## 5 Analysis of Results
### Numerical Calculation Analysis before and after Optimization  
To gain a deeper understanding of the uncertainty factor, the mechanism of influence on the overall aerodynamic performance of the fan, as well as the impact of the changes in the fan performance before and after the optimization of the rear guide vane of the inclined flow fan, the following study of the uncertainty flow phenomenon inside the rear guide vane is carried out. For a more transparent analysis and comparison, a 0.5 blade height section of the rear guide vane is selected for investigation. The velocity distribution, flow field and turbulent energy distribution in the fan before and after optimization are analyzed separately at the operating design point. Figure 16: A noise test diagram. Figure 17: The noise performance testing. As Figure 18 above shows the 3D surface plot of the velocity distribution at the 0.5 lobe height section before optimization, it can be seen from the plot that the velocity distribution of the section is uneven, and the velocity fluctuation difference is significant; there is a specific abrupt change phenomenon, apparent velocity distribution. Using the Turbo module in the CFD-POST software, the blades are expanded into the fluid domains of the moving and guide vanes. As shown in Figure 19, the 0.5 leaf height cross-section (blue face) of the moving and guide vanes is expanded into a 2D plane. As shown in Figure 20, the velocity distribution around the guide vane is at 0.5 leaf height before and after optimization. The flow capacity of the guide vane area is indirectly reflected by the velocity distribution in this area, the velocity distribution gradient between  
Figure 19: A leaf expansion diagram. Figure 18: The velocity distribution of the cross-section before and after optimization. the guide vane before the modification is uneven, and the velocity difference between the pressure surface and the suction surface is significant. After the modification, the pressure surface forms a larger high-speed zone, which enhances the circulation capacity of the guide vane section, and the flow velocity increases significantly at the guide vane outlet, improving the efficiency of the guide vane. Figure 21 above shows the flow distribution around the guide vane at 0.5 leaf height before and after optimization. Before optimization, the negative angle of attack of the airflow is more prominent, and the pressure surface of the guide vane is prone to the backflow phenomenon, causing detachment loss and reducing the effective circulation area. The increase in the angle of attack of the surface layer into the flow is prone to the phenomenon of airflow separation; the formation of the blockage zone inhibits the flow instability of the mainstream inside the fan. After the blade transformation, the bending angle is reduced on the incoming flow of the dynamic lobe, playing a slowing role, effectively curbing the phenomenon of reverse flow and reducing the flow loss of the guide lobe. The modification scheme in this paper confirms the conclusions of [PERSON] [19], shown in Figure 22, who suggests that the onset of stall is related to the blade inlet impulse angle and explains the mechanism by which rotational stall propagates between the guide vanes. As the impeller flow decreases and the guide vane inlet impulse angle increases, any slight asymmetry in the flow path structure will first cause flow separation in one or  
Figure 21: The streamline distribution at 0.5 blade height before and after optimization. Figure 20: The velocity distribution at 0.5 blade height before and after optimization. more fluid domains and further block the fluid domains. The incoming flow will change direction towards the adjacent impeller. The incoming flow will change direction towards the neighbouring impellers, which corresponds to a change in the direction of the incoming flow of the neighbouring impellers.