# Mechanisms of Sweep on the Performance of Transonic Centrifugal Compressor Impellers
## 5 Aerodynamic Effects of Sweep Feature  
### Effect on the Front Loading  
The effect of sweep on the front loading of main blade can be observed in Figure 14. It is illustrated that the front loading near the tip is decreased with the enlarged sweep angle, while that near the hub is increased. Because the front of main blade locates at the axial part of the impeller, the loading redistribution effect is similar with that in the axial case [12]. Such effect is also reported in a former centrifugal case [25], but the extent in the current case is much stronger due to the significant change of sweep angle. The effect of sweep on the front loading of splitters is also examined in Figure 15. For splitter I, both the front loading near the hub and the tip has been decreased. While for splitter II, the front loading near the tip has been slightly decreased, and that near the hub has been increased. The effect on the front loading of splitters is therefore different from the main blade. To understand sweep effects on the front loading of different blades, one of the explanations is given based on the streamline twist. For axial turbomachines and axial parts of centrifugal compressors, illustration of streamlines is shown in Figure 16a. When the blade is desi  
Figure 14: Static pressure distribution of main blade at **(a)** 95% span and **(b)** 5% span. Figure 15: Static pressure distribution of splitters at **(a)** 95% span and **(b)** 5% span. flow that drives the fluid from the pressure side (PS) across the leading edge to the suction side (SS) exist, which works in similar fashion with the tip leakage flow. The leading edge secondary flow is generated by the local pressure gradient, where the incoming flow pressure is lower than the pressure side pressure and higher than the suction side pressure near the leading edge. As a result, the suction side and pressure side streamline will be twisted radially inward and outward, respectively. The axial velocity at suction side near the hub will therefore be higher, which leads to a lower incidence and a decreased loading. The axial velocity at suction side near the tip will be lower, which results in a higher incidence and an increased loading. When the blade is designed forward swept, it can be deduced in a similar manner that the front loading near the hub will be increased, while that near the tip will be decreased. For axial-to-radial parts of centrifugal compressors, the streamline twist also exists as illustrated in Figure 16b. For the front loading near the hub, similar conclusion is obtained due to the streamline twist effect. For the front loading near the tip, however, the tip incidence is also altered through the incoming flow swirl. When increasing the sweep angle, the blade tip encounters flow more upstream with a higher relative flow angle, which tends to increase the incidence and the loading near the tip, and thus counteracts with the streamline twist effect. Therefore, limited change in streamwise velocity, incidence and loading is obtained near the tip. It should be noted that the above incoming flow swirl pattern is only true when there are blades upstream, which gradually turns the flow from high relative flow angle at inlet to low relative flow angle at exit. This is the case for most impellers with splitters. Another approach to understand sweep effects on front loading is based on the loading definition, which is the pressure difference between pressure side and suction side of the blade. For axial turbomachines and axial part of centrifugal impellers, illustration of loading is shown in Figure 17a. The front loading at the blade tip (point \(A\) and \(A^{\prime}\)) can be expressed by the loading perpendicular away from the tip (point \(B\) and \(B^{\prime}\)) with an integral term:  
Figure 16: Understanding sweep effects on the front loading of (**a**) axial compressors and (**b**) centrifugal compressors through streamline twist.