# Flow Control of Radial Inlet Chamber and Downstream Effects on a Centrifugal Compressor Stage
## 3 Flow Analysis  
The flow characteristics inside the radial inlet chamber were analyzed at the design conditions. The exit of the radial inlet chamber (i.e., 0 in Figure 1) was a key section analyzed in this paper, since the flow condition here is an important factor affecting the performance of the downstream components. Figure 8 shows the flow distributions of this section of the radial inlet chambers with and without guide vanes, including the Mach numbers, velocity vectors and total pressure contours. This revealed that a pair of vortices opposite to each other were captured on the left side of this section, and the distributions of the Mach numbers and total pressure in the two schemes had the same trends from the left to the right. The numerical results of the scheme without guide vanes in Figure (a)a reveal a low-pressure and low-speed separation area on the side, close to the intake channel of the radial inlet chamber. When the guide vanes were arranged in the annular convergent channel, this flow separation disappeared, as shown in in Figure (b)b, and instead, two vortices of a larger size were clearly observed in this area. This was mainly because when the high-speed airflow from the intake channel turned 90\({}^{\circ}\) in the axial direction, it impacted the guide vanes and slowed down, which greatly weakened the flow separation caused by the 90 degree turning. Meanwhile, the flow impacting on the leading edges of the guide vanes induced extra separations and wake flows, resulting in more obvious vortices downstream of the 90 degree turning at the exit. The three-dimensional streamlines of the radial inlet chambers are shown in Figure 9. It can be found that, when the impeller was absent, the flow field was symmetric with respect to the \(X\)-\(Z\) plane. According to the streamlines, the fully developed incoming flow from the intake channel entered the spiral channel and was divided into different paths. Part of the airflow kept the direction almost unchanged and flowed into the annular convergent channel along the shortest path, while the rest of the fluid expanded at the spiral channel inlet and generated two pairs of sudden expansion vortices on both sides, which continued to develop toward the downstream. When the airflow turned 90\({}^{\circ}\) to the convergent channel, flow separations were easier to form at the inner radius due to the large curvature. As the fluid flowed along the tangential direction in the spiral channel, it was guided by the convergent structure of the bottom walls in the radial inlet chamber  
Figure 7: Comparison of the numerical and experimental performances for the compressor stage. and was gradually redirected into the annular convergent channel without generating more separations. It can be found that the flow separations and vortices were mainly formed in the spiral channel and the convergent channel, which were the major causes of the flow loss in the radial inlet chambers. Figure 8: Flow distributions at the exit of the radial inlet chamber (**a**) without guide vanes and (**b**) with guide vanes. Figure 9: Three-dimensional streamlines inside the radial inlet chambers. In order to further analyze the flow mechanism, detailed distributions of the vorticity and entropy in the middle section of the spiral and annular convergent channels are provided in Figure 10. In the inlet chamber without guide vanes, it was observed that the entropy productions mainly occurred on both sides of the spiral channel and the 90 degree turning of the convergent channel where a high vorticity magnitude appeared, while the entropy was quite low on the upper side close to the intake channel due to the low vorticity magnitude in this area. This resulted in uneven flow distributions along the circumferential direction, which would deteriorate the flow condition in the downstream compressor. Therefore, guide vanes were employed to reduce the unevenness at the exit of the radial inlet chamber.