• Energy Research

Tip leakage vortex (TLV) in an axial flow pump was simulated by using the shear-stress transport (SST) k-ω turbulence model with a refined high-quality structured grid at different flow rate conditions. The TLV trajectories were obtained by using the swirling strength method corresponding to the cross-sections of streamlines of the TLV. High-speed photography experiments were conducted to observe the TLV trajectory based on cavitation tracing bubbles in an axial flow pump with a transparent casing. The TLV trajectories predicted by the SST k-ω turbulence model agreed well with the visualization results. The numerical and experimental results show that the starting point of the TLV is near the leading edge at part-load flow rate condition (Q/QBEP=0.85), and it moves towards the trailing edge to approximately 20% blade chord at the design flow rate condition (Q/QBEP=1.0). At large flow rate conditions (Q/QBEP=1.2), the starting point of the TLV shifts to about 40% blade chord, and the relative angle between the TLV trajectory and the blade chord is gradually reduced with the increased flow rate. Detailed statistics of the fluid dynamics of the end-wall shear layer and the TLV at design and off-design conditions were discussed based on the numerical results. The shear layer and jetting flow in the tip gap are highly affected by the pressure difference between the pressure side (PS) and suction side (SS). It was also found that the distributions of static pressure, turbulent kinetic energy (TKE) and vorticity inside the TLV core are associated with the TLV structure which is affected by blade loading and operation conditions of the axial flow pump.

Axial liquid hydrogen pumps, essential for aerospace and energy sectors, offer high efficiency and reliability at extremely low temperatures. The significance of the tip clearance flow of axial liquid hydrogen pumps, in its interaction with the main flow, wall boundary layer, and wake flow, is paramount in determining the overall stability of the system. This study looked into the characteristics of axial liquid hydrogen pumps' tip leakage using numerical modeling methods. Regarding the leakage characteristics of hydrofoils (as a simplified form of pump blade), the predictive capabilities of three turbulence models—Shear Stress Transport (SST) k-ω, Stress-Blended Eddy Simulation (SBES), and Large Eddy Simulation (LES)—were compared. In terms of predicting the location of downstream tip leakage vortices, both the SBES and LES models align relatively well with experimental data. While the SST k-ω model offers computational efficiency, the SBES model demonstrates higher accuracy in predicting the velocity field near the core of the leakage vortex flow field, combining both efficiency and accuracy advantages. For the research on the pumps, with increasing flow rates, the leaky vortex's strength and effect range gradually decreased, and the vorticity value in the core region also decreased. This shift shows that as the flow rate increases, the leakage vortex becomes more stable. Additionally, the investigation was conducted on the pressure fluctuations on the blade surface. Under design conditions, the pressure fluctuations exhibited periodic changes, with their frequency primarily determined by the rotation frequency of the impeller. By establishing monitoring points on the blade surface, the study found that the amplitude of pressure fluctuations near the leading edge increased with increasing flow rates, whereas decreased at the suction and pressure sides. Under low flow conditions, due to the large amount of leakage and the poor stability of the leakage vortex, the pressure fluctuations on the back of the blade were more significant. This work offers theoretical assistance and technological direction for understanding and predicting the leakage characteristics and blade pressure fluctuations of axial liquid hydrogen pumps.

Tip leakage vortex (TLV) usually exists at a small tip clearance in axial-flow energy conversion machines, which may induce flow loss, vibration, noise and vortex cavitation. In this work, the double-control-hole (DC hole) at the leading edge of a hydrofoil is used to suppress TLV by inducing a passive jet. The SST-CC model by adding a rotation-curvature correction (CC) term into the original shear stress transport (SST) k-ω and Zwart-Gerber-Belamri (ZGB) model were applied to investigate the controlling effect under different Reynolds numbers (Re). The results show that TLV is decomposed and new vortices are generated when changing Re. By guiding the high-pressure flow from the pressure side (PS) to the suction side (SS) of hydrofoil vertically, the TLV core pressure is increased and local cavitation is suppressed. When the inflow velocity gradually increases to 7.5 m/s, 10 m/s and 12.5 m/s, the velocity swirl intensity at the TLV core is suppressed by nearly 50%, 36.4% and 28%, respectively, showing a downward trend. Based on the vorticity equation, it is observed that the distribution of TLV vortex bending terms is changed. Also, the vortex bending term and the vortex stretching term are suppressed in specific directions. Further analysis of the suppression mechanism of DC hole indicates that this structure can increase the turbulent kinetic energy of TLV, which destabilizes the flow field. Besides, the velocity circulation in the downstream region of TLV can be reduced, which can suppress the TLV effectively.

