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Noise and vibration reduction technologies: Fans are a type of widely used general-purpose mechanical equipment, employed extensively in industries such as chemicals, petroleum, metallurgy, mining, and machinery. The noise generated by fans during operation is particularly noticeable, especially at high speeds and under light load conditions. Noise is generated as a result of the interaction between the asymmetric structures surrounding the blades and the circumferential uneven flow field created by the rotational motion during design tests of the blade openings. It is generally believed that there are several types of such noise: (1) intake interference noise caused by the presence of guide vanes or metal mesh covers in front of the air inlet; (2) rotational frequency noise produced by the blades within a rough or asymmetric casing; (3) outlet interference noise resulting from the presence of volute tongues at the centrifugal outlet or from the rear guide vanes in axial flow fans. Discrete noise has discrete spectral characteristics, with the fundamental frequency (the frequency corresponding to i=1) being the strongest, followed by higher harmonics in decreasing order of intensity. Fan vortex noise: It is generated by various separated vortices resulting from airflow movement. Generally, there are 4 causes for this phenomenon: (1) The incoming flow turbulence noise that occurs when air with a certain level of turbulence flows towards the blades ； (2) Turbulent boundary layer noise generated by the pulsating turbulent boundary layer as the airflow passes over the blade surface ； (3) Separation vortex noise generated by the separation of the turbulent boundary layer on the blade surface at the blade trailing edge ； (4) Tip vortex noise generated in axial flow due to the secondary flow from the concave side to the convex side at the blade tip, where the pressure on the concave side is greater than that on the convex side, and which is carried away by the main airflow. 1. Causes of noise: Piercing the fan blades to reduce vortex noise in fans. To reduce the vortex noise generated by fans, it is common to use the method of piercing the blades of the rotating element; this is because vortex separation often occurs at the outlet of the blades. By piercing the blades, some of the airflow can flow from the high-pressure side of the blade to the low-pressure side, which helps to shift the point of separation downstream. The mechanism behind this is similar to that of boundary layer blowing. This reduces the separation region at the blade exit section; the intensity and size of the vortices in the separation region decrease, and thus the noise is reduced as well. However, a high perforation coefficient causes the pressure difference to decrease too rapidly, resulting in an insufficient energy head. Therefore, the key to the blade perforation method lies in the design of the number of perforations per row, the perforation area, the perforation coefficient, the perforation diameter, and the perforation angle. The specific methods are as follows: (1) Increase the aerodynamic load on the cascade and reduce the circumferential velocity. For fans, using strongly forward-tilted blades, along with multi-blade impellers, helps to increase the aerodynamic load on the cascade; this allows the circumferential velocity on the outer surface of the impeller blades to be reduced, thereby significantly lowering the noise level of the fan under the same conditions of air volume and pressure. (2) Appropriate vane tip clearance and vane tip radius: When the airflow moves relative to the blades, the velocity and pressure in the flow wake behind the blade tips are lower than those in the main flow area, resulting in an uneven distribution of velocity and pressure downstream of the cascade. This uneven airflow rotates, and due to the presence of vane tips at the outlet of the moving blades, this unstable flow interacts with the vane tips to generate noise. The noise intensity increases as the distance to these sources of noise decreases. Generally, choosing a larger radius for the front end of the vane tips can help reduce the rotational noise and vortex noise of centrifugal fans. (3) Vortex tip inclination: The periodic fluctuating aerodynamic forces generated by the periodic fluctuations in the flow velocity within the fan impeller blades also cause rotational noise due to the interaction between the vortex tips. The magnitude of this noise is related to the intensity of these fluctuating aerodynamic forces as well as the windward area of the vortex tips. By tilting the vortex tips, the area over which the aerodynamic forces act in phase is reduced, thereby decreasing the noise generated. (4) Installation of turbulence generators