# Effect of Vaneless Diffuser Shape on Performance of Centrifugal Compressor
## 4 Conclusions  
The centrifugal compressors with four different shape vaneless diffusers were numerical simulated in this paper. The shapes of the diffuser were modified by proper pinch on the shroud side and hub side of the vaneless diffuser wall. Based on the CFD simulation results, the aerodynamic characteristics and internal flow characteristics of the centrifugal compressor were studied. The conclusions were summarized as follows:  
1. Three convergent vaneless diffuser shapes mentioned in this article can improve the overall stage pressure ratio and isentropic efficiency of centrifugal compressors under design condition and small flow rate condition. The pinch on the diffuser shroud side can achieve the optimal expansion effect of pressure ratio and isentropic efficiency under small flow rate condition and design condition, and the expansion effect is concentrated on the impeller, while the pinch on the diffuser in the near choking condition produces negative effect for the pressure ratio of compressor. As for the isentropic efficiency, convergence on the shroud side is the best way to improve efficiency under small flow rate condition, while convergence on the hub side is the best way to improve efficiency under design condition and near choking condition.
2. By analyzing the internal flow field of the vaneless diffusers, the convergence of the diffuser width can optimize and improve the internal backflow phenomenon, thereby increasing the efficiency of compressors. Although the effect of the diffuser modification on the pressure ratio of the diffuser is not obvious, the corresponding change can make the pressure change of diffusers along the length direction more uniform and reduce the internal reverse pressure gradient.
3. The internal energy loss is changed significantly with the change of shapes of the vaneless diffuser. Therefore, the change of the static entropy distribution range in the contour is observed to judge the variation of the energy loss. The effect of convergent diffuser width is not obvious in the near choking condition, while the convergent diffuser width under the design condition and small flow rate condition can effectively reduce the area of entropy rise within the compressor, so as to reduce energy loss and improve efficiency. There is no significant difference of the three convergent forms mentioned above on the influence of energy loss.  
Conceptualization, Q.Z.; methodology, Q.Z., L.Z. and J.Y.; writing--original draft, Q.H. and Q.Z.; writing--review and editing, L.Z.; software, L.S. All authors have read and agreed to the published version of the manuscript.  
This research was supported by National Natural Science Foundation of China (Grant No.11602085), the Fundamental Research Funds for the Central Universities of China (Grant No. 2018 MS107), and the Natural Science Foundation of Hebei Province, China (Grant No. E2016502098).  
Figure 16: Meridional surface static entropy distribution at 0.6 relative mass flow: (**a**) parallel; (**b**) convergence from shroud; (**c**) convergence from hub; (**d**) equal convergence from shroud and hub.