Stall flutter for incomplete synchronization of turbocompressor-blade vibrations

摘要

In [1], flutter evolution under vibration synchronization of rotor blades of turbocompressors is considered on the basis of the spectral analysis of their trial record. In this study, we investigated the features of stall flutter for incomplete bladevibration synchronization, which are mapped by the transient process for the rotorrotation frequency (Fig. 1). Rotational stall is the located separation (or reversal) cell, which rotates with the frequency of the order of magnitude from 30 to 70% of the rotorrotation frequency fR in the direction of the compressor blades [2, 3]. As a rule, the rotational stall arises on the compressor stator consisting of the fixed blades. In the compressor, both for the rotational stall and for the general phenomenon of rotation instability, there is almost periodic airpressure pulsation with the frequency < fR ( fR is the rotorrotation frequency) [4, 5]. In the strongly loaded fan (compressor), the rotationalstall frequency ≈ 62 Hz. As can be seen in Fig. 2, the rotational stall arises at t = 1.6787 s (the moment of the sudden change in the dampingfactor sign (logarithmic decrement) from “–” to “+” at the frequency ). In the coordinate system related to the compressor stator, the flowpressure pulsation is characterized by the almost harmonic discrete spectrum of frequencies ≈ n , n = 1, 2, …. The frequency components of the flowpressure pulsations are excited at different moments of time, but the time change in the damping on harmonics , n = 2, 3,…, reproduces the dependence on the frequency ≈ 62 Hz (Fig. 2). In this case, the frequency (the harmonic of the order 1) is first excited. In the table, we listed the spectral characteristics: is the frequency, and is the damping factor for the frequency components of airflowpressure pulsations at t = 4.65 s. The parameters of the frequency components are identified individually. The table shows the accuracy with which the condition of the multiplicity of their frequencies is satisfied. On average, ≈ 433 Hz. At t = 4.65 s, the frequency of the seventh harmonic accepts the highest value (see table) and, after that, decreases sharply. The rota tional stall is mapped in the frequency spectrum and the damping of the rotorblade vibrations; however, these vibrations are not synchronized. For example, the moments of change in the damping sign from “–” to “+” at the frequency of 229 Hz for the rotor blades nos. 8, 18, and 31 amount to 2.23, 2.27, and 2.25 s.