## 3 Results and Discussions
### Analysis of Flow Field in Rotor  
When the air flow reaches the exit of the bend section of the interstage pipeline, the axial velocity corresponding to the total pressure distortion area decreases obviously. The flow field difference at the inlet of the impeller caused by the bend pipe will inevitably affect the flow state inside the impeller. Figure 10 shows the circumferential total pressure and total temperature distribution at the inlet section of the impeller of the three models. It can be seen that the high-pressure region of the three models is located between the main blade and the splitter blade, and the extreme value region is close to the leading edge of the main blade and distributed along 50% blade height to the tip. On the suction side of the blade near the root, a low-pressure area is generated, which is mainly affected by the flow of the bend pipe. Comparative analysis shows that the SPC model has higher circumferential total pressure at the impeller entrance than the other two models, and the EPC model has the smallest high-pressure zone. It can be seen more intuitively from the total temperature nephogram of the three models that a high temperature area is formed on the suction surface side of the main blade, which corresponds to the high-pressure area in the total pressure nephogram. It is found that the high-pressure zone formed in the SPC model at the leading edge of the main blade is larger than that of the other two models, while the high-temperature zone is smaller than that of the other two models, which indicates that the SPC model has the smallest flow loss at the impeller inlet, while the EPC model has the highest flow loss, which is consistent with the law corresponding to the total pressure cloud map of the intermediate pipeline mentioned above. Figure 11 shows the entropy distribution of the three models at 10%, 50% and 90% blade heights. It can be seen from the analysis that the entropy of the three models at different blade heights show the same change pattern. At 90% blade height, a leakage vortex formed on the suction surface of the blade and caused significant flow loss. Compared to  
Figure 10: Circumferential total pressure and total temperature at impeller inlet. flow losses that occur at other locations, the entropy increase caused by the leakage flow loss is the most significant, which becomes the main source of flow loss. By comparing the three models, it can be seen that the EPC model with the stronger total pressure distortion has the greatest loss at this position, while the SPC model with the smallest total pressure distortion has the least flow loss. In addition, the entropy mutation region caused by the wake can be seen at the trailing edge of the blade. The loss at 50% blade height is mainly caused by the tip leakage flow combined with the wake loss, but the value decreases compared with the high entropy area at 90% blade height. At 10% blade, the entropy increases and mutation area caused by the leakage vortex has disappeared, and the wake is the main cause of the flow loss. From the entropy distribution at different blade heights, it can be seen that from the leading edge to the trailing edge, the loss caused by leakage vortex accounts for a relatively large proportion, and the wake loss is small. The blade clearance leakage coupled with intake distortion becomes the main source of loss. By comparing the entropy values of the three models in Figure 11, it is found that at 90% blade height, the high entropy area of EPC model is more prominent than that of TPC and SPC model, and the SPC model has the lowest entropy. It can be seen that the change of the pipe's inner diameter has a significant impact on the flow loss along the blade height: the more uniform the inner diameter of the bend section, the smaller the leakage loss. At 50% blade height, the entropy distribution is the same as the 90% blade height. The area of entropy increase caused by leakage vortex decreases and the flow field becomes more stable.