The following analysis of the causes of fan noise:

Fan discrete noise (rotating noise): It is related to the rotation of the impeller. In particular, the noise of high flow exhaust fan is particularly prominent under the condition of high speed and low load. Discrete noise is caused by the interaction between the asymmetric structure around the blade and the circumferential uneven flow field formed by the design test rotation of the blade port. Generally, it is considered that there are the following types:

(1) Intake interference noise due to the presence of front guide vanes or metal mesh cover in front of the air inlet;

(2) Rotating frequency noise generated by blades in unsmooth or asymmetric casing;

(3) The outlet interference noise generated by the existence of volute tongue at centrifugal air outlet or the existence of rear guide vane of axial flow fan. The discrete noise has discrete spectrum characteristics, and the fundamental frequency (the corresponding frequency when i=1) has the strongest noise , And the higher harmonics decrease accordingly.

Fan vortex noise: it is caused by various separated vortices during air flow. It is generally considered that there are four causes:

(1) When the airflow with a certain degree of inflow turbulence flows to the blades, the inflow turbulence noise is generated;

(2) The turbulent boundary layer noise caused by the turbulent boundary layer of the pulsating exhaust fan when the air flows through the blade surface;

(3) The detached vortex noise caused by the falling off of the turbulent boundary layer on the blade surface at the blade trailing edge;

Because the concave pressure of axial ventilation fan is greater than the convex pressure, the secondary flow from the concave to the convex at the top of the blade is taken away by the main air flow to form the top vortex noise.