## 5 Result Analysis
### Experimental Result Analysis  
As shown in Figure 24, a diagram of OGVs before and after optimisation, the optimised fan was proofed to facilitate the analysis of the results before and after optimisation. The modified axial flow fan was subjected to aerodynamic performance tests, noise tests, and triaxial acceleration tests. Figure 25 shows the fan performance curves before and after optimisation. It can be seen that the air performance curve of the optimised fan follows the same trend as the original fan performance curve. The complete and static pressures of the optimised fan have been increased at all operating points. At the operating design point, the total pressure of the fan is increased by approximately 154 Pa and the efficiency rises by 3.4%. There is a slight increase in fan efficiency at low flow rates, and only a 1.14% increase in efficiency. However, at high flow rates near the operating point, efficiency increases by 2-3.2%.  
Table 8 shows the noise values of the axial flow fan structure before and after optimisation under different working conditions. From the table, it can be seen that the noise of the axial flow fan has been significantly reduced after optimisation, with optimisation percentages in the range of 65,000 m\({}^{3}\)/h to 85,000 m\({}^{3}\)/h, between 3.7% and 3.9%; under minor flow conditions, the noise value of the fan is only reduced by 3.7 dB, and with the increase in flow, the noise of the fan can be reduced by 4.3 dB after optimisation. When the air volume exceeds 57,000 m\({}^{3}\)/h, the change in noise with the air volume is more minor. At the design working point, the fan noise was reduced from 110.4 dB to 106.3 dB, a reduction of 4.1 dB, with a noticeable noise reduction effect.  
Figure 23: Comparison of sound pressure amplitude before and after optimisation. (**a**) Original. (**b**) Optimized.  
Figure 24: The proofing model before and after optimisation. (**a**) Original. (**b**) Optimized.  
Figure 25: Fan performance curves before and after optimisation.  
For a more accurate analysis of the noise, B&K microphones were used to obtain the noise spectrum; Figure 26 shows the B&K experimental equipment. The test results show that the original and optimised SPL spectral distribution peaks at the fundamental frequency and its multiples. This is because the discrete component of the fan noise contributes less to the far-field noise than the turbulent broadband noise in the frequency range tested, and therefore the fan noise changes from discrete to turbulent. The reduction in noise is more pronounced in the low frequency range of 20-160 H. Waterfall plots of the noise characteristics are shown in Figure 27a,b. The motor rotates at 25 Hz, the moving blades BPF at 545 Hz, and twice that at 1090 Hz. Figure 27a,b compare the noise waterfall plots for the original fan and the optimised one. After optimisation, the noise of the fan is reduced in the low frequency region around the motor frequency and the noise level at the blade passing frequency of 545 Hz is also significantly reduced, indicating that the fan is well optimised for noise.  
As shown in Figure 28, optimised vibration acceleration is still dominated by radial acceleration. In the range of 450-600 Hz, the optimisation of the OGVs resulted in a significant reduction in the magnitude of pulsations in the OGVs duct, with a decrease in amplitude from 0.47 to 0.28 mm/s\({}^{2}\) at the vane-passing frequency and an unsatisfactory maximum magnitude of 0.52 mm/s\({}^{2}\) at the rotational frequency multiplier. The axial and circumferential vibration acceleration magnitudes in the dynamic-static coupling duct are significantly reduced. The vibration acceleration at the rotational frequency is reduced from 0.31 to 0.17 mm/s\({}^{2}\), while the pulsations are decreased considerably. The rest of the monitoring points showed no significant change in acceleration vibration.