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Noise analysis of axial fans

- Aug 01, 2018 -

The total noise level of the fan is proportional to the sixth power of the blade speed. According to the analysis, the fan noise source is basically dipole-like. Further, the noise is caused by the action of the blades on the pulsating power of the air flowing through the fan. It can be considered that there are two discrete frequency noise sources of the fan, one is the propeller type noise caused by the pressure field of the rotor blade movement, and the other is the blade pulse dynamic noise caused by the aerodynamic interference. The distance between wind maneuver and static blades is an important factor in interference noise.

When this distance is small, changes in the bit stream and the wake will have an effect, and the blade may also act as a sound barrier, reinforcing the acoustic radiation generated by the lift pulsations on the blades adjacent to the row of blades. This effect depends on the ratio of the wavelength of the acoustic wave associated with the lift pulsation to the size of the blade as a barrier. In the frequency range where the ratio is greater than 2, the change in the intensity of the radiation due to this effect is most pronounced. Therefore, when a radiation noise blade has the same number of blades upstream and downstream, and each of the two rows of blades simultaneously meets one rotor blade to form a sound barrier on both sides of the source, the effect will be stronger.

When the distance between the moving and stationary blades increases, the influence of the interference of the bit flow is much faster than the change of the wake velocity, and the effect of the blade as a sound barrier also decreases with the increase of the distance. It can be seen that at least three parameters affect the magnitude of the interference noise: the blade shape of the velocity field waveform (ie, the blade load), the distance between the blade rows, and the blade radiation area as the sound source. Very small spacing can have two acoustic effects. If the force pulsation established by the stationary blade interference field on the moving blade makes the moving blade become a sound source, the stationary blade is a sound barrier.