# CFD and Energy Loss Model Analysis of High-Speed Centrifugal Pump with Low Specific Speed
## 3 Results and Discussion
### Loss Analysis Based on ELM/CFD Method  
Based on the numerical results, the energy power loss and the proportion of energy loss power are calculated by the equations in 2.4. The variations of different power losses with the flow rate are shown in Figure 8. Except for the useful power, the hydraulic loss of impeller is the largest loss in the pump, followed by the volume loss power, the hydraulic loss of motor channel, and the mechanical loss power. With the increase of flow, both the total power and the top curve first increase and then decrease after \(Q\) = 4.2 m\({}^{3}\cdot\)h\({}^{-1}\). Meanwhile, the lowest curve representing the useful power increases and finally tends to keep constant. The disc friction loss power basically maintains constant and the change of flow almost has little effect on it. The hydraulic power loss, the hydraulic loss of the inducer is the smallest, while that of the impeller is the largest. Moreover, the hydraulic loss of the impeller decreases significantly when the flowrate is greater than 4 m\({}^{3}\cdot\)h\({}^{-1}\). The hydraulic loss in the diffuser is almost zero under smaller flowrates and it increases rapidly under large flowrates. The hydraulic loss in the motor channel increases slightly as the flow increases. The analysis of the proportion of energy loss under the design condition and the deviation condition is shown in Figure 9. At \(Q\) = 3 m\({}^{3}\cdot\)h\({}^{-1}\), the useful power accounts for the highest proportion, 34.2%, which means the pump efficiency is 34.2%. The largest loss is the hydraulic loss of the impeller at 183.0 W, accounting for 27.3%, followed by the volume loss at 93.1 W, accounting for 13.9%. The disc friction loss is 52.2 W and the hydraulic loss of the motor channel is 50.1 W, which are relatively small. Meanwhile, the others can be ignored for they are all below 4%. To further improve the pump efficiency, the hydraulic loss of the impeller should be reduced. Increasing the number of blades and modifying the inlet and outlet angle of the blades can improve the distribution of the impeller flow field, but it will also increase the impact loss at the inlet of the impeller. Figure 8: Relationship between various lost power and flow. Figure 7: Velocity vector diagram of the interface between impeller and vaned diffuser. (**a**) Velocity vector diagram of the interface between impeller and vaned diffuser at \(Q\) = 1 m\({}^{3}\cdot\)h\({}^{-1}\); (**b**) velocity vector diagram of the interface between impeller and vaned diffuser at \(Q\) = 3 m\({}^{3}\cdot\)h\({}^{-1}\); (**c**) velocity vector diagram of the interface between impeller and vaned diffuser at \(Q\) = 5 m\({}^{3}\cdot\)h\({}^{-1}\). When the pump works in the low flowrate, the hydraulic loss of the impeller is up to 177.8 W, accounting for 35.4%, and the volume loss is up to 115.4 W, accounting for 23.0%, which is much higher than the useful power at 85.4 W. Under the working condition, the pump efficiency is only 17.0%. The hydraulic loss power and its proportion of other flow channels are lower than that under the design conditions, indicating that the factors in restricting the efficiency of the centrifugal pump under small flowrates are the \"blockage\" phenomenon of the centrifugal impeller and the large leakage flow. To optimize the performance of the pump, the impeller structure should be improved. Increasing the blade wrap angle and the number of blades and adopting the composite impeller structure works. Moreover, reducing the clearance between the tip and the hub as much as possible is also a choice. However, the methods above will be limited by the processing conditions and there will be a risk of strong pressure pulsation. When the pump works in a high flowrate, the useful power is 271.9 W, accounting for 36.4%, which is much higher than other losses, and the pump efficiency is 36.4%. The hydraulic loss of the impeller is 104.0 W, accounting for 13.9%, and the volume loss is 68.2 W, accounting for 9.1%, both of which are much lower than that under the design condition.