Mixed-flow pumps are characterized by their wide operating range and high-efficiency region in the fields of agricultural irrigation, hydropower engineering, and urban water supply systems. The stability of the transient processes, such as start-up and shutdown, has drawn great attention to the increasing capacity and impeller diameter of mixed-flow pumps in the industrial field. Particularly, the start-up process of a mixed-flow pump is an extremely complex transient process. The mixed-flow pumps can experience shock loading, hydraulic vibrations, and cavitation damage during the start-up process, leading to a negative impact on the stable operation of the pump. The evolution of noise characteristics at the pump section is of great significance for stability monitoring during the start-up process of mixed-flow pumps. This study aims to investigate the variation and influencing factors of noise under different start-up modes. An acoustic vibration test system was selected to collect the noise, shaft vibration, and pressure fluctuation signal of mixed-flow pumps under linear and non-linear starting modes. Then, the collected noise signals were analyzed using acoustic signal processing, including the energy entropy ratio analysis, spectral chart analysis, and A-weighted sound pressure calculation. A systematic analysis was made to clarify the influence of start-up on the sound pressure level and spectral characteristics of noise. The energy intensity in the noise was also analyzed. A coherence analysis was then implemented to determine the correlation among the impeller outlet pressure fluctuation, shaft vibration, and pump noise during the start-up process. The results showed that there was a concentrated region in the high amplitude sound pressure of the noise starting at the end of the start-up process in the three start-up modes. The shortest time scale was achieved in the high amplitude noise region under the concave exponential function start-up mode, indicating the least persistent impact on the environment. There was a relatively minimal impact noise generated by the concave exponential function start-up mode, indicating that this start-up mode was beneficial to avoid the severe mechanical impact during start-up. In addition, the formant analysis of the noise spectrogram showed that there was the lowest probability of mixed-flow pump’s instability induced by the concave exponential function start-up mode among the three start-up modes. The highest A-weighted sound pressure level of the noise generated by the pump was located at the frequency band with the center frequency of 250 Hz. The maximum A-weighted sound pressure level and the total effective sound pressure level of the noise generated by the pump under the linear start-up mode were less than those of the other two start-up modes. Compared with the linear start-up mode, the concave exponential function start-up mode improved the sound pressure level of the medium- and low-frequency noise. The wavelet partial coherence analysis showed that the impeller outlet pressure fluctuation was the main factor affecting the dominant sound pressure level (center frequency was 250 Hz) of the noise during the start-up process. The radial vibration of the shaft only affected the noise at the very low-frequency band.

Pumping stations can serve as crucial hydraulic facilities in various applications. However, their operating conditions can often lead to the presence of suction vortices in the sump. There are some serious impacts on the safe and stable operation of the pumping station. This study aims to explore the effect of the suction vortex morphology and its evolution on the stability of the centrifugal pumps under different operating conditions. A full-channel test platform was constructed for the vertical centrifugal pump. The research object was taken as the suction pipe on the horizontal side. The volume of fluid (VOF) was used to simulate the flow characteristics. A systematic analysis was made to explore the effects of the unsteady flow on the steady operation of a centrifugal pump during the evolution of the suction vortex at high flow rates. The results show that the steady duration of the suction vortex increased with the flow rate increased during testing. The shape of the vortex was greatly varied from the surface depression to the continuous suction. The strength and diameter of the suction vortex increased continuously at the high flow rates. The whole observation time was in the continuous phase of the suction vortex. Among them, the projected area remained at a high value. At the same time, the shedding of the large-scale bubble also caused the projected area to fluctuate violently. Excellent agreement was found in the development paths of the suction vortices that were captured by numerical simulations and the experimental ones (Q=15.6 m³/h). The suction vortices first appeared near the wall of the tube, then broke off into the suction tube under the action of the incoming flow after being destroyed, and flowed along the bottom of the suction tube to form two air paths and large bubble clusters at the inlet of the centrifugal pump. The air moved constantly