NON-SYNCHRONOUS VIBRATION AND RELATED TIP CLEARANCE FLOW CHARACTERISTIC IN A LOW-SPEED AXIAL FAN

An experimental study investigating non-synchronous vibration due to rotating instability was conducted on a single-stage axial fan. The experiment monitored high-amplitude vibrations that could lead to high-cycle fatigue of the blades, and a wire mesh casing treatment was successfully employed to suppress the vibration. Two blade materials, metal and nylon, were used in the experiment under two different tip clearance conditions. Under the small tip clearance condition, no vibration causing high-cycle fatigue was observed, and there was no indication of rotating instability flow. For the case of enlarged tip clearance with nylon blades, as throttling proceeded, the azimuthal mode decreased. When the frequency in the rotating frame of reference closely matched the blade vibration frequency, it induced high-amplitude non-synchronous vibration. In response to this phenomenon, the paper conducts a mechanism analysis on the mode and frequency of the rotating source. Under the same enlarged tip clearance configuration, a wire mesh casing treatment was tested. While expanding the stability margin, it successfully suppressed the high-amplitude vibration phenomenon occurring in the near-stall area.