# Optimal Design and Fish-Passing Performance Analysis of a Fish-Friendly Axial Flow Pump
## 4 Optimal Design and Hydraulic Performance Analysis of a Fish-Friendly Axial Flow Pump
### Optimal Design of Leading-Edge Guiding Characteristics of the Blade  
When the runner rotates, the linear velocity of the leading edge of the blade is proportional to the radius. When the speed of the runner is constant, the position of the fish entering and leaving the runner is closer to the hub side, and the impact velocity is smaller. Therefore, the effective way to reduce the impact speed of fish is to make the fish enter the flow channel from the inside of the runner as far as possible. For the runner, the cutting blade can make the entrance of the runner have a certain radial velocity, so that the position of the fish entering the runner is closer to the hub side, thus reducing the death rate in the event of an impact. Aiming at an axial flow pump whose blade leading-edge thickness was 11.4 mm, the leading edge of the blade was obliquely cut to different degrees, and three kinds of runners with different blade cutting degrees were designed. The angles between the leading edge and the radial direction of runner blades in different schemes were \(6^{\circ}\), \(12^{\circ}\), and \(18^{\circ}\). When the blade angle becomes larger, the total area of the blade decreases, the throat area at the entrance of the blade increases slightly, the length of the airfoil bone line on the hub side increases, and the length of the flange wing bone line decreases. As the output power of the runner changes with the change in the total area of the blade, it brings an unnecessary characteristic offset to the runner. To keep the output power of the runner stable, other control parameters should be considered at the same time to avoid great changes in the effective work area of the blade. To control the blade area, the cutting blade was properly extended to both sides of the inlet and outlet on the axial projection, and the gap between the total blade area of each scheme was narrowed. Finally, the total leaf area difference of each scheme was controlled within 0.01 m\({}^{2}\) (about 0.4% of the total area). The appearance after cutting and the adjusted results are shown in Figure 10.  
According to Table 6, We used the comparative analysis method to comprehensively analyze the operation characteristics and flow field characteristics of the axial flow pump under different blade cutting angles and determine the better blade cutting angle. Figure 10: Different blade cutting angles and schematic diagram of axial projection. After the blade was cut at different degrees, the head of the axial flow pump increased at first and then decreased. This showed that the hydraulic loss in the runner before and after blade cutting was not linearly coordinated with the design's cutting angle. With the increase in blade cutting angle, the blade inlet pressure first decreased and then increased, and the outlet pressure increased gradually, but the range of change was not more than 1.2 kPa, and the total pressure difference between the inlet and outlet of each scheme changed little. In terms of output power and efficiency, the runner with a blade cutting angle of 12\({}^{\circ}\) had the highest output shaft power and the lowest runner efficiency, but the overall efficiency was slightly higher than that of 18\({}^{\circ}\) runners. The results showed that there were more significant differences in different parts of the internal flow field. As shown in Figure 11, the negative pressure area below \(-40\) kPa increases at first and then decreases with the increase in blade cutting angle. The excessively high negative pressure area in the suction chamber increases the negative pressure injury time of fish, which is not conducive to the passage of fish. From the velocity distribution map, it can be seen that the leading edge of the cutting blade has a negative effect on the downstream flow pattern, and the fluid enters the state of separation from the mainstream earlier, which widens the downstream area surrounded by the velocity gradient line.