## 5 Conclusions  
This paper focuses on a large, high-speed centrifugal fan and analyzes the impact of incoming flow distortion on the internal flow evolution of the impeller. To address the design challenges of large, high-speed centrifugal fans under incoming flow distortion, a blade modification method based on the suppression of internal secondary flows is proposed. Experimental tests were conducted on the entire vacuum quenching furnace system before and after impeller optimization. The results show significant improvement, leading to an increase in the centrifugal fan's operating efficiency. The main conclusions of this paper are as follows:  
1. The SST k-\(\omega\) turbulence model, modified with the eddy viscosity coefficient, was compared with experimental results of flow in a U-shaped pipe. The modified SST k-\(\omega\) model effectively captures the non-uniform velocity distribution characteristics, with a maximum error of only 4.7%. The refined computational model improves the accuracy of the results, thereby providing a more precise analytical tool for evaluating the impact of subsequent impeller structural optimizations on performance improvements. 2. Under the influence of the complex flow channel structure within the gas quenching furnace, the airflow parameters at the fan inlet cross-section undergo distortion. The maximum swirl angle appears at the circumferential position of 60\({}^{\circ}\), corresponding to downstream passage A. In this passage, the mid-span tip leakage flow along the suction side is affected by vortices, eventually mixing with them. This interaction forms a large vortex in the rear section of the passage. Additionally, a stall vortex perpendicular to the blade and spanning the entire passage develops in the downstream section, resulting in an average blockage coefficient of 0.29. At the circumferential position of 300\({}^{\circ}\), where the minimum swirl angle occurs, the corresponding downstream impeller passage B exhibits strip-like leakage flow at the rear of the blade tip. However, no large-scale vortex structure is formed, and the average blockage coefficient decreases to 0.27. 3. The impeller flow passage optimization design method based on the suppression of secondary flows effectively improves the secondary flow phenomena and the intensity of tip leakage flows within the impeller. The modified impeller achieves an 11.7%  
Figure 23: Comparison of temperature before and after optimization. increase in maximum efficiency compared to the original design, with the blockage coefficients in passages A and B reduced to 0.07 and 0.04, respectively. Additionally, flow field and temperature field experimental tests were conducted on the entire vacuum quenching furnace system. The test results show that the standard deviations of the velocity at the center of the test sections at Y = 400, Y = 600, and Y = 800 were reduced by 62.97%, 60.69%, and 57.14%, respectively, compared to the original impeller. The temperature fluctuation range at the surface measurement points was reduced by 53.09%, indicating a significant improvement in the uniformity of the flow and temperature fields. It should be noted that the proposed model has been validated specifically for the VHPGQ centrifugal fan under high rotational speed and strong rotational flow conditions. The applicability of this model may be limited to similar fan configurations, and for other types of fluid machinery with significant differences in fan geometry or operating conditions, appropriate adjustments may be required. Furthermore, the modified SST k-\(\omega\) turbulence model, on which the proposed model relies, performs well under these conditions but may need to be adjusted for more complex turbulent flow states. Although the proposed model was developed and validated specifically for the centrifugal fan in the VHPGQ gas quenching furnace, it may potentially be extended to similar centrifugal fans or other fluid machinery operating under comparable conditions. Conceptualization, S.Z. and T.W. ; methodology, Z.M.