Numerical investigation on the suppression mechanism of tip leakage vortex and cavitation via tip water injection

The purpose of this paper is to suppress the tip leakage vortex and cavitation of fuel pump by using the casing slot. The flow fields of the blades with different casing slots were studied by numerical simulation under the conditions of no cavitation and cavitation respectively. The results show that the loss with leakage vortex cavitation increases by 37 % compared with the condition without cavitation. Under cavitation conditions, the optimal scheme is c6 scheme, and the loss is reduced by 9.3 %. The tip leakage vortex and vortex cavitation can be effectively reduced by casing slot. Under the condition of no cavitation, the tip leakage flow loss can be effectively reduced by the casing slot in a wide range of axial positions. After the casing slot is applied, the shape of the blade tip leakage vortex is significantly changed, the size of the leakage vortex is significantly reduced, and the loss is reduced. Under cavitation conditions, the casing slot can also effectively improve the tip flow field and inhibit leakage vortex cavitation. In a wide range of axial positions, the casing slot can effectively inhibit leakage vortex cavitation.

The tip leakage vortex (TLV) cavitating flow in an axial flow pump was simulated based on an improved shear stress transport (SST) k-ω turbulence model and the homogeneous cavitation model. The generation and dynamics of the TLV cavitation throughout the blade cascades at different cavitation numbers were investigated by the numerical and experimental visualizations. The investigation results show that the corner vortex cavitation in the tip clearance is correlated with the reversed flow at the pressure side (PS) corner of blade, and TLV shear layer cavitation is caused by the interaction between the wall jet flow in the tip and the main flow in the impeller. The TLV cavitation patterns including TLV cavitation, tip corner vortex cavitation, shear layer cavitation, and blowing cavitation are merged into the unstable large-scale TLV cloud cavitation at critical cavitation conditions, which grows and collapses periodically near trailing edge (TE).

To better understand the characteristics of tip leakage flow and interpret the correlation between flow instability and tip leakage flow, the flow in the tip region of a centrifugal impeller is investigated by using the Reynolds averaged Navier–Stokes solver technique. With the decrease of mass flow rate, both the tip leakage vortex trajectory and the mainflow/tip leakage flow interface are shifted towards upstream. The mainflow/tip leakage flow interface finally reaches the leading edge of main blade at the near-stall condition. A prediction model is proposed to track the tip leakage vortex trajectory. The blade loading at blade tip and the averaged streamwise velocity of main flow within tip clearance height are adopted to determine the tip leakage vortex trajectory in the proposed model. The coefficient k in Chen’s model is found to be not a constant. Actually, it is correlated with h/b (the ratio of blade tip clearance height to blade tip thickness), because h/b will significantly influence the flow structure across the tip clearance. The effectiveness of the proposed prediction model is further demonstrated by tracking the tip leakage vortex trajectories in another three centrifugal impellers characterized with different h/b (s).

The tip leakage vortex (TLV) cavitation mechanism of axial flow pump was investigated with the results of high speed photography and pressure pulsation measurement. The tip leakage vortex cavitation morphology and the transient characteristics of the TLV-induced suction-side-perpendicular cavitating vortices (SSPCV) were analyzed under different flow rates and different cavitation numbers which were combined with the time domain spectrum of pressure fluctuation to elucidate the relationship between the tip cavitation and pressure pulsation. The results showed that cavitation inception occurs earlier with more unstable tip leakage vortex cavitation shape under part-load flow rate condition, and the cavitation is more intense with the decrease of the cavitation number. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, moving toward the adjacent blade perpendicular to the suction surface, resulting in a flow blockage. With further decrease of pressure, the SSPCVs grow in size and strength, accompanied with a rapid degradation in performance of the pump. The cavitation images and the corresponding circumferential pressure distribution with the same phase showed that the lowest pressure coincides with the suction surface (SS) corner, The pressure was found to decrease along with the occurrence of the cavitation structure.