toward the centrifugal pump at the high flow rates, as the suction vortex evolved. The content of air then reached a peak in the flow passage components during the continuous phase. As the suction vortices evolved from the development phase to the continuous phase, the void fraction rapidly increased, and the bubbles clustered in the flow channel of the impeller, leading to a sudden change in the pressure pulsation on the suction surface of the blades. The pulsation range of pressure in the continuous phase was 2 times that of the development phase. The main frequency of the pressure pulsation was shifted from f_n（rotating frequence）to 2f_n. While the amplitude of the pulsation at 2f_n increased along the flow direction. The gas action was also enhanced dramatically by the non-uniform inflow. Large-scale vortices were observed within the channel of the impeller. The greatest velocity of the gas was observed at the leading edge of the blade near the suction pipe side. In the rest of the channel, the gas velocity varied from increasing first to decreasing, leading to the increasing amplitude of the radial force on the impeller and the radial force vector to eccentric. The gas was distributed unevenly in the impeller flow passage and then converged at the outlet of the volute after rotating. The high peak-and-peak values of the pressure pulsations were found in the three specific regions (vaneless region, guide vane, and volute) of the centrifugal pump. The peak-to-peak values of pressure pulsations exhibited the most significant increase near the volute tongue, with increments of 59%, 57%, and 58% observed in the vaneless space, guide vane, and volute respectively. Furthermore, the bubble moved with the main flow towards the tongue in the continuous phase of the suction vortex. The density of the gas was much lower than that of the liquid. The bubble cluster was prevented from a pressure gradient to occupy the flow channel. There was a significant increase in the turbulent pulsation. The region was then expanded into the highly turbulent kinetic energy. Consequently, there was an increase in the peak-to-peak value of the pressure pulsation at the tongue, thus affecting the operational stability of centrifugal pumps.

Speed conversion has posed a significant impact on the stability of the mixed flow pump in the operation of variable speed. Taking the mixed flow pump as the research object, this study aims to collect the displacement signals of shaft vibration and pressure pulsation inside the pump under different speed conversion modes using a synchronous acquisition system. A systematic analysis was made to clarify the influence of speed conversion modes on the stability of mixed-flow pumps. The results show that the position of the shaft center was inconsistent with before the start of speed conversion at the end of speed conversion. There was the smallest offset distance of the quartic power speed conversion, which was 0.075 mm. The vortex motion of shaft vibration was the most stable under the speed conversion to the quartic power, compared with the rest. In the quartic power speed conversion, the ratio of S_(p-p) to ∆D was only greater than 0.33 at the end of the speed conversion. The maximum kurtosis of the two-dimensional joint was 5.67 (quarter power), 5.61 (linear), and 5.4 (quartic power), respectively, for the three-speed conversion modes. Among them, the proportion of positive joint kurtosis for the shaft vibration was 70% (quarter power), 44% (linear), and 4% (quartic power), respectively. The quartic power speed conversion effectively improved the mechanical impact of shaft vibration. The main frequency of shaft vibration was the rotational frequency f_n under different speed conversion modes. The proportion of high amplitude region was 87.5%, 62.5%, and 12.5% in the descending order of the index under the speed conversion mode. The high amplitude and high-order harmonic frequencies of shaft vibration were concentrated mainly in 2f_n among the three-speed conversion modes. Phase resonance caused the amplitude of higher-order harmonics of pressure pulsation inside the impeller to be stronger than f_BPF(passing frequency of impeller blades) during speed conversion. The proportions of high amplitude areas in 12f_n under the three-speed conversion modes were 75%, 50%, and 5% in the descending order of their indices. The main frequency of shaft vibration velocity still remained in the time-frequency of pressure pulsation inside the pump during speed conversion, indicating the high impact of shaft vibration on pressure pulsation. Finally, the improved TOPSIS was used to quantitatively evaluate the operational stability of mixed-flow pumps under speed conversion modes. The results showed that the operational stability of the mixed flow pump gradually deteriorated, as the speed increased. There was the smallest stability index of the quartic power speed conversion. The mixed flow pump has the best stability during the quartic power speed conversion, only considering shaft vibration and pressure pulsation. The findings can provide a strong reference to improve the stability of mixed flow pumps during speed conversion.