Causes of high noise during operation of counter-rotating fan in coal mine

1. Vortex noise of counter-rotating fan in coal mine: it is generated by various separated vortices when the airflow flows. It is generally believed that there are 4 causes
① The turbulent boundary layer noise generated by the turbulent boundary layer on the surface of the blade due to the pulsating turbulent boundary layer

② The debody vortex noise caused by the shedding of the turbulent boundary layer on the blade surface at the trailing edge of the blade.

③ The top vortex noise formed by the secondary flow from the concave surface to the convex surface caused by the concave surface of the coal mine counter-rotating fan at the top of the blade due to the concave surface pressure greater than the convex surface

④ When the airflow with a certain degree of incoming turbulence flows to the blade, the incoming turbulence noise is generated.

2. Discrete noise (rotating noise) of coal mine counter-rotating fan: It is related to the rotation of the impeller, especially in the case of high speed and low load, this noise is particularly prominent. Discrete noise is the noise generated by the interaction between the asymmetric structure around the blade and the circumferential uneven flow field formed by the rotation of the blade mouth design test. It is generally believed that there are the following three types:
① Rotation frequency noise generated by blades in unsmooth or asymmetric casing

② The outlet interference noise generated by the existence of the guide vane behind the fan, the discrete noise has discrete spectral characteristics, the fundamental frequency (the corresponding frequency when I = 1) has the strongest noise, and the higher harmonics decrease accordingly.

③ Air intake interference noise due to the presence of the leading vane or metal mesh cover in front of the air inlet