Axial compressors with variable stator vanes require annular gaps and radial gaps from the endwalls for smooth adjustment, which induces complex secondary flows such as the penny leakage vortex and tip leakage vortex, leading to a negative impact on the aerodynamic performance. To better understand these mechanisms, numerical investigations were conducted on four different clearance configurations. The results show that the penny leakage vortex moves toward the suction side under the transverse pressure gradient and mixes with the hub corner stall vortex. This causes the corner separation to be further developed, leading to an increase in total pressure loss by 13.6%. However, the tip clearance leakage flow could reduce the transverse pressure gradient, which prevents penny leakage vortex from mixing with low-energy fluid in the corner region. Moreover, the hub corner stall vortex is also replaced by the tip leakage vortex, which effectively suppresses the range of corner separation. Under the comprehensive effects of the penny leakage vortex and the tip leakage vortex, the total pressure loss coefficient is increased only by 7.6%. Therefore, the mixing effect between the penny leakage vortex and low-energy fluid in the corner separation is the main reason for higher loss production of the cascade, and these findings provide theoretical support for the future application of flow control technology to reduce secondary flow loss.

Influence of C groove on suppressing vortex and cavitation for a NACA0009 hydrofoil with tip clearance in tidal energy

The influence of circumferential grooves on the tip flow field of an axial single-stage transonic compressor rotor has been examined experimentally and numerically. The compressor stage provides a strongly increased stall margin with only small penalties in efficiency when the casing treatment is applied. Due to the complex interactions of the grooves with the rotor flow, unsteady measurement techniques have been chosen as an attempt to identify the aerodynamic effects responsible for the operating range extension. Therefore, the casing treatment has been instrumented with piezoresistive pressure sensors in the land between the grooves providing high-resolution static wall pressure measurements at different operating conditions. Data acquisition worked at a sampling rate of 125kHz, providing around 23 static pressure values per blade passage at 11 axial positions at the nominal speed of 20,000 rpm. A comparable dataset, but with 14 sensors, was obtained for the smooth casing. The results show the fluctuation of the tip leakage vortex and shock-vortex-interactions as well as the changed situation with casing treatment. Ensemble-averaged data shows tip leakage vortex trajectories. At near stall conditions with the smooth casing, the vortex hits the front part of the adjacent blade, which indicates the possibility of a spill forward of low momentum fluid into the next passage. Standard deviation values prove a high fluctuation of the pressure field over the tip gap. When the casing treatment is applied, the vortex trajectory maintains alignment along the blade’s suction side, thus preventing the onset of rotating stall. Results are presented as a back-to-back comparison of the smooth casing versus the treated casing at three operating conditions: peak efficiency at a mass flow rate of m˙pe = 16.2kg/s, near stall of the smooth casing at m˙nssc = 14.0kg/s and near stall of the treated casing at m˙ns = 12.6kg/s. Steady and unsteady numerical simulations of the rotor-only flow field have been calculated with and without grooves. These calculations aim at a broad analysis of the occurring flow phenomena at the rotor tip. Tip leakage flow behaviour and vortex trajectories are discussed in detail by summarizing the congruent findings of both numerical and experimental investigations.

High flow rate aeroengines typically employ axial flow compressors, where aerodynamic loss is predominantly due to secondary flow features such as tip leakage and corner vortices. In very high altitude missions, turbomachinery operates at low density ambient atmosphere, and the recent trend toward more compact engine core inevitably leads to the reduction of blade size, which in turn increases the relative height of the blade tip clearance. Low Reynolds number flowfield as a result of these two factors amplifies the relative importance of secondary flow effects. This paper focuses on the behavior of tip leakage flow, investigating by use of both experimental and numerical approaches. In order to understand the complex secondary flow behavior, cascade tests are usually conducted using intrusive probes to determine the loss. However relatively few experimental studies are published on tip leakage flows which take into account the interaction between a rotating blade row and its casing wall. Hence a new linear cascade facility has been designed with a moving belt casing in order to reproduce more realistic flowfield as encountered by a rotating compressor row. Numerical simulations were also performed to aid in the understanding of the complex flow features. The experimental results indicate a significant difference in the flowfield when the moving belt casing is present. The numerical simulations reveal that the leakage vortex is pulled by the shearing motion of the endwall toward the pressure side of the adjacent blade. The results highlight the importance of casing wall relative motion in analyzing leakage flow effects.

