# Flow Control of Radial Inlet Chamber and Downstream Effects on a Centrifugal Compressor Stage
## 4 Performance Improvement and Downstream Effects
### Effects on Downstream Components  
In order to investigate the influence of structure modification on the downstream components, all the compressor stages with the improved and original inlet chambers were numerically analyzed and compared under the design conditions, as shown in Table 6. It was found that, although the efficiency of the inlet chamber itself increased by 5% after the modification, the performance of the entire compressor stage almost remained unchanged, even being slightly lower than that of the original stage. According to the comparison, the decrease in the impeller's efficiency was the main reason for the performance decrease of the stage with the improved inlet chamber. As is well known, impellers are the only working component in the compressors, and the impeller in the first stage generally operates with a medium or high-pressure ratio, which is crucial for the working capacity and performance of the whole machine. Therefore, besides the performance of the inlet chamber, the flow condition at the impeller inlet is also important for the performance of downstream components and the entire compressor. Figure 14 shows the distributions of the major flow parameters at the impeller inlet, which is also the exit of the inlet chamber, including the total pressure, velocity magnitude and the tangential component of the velocity. It was found that, according to the improvement in the present study, not only the performance of the radial inlet chamber but also the uniformity of the flow condition at the impeller inlet was improved. In the original structure, a high pressure and high speed appeared in the region close to the intake channel, while a low total pressure with low speed occurred on the side away from the intake. In the results of the improved structure, high total pressures were evenly distributed at the impeller inlet, the unevenness of the velocity distribution almost disappeared in the circumferential direction, and the regions with greater tangential velocities were also stretched and weakened in this section. This indicates that part of the airflow which directly turned \(90^{\circ}\) to the annular convergent channel in the original structure was turned to the other side along the tangential direction of the spiral channel, realizing the redistribution of the flow rate inside the radial inlet chamber. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Performance Parameter** & **Original** & **Improved** \\ \hline Stage efficiency \(\eta_{in-out}\) & 80.38\% & 79.79\% \\ Impeller efficiency \(\eta_{0-2}\) & 95.27\% & 93.36\% \\ Total pressure ratio \(\varepsilon_{in-out}\) & 1.2578 & 1.2561 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Performance comparison of the compressor stages with different inlet chambers. Figure 13: Performance comparison of the radial inlet chambers under different conditions. Generally, it was considered that an important reason for the performance decrease of the compressor stage with the radial inlet chamber was the non-uniformity of the flow distributions, since it was deviated from the ideal design condition of the impeller, which required an axial and uniform incoming flow. Although the structure modification in the present study improved the performance of the radial inlet chamber and increased the flow uniformity at the impeller inlet, the impeller efficiency was not improved. According to the flow distributions in Figure 14, it can be observed that the distributions of the total pressure and velocity magnitude in the improved scheme were closer to the axial inlet condition, and the distribution of the tangential velocity at the impeller inlet might be an essential factor for the whole compressor's efficiency. The values of the positive and negative tangential velocities at the exit of the original inlet chamber were greater, but  
Figure 14: Flow contours at the impeller inlet. (**a**) Comparison of total pressures. (**b**) Comparison of velocity magnitudes. (**c**) Comparison of tangential velocities.