# Effect of Vaneless Diffuser Shape on Performance of Centrifugal Compressor
## 3 Result and Discussion
### Flow Field Analysis
#### 3.2.2 Meridian Plane Radial Velocity Distribution  
Through the analysis of Figure 4, the shape of the vaneless diffuser has a great influence on the flow stability under the design conditions of the centrifugal compressor, especially the internal flow field of the vaneless diffuser. In order to analyze the internal flow characteristics of the diffuser in detail, the method of analyzing the radial velocity distribution in the meridian plane is adopted.  
Figure 5 shows the radial velocity distribution on the meridional surface of the vaneless diffuser at 1.125 relative mass flow operation condition. Both the minimum and maximum radial velocity of the meridional surface are observed in the parallel wall vaneless diffuser. The low-speed backflow area appears on the shroud side of the parallel wall diffuser is suppressed after converging the width, and the backflow phenomenon is improved. The high-speed core flow appearing on the hub side of the parallel wall diffuser is also affected along with the convergence of the width. The smaller the high-speed region indicates that the internal speed of the diffuser changes to pressure and the efficiency of the diffuser becomes higher. The backflow area at the position where the impellers are connected to the vaneless diffusers is unaffected as the width convergence. From Figure 5, it can be concluded that converging the wall of the vaneless diffuser from the shroud side is the best way to improve the low-speed return flow area near shroud.  
Figure 4: Radial velocity streamline distribution of the entire meridian at the design mass flow: (**a**) parallel; (**b**) convergence from shroud; (**c**) convergence from hub; (**d**) equal convergence from shroud and hub.  
Figure 6 shows the radial velocity distribution of the meridional surface of the four vaneless diffusers under design conditions. Compared with the relative mass flow of 1.125, the low-speed return flow area on the shroud side of the parallel wall diffuser extends from the shroud side entrance to the exit and another high speed zone is located near the shroud side exit. The flow field in the three types of diffuser is changed similarly to that when the relative mass flow is 1.125. The backflow area on the shroud side basically disappears and the high-speed flow area on the hub side becomes smaller.  
Figure 5: Radial velocity distribution on the meridional surface at 1.125 relative mass flow: (**a**) parallel; (**b**) convergence from shroud; (**c**) convergence from hub; (**d**) equal convergence from shroud and hub.  
Figure 6: Radial velocity distribution on the meridional surface at 1 relative mass flow: (**a**) parallel; (**b**) convergence from shroud; (**c**) convergence from hub; (**d**) equal convergence from shroud and hub.  
Figure 7 shows the radial velocity distribution of the meridional plane in four vaneless diffusers at 0.6 relative mass flow operation condition. As the relative mass flow decreases, the instability of the flow intensifies, and the value of the backflow area at the connection between the vaneless diffuser and the impeller is lower, and the backflow area increases along the span direction. What is different from the relative mass flow of 1.125 and 1.0 is that the convergence of the width can significantly suppress the backflow phenomenon namely reduce the backflow area at the connection between the impeller and the vaneless diffuser.  
The backflow area near the shroud side is extended almost from the inlet to the near exit of the vaneless diffusers along the length direction. Various shapes of diffuser convergence cannot completely eliminate backflow area on the shroud side, but can suppress the expansion of the backflow area from the length of the vaneless diffuser.