Investigation of stall flutter of fan blades by Blade Tip Timing

Proceedings of the 15th International Symposium on Unsteady Aerodynamics,

Aeroacoustics & Aeroelasticity of Turbomachines
ISUAAAT15
24 – 27 September 2018
University of Oxford

Abstract: Flutter is a system mode that all blades vibrate at the same frequency but with a different phase angle, which can form a nodal diameter (ND) pattern. One of the advantages of blade tip-timing (BTT) technique is to measure the vibrational displacements of all blades, which makes BTT a potential method to study flutter. This paper investigates the stall flutter of fan blades during an acceleration process through a BTT system with eight casing-mounted optical probes and one OPR (once per revolution) sensor. By applying the traveling wave plot, the non-integral vibration with three different frequencies are observed. In order to calculate the ND, a cross-power spectral analysis based on the data of two probes is used. Results indicate that it’s a subsonic stall flutter with three nodal diameters (2, 3, 4) at engine order (EO) 1.5 approximately, however, ND 2 is dominant. Combined with Campbell diagram, EO1.5 crossing M1 was found at around 92% design speed, which is consistent with experimental results. During flutter, as the increase of rotor speed, the amplitudes of all blades experience a rapid growth at first. Then they level off into a steady oscillating condition known as the limit amplitude region. When the rotor speed increases further, the blade amplitude keeps growing and go into another limit amplitude region. The development of nodal diameter as a function of rotor speed explains the formation of flutter. Furthermore, the change of phase difference between two blades shows a particular “rhombus” shape before flutter, which provides a new perspective to predict it.