Generally speaking, the sound of centrifugal fans and axial fans is relatively high. The reasons for their noise can be summarized as the following three:
1. Noise caused by aerodynamics.
2. The sound produced by mechanical vibration.
3. The noise generated by the interaction between the two.
Whether it is a duct fan or any other machine, understanding and mastering the noise source that produces its noise is the most fundamental and effective method of noise control.
1. Aerodynamic noise
1. Impact noise
When the impeller rotates at a high speed, the blades move periodically, the air particles are subjected to periodic force, and the impact pressure wave impacts the noise at the speed of sound.
The intensity of this noise varies with the geometry and size of the blade.
If the rotation speed of the impeller per second is n(rpe), when the number of blades is z, its basic frequency fc is determined by the following formula: fc=nz, Hz
The higher the pressure, the greater the circumferential speed of the impeller, and the greater the noise.
2. Eddy current noise
The noise caused by the vortex generated by the separation of the gas boundary layer when the impeller rotates at a high speed.
The frequency of eddy current noise: f=K(v/d), Hz
formula:
K-constant 0.15~0.22;
v- the velocity of the blade relative to the gas, m/s;
D- the width of the blade in the direction of the gas inlet, m.
Due to the irregular movement of the disaster current, it sometimes grows and sometimes attenuates, so that the flow noise has a wide range of rates.
2. Mechanical vibration voice
The vibration caused by the unbalance of the rotating body and the wear and damage of the bearing must be accompanied by noise. When the blade rigidity is insufficient, the vibration of the blade due to the airflow will also produce noise.
3. Noise generated by the interaction between the two
The rotation of the blades causes its own vibration to be transmitted through the pipeline, and often shocks and eddy currents occur in the bend of the pipeline, causing vibration to increase the noise, especially when the air pressure sound wave frequency is consistent with the natural vibration frequency of the pipeline, strong resonance will occur, and the noise will suddenly occur Increase, and cause damage to the machine in severe cases.
In addition, when the main shaft of the centrifugal fan and the axial fan is driven by the motor, gear speed increasing device or belt, it will also cause vibration and generate secondary noise. The noise of centrifugal fan and axial fan, from the characteristics of sound source, belongs to dipole source radiation noise (or called dual-source radiation sound).
The directivity pattern is a figure of eight, and the sound waves in the axial direction are stronger, and the sound waves in the direction perpendicular to the axis are weaker.