The aim of this work is to investigate the unsteady characteristics of tip leakage vortex (TLV) structure in an axial flow pump experimentally and numerically. Experiments were carried out to study the performance of an axial flow pump under cavitating cavitation. The numerical simulation was conducted by employing the Large Eddy Simulation turbulence and Zwart cavitation models, and the results show a good agreement with the experimental ones. The evolution mechanisms and influencing factors of the transient TLV morphology was revealed, and the correlation between pressure difference and leakage velocity were investigated in a systematic way. The TLV evolution appears periodic development, coinciding with the variation of leakage velocity driven by the pressure difference. The cylindrical coordinate system is more suitable for analyzing the TLV dynamics, due to the impeller rotation. The vorticity in the tip region is dominated by tangential vorticity ωθ and axial vorticity ωz. The transport terms of ωθ and ωz present a great relationship with TLV and TLV-induced cavitation. Relative vortex stretching dominants the vorticity generation in the tip region, and relative vortex dilation and Coriolis force are also important sources. In contrast, baroclinic torque has the least influence on the vorticity in the flow passage.

The energy recovery and utilization of high-pressure brine are the key to reduce the cost. Reducing flow rate method (RFM) was put forward firstly to adapt pump as turbine (PAT) to a wide range of operating conditions in this paper. In order to validate advantages of the new method, three different designs of PAT were obtained, respectively, based on three different methods, which were, respectively, selection design method, forward curved method and RFM. Based on numerical method, the external energy characteristics, axial force and internal flow patterns of PATs were analyzed. Results show that matching of the blade inlet angle and flow angle at the blade leading edge has a great influence on the turbine flow patterns which are directly related to the efficiency of PAT. A PAT designed by the new design method RFM improves both the hydraulic efficiency and shaft power under design and partial load conditions. Moreover, the axial force of the PAT designed by RFM is reduced, which can improve the reliability of the PAT operation with high rotating speed.

Tip leakage vortex (TLV) frequently occurring in bladed energy conversion machines may induce flow loss as well as cavitation. The double-curved-hole (DC hole) structures located at the 10%–25% chord near the leading edge of the foil are adopted to induce a passive jet thus suppressing the tip leakage vortex structures in this paper. The control effects and the control mechanisms of three DC hole positions are illustrated by numerical analyses based on the rotation-curvature corrected shear stress transport (SST) k-ω turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model. The results demonstrate that the passive jet from the DC hole could divide the TLV into several parts and induce new vortical structures. The newborn vortical structures can be classified into three types: weakened tip leakage vortex (WTLV), newborn secondary tip leakage vortex (NSTLV) and hole separation vortex (HSV). The control effects for different tip clearance are greatly related to the DC hole location. Once the DC hole locates near the merger point of the tip leakage vortex and the tip separation vortex, it could achieve the best control effects. Besides, TLV core pressure could be improved thus suppressing the local cavitation, and the lift-drag ratio for different inlet velocities can be slightly reduced.

In bladed energy conversion machines, the tip leakage vortex (TLV) is a prevalent phenomenon, which can lead to both flow losses and cavitation. Numerous scholars in recent years have proposed effective methods for TLV suppression, utilizing either active or passive control strategies. This work focuses on the implementation of the hole-pit (HP) structure located at the leading edge 2%–25% chord position of the hydrofoil to induce passive jets and separate flows for suppressing TLV structures. The rotating curvature correction (CC) shear stress transport (SST) k-ω turbulence model and Zwart-Gerber-Belamri (ZGB) cavitation model are employed to clarify the control effect and mechanism of HP structure through numerical analysis and experimental verification. The results reveal that the passive jet and pit-separated flow from the HP structures effectively divide the TLV into distinct segments, giving rise to new vortical structures. These vortical structures are categorized hole separation vortex (HSV), pit separation vortex (PSV), newborn tip separation vortex (NTSV), and weakened tip leakage vortex (WTLV). The control ability of HP structures in different schemes is strongly correlated with their number and arrangement. Optimal control effects are achieved when the HP is positioned near the separation vortex over the top of the foil. Specifically, when the number of drainage holes is 3 or 4, the theoretical TLV cavitation length is suppressed by at least 63.93%. The passive jet induced by the HP structure alters the distribution of different kinds of energy losses in tip clearance and TLV core through the effect gap leakage flow, leading to a sharp rise in the value of total energy loss. This results in a weakening of gap leakage flow energy and TLV intensity, accompanied by a raised vortex core pressure. Thus, the research successfully realizes the purpose of suppressing both TLV and TLV cavitation.

Pump as turbine (PAT) is one of the economical and effective energy recovery devices in small hydropower stations. A back-curved PAT and a front-curved PAT were designed, and performance characteristics were studied, the accuracy of the numerical calculation was verified by comparing with the experimental results. The entropy generation theory was used to compare performance and energy loss of PATs. The results show that the high efficiency range of front-curved PAT is significantly wider than that of back-curved PAT. Under part-load condition (0.8Qd), design flow condition (1.0Qd) and over-load condition (1.2Qd), the efficiency of the front-curved PAT is 0.6%, 5.9% and 7.9% higher than that of the back-curved PAT, respectively. The energy loss in the PAT impeller mainly comes from the turbulent entropy generation rate which is mainly concentrated on the blade leading edge and trailing edge. Flow separation and flow impact caused by the mismatch between the relative flow angle and the blade setting angle are the main mechanisms of energy loss in impeller. In addition, the loss caused by the wall friction in the front-curved impeller is less than that in the back-curved impeller. Therefore, the entropy generation theory can provide guidance for the performance optimization of PAT.