Fluid-Structure Coupling Features Analysis And Fault Diagnosis Of Fan Blade

The stall flutter is one of the common faults which exists in fans, at the same time, the blade crack is another hidden danger, fan blade crack is also prevalent in a serious security risk while fans are working. So, it is important for fans'safe operation to prove the theory of stall flutter and detect as early as possible the existence of the crack. The paper systematically researches the relationship between blade natural frequency and flow fluctuation frequency, then studies the fluid-structure coupling feature of embodiment, as well as carries on the fault diagnosis of blade according to above two points. The detailed work is as follows:Firstly, for exploring the generation mechanism of fan stall flutter, the method of seeking the stall flutter frequency is put forward according to the relationship between flow fluctuation and blade structure. The method analyses the fan blade's modal and the spectrum of flow fluctuation signal. The result shows that the flutter caused by flow fluctuation relates with natural frequencies of blade within the scope flow velocity. That is, the frequency of flow fluctuation and the blade natural frequency tend to be the same. The frequency of flow fluctuation will escape the influence of natural frequency when flow field is far way from stall attack angle. The natural frequency of the blade still affects the frequency of separation flow in the large attack angle, but there is not an obvious frequency lock region. The result provides the experimental basis for exploring the theory of fan stall flutter, and proves that the blade structure influent the flow fluctuation frequency.Secondly, a multi-feature analysis method is proposed to study embodiment feature of the fluid-solid coupling. This method analyses the vibration signals of axial impeller and flow fluctuation collected in different working condition using the phase-space reconstruction, correlation dimension, approximate entropy and L-Z complexity. Experimental results prove that two kinds of signals have coupled characteristics of embodiment; it is the fundamental of diagnosing the blades'crack fault using by the flow fluctuation signals.Thirdly, the research is performed to detect the existence of the blade crack. The signal energy mainly concentrates in low-frequency and decreases with the increasing of frequency. Coupling vibrations are more nearer more embodied each other. For detecting the blade crack fault, the method which combines M-band wavelet and 2-band wavelet analysis is proposed.Aerodynamic signals collected on different working conditions are decomposed and reconstructed. Then the energies of every frequency band are calculated as the character vectors. Finally, the classifier is trained to class the character vectors and recognize the fault. The experimental results indicate that this method can recognize the fault of early blade crack effectively.Finally, for finding out the changes of the flow fluctuation signal with the blade crack spreading, the statistical analysis on the wavelet sub-band energy of the flow fluctuation signal collected on the different working condition is carried out. Analysis results show that high frequency band energy reduces and the low frequency band energy increases with the spread of blade cracks. So, the flow fluctuation frequency range gradually expands because of the spread of blade cracks. The results provide the basis for blade crack fault monitoring and diagnosis. At the same time, the reducing of blade stiffness and the low-frequency of flow fluctuation signal with the spread of crack indicate that the separated flow pulse frequency will be consistent with blade natural frequency.