## 5 Result Analysis
### Numerical Analysis before and after Optimisation
At the same time, the size of the vortices in the dynamic-static coupling duct at 0.1 span is also considerably reduced because of the increased spacing between the moving blades and OGVs. As shown in Figure 12, the pressure gradient on the pressure surface before the optimisation is more significant, the pressure pulsation on the surface of the OGVs caused by the turbulent boundary layer on the surface of the OGVs itself will produce aerodynamic noise, and the trailing edge of the blade will create shedding vortexes, which will also cause random pressure pulsations. As shown in Figure 21, after optimisation, the pressure gradient on the pressure surface of the OGVs is more stable, the separation point is shifted back to the rear of the OGVs, and the impact of the impeller wake is on the leading edge of the OGVs position. Figure 20: Comparison of vorticity before and after optimisation. (**a**) Original 0.9 span. (**b**) Optimized 0.9 span. (**c**) Original 0.5 span. (**d**) Optimized 0.5 span. (**e**) Original 0.1 span. (**f**) Optimized 0.1 span. The pressure drops smoothly, and the pressure pulsation on the surface of the OGVs is reduced. The smooth pressure gradient enables the boundary layer of the OGVs to be damaged less significantly, the separation vortex is diminished, and the aerodynamic noise generated by the pressure pulsation on the surface of the OGVs is reduced. At the same time, the optimised OGVs increase the airflow's kinetic energy into the fan's pressure energy, reducing the energy loss of the airflow's frictional collision with the OGVs, thus increasing the total pressure and efficiency of the fan. The frequency domain pulsation of the pressure before and after optimisation is shown in Figure 22. Comparing the frequency domain plots before and after optimisation, it can be seen that there is no significant change in the pressure pulsation in the inlet duct of the OGVs, with the blade frequency still being the dominant frequency. The amplitude at points P5-P9 is significantly reduced after optimisation, with the amplitude at points P7-P9 being particularly pronounced, decreasing from a maximum of 0.11 to 0.03. At the same time, the pulsation at 200-600 Hz is also significantly reduced. There is no significant change in the fluctuation of the pressure pulsation at P1-P4, and the pulsation and amplitude at P10-P12 at 0-600 Hz are reduced, from 0.02 to 0.01. In summary, the analysis indicates that the matching of the fan moving blades and OGVs before the optimisation is worse than the optimised fan, resulting in the rear guide vane being affected by the airflow from the automatic vane, thus increasing the noise of the fan and reducing the efficiency of the fan. After optimisation of the OGVs, the impact of the airflow on the OGVs is effectively reduced, and the flow of the nuclear grade axial flow fan is improved. Figure 21: Static pressure distribution curve after optimisation. According to GB/T 3767-1996, \"Acoustics-Determination of sound power levels of noise sources using sound pressure-Engineering method in an essentially free field over  
Figure 18:0 Frequency domain diagram of pressure pulsation before and after optimisation. (**a**) Original. (**b**) Optimized. a reflecting plane\" [30]. For noise analysis modelled by LMS Virtual.Lab software, a hemispherical domain field unit containing 19 domain points was established, as shown in Figure 23; for the rated operating conditions 19 domain points in the centre frequency point near the sound pressure distribution cloud, the location of the ring line A in the figure is the fan outlet 45\({}^{\circ}\) doubled diameter in the region. From Figure 23b, it can be found that the sound pressure value in the area where the ring line A is located after optimisation is reduced more obviously by about 3-4 dB, indicating that the optimised fan structure has a better noise reduction effect.