## 4 Results and Discussion
### Time-Averaged Unsteady Simulation Results
#### 4.1.3 Investigation of the Flow Field in the Different Diffusers  
The flow fields for various diffusers are considered to explore the performance improvement mechanism of different diffuser configurations. It is well known that the operating range of a centrifugal compressor is usually limited by a rotating stall or surge. Therefore, this section focuses on the internal flow conditions in the diffuser at a low mass flow rate point. Figure 8 shows the meridional velocity distribution in different diffusers at 0.7 \(Q_{\mathrm{d}}\). A sizeable low-momentum flow region is formed near the shroud side in VD. The low-momentum flow region decreases significantly but remains on the shroud side, and the mainstream is accelerated near the hub side in TVD. However, the accelerated mainstream on the hub side is not fully developed downstream of the TVD vane. Similar to VD, the low-momentum flow near the shroud side and the accelerated main steam on the hub side of HVD extend to the diffuser exit. However, the flow in SVD is significantly better than that in the other three diffusers. The low-momentum flow region near the shroud side in VD, TVD, and HVD is well controlled within SVD. Figure 9 shows the static pressure distribution in the meridional direction. Various degrees of low-pressure regions are found at the entrance of different diffusers at 0.7 \(Q_{\mathrm{d}}\). Similar to the distribution of meridional velocity, HVD and VD have similar static pressure distribution, with a slight improvement. Only a tiny zone of low pressure is found at the entrance of SVD near the shroud side, and the high-pressure region in the meridional direction of SVD is larger than that of the other three types of diffusers. That is the main reason why SVD has the highest static pressure recovery coefficient. Figure 8: Distribution of meridional velocity at low mass flow rate operating point (\(\varphi=0.7\)\(Q_{\mathrm{d}}\)). Figure 7: Performance comparison for different diffusers: (**a**) total pressure loss coefficient; (**b**) static pressure recovery coefficient. To further investigate the performance improvement of SVD compared to TVD, the Mach number distribution at numerous cross-sections of the diffuser passages is shown in Figure 10. As one can see from the TVD, the low-velocity zones formed by large-area flow separation occur in the suction side of the diffuser vane and the diffuser passage. This phenomenon reduces the effective flow area in the diffuser passage, which leads to the acceleration of local flow. However, there is no flow separation phenomenon similar to TVD in the SVD passage. Part of the high-momentum fluid in the SVD passage migrates from the hub side to the shroud side, which suppresses the generation and development of flow separation on the suction side of the diffuser vane. Therefore, SVD can improve the local flow separation phenomenon, and favorable flow characteristics can be attained. Furthermore, the static pressure distribution in TVD and SVD passages is shown in Figure 11, where the location of the cross-sections in the flow passage is consistent with that in Figure 10. As can be seen from the results of SVD, the low-pressure region mainly appears near the suction side of the diffuser vane. However, the large-scale low-pressure zone appears in the flow passage of TVD. When the fluid flows from the entrance to the diffuser exit, the low-pressure zone in the TVD passage migrates from the middle of the flow passage to the suction surface of the vane. However, the area of the low-pressure region is more prominent than that of SVD. In summary, this is one of the reasons why SVD has favorable pressure recovery characteristics. The \(Q\)-definition [21] is used to visualize the vortex structures, and the nondimensional helicity is used to color the vortex structures. The nondimensional helicity is defined as the  
Figure 11: Distribution of static pressure in diffuser passages (\(\varphi=0.7\ Q_{\mathrm{d}}\)).