Transonic Fan Blade Redesign Approach to Attenuate Nonsynchronous Vibration

Due to manufacturing tolerance and deterioration during operation, different blades in a fan assembly exhibit geometric variability. This leads to asymmetry which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternative Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference. After the formation of alternating tip stagger pattern, APD’s unsteady effect, Non-Synchronous Vibration (NSV), can cause the blades from one group to switch to the other creating a travelling wave pattern around the circumference. Thus, it can potentially lead to high cycle fatigue issues. More importantly, this phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. Therefore, it is vital to attenuate the NSV behaviour.

It was found from a previous study (Lu et al, 2019) that the dominant blade design feature leading to NSV is the covered passage geometry. Thus, in this study, a blade redesign with modified passage geometry is evaluated. It can be demonstrated through the aeromechanical comparison of the original blade and the fully modified blade that such redesign approach could efficiently minimise the APD behaviour. More importantly, it can eliminate NSV behaviour completely.

The NSV phenomenon is examined numerically by a partially coupled aeroelastic solver. The solver, AU3D, is developed by Imperial College London and has been proven to be proficient in conducting aerodynamic and aeroelastic prediction at off-design conditions for numerous fans and compressors.

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