Development Of Tip Leakage Vortex Research Articles

Analysis of hydrofoil tip leakage vortex dynamic characteristics

The clearance exists inevitably between the turbine guide vanes and the fixed wall, and the resulting tip leakage vortex leads to the increase of hydraulic loss. Based on the fluid simulation software ANSYS CFX, the hydrofoil was numerically simulated under different flow velocity conditions. The trajectory and energy dissipation of the tip leakage vortex are discussed. The results show that the jet entrainment with the main flow to form the tip leakage vortex, which is mainly divided into main leakage vortex and separation vortex. The flow evolution of tip leakage vortex can be divided into three stages. The evolution of tip leakage vortex can be divided into three stages: independent development of main leakage vortex and separation vortex, fusion stage, and dissipation of main leakage vortex. The strongest turbulent kinetic energy dissipation occurs in the clearance, which confirms that the clearance causes non-negligible energy dissipation for hydraulic machinery. The results provide guidance for tip leakage vortex control of hydraulic hydrofoil.

The investigation on the flow characteristics during the continuous development of the tip leakage vortex cavitation in an axial pump-jet propulsion

The continuous deterioration and development of tip leakage vortex (TLV) cavitation in the pump-jet propulsion significantly affect propulsion performance and operational stability. Larger eddy simulation and cavitation tunnel experiment are utilized to investigate the temporal and spatial evolution characteristics of TLV cavitation under varying cavitation conditions. The results reveal that the continuous development of TLV cavitation prompts the TLV to gradually move away from the blade suction surface due to increasing pressure difference at the blade tip surface. Furthermore, the development of TLV cavitation amplifies the effect of the radial outward Coriolis force and makes the TLV even more unstable. Under the influence of the tip leakage flow, primary generation of turbulent kinetic energy (TKE) persistently migrates to the TLV core center and subsequently travels downstream. Despite the large magnitude of TKE that occurs at the TLV core center, the TKE generation remains low. With the inception of TLV cavitation, the transport of TKE between the TLV core center and the surrounding flow gradually intensifies, followed by a subsequent weakening of this transport effect. It increases again as the breakdown of TLV becomes more severe.

Investigation on the entropy production distribution in a multiphase pump considering gas–liquid two-phase velocity slip

To study the energy loss characteristics of a semi-open mixed-flow multiphase pump, an improved entropy generation theory considering the slip velocity was established to locate local areas with high energy loss. The relationships among local entropy generation, phase interface entropy generation, wall entropy generation, and unstable flow were analyzed for each component. The results showed that magnitude of interface entropy generation was similar to turbulent entropy generation and wall entropy generation, which could not be ignored. The interface entropy generation was mainly distributed at the leading edge, trailing edge, hub, and blade tip clearance. With an increased inlet gas volume fraction, the proportion of interfacial entropy production loss to total entropy production loss increased. As the inlet gas volume fraction increased to 30%, the interface entropy generation loss accounted for 70% of the local entropy generation loss at leading edge and 63% at trailing edge. The high interface entropy generation zone at the tip clearance region began to extend from the pressure side of the blade to the suction side of the blade. During the evolution of tip leakage vortex, the generation, unstable stretching, and breakup–regeneration stages were accompanied by a large loss of interface entropy generation.

Investigation of Vaned-Recessed Casing Treatment in a Low-Speed Axial Flow Compressor, Part I: Time-Averaged Results

This paper investigates the effects of two modifications to a vaned recessed casing treatment. First, the shape of a circular curve was used in the top of the treated casing. Second, a fully curved guide vane was also applied. The goals of the modifications are to enhance the flow recirculation as well as to relieve the low-speed flow, which is normally accumulated within the corners of the vaned recessed casing treatment. The solid casing in addition to the two vaned recessed configurations with 23.2% and 53.5% rotor blade tip axial chord exposure have been studied numerically. The results indicated that two mechanisms are involved in the stall margin enhancement. First, the circumferential pressure gradient is reduced for both configurations. The reduction in pressure gradient largely reduces the development of tip leakage vortex and, thus, the generation of low-speed fluid is diminished. Second, the main flow/tip leakage interface moves toward downstream and the movement of interface toward the leading edge is delayed. The second configuration with a greater rotor blade tip exposure enables extra flow recirculation due to decreasing surface area and, therefore, could be superior to the application of the first casing treatment configuration. The major streamlines within the casing treatment are also discussed. The time-averaged results are presented in this paper, while the unsteady results including instantaneous flow fields, origins of the unsteadiness and frequency analysis are discussed in part II.