Tip leakage flow and induced unstable cavitation can significantly damage the performance of axial waterjet pumps. This study investigated the impact of blade numbers on cavitating conditions in an axial waterjet pump by conducting tests of performance characteristics and high-speed photography experiments on three-blade and four-blade impellers. The results showed that the critical cavitation number σc of the three-blade impeller was larger, while the four-blade impeller flow pattern deteriorated more rapidly after σc. Various cavitation structures in the tip region were observed under different conditions, including clearance cavitation, shear layer cavitation, tip leakage vortex cavitation, and suction-side-perpendicular cavitating vortices (SSPCVs). Tip cavitation maps of the test impellers were drawn based on the flow rate coefficient and cavitation number variation. The three-blade impeller exhibited a wider range of severe cavitation, particularly with an increased occurrence of SSPCVs. With the cavitation number and flow rate coefficient decreased, the SSPCV generated from triangular cavitation cloud shedding presented an increased trend in scale and quantity. Conversely, in the case of the four-blade impeller, SSPCVs were often disrupted by the adjacent blade during migration and interfered with the tip cavitation in the neighboring flow passage.

Tip leakage cavitating flow under a dynamic boundary remains a critical challenge in axial hydraulic machinery; research characterizing such cavitating flow with acceptable accuracy is currently limited. In this study, a flat plate hydrofoil undergoing sinusoidal pitching motion around the mid-chord point is employed to simulate tip leakage cavitating flow under dynamic boundaries. The shear stress transport k-ω model and the Zwart–Gerber–Belamri model are applied to investigate the cavitation patterns and vortex structures within the tip clearance under pitching motion. The results indicate that the tip leakage cavitating flow involves the tip leakage vortex (TLV), the tip separation vortex, and the induced vortex, with the TLV being the dominant vortex structure. The TLV-induced tip leakage vortex cavitation (TLVC) is the only cavitation pattern within the tip clearance. The TLV and TLVC exhibit significant periodicity during the pitching process. During the upstroke phase, the TLVC intensity gradually decreases, the streamwise vorticity near the TLVC gradually increases, and the velocity circulation initially decreases and then increases. However, during the downstroke phase, the changes in these features are opposite to those in the upstroke phase. In addition, at the same angle of attack for upstroke and downstroke phases, the TLV and TLVC intensity are greater in the downstroke phase. Moreover, a wandering cavity appears in the TLVC during the upstroke phase, while the TLVC has more integral shape during the downstroke phase.

Cavitation inception and the associated flow structure in the tip region of a ducted propeller are investigated experimentally at varying advance ratios (J) using high-speed imaging and stereoscopic particle image velocimetry (SPIV) measurements in a refractive index-matched facility. At design and higher J values, inception occurs in axially aligned secondary vortices, located between the blade suction side and the tip leakage vortex (TLV), circumferentially after the trailing edge. With decreasing J, the inception shifts first to the TLV, and then along its core towards the leading edge. High-resolution SPIV data follow the evolution of TLV, tip leakage flow, near wake and several secondary vortices. Time-resolved SPIV at 30 kHz enables calculation of all three mean vorticity components, hence capturing axial vortices, and identifies the origin of flow structures. At high J values, inception occurs when quasi-axial vortices are stretched by the circumferential TLV and co-rotating secondary vortices located in the shear layer connecting the TLV to the suction side blade tip. With decreasing J, inception shifts to the TLV and towards the leading edge owing to earlier rollup and higher vortex strength, along with earlier breakup, evidenced by high core turbulence and a decrease in peak vorticity despite an increase in circulation.

For some axial flow compressors, the compressor stall is a result of the blade tip blockage caused by the complex flows, which include the boundary layer flow separation (BLFS), tip leakage flow (TLF), and shock wave. Owing to the difference of the design rotating speed and aerodynamic load in the axial flow compressor, these complex flows might exist in isolation or occur at the same time in practical application. Aiming at the stall mechanism in the axial flow compressors, a great deal of experimental and numerical investigations have been carried out at the design rotating speed. However, the investigation for off-design rotating speed in the axial flow compressors is seldom. Therefore, a transonic axial flow compressor rotor, which is NASA Rotor67, was chosen to investigate the stall mechanism at 100%, 80% and 60% design rotating speeds with the help of the numerical method. Moreover, the guiding suggestions for selecting the measures of increasing the transonic axial flow compressors stability are presented for the later investigation. The compared results show that the variation tendency of the experimental total performance lines are finely repeated by the numerical results at the three design rotating speeds. The fundamental flow mechanism of the rotor is obtained by analyzing the flow field in the blade passage in details. With the decrease of the rotor mass flow at the three design rotating speeds, the starting position of the tip leakage vortex (TLV) moves to the blade leading edge gradually, and the tip leakage vortex also deviates to the pressure surface of the adjacent blade. The deviated angle, which is the angle between the trajectory of the tip leakage vortex core and rotor rotating axis, for near stall point (NS) are about three degree, five degree and nine degree than that for near peak efficiency point (NPE) at 100%, 80% and 60% design rotating speeds respectively. The blockage resulted from the interaction between the tip leakage vortex and shock wave is the cause of the rotor stall at 100% and 80% design rotating speeds. Besides, the breakdown of the tip leakage vortex and leading edge spilled flow (LESF) occur at 80% design rotating speed. At 60% design rotating speed, the blockage caused by the leading edge spilled flow resulted from the tip leakage vortex is the main cause of bringing about the compressor stall, and the boundary layer flow separation (BLFS) in a small scope appears at the blade tip suction surface near the trailing edge.

