In this paper, the S-CLSVOF (simple coupled level-set and volume of fluid) and BM (bifurcation model) models are used for the accurate prediction of the flow in an open pump intake with a vertical pipe. The continuous wavelet transform, which is suitable for the vortex detection is applied to the pressure signals near both air-entrained and subsurface vortices. Low-frequency with long duration for air-entrained vortex due to the vortex wandering and broadband with short duration for floor vortices (dominant in subsurface vortices) due to the generation of extreme strong vortex are observed. The vortex motion mechanism is revealed by the analysis of the transport equation of the vertical vorticity's enstrophy. Different from previous results in which the tilting effect controls the vortex motion, the stretching effect is found to be dominating when it is large enough. When going through the bell mouth, the air-entrained vortex's vorticity changes the sign and strengthens the vortex with the same sign. On the plane near the bell mouth, three vortex patterns including co-rotating pair, merging and counter-rotating pair are observed. The onset criterion of the vortex merging at a/b = 0.29~0.32 is found to be applicable to the present case. The counter-rotating pair is found to be more stable. Air-entrained vortex serves as an amplification of the strong vortices generated from subsurface.

This study aims to investigate the critical state of the saddle-shaped unstable region of the axial-flow pump and propose a suitable criterion for identifying this state. The bifurcation SST $k–\omega$ model considered the rotation effect is used in the present work and verified in the numerical calculation of a water jet pump. Then, it is used to simulate the critical state of the axial-flow pump. Results show that the leading-edge separation vortex generates at 0.6Q_d, while the head declines only at 0.55Q_d. Therefore, using the inflection point of the head-flow curve as the critical state criterion is unsuitable. In addition, the fixed monitoring point is unsuitable for identifying the critical state due to the insensitivity to the amplitude, main frequency, and periodicity changes at the critical state. Finally, to identify the critical state, it is essential to arrange a monitoring point at the leading edge of the blade suction near the shroud, which should rotate with the impeller. The critical state criterion is that the main frequency position of the pressure fluctuation signal is offset at the monitoring point, and the amplitude is increased by 10 times.

Up to now, there is no appropriate method to quantitatively evaluate the effect of local flow on the fluid machinery's performance, especially the moment and efficiency. In this paper, a moment decomposition method (MDM) is developed for this purpose. The major advantage of this method is the ability to calculate the contribution of local flow to the blade's moment and the applicability to simulations with non-constant density flows. The method is applied to a centrifugal pump under different flow rates. The relative error is around 10% under 0.5Qd∼1.0Qd (Qd is the design flow rate), showing satisfying accuracy. By analyzing the flow angle, it is found that the flow impact at blade inlet triggers the flow rate imbalance and alternative vortex generation in adjacent passages. The MDM results show that vorticity effect dominates under different flow rates. Negative kinematic effect in the passage with stronger vortices leads to lower moment contribution compared to another passage. By decomposing the passages, it is found that the wake region is the main source of moment. In addition, the hydraulic efficiency decrease in the impeller due to flow rate decrease is the result of wake suppression in the passage with weaker vortices.

Gas–liquid two-phase flow is very common in fluid machinery and has complex multiphase flow characteristics. Under the gas-containing conditions, common issues in fluid machinery include the transport of liquid, bubble variations, and pressure drop characteristics; these can be analyzed using a simplified venturi tube. In order to investigate the influence of incoming gas on the gas–liquid flow, a venturi tube with the range of inlet gas volume fraction (IGVF) from 0 to 16% was used in this experiment. The development of a two-phase flow was recorded by using high-speed photography. Under different initial liquid flow rates and gas content conditions, the evolution of the two-phase flow was similar, with the main difference being the rate of evolution. The incoming gas mainly underwent a process from column shape to expansion and then to fragmentation passing through the venturi tube. In the experimental images, the projected area of the main bubble increased linearly with the increase in IGVF. Meanwhile, the transporting ability of the venturi tube was weakened due to the blockage caused by high gas content, especially when the IGVF exceeded 10%. The pressure drop characteristics indicated an increase in losses with the increase in gas content.

This study elucidates the effect mechanism of stall vortex on the hump region of the Q–H (Flow rate-Head) curve in centrifugal pumps by using both experimental and numerical simulation methods. Key characteristic parameters related to stall vortex are extracted under two types of impeller inlet boundary conditions. By comparing the blade torque and flow rate in both stall and non-stall channels during flow rate reduction, it is demonstrated that prior to the appearance of stall vortex in the impeller, the reduction rates of impeller torque and flow rate consistently maintain a consistent ratio, resulting in a linear trend of the Q–H curve. Following the emergence of stall vortex near the blade suction side, the reduction rate ratio of impeller torque to flow rate initially increases and then decreases. The hump peak point on the Q–H curve emerges when the reduction rates of these two parameters become approximately equal. A significant drop in the Q–H curve and the formation of a hump valley point occur when a large-scale stall vortex occurs near the blade pressure side. By comparing energy loss values at the hump peak and valley points, it shows that although stall vortex in the impeller channels intensify energy loss, the pressure drop due to the emergence of stall vortex near the blade pressure side is the primary cause of hump valley formation. This paper reveals the mechanism that the emergence of stall vortex results in hump formation on the Q–H curve, which is crucial for the optimization design of centrifugal pumps.