Influence of cavitation on radial force and pressure pulsation of a centrifugal pump

Cavitation is a kind of complex multiphase flows. When unstable cavitation flow occurred inside the centrifugal pump, it could induce high amplitude pressure pulsation inside the centrifugal pump and intensify pump body vibration, which is posing a threat to the safety and stability of the pump operation. To exploring the variation laws of the radial force and the pressure pulsation of the impeller as cavitation development, this study took a closed centrifugal pump as the research object to investigate the cavitation flow in centrifugal pump under design flow rate by unsteady numerical simulation. The homogeneous Eulerian-Eulerian two-fluid model was used to simulate the cavitation flow. The two equation SST (Shear Stress Transmission) k-ω model was adopted as turbulence model to closure the URANS (Unsteady Reynolds Averaged Navier-Stokes) equations and the Schnerr-Sauer cavitation model was used to solve the volume fraction of vapor phase. And the reliability of the model and numerical simulation settings in this study was verified by comparing with experimental values. The influence of vapor bubble changes on radial force and pressure pulsation was analyzed. The research results indicated that the time averaged radial force on the impeller first slightly decreases and then suddenly increases as cavitation develops. The turning point of the change was the condition that the sheet cavitation appeared near the shroud. Under the complete cavitation condition, the value of the time averaged radial force can reach as high as 60.5N, which is 1.4 times higher than under non cavitation condition. And the peak value of radial force can reach up the maximum of 120N, which is 1.69 times that of the no cavitation stage. The high amplitude radial force could make the pump body more prone to instantaneous large vibration. And the presence of bubbles disrupted the symmetry of pressure distribution, resulting in a polygonal distribution of radial force. Under different cavitation states, the frequency of radial force variation is mainly determined by the impeller rotational frequency and its multiplication. When cavitation develops to a certain extent, the amplitude of radial force corresponding to the impeller rotational frequency no longer changes, only the amplitude of its multiplication increases. When the net positive suction head was 1.9 m, the amplitude of radial force corresponding to the impeller rotational frequency was 41N, about 1.4 times that under non cavitation condition. And the amplitude of radial force corresponding to the sub frequency of 2 times of the impeller rotational frequency was 10N, which was 1.7 times that under non cavitation condition. The influence of the breaking of vapor bubbles on pressure fluctuations had a certain degree of global significance. However, the degree of influence depended on the relative position of the monitoring points and bubble clusters. When the monitoring point was located inside the bubble cluster, the pressure coefficient and standard deviation basically were 0. When the monitoring point was located near the bubble clusters, its pressure would undergo a significant increase and decrease process as the vapor bubbles break and aggregate. The pressure wave generated by the breaking of vapor bubbles also would develop downstream along the flow direction. Under different cavitation conditions, the main frequency of pressure pulsation at each monitoring point is the impeller rotational frequency. The slow evolution of cavity volume in impeller will induce low-frequency pressure pulsations of 0.5 times impeller rotational frequency during the complete cavitation stage. From the comprehensive comparison of the changes in pressure pulsation, the magnitude of the main frequency amplitude of radial force is more suitable for judging the development of cavitation flow field in centrifugal pumps. The research results can provide theoretical references for the monitoring and diagnosis of cavitation in centrifugal pumps.