## 7 Conclusions  
By comparing the flow pattern, impeller performance, internal flow field distribution, and pressure pulsation, the following conclusions can be drawn:  
1. At partial-load (0.25 \(Q_{d}\) in this study), incoming flow struck on the blade leading-edge on the pressure surface. Flow separation happened on the blade suction surface and induced the passage blockage. In this case, an alternating stall pattern was found in the impellers. In the six blade passages, three passages were well-behaved, and three passages were stalled (flow-blockage caused by backflow structures). The well-behaved and stalled passages distributed alternately and cite-fixed during rotation. 2. Leading-edge shape strongly influenced the alternating stall pattern and showed differences. In the blunt and sharp leading-edge impellers, the leading-edge geometry had sudden turned corners. Flow direction suddenly changed at the corner with large-scale separation. The stalled passages were completely blocked. Fluid went into the well-behaved passages. The flow pattern in the well-behaved passages was very smooth, without undesired flow structures, meaning that only three passages were accessible. In the ellipse and round leading-edge impellers, the geometry continually changed on the arc or elliptical-arc without sudden turning. The backflow scale in the stalled passages was smaller with slight accessibility because the leading-edge separation was \begin{table}
\begin{tabular}{c c c} \hline \hline
**Leading-Edge Shape** & **Main Frequency \(f_{m}\) (Hz)** & \(f_{ml}\)/\(f_{rt}\) (Rotating Frequency) \\ \hline Blunt & 9.67 & 0.8 \\ Sharp & 0.81 & 0.067 \\ Ellipse & 2.41 & 0.2 \\ Round & 1.00 & 0.083 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main/dominant frequency value in the impellers with different leading-edge shape. \begin{table}
\begin{tabular}{c c} \hline \hline
**Leading-Edge Shape** & **Obvious Frequencies/Frequency Bands** \\ \hline Blunt & \(f_{mb}\), 3\(f_{mb}\), 5\(f_{mb}\), 14.5–20.7 \(f_{mb}\) and 36.2–39.3 \(f_{mb}\) \\ Sharp & \(f_{ms}\), 2\(f_{ms}\), 3\(f_{ms}\) and 4–6\(f_{ms}\) \\ Ellipse & \(f_{me}\), 2\(f_{me}\), 3\(f_{me}\) \\ Round & 0.33–1.35 \(f_{mr}\) (including 1.0 \(f_{mr}\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Obviously seen frequencies and frequency bands in different leading-edge shape impellers. not so strong. The flow rate in the well-behaved passages became smaller, and some backflow \"vortex\" structures occurred on the blade suction side. 3. Because of the difference of the leading-edge shape, the internal flow field became different. In the blunt and sharp leading-edge impellers, an extremely low-pressure region occurred at the inlet of the stalled passage. The pressure at the inlet of the wall-behaved passage was on a higher level. A high turbulence kinetic energy region occurred mainly in the stalled passage due to strong leading-edge separation. In the ellipse and round leading-edge impellers, a small-scale low-pressure region occurred on each blade's leading-edge. The pressure distribution near the blade inlet was somehow averaged. A high turbulence kinetic energy region occurred in both the well-behaved passages and the stalled passages. It revealed the flow instability in all the blade passages of the ellipse and round leading-edge impellers. 4. The flow pattern in the blunt leading-edge impeller was similar to that in the sharp leading-edge impeller. Moreover, the flow pattern in the ellipse leading-edge impeller was similar to that in the round leading-edge impeller. However, the pressure pulsation characteristics in the four types of impellers were completely different. In the blunt leading-edge impeller, frequency of 9.67 Hz dominated. The amplitude of pressure pulsation on the monitoring points was up to about \(6\times 10^{-6}\) Pa. In the sharp leading-edge impeller, a frequency of 0.81 Hz dominated. The amplitude of pressure pulsation on the monitoring points was up to about \(1.2\times 10^{-3}\) Pa.