Abstract

Due to imperfection in the manufacturing process and in-service wear, fan blades in a turbofan engine do not have the same geometry. This lack of symmetry inevitably leads to difficulties in predicting the fan blades’running geometry in an assembly as blade variability is amplified by the aerodynamic and centrifugal loading. This variability can lead to an aeromechanical phenomenon termed ‘Alternate Passage Divergence’ (APD). As the name suggests, it manifests itself as alternating geometric and aerodynamic patterns in a fan assembly during operation. APD can potentially influence fan performance, stability, and Multiple Pure Tone (MPT)/’Buzz Saw’ noise and is therefore an important area of research.

For this study, the APD phenomenon is purposely triggered on a transonic fan blade. A full assembly computational domain with one mis-staggered blade is used to examine whole assembly performance and untwist behaviour with APD. In particular, the influence of fan blade stiffness on APD behaviour is examined. The behaviour of the current blade is compared to that of a blade used in a precursor study and it is found that, under certain conditions, the blades show similar behaviour even though they have distinctly different geometry features. This illustrates that it is important to understand the phenomenon as accurate prediction of running geometry is vital at early design stage.

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