Rotating instabilities (RIs) have been reported in axial flow fans as well as in low-speed and high-speed axial compressors during recent years. They can cause the non-synchronous vibrations (NSV) and generate noise.
In this paper, numerical simulations are performed on an axial flow fan to investigate the influence of RIs on aerodynamic damping of fan blades. The phenomena of RIs in the axial fan were first observed during an experimental test. The spectrum of the pressure signal captured by the case-mounted unsteady pressure transducers near blade tips contains narrow frequency bands below the blade passing frequency, of which the dominant frequency does not equal harmonics of rotor shaft frequency.
The numerical investigation is performed with an unsteady 3D Navier-Stokes method that solves for the entire blade row at near-stall point to confirm the phenomena. Flow separation and blade tip vortex shedding are captured at the tip region of the blades in the simulation. Although the spectrums are different between numerical simulation and experimental results, the simulation predicts a dominant pressure fluctuation frequency of RI that is almost the same as the experimentally measured value, which verified the accuracy of the numerical simulation. Aeroelasticity of the fan blade is investigated by influence coefficient method based eigenvalue method. At near-maximum-efficiency point and near-stall point, aerodynamic modal damping ratios (AMDRs) and distribution of local AMDR of first two modes (first and second flapping mode) are computed respectively. The separation region improves the stability of both modes. However, the vortex shedding area on suction side reduce the AMDRs of 1F mode while the vortex impingement region on pressure side reduces local AMDR for both modes.