Research on tip leakage vortex characteristics of axial-flow circulating pumps under unpowered driven conditions

To explore the flow characteristics around the tip clearance and the evolution of tip leakage vortex, this paper adopted the delayed detached eddy simulation model to calculate the blade tip region and employed the vorticity transport equation to analyze the forms and strength of tip leakage vortex. It is found that under the effect of tip leakage vortex, the unsteady flow at blade edges are serious under unpowered driven condition. Tip leakage vortex is generated from the leading edge of the blade. From the leading edge to the trailing edge of the blade, the intensity of tip leakage vortex gradually decreases and the generation frequency gradually increases. The core intensity of tip leakage vortex at L1 to L5 points decreases gradually, with a decreased range of about 24.8%, 34.4%, 55.03% and 60.48%.

Mechanism Affecting the Performance and Stability of a Centrifugal Impeller by Changing Bleeding Positions of Self-Recirculating Casing Treatment

This study aimed to investigate the influence of the bleeding position of a self-circulating casing on the aerodynamic performance of a transonic centrifugal compressor. Three types of self-circulating structures with the bleeding positions of 11% Ca (the axial chord length of the blade tip), 14% Ca and 20% Ca from the leading edge of the blade were studied by using the numerical simulation method, with the Krain impeller taken as the research object. It was found that all three types of self-recirculating casing treatments can expand the stable operating range of the impeller, and that at medium and small flow rates, the total pressure ratio and efficiency of the impeller increase gradually with the backward movement of the bleeding position. The self-circulating casing treatment can restrain the development of tip leakage vortex, reduce the blockage area, and improve the stability of the impeller by sucking low-energy fluid. The farther back the bleeding position is, the greater the bleeding mass flow rate of the self-circulating casing for the low-energy fluid in the blade-tip passage becomes. Additionally, a greater inhibition effect on the tip leakage vortex, and a better effect of improving the performance and stability of the impeller, can be obtained. The best air bleeding position is 20% Ca, but it is not directly above the blade-tip blockage center of the solid wall casing passage. Instead, it is downstream of the blockage area.

Unsteady characteristics of tip leakage vortex structure and dynamics in an axial flow pump

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.

Tip Clearance Effect on The Tip Leakage Vortex Evolution and Wake Instability of a Ducted Propeller

The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TLV became thicker, indicating damage to the energy recycling process. The fast Fourier transform (FFT) of several wake points showed pressure pulsations of the wake ranging from the blade-passing frequency to the shaft frequency, and the evolution process accelerated with the increase in the GSR. The power spectral density (PSD) analysis showed that the energy of the wake enhanced with the increase in the GSR. In particular, the vortex interactions could cause pulses in low-GSR conditions, which could intensify the excitation force of the propeller and also have a certain impact on the noise level.

Effects of Outer Edge Bending on the Aerodynamic and Noise Characters of Axial Fan for Air Conditioners

Outer edge bending is already used on the axial fan blades of air conditioners, reducing the leakage flow loss at the blade tip and suppressing the tip vortex development, thereby improving fan aerodynamic and acoustic performance. However, there are few studies on the multi-parameter design and optimization of this complicated structure, and most studies only focus on the overall sound pressure level rather than the sound quality when evaluating the fan noise. This study investigated the effects of outer edge bending structure on the aerodynamic performance and sound quality of air conditioners’ axial fans by experiments and numerical methods. Based on the orthogonal design method, the effects of three bending parameters, the circumferential starting angle, radial relative position, and the bending degree effects on the performance of the axial flow fan blade were analyzed, and the best efficiency scheme was selected. A comparative analysis of the preferred and the original bending schemes shows that the bending towards the blade suction surface successfully inhibits the development of tip leakage vortex at the blade tip, thereby achieving efficiency enhancement and noise reduction. The experimental results show that the preferred bending scheme with a 10° circumferential starting angle, 90% radial relative position, and 8% bending degree can effectively reduce the fan’s broadband noise within 200~1000 Hz by 0.54~2.68 dB (A) at different operating conditions. Additionally, the preferred bending blade with reasonably designed bending effectively reduced the loudness and roughness of the fan noise in the rated conditions, and the sound quality of the studied fan was correspondingly improved.

