Experimental investigation of aeroelastic instabilities in an aeroengine fan: Using acoustic measurements

Abstract

Aeroelastic instabilities will occur to the aero-engine fans inevitably during transient operations, causing excessive blade vibration and threatening the structural integrity. However, the complicated fluid-solid coupling interactions in aero-engine are difficult to be described precisely using current numerical or analytical models. Instead, experimental investigations on the flow induced blade vibration can reveal the aeroelastic performance of aero-engines, which is regarded as an effective approach to provide instructive guidance in the design of aero-engine. In this paper, experiments on a realistic aero-engine fan with 3.5 stages is conducted to study the typical aeroelastic instabilities, including the blade forced response, flutter, and acoustic resonance. In addition to the conventional blade strain measurements, the duct acoustic array measurements are also implemented to characterize the distinguishing acoustic features of the aeroelastic instabilities. Particularly, the acoustic mode decomposition technique is employed to provide insight into the unsteady flow as the aeroelastic instabilities occur, by which the blade vibration and aerodynamic excitation are bridged. The run-up test is conducted firstly to investigate the blade forced response and flutter; thereafter, a transient test is designed to encounter the acoustic resonance. The results illustrate the acoustic features of aeroelastic instabilities properly, validating the feasibility of acoustic measurement for instability identification. When the blade forced response occurs in the run-up test, the amplitude of Tyler-Sofrin mode increases significantly. The flutter also occurs in the run-up test and is believed to be excited by the rotating instability, where a sloping strip in the acoustic mode spectrum can be observed. In terms of the acoustic resonance, a nonsynchronous tonal component is observed below the blade passing frequency, where the dominant acoustic mode order matches the nodal diameter number. This paper presents an alternative way to study vibrations using acoustic measurements and the acoustic mode decomposition, which is validated by experiments on a realistic aero-engine fan.