at the inlet/outlet of the impeller: Installing turbulence generators (metal meshes) at the inlet or outlet of the fan impeller blades can transform the laminar boundary layer on the back side of the blades into a turbulent boundary layer immediately, delaying or even preventing the separation of the boundary layer there. By installing a mesh at the trailing edge of the blades, the flow velocity and pressure gradient behind the mesh can be quickly made more uniform; if the mesh is placed in the vortex region, it can reduce the size of that vortex region, thereby further reducing noise. (5) A serrated structure is provided on the inlet and outlet edges of the moving blade. This serrated structure enables the laminar boundary layer of the airflow over the blade to transition to a turbulent state at an earlier stage, thereby preventing unstable waves within the laminar boundary layer from causing vortex separation and reducing noise. (6) An acoustic resonator is installed at the throat. When sound waves reach this resonator, the aperture of the small hole and the gas in the cavity move back and forth as a result of the action of the sound waves. This moving gas has a certain mass, which resists the motion caused by the sound waves. Meanwhile, when sound waves enter the aperture of the small hole, a considerable amount of sound energy is lost due to friction and damping on the walls of the throat, resulting in heat generation. Furthermore, the gas-filled cavity has the property of resisting pressure changes from small holes; due to the combined effect of these factors, noise is reduced as the gas passes through the resonator. 2. Noise control measures: (1) Rotating components such as fan impellers, fan shafts, pulleys, and couplings must undergo strict static and dynamic balance adjustments; only after passing these tests can they be assembled into a complete unit. Approval for release from the factory should be granted while also selecting appropriately the gap between the motor cooling fan blades and the air guide rings, in order to effectively reduce the rotational noise of the motor cooling fan blades. (2) Regularly check whether the connection bolts and foundation bolts of the fan’s various components are loose, and whether the bearings are abnormally worn or poorly lubricated. Whether the conveyor belt is tensioned, etc. If any abnormality is detected, the machine should be stopped immediately to address it. (3) During installation, soft materials such as rubber, cork, or felt should be placed between the fan and the reinforced concrete foundation. To minimize or eliminate the vibrations transmitted from the centrifugal fan to the reinforced concrete foundation. (4) By installing a section of rubber hose at the inlet and outlet of the fan, the vibrations transmitted from the centrifugal fan to the air duct can be greatly reduced or eliminated at that rubber hose. (5) Select the shape and diameter of the motor cooling fan blades, as well as other parameters, appropriately. Effectively reduces the eddy current noise of the motor cooling fan. (6) Silencers are installed at the air inlet and outlet of the fan. Silencers utilize porous materials to absorb sound energy; this occurs when sound waves pass through the inlet and outlet areas covered with such porous materials. Sound waves cause the air molecules in the numerous tiny pores within the porous material to move vigorously; most of the sound energy is used to overcome frictional and viscous resistance, and is thus converted into heat energy and lost. This helps to reduce the aerodynamic noise generated by the centrifugal fan. Practice has shown. Silencers are installed at the inlet and outlet of centrifugal fans. They can typically reduce the aerodynamic noise generated at these locations by about 20–30 decibels (A). (7) Since the size of the exhaust opening at the impeller blades of a centrifugal fan is usually larger than that of the inlet opening at the front disk, as the air flow moves through the fan, many vortices are formed at the curved section of the inlet opening. The eddies will collide frequently with the fan volute and the components of the air inlet, thereby generating aerodynamic noise. By designing and installing rectifying rings and baffles at the periphery inside the fan volute at the fan’s inlet, it is possible to effectively prevent vortices from forming at the fan inlet, thereby reducing the aerodynamic noise generated by the centrifugal fan. (8) The fan impeller blades are designed as swept and twisted blades; that is, they tilt slightly forward at the exhaust opening and slightly backward at the inlet opening, which helps to avoid sudden changes in the gas flow path. It prevents the gas from forming vortices, thereby reducing the aerodynamic noise generated by the centrifugal fan.