Control strategies for tip leakage vortex using inclined squealer rim in axial turbines

In a typical gas turbine, due to its complicated blade geometry, complex vortex structures appear and cause significant aerodynamic loss. Vortex systems dominated by a tip leakage vortex near the tip region are the primary source of this loss. In this paper, to improve the aerodynamic performance of the turbine, two novel control strategies for tip leakage vortex and the tip leakage flow of the cavity tip are proposed, and their coupling control effects are numerically investigated. The first control strategy is intended to control the loss caused by the breakdown of tip leakage vortex. By inclining the external wall of the suction side rim toward the passage, the emergence of a trailing edge pressure spike is delayed. This significantly reduces the adverse pressure gradient, suppressing the breakdown of tip leakage vortex and reducing tip leakage loss. The second control strategy controls tip leakage flow using the inclined inner wall of the suction side rim, which enhances the separation bubble on the top of the rim of the suction side and reduces the leakage rate by 7.7%. In this way, the formation and development of tip leakage vortex are indirectly manipulated, inhibiting the tip leakage loss. The coupling of the two strategies reduces the blocking effect on tip leakage flow slightly compared to the second strategy. However, the stage efficiency of the turbine is still improved by 0.24% because of the effective suppression of tip leakage vortex breakdown.

Large-eddy simulation of three-dimensional aerofoil tip-gap flow

Tip-gap flow is inevitable between the rotor and endwall in rotating machinery, which can lead to operation instability, blockage effects, energy loss, and even cavitation in water. In this study, large eddy simulation of the tip-gap flow of a three-dimensional aerofoil in a linear cascade with a moving endwall are performed. A series of aerofoils are designed with different section parameters (chord, thickness and camber), pitch angles, and tip-gap sizes. Detailed analysis of mean flow field, turbulence statistics and tip-leakage vortex (TLV) centre trajectory is performed to determine the parameter effects. The section parameters with the main influence on TLV is found to be the gap width. Larger chords and thicknesses can effectively suppress the TLV, whereas the pitch angle has small influence. The gap size has a significant effect on the TLV evolution, and in particular, small gaps lead to completely opposite characteristics compared with large gaps by restraining the TLV close to the suction surface and endwall. Quantitative analysis of the TLV centre wandering phenomenon demonstrates the unsteady nature of TLV. Furthermore, a duct propeller is simulated with the rotor tip designed to suppress the TLV, which turns out to improve the whole hydrodynamic performance of duct propeller.

Numerical Investigation on a Axial Slot Casing Treatment of a Large Circumferential Interval and Small Opening Area

Axial slot casing treatment is a common method to extend the stall margin of a compressor. Based on the mechanism of unsteady flow control, this paper redesigns axial slots with large circumferential interval and small opening area. To test the effect of this axial slot structure, unsteady numerical simulations were carried out with different slot areas and circumferential intervals. The results show that this novel axial slot casing treatment can significantly improve compressor stall margin. Meanwhile, compared with the traditional axial slot, the efficiency loss is greatly reduced. The flow field analysis shows that the new axial slot structure proposed in this paper can suppress the development of tip leakage vortex and unsteadiness in the tip region at the near stall condition through decreasing the tip loading periodically. Moreover, we find that the slot area is proportional to the improvement of stability margin. Under the same slot area, an excessive number of slots is not conducive to the improvement of the stability margin.

Experimental Investigation of the Transient Patterns and Pressure Evolution of Tip Leakage Vortex and Induced-Vortices Cavitation in an Axial Flow Pump

Abstract Cavitating flow is extremely complex in axial and mixed flow pumps, resulting in several adverse effects on pump performance. In this paper, the tip leakage vortex (TLV) cavitation patterns in an axial flow pump model were studied based on high-speed photography and transient pressure measurements. The TLV cavitation morphology and transient development of the induced suction-side-perpendicular cavitating vortices (SSPCVs) were investigated at multi-operating conditions. The time-domain of the transient pressure was employed to clarify the relationship between the tip cavitation and the pressure field. The results showed that cavitation inception occurred earlier with an unstable TLV cavitation shape at part-load conditions. Cavitation was more intense with a decrease of the cavitation number, presenting a larger area of triangular cavitation with the shedding of SSPCV. The inception of SSPCV was attributed to the tail of the shedding cavitation cloud originally attached to the suction surface (SS) of the blade, moving in the direction of the adjacent blade perpendicular to the SS, resulting in a flow blockage. With a further decrease in pressure, the SSPCVs grew in size and strength, accompanied by a rapid degradation in performance of the pump. The cavitation images and the corresponding circumferential pressure distributions showed that the lowest pressure point coincided with the SS corner. After this position, the pressure fluctuated as the cavitation intensity changed. The transient characteristics of SSPCV are a basis for revealing the instability mechanism of its evolution in the axial flow pump.

