Concept and hydraulic optimization of a dual-layer runner for ultra-high head ultra-large capacity Francis turbines

For large-scale hydropower stations with multiple generating units, increasing unit capacity is beneficial for reducing investment, shortening construction periods, and improving overall efficiency. In this study, based on the concept of “dual-layer runner for capacity doubling”, a Francis turbine hydraulic design scheme for a hydropower station with the rated head of approximately 700 m and the unit capacity exceeding 700 MW was proposed and verified. This paper introduces the dual-layer design concept and structural configuration of the new turbine. It also examines how runner blade control parameters influence efficiency, proposes measures to reduce pressure pulsations and improve operational performance, and recommends and validates the final optimized runner design. The results show that: (1) under the design head of 700 m, the turbine unit capacity of the preliminary scheme can reach 700 MW with an efficiency of 91.79 %; (2) by positioning the load peaks of the hub and shroud near the mid-blade region, introducing a negative blade inlet stacking, and reducing the blade trailing edge sweep, the blade optimized scheme can achieve an efficiency of 93.25 %; and (3) a 20° stagger arrangement between the upper and lower layers of the dual-layer runner can effectively reduce pressure pulsations by 20–30 %, accompanied by only a slight decrease in turbine efficiency. The gradually improved dual-layer runner scheme successfully accommodates both ultra-high head and ultra-large capacity requirements, demonstrating excellent comprehensive performance in efficiency and pressure pulsation suppression, thereby providing an innovative Francis turbine hydraulic design solution for ultra-high head and ultra-large capacity hydropower stations.