Vibration Analysis of a Micro Turbojet Engine under Two Impeller Structural Faults

Disk fracture and locking nut loosening of centrifugal impeller leads to severe vibration of micro turbojet engines, posing significant threats to engine stability and safety. In order to identify the engine vibration characteristics induced by the above two faults in a practical micro aero-engine, vibration acceleration of the engine casing is collected and analysed comprehensively both in time and frequency domain. The order tracking resampling method is applied to analyse the transient vibration response. The vibration trend during the pre- and post-fault phases is assessed from the perspective of root mean square (RMS) value of the vibration velocity. In time domain, a transient phase is observed when the structural faults occur, followed by a steady state phase. For the disk fracture case, the vibration amplitude at the rotating frequency (RF, 1X) is increased significantly. On the contrary, the amplitude at the blade passing frequency (BPF, 7X) is decreased slightly. Furthermore, the presence of subharmonic components is captured in the transient phase, which reflects the rebalance of flow in the impeller. A frequency modulation between the BPF and the RF is detected in the steady state. As for the nut-loosening fault, the vibration of 1X and 3X components is increased, while the 7X amplitude remains almost unchanged. The RMS value of vibration velocity in the post-fault steady state is much higher than that of normal state for both fault conditions.