Nonlinear acoustic diametral modes of pressure pulsations during flutter of rotor blades of a turbocompressor

Abstract

The feature of the occurrence and development of classic flutter for rotor stage blades of the blisk (bladed disk) type for an axial turbocompressor in aircraft engines is that flutter at the natural frequency of blades is preceded with the resonance on the other natural frequency of the blades, which is caused by the torsion frequency harmonic of the rotor. With increasing the rotor speed, the resonance development interrupts as a result of the occurrence of flutter-synchronous vibrations of blades at their natural fre quency, which is not connected by any numerical relationships with the rotor speed. The synchronization of vibrations of rotor blades in the aeroelastic system occurs because of the elasticity of the compressed air flow, in which acoustic diametral modes of pressure pulsations appear during flutter. The flutter of tur bocompressor rotor blades is described by the model of circular modes of synchronous vibrations:,in which blades of the rotor stage vibrate with natural frequency f B of blades and constant phase shiftwhere N b is the number of blades in the rotor stage of the tur bocompressor. The precedence of resonance to flutter, however, with the competition of resonance and flutter for only one vibra tion mode of rotor blades, is established in [1]. The dependence of rotor speed f R on the time to occurrence of the bending flutter of blades of the first stage at f R ≈ 60 Hz dur ing the test of the blisk type rotor of the turbocompressor consid ered is represented in Fig. 1. It is established [2] that torsion modes of blades with frequency f T ≈ 178 Hz, which is close to harmonic 3f R of the rotor speed, are primarily excited in the time interval t = 4, …, 20 s. With increasing rotor speed, the reso nance departure at frequency f T of the torsion mode of blades occurs, and in time interval t = 17.4, …, 24.6 s flutter with a pos itive damping factor (logarithmic decrement with the opposite sign) occurs at a lower bending frequency of the blades,Abstract-Features of pressure pulsations of flow during flutter on the bending frequency of rotor blades of an axial turbocompressor, which occurs after incomplete resonance at the torsion frequency of blades with a harmonic of rotor speed, are investigated. The frequency spectrum of nonlinear acous tic diametral modes is predicted, based on Navier-Stokes equations ensemble averaged. This makes it possible to identify characteristic series determined by the bending frequency of flutter and its sec ond harmonic in the frequency spectrum for recording the pressure pulsation of flow during flutter along with harmonics of rotor speed. The occurrence the second harmonic of the bending frequency is the nonlinear reaction of flow, which locally limits the flutter amplitude in time.

Initiation and development of flutter of axial turbo compressor rotor blades

“…3 the growth of flow pressure pulsation energy of and oscillations of the 14th blade happens almost simultaneously. This is confirmed by the simultaneity of the damping factor peaks at frequency f B ≈ 81 Hz of flexural oscillations of 14th blade and damping factor for the frequency of acoustic mode ≈ 201 Hz [7]. Analysis of time dependencies of short term energy oscillations of blades and airflow pressure pulsa tions using the CPM models shows that there is an excitation delay of flexural oscillations of the separate rotor blades in comparison with excitation of pulsation modes of airflow pressure.…”

“…In addition, the formulas for frequencies of acoustic diamet rical modes of flow pressure pulsations accompanying the flatter have been obtained. [7]. For frequencies and under t = 22 s (setting of flutter) the damping factor sharply changes its sign to plus.…”

“…The work [7] presents a record of flow pressure pulsations under a flutter in the studied axial turbocharger, which is simultaneous to records u i , i = , of unstable oscillations of blades 3, 5, and 14. In addition, the formulas for frequencies of acoustic diamet rical modes of flow pressure pulsations accompanying the flatter have been obtained.…”