## 1 Introduction  
Large size and significant flow rate are the characteristics of nuclear grade axial flow fans; their defect is in their flow field because the dynamic-static coupling duct, rotation duct, and guide vane structure are more complex and cause backflow, vortexes, and other flow phenomena. The blade root of the flow appears separated. In the design phase of the OGVs, less attention was given to the degree of OGVs [1], and the design of the OGVs because the axial distance of the moving blades is unreasonable, which is due to the pattern of flow at the axial distance [2], resulting in a decline in fan performance. This has a negative impact on noise. Therefore, it is essential to optimize the design of the OGVs. The performance of axial fans is influenced by many factors [3; 4; 5]. [PERSON] et al. [6] presented their experimental investigations and numerical simulations of the performance of a deflected-blade axial flow fan. They showed that compared to radial blades, forward-swept blades could increase pressure by 13.1%, boost flow by about 5%, reduce noise by 2-4 dB, and increase efficiency by 3%. [PERSON] et al. [7; 8; 9] studied the flow fields of two subsonic axial fans with 35\({}^{\circ}\) forward-swept blades and unswept blades. The results demonstrated that the forward-swept blades were more stable, especially at low flow speeds, with a stall delay effect. [PERSON] results show that forward-swept blades on low-speed axial flow fans can improve aerodynamic performance and the potential for a wide range of applications. The noise emitted by the axial flow fan is a key area of research [10]. Therefore, in addition to obtaining the required aerodynamic performance, operating with low noise is one of the design considerations. [PERSON] [11] investigated the relationship between turbulent boundary layer noise and trailing edge noise. [PERSON] [12] predicted the combined model of the turbulent boundary layer noise and trailing edge noise. [PERSON] et al. [13] explored the interrelationships between noises and found that turbulent boundary layer noise was negligible compared to other noise sources. For the optimisation of axial flow fans, researchers have used various optimisation techniques to reshape and optimise the impeller blades. [PERSON] et al. [14] investigated the internal flow problem in an axial flow fan with front and rear guide vanes using numerical calculation methods and obtained a relatively reasonable position for the front guide vanes. [PERSON] et al. [15] used the spoke-flow control technique of evenly spreading strips on the inner sidewall of the inlet tapered tube of an axial flow pump, carried out simulations and analyses based on the FBM-CC model, and found that the efficiency of the axial pump could be improved to a certain extent and the formation of the \"saddle area\" could be effectively suppressed. [PERSON] et al. [16] used the response surface method (RSM) to obtain the optimum blade bending angle for a low-speed axial fan to optimize the fan's efficiency. [PERSON] et al. [17] used a multi-objective genetic algorithm combined with an approximate model to optimise the blade shape of an axial fan, resulting in a 10% increase in fan efficiency. [PERSON] [18] used a Bezier surface as a parametric method to control the suction surface of the blade, significantly reducing the design variables, ensuring a smooth blade surface, and successfully improving the aerodynamic performance of an axial compressor. [PERSON] [19] proposed a combined optimisation method for axial flow blades that included experimental design and velocity gradient algorithm, which improved the optimisation efficiency by 3.07% and 0.87%, respectively, compared to the conventional direct experimental design or sequential quadratic programming algorithm. The analysis was completed and the results concluded that the optimised vanes increased the pump's efficiency by 2.91% at the optimum operating point.