1. The vibration caused by the eddy current pulsation of the bellows. The structural design of the inlet bellows is unreasonable, causing the airflow in the inlet bellows to generate violent vortices, which are accelerated and enlarged in the inlet collector of the centrifugal fan, thereby stimulating a large pulsating pressure wave. Its vibration characteristics are: the pressure wave is often irregular, and the amplitude increases with the increase of the flow rate.


2. Vibration caused by local eddy current in the air duct. The unreasonable design of some parts of the air duct (elbow, diffuser pipe section) results in poor air flow, local eddy currents in the air duct or mutual interference and collision of the air flow, causing pressure pulsation of the air flow, thereby stimulating noise and vibration of the air duct . Its vibration characteristics: the vibration is irregular, and the amplitude increases with the increase of the load.


3. Insufficient rigidity of centrifugal fan casing and air duct wall causes vibration. Where the stiffness is weaker, the amplitude is larger.


4. Rotational stall. When the angle of attack of the airflow reaches the critical value, the airflow will leave the convex surface of the blade, and the separation of the boundary layer will occur, thereby generating vortices in a large number of areas, resulting in a drop in the wind pressure of the fan. Rotating stall mainly occurs in axial flow fans. In the impeller and blade diffuser of centrifugal fan, due to the decrease of flow rate, rotating stall will also occur.


Characteristics of vibration caused by rotating stall:


(1) The vibrating parts are often in the air inlet box and outlet air duct of the fan;


(2) Vibration mostly occurs on fans with imported louver-type adjustment baffles and backward-curved blades;


(3) Strong vibration occurs when the opening of the baffle is 0-30%, and drops to the normal value when the opening exceeds 30%;


(4) When the rotating stall occurs, the flow and pressure of the centrifugal fan will pulsate strongly.