A three-dimensional state-of–the-art block structured Navier-Stokes solver has been developed to investigate the complex flow phenomena in the tip gap of axial flow compressor cascades. First, Numerical investigations into a linear compressor cascade have been conducted to analyze the tip leakage flow fields both in stationary and moving end-wall condition. The good agreements between the calculated results and the available experimental data demonstrate the validity of the computations. Both experimental and numerical results indicate that the moving boundary layer entrain and diffuse the tip leakage vortex from the suction surface of one blade across the blade pitch to the pressure surface of the adjacent blade, and the tip leakage vortex with its smaller and tighter core moves closer to the pressure surface of the adjacent blade with a reduction in tip clearance size. Then, numerical analysis is carried out to investigate the moving end-wall effects on tip clearance flow in the stator of a subsonic one-stage axial flow compressor. It is found that the corner separation region on the blade suction surface diminishes remarkably under moving endwall condition. The high loss region, which is located at the corner of the inner wall and blade suction surface under stationary endwall condition, contracts and extends toward the pressure side of the adjacent blade, and the loss level associated with the leakage vortex core decreases under moving endwall condition. These changes will result in a reduction in passage losses with a corresponding reduction in overall blockage in the passage, which is favorable not only to raise the efficiency, but also to improve the stall margin of the compressor.

Unsteady flow in the blade tip region of modern axial flow compressors is one of the sources of loss, noise, and blade vibration. In some cases, it is potentially linked to stall inception. In this paper, the complex flow fields in the blade tip region of a transonic axial flow compressor rotor have been numerically investigated. The predicted results were validated by experimental data. Analyses of monitoring results of numerical probes showed that three typical flow characteristics occurred as the operating condition approached the stability limit: no flow fluctuation at the first operating point; flow fluctuation with high frequency and low amplitude at the second operating point; flow fluctuation with low frequency and high amplitude at the third operating point. Further analysis of the tip flow field showed that the evolution of the tip leakage vortex experienced three stages as the rotor was throttled. At the first stage, the TLV did not breakdown. At the second stage, a bubble-type breakdown of the tip leakage vortex occurred. At the third stage, a spiral-type breakdown of tip leakage vortex occurred. The current study demonstrated that the flow unsteadiness that appears within the test rotor was induced by the tip leakage vortex breakdown. Furthermore, with the transformation of the vortex breakdown form, the characteristic frequency and amplitude of the flow oscillation substantially changed.

A numerical investigation on the self-induced unsteadiness in tip leakage flow is presented for a transonic fan rotor. NASA Rotor 67 is chosen as the computational model. It is found that under certain conditions the self-induced unsteadiness can be originated from the interaction of two important driving “forces:” the incoming main flow and the tip leakage flow. Among all the simulated cases, the self-induced unsteadiness exists when the size of the tip clearance is equal to or larger than the design tip clearance. The originating mechanism of the unsteadiness is clarified through time-dependent internal flow patterns in the rotor tip region. It is demonstrated that when strong enough, the tip leakage flow impinges the pressure side of neighboring blade and alters the blade loading significantly. The blade loading in turn changes the strength of the tip leakage flow and results in a flow oscillation with a typical signature frequency. This periodic process is further illustrated by the time-space relation between the driving forces. A correlation based on the momentum ratio of tip leakage flow over the incoming main flow at the tip region is used as an indicator for the onset of the self-induced unsteadiness in tip leakage flow. It is discussed that the interaction between shock wave and tip leakage vortex does not initiate the self-induced unsteadiness, but might be the cause of other types of unsteadiness, such as broad-banded turbulence unsteadiness.