio of the volume of the released air and water vapor mixed gas, up to reaching an equilibrium state in the static pressure in which the effective suction head, is 1.15 m to the volume of water is theoretically 0.14. This result was also shown to influence the evaluation of the cavitation inception concerning the water flowing through the turbine.