Alternate passage divergence of wide chord transonic fan blades

Abstract

Due to manufacturing tolerance and deterioration during operation, fan blades in the same engine exhibit geometric variability. The absence of symmetry will inevitably exacerbate and contribute to the complexities of running geometry prediction as the blade variability is bound to be amplified by aerodynamic and centrifugal loading. In this study, the fan blade untwist (which is the blade deformation between its static condition and running condition) related phenomenon known as Alternate Passage Divergence (APD) is addressed. As the name suggests, APD manifests as alternating passage geometry (and hence alternating tip stagger pattern) when the fan stage is operating close to/at peak efficiency condition. APD can introduce adverse influence on fan performance, aeroacoustics behaviour, and high cycle fatigue characteristics of the blade. In this study, the APD behaviours of two transonic fan blade designs are compared. The main objective of the study is to identify the parameters contributing to the APD phenomenon.

After the formation of alternating tip stagger pattern, APD's unsteady effect can cause the blades from one group (segmented by tip stagger angle) to switch to the other, creating a travelling wave pattern around the circumference. It was found from numerical assessment on a randomly mis-staggered assembly that real engines can potentially experience such travelling disturbance and suffer fatigue damage. The phenomenon is termed APD-induced Non-Synchronous Vibration (NSV) and is abbreviated as NSV in this study. An idealised case is used to capture the bulk behaviour from the more complex cases in real engines and to decipher the underlying mechanism of this travelling disturbance. The results indicate that the driving force originates from the interaction between passage shock displacement and the passage geometry.

Based on the findings on APD & NSV, vibration attenuation methods are explored. Using machine learning techniques, a passive attenuation method is found to minimise the chance of NSV manifestation for a given set of fan blades. Alternatively, active attenuation method is implemented through blade redesign which modifies the passage geometry.