The vibration induced by flow is one of the important factors to the instability of the mixed-flow pump. With the increase of the capacity and the size of mixed-flow pumps, the vibration of the hydraulic components attracts more attention from researchers and engineers. In order to study the vibration of mixed-flow pump induced by hydrodynamic force, based on the Bentley 408 data acquisition system, vibration signals in 3 directions (X, Y and Z) on the base under unloaded and designed flow conditions of the mixed-flow pump are tested and then analyzed using Hilbert-Huang transform (HHT). The original vibration signal is decomposed by EMD (empirical mode decomposition) using the Hilbert-Huang transform, and the spectral distribution of the different mode function components is obtained. The decomposed signal contains intrinsic modulus with 11 different orders and one residual. The acquired intrinsic modulus represents vibration signal with different frequencies, except the Intrinsic mode function 11 and the residual, which show no periodic characteristics. The results show that the vibration under loaded condition is significantly increased compared with the unloaded operating condition, but the vibrations caused by the hydraulic excitation differ in different directions. When the mixed-flow pump is working under designed operating condition, the low frequency vibration occupies the main vibration energy distribution, making the main mode of the different mode function components move to the low frequency direction, and the hydraulic induced vibration is dominated by the middle and low frequency vibration. In the X direction, the vibration spectrum distribution is similar in the 2 cases, while in the Y and Z direction and on the base of mixed-flow pump the frequency distribution of the waveform is narrowed and the energy distribution is concentrated under loaded condition. Under both loaded and unloaded conditions, the main frequency of the spectrum moves toward the low frequency region with the increase of the modulus order. The difference of the frequency spectrum under loaded and unloaded conditions is more obvious for the intrinsic modulus with lower order, namely Intrinsic mode function 1-3. Under unloaded condition, frequency spectrum of the Intrinsic mode function 1-3 reaches the highest value near 600 Hz; while under loaded operating condition, the position for the peak values moves from 900 to 300 Hz for the spectrum of the intrinsic modulus of the first 3 orders. For the other intrinsic moduli, the peak of the frequency spectrum is distributed below 200 Hz, which indicates that the vibration induced by flow is mainly composed of low and middle frequency vibrations. Compared with the frequency spectrum of vibration signal under unloaded condition, the distribution of vibration spectrum under loaded operating condition is narrower, and the power distribution of the vibration is more concentrated, indicating that the vibration induced by flow increases the vibration of the pump and the hydraulic vibration makes the main frequency of the pump vibration move toward the low frequency region in the frequency spectrum. In the original spectrum under loaded operating condition, the amplitude of the vibration spectrum is higher in the region of 200-600 Hz, which indicates that the vibration power is high in this frequency section, and the vibration caused by hydraulic force is greater there. According to the original vibration signal on the base of the pump, it was found that in the frequency region from 0 to 400 Hz, the amplitude of the vibration spectrum under loaded operating condition is 2 times higher than that of other frequencies, which indicates that low frequency vibration occupies the vibration on the base of the mixed-flow pump. The research results have important engineering application value and theoretical guidance for effectively reducing or preventing the vibration of mixed-flow pumps. © 2018, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.
