A partial cross section (meridional cross sectional view) illustrating a hydroelectric power station provided with a Kaplan water turbine which is a typical axial flow water turbine is shown in FIG. 8. Water current that flows from the upstream into a casing 1 passes through a stay vane 2, and flows through a guide vane 3 having the open/close function for adjusting the flow rate of water, and reaches a runner that is coupled with an electric power generator by means of a main shaft. This flowing water causes a runner to rotate about a water turbine rotation axis 5. The runner has a runner boss 10 and multiple runner vanes 4 attached thereto. Accordingly, the electric power generator is rotated and electric power is generated. The flow that has flown out of the runner passes through a draft tube, and is discharged to the downstream or a lower reservoir. The runner vanes 4 of the Kaplan water turbine rotate about the vane rotation axis 6 in accordance with the flow rate of water.
There is a gap between a runner vane 4 and a discharge ring 9, and therefore, there is leakage flow passing through the gap. Because of the rate of the leakage flow, the fluid force exerted on the runner vane 4 cannot be collected, and when the leakage flow is large, the loss increases.
When the runner vanes 4 do not overlap each other when the runner vane is seen in the section perpendicular to the water turbine rotation axis 5, through flow that does not exert on the runner vane 4 is generated at the outer peripheral side (chip portion side) where the velocity of the flow is particularly fast. Because of the influence of the leakage flow due to the gap between the runner vane 4 and the discharge ring 9 and the through flow, the velocity of the flow in the exit of the runner vane 4 is likely to be disturbed at the chip portion side, the disturbed flow cannot sufficiently recover pressure in the draft tube, and the performance decreases. In FIG. 8, hatching in the chip portion of the runner vane 4 indicates disturbance of vane surface flow caused by the leakage flow.
In FIG. 8, the posture of the runner vane 4 corresponding to the design point is indicated by a chain double-dashed line, and the posture of the runner vane 4 corresponding to high flow rate operation point is indicated by a solid line. In FIG. 8, the external peripheral end surface 7 (this is a spherical surface) of the runner vane 4 that is at the posture in the design point when seen in the direction of the vane rotation axis 6, is indicated by a chain double-dashed line (denoted with reference numeral 7A), and the external peripheral end surface 7 that is at the posture in the high flow rate operation point is indicated by a solid line (denoted with reference numeral 7B). When the operability in the disassembly and assembly of the runner vane 4 is considered, the discharge ring 9 is manufactured, in most cases in recent years, so that the upstream side inner peripheral surface is a cylindrical surface, and the downstream side inner peripheral surface is a spherical surface. Therefore, the size of the gap G between the runner vane 4 and the discharge ring 9 is large at the upstream side, and is larger when the runner vane 4 is at the posture in the high flow rate operation point than when the runner vane 4 is at the posture in the design point. For this reason, the loss due to the leakage flow explained above is high in the high flow rate operation point, and this is the main cause why the efficiency is reduced in the high flow rate operation point.
Due to the influence of the centrifugal force, the flow is likely to deviate to the external peripheral side (chip portion side), and in addition, the velocity of the flow is high at the external peripheral side, and therefore, the pressure is reduced at the negative pressure surface (back surface) of the runner vane 4. For this reason, cavitation is likely to be generated at this portion where there is the gap between the runner vane 4 and the discharge ring 9, and cavitation erosion is likely to be generated. In order to extend the life of the runner vane 4, it is important to suppress the cavitation and to reduce the loss that causes the reduction of the performance.