Axial fans have gradually replaced centrifugal fans and become the mainstream due to their advantages of large flow, small starting torque, and strong adaptability to changes in the air duct system. Air pulsation is the product of ubiquitous air separation and vortex development.
Ⅰ. Reasons for fluid pulsation in axial flow fans
For the axial flow fan, except for the fluid pulsation caused by the unreasonable design of the flow structure such as the inlet and outlet flow channels and baffles in the original design and the later transformation, the reasons for the fluid pulsation of the axial flow fan in operation are as follows:
1. The blade opening of the adjustable axial flow fan has a large redundancy. The change of the opening of the stator under low load can easily cause the working point of the axial flow fan to fall into the unstable operation area, resulting in fluid pulsation or even surge, causing strong vibration.
2. Due to welding stiffness, local stress, corrosion or the entry of foreign matter, the moving blades and guide vanes of the axial flow fan are severely worn or even partially fallen off, causing fluid pulsation.
3. Due to abnormal baffles and blockage of foreign bodies at the inlet of the axial flow fan, the system resistance increases and the flow is insufficient, causing fluid pulsation, stall and even surge.
Ⅱ. Vibration phenomenon of axial flow fan
The actual test data on site show that the vibration phenomena and characteristics of the axial fans caused by the above-mentioned fluid pulsation are similar, mainly including the following points:
1. Air pulsation mostly causes the vibration of the casing, inlet and outlet pipes and the foundation of the casing of the axial flow fan, which has little impact on the mechanical vibration of the bearing and rotor. The onset frequency is mainly low-frequency components unrelated to the main frequency of the rotor.
2. When the moving blades or stationary blades are worn and ruptured to generate airflow pulsation, the airflow pulsation will be coupled with mechanical vibration. At this time, a larger blade passing frequency and its harmonics will appear in the airflow pulsation fault frequency, and the bearing and rotor will also have failure frequency.
3. Air pulsation not only causes vibration changes of the axial fan, but also causes the current and flow of the axial fan to be unstable and even fluctuate greatly, resulting in a large difference in current between the two axial fans running in parallel under the same air volume, and there is obvious airflow noise on site.
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