Aeroelastic Stability of Axial Compressor Blades Under Different Operating Conditions

    Flutter is one of the important issues in turbomachinery analysis. When the compressor is operating at 100% speed, the flutter may occur in both cases where the  incidence is too large, supersonic stall flutter, or too small, supersonic non-stall flutter. So it is significant to study the aeroelastic stability when the compressor operating under different operating conditions.

    Based on the energy method proposed by Carta, this paper investigates the aerodynamic work on the second stage blade of axial flow compressor under different working conditions, including near-stall point A, near-maximum-efficiency point B, negative-incidence point C, and near-choke point D. The operating conditions mentioned above are based on the steady results of a 3.5-stage compressor. Therefore, the phenomenon associated with stall and choke, such as flow separation, is not obvious for the second stage blade and the aerodynamic work is mainly caused by shock.

     From point A to point D, it is found that the distribution of aerodynamic parameters, such as pressure and mach number, changed gradually with the reduction of back pressure. In order to analyze the variation law of flow and aerodynamic work in detail, 90% blade height section is selected as the characteristic section. The supersonic region at the leading edge of the suction surface move backwards into the blade channel with the supersonic zone appears at the leading edge of the pressure surface and expands to merge with the supersonic region at the suction surface. Comparing the results of aerodynamic work, pressure and mach number on the blade surface, it is found that the aerodynamic work region also changes with the move of the supersonic region, scilicet the move of the shock, and the distribution of the aerodynamic work is consistent the first harmonic of the pressure.

    Finally, the influence coefficient method (ICM) is used to study the aerodynamic damping of the blade at point B and point D and compare it with the results of the energy method. The relationship between the aerodynamic work of the blade and the vibration of the adjacent blade and itself is studied by combining the influence coefficient method and the energy method. It is found that the positive work, resulting in aeroelastic instability, is mainly caused by adjacent blades. Therefore, it can be concluded that one of the ways to suppress the flutter of the blade is controlling the unsteady aerodynamic force caused by the vibration of the adjacent blade. Based on the above results, the reason why blade mistuning can suppress flutter is that the work done by the unsteady aerodynamic forces generated by adjacent blades on the target blade cancels each other out over a large period when the frequency between the target blade and its adjacent blade is different.