Research on Failure of Semi-Open Centrifugal Impeller Under Aerodynamic Load

The designs of centrifugal compressors are pushed towards higher pressure ratios, higher mass flow rates, and wider operating conditions. As the change of the actual condition, compressors often operate at low flow rates. There are some important unstable flow conditions at low flow rates such as rotating stall. The exciting forces may cause blade resonance and high dynamic stress level. High cycle fatigue failure is one of the main damage form of compressor impellers. Therefore, the dynamic stress prediction of impeller is an important part of compressor design and failure analysis. This paper is concerned with the prediction of dynamic stress of an actual damaged semi-open centrifugal impellers under unsteady aerodynamic load using a full nonlinear damping model which includes material and aerodynamic damping. Material damping is predicted based on an empirical equation and expressed as a function of stress amplitude. Aerodynamic damping is predicted through unidirectional fluid-structure interaction analysis. In this paper, the aerodynamic damping of the semi-open centrifugal impeller with various vibration amplitudes, modes and operation conditions is estimated. The numerical result indicates that, the material damping increases with the increasing stress amplitude while the aerodynamic damping is independent of the blade vibratory amplitude for a given blade mode. A nonlinear total damping model is then proposed, including both material and aerodynamic damping. The contribution of material damping plays an important part in total damping estimation as well as the aerodynamic damping. With this model, a procedure for dynamic stress estimation is proposed. Aerodynamic load on the surface of the impeller obtained by transient CFD calculation and the load in frequency domain obtained by Fast Fourier Transformation (FFT). Comparing normal condition with low flow condition, the main frequencies of the aerodynamic load are basically coincident and the load amplitude increases significantly under low flow rate. The main frequencies at the leading edge of blades are 4 times, 5 times and 6 times rotation frequency. They may be caused by rotating stall and excite the impeller. As the load of 5 times rotation frequency is maximum, harmonic analysis is performed to estimate the dynamic stress of the semi-open centrifugal compressor blades under the load. The result shows that the resonance stress at the leading edge of the blade under low flow condition is 22 times up on the stress under normal condition. The result of fatigue strength assessment shows that fatigue damage may occur at the leading edge of blades and it is consistent with the actual damaged position of the impeller. Therefore, fatigue damage is likely to happen under low flow rate condition. It is necessary to consider the situation carefully during the design of compressors.