Experimental and Numerical Investigation on the Tip Leakage Vortex Cavitation in an Axial Flow Pump with Different Tip Clearances

The tip gap existing between the blade tip and casing can give rise to tip leakage flow and interfere with the main flow, which causes unstable flow characteristics and intricate vortex in the passage. Investigation on the tip clearance effect is of great important due to its extensive applications in the rotating component of pumps. In this study, a scaling axial flow pump used in a south-north water diversion project with different sizes of tip clearances was employed to study the tip clearance effect on tip leakage vortex (TLV) characteristics. This analysis is based on a modified turbulence model. Validations were carried out using a high-speed photography technique. The tip clearance effect on the generation and evolution of TLV was investigated through the mean velocity, pressure, and vorticity fields. Results show that there are two kinds of TLV structures in the tip region. Accompanied by tip clearance increasing, the viscous loss in the tip area of the axial flow pump increases. Furthermore, the tip clearance effect on pressure distribution in the blade passage is discussed. Beyond that, the tip clearance effect on vortex core pressure and cavitation is studied.

Direct numerical simulation of a tip-leakage flow in a planar duct with a longitudinal slit

A planar duct flow configuration with a cross-flow injected from a longitudinal slit close to the upper wall of the duct is studied by using a direct numerical simulation approach to explore the underlying flow mechanism in relation to the tip-leakage vortex (TLV), which is one of the most important flow phenomena in turbomachinery. Major characteristics of TLV in a rotor of turbomachinery are identified in the current flow model. The analysis of mean and instantaneous flow fields reveals that the interaction between the main (axial) flow and jet (cross) flow is the primary source of the generation of the TLV. The evolution of the TLV is then investigated, and a vortex breakup phenomenon is identified. The evolution of TLV can be divided into three phases, i.e., the formation phase, the breakup phase, and the diffusion phase. Mean streamlines and turbulence kinetic energy (TKE) budgets are analyzed, showing that the high TKE central spot in the formation phase is due to the interaction between highly swirling vortex filaments and mean velocity gradient. In the outer part of the TLV, the TKE is mainly produced in the shear-layer and transported toward the center by the turbulence transport.

Coupling method of stability enhancement based on casing treatments in an axial compressor

Casing treatment utilized in aircraft compressors is characterized by its powerful capacity of enhancing stability but causes remarkable efficiency penalty. To deal with this problem, we tested several types of casing treatments, including slots, grooves, recirculating casing treatments, and a composite structure to understand their characteristics. The test results show that different casing treatments can be combined to obtain a higher compressor stability, and the efficiency penalty can be decreased by implementing a novel flow management technology. Based on the results, a coupled casing treatment (CCT) that is constructed with slots, injectors, and a plenum chamber is proposed and optimized in the present study. The optimized CCT improves compressor stability by 16.7% with no penalty on the compressor efficiency. Two flow circulations, namely, an inner circulation in the slots and an outer circulation from the slots to the injectors, which dampen the development of tip leakage vortex, account for the excellent stability enhancement. The outer circulation, defined as the coupling effect, has generally a positive effect on compressor stability and a negative effect on the compressor efficiency. The effects of the length and numbers of the slots on the coupling effect are more significant compared to the slot location. The hysteresis effect that the recovery of tip blockage lags behind that of the tip leakage vortex cannot be established due to inadequate injection energy actuated in the CCT. As a result, a circumferentially discrete distribution of recirculating loops cannot improve compressor stability satisfactorily. As the CCT covers the full annulus, a much higher stability is obtained, and the boundary layer at the rear part of blade passages is severely separated when approaching the stall limit, which ultimately triggers compressor stall rather than the tip leakage vortex. The stall inception behaves as a full-annulus-dimension collapse with the flow phenomena of backflow at the trailing edge and forward spillage at the leading edge of the blade.

Investigation of the Unsteady Disturbance in Tip Region of a Contra-Rotating Compressor near Stall

The present study investigated the spectrum characteristics of unsteady disturbance and the tip leakage vortex evolution during pre-stall process for a contra-rotating axial compressor (CRAC). Transient numerical simulation was carried out in a single passage of the CRAC. The original transient fluctuation and oscillation of the tip leakage vortex structure with varying flow capacity of the CRAC were revealed using circle-like pattern figure and phase-locked root mean square (PLRMS). Additionally, the tip leakage flow in terms of vortex structure evolution was visualized for the sake of revealing the flow mechanism during pre-stall process. Results show that the unsteady fluctuation first appears at φ=0.3622, and the fluctuation frequency is 2.86 BPF. Unsteady disturbance source is mainly located at the tip side of the downstream rotor leading edge. From the choking point to the near stall condition, tip leakage vortex is always found in the tip leading edge of the upstream rotor. In addition, the tip leakage vortex of upstream rotor remains in the same place over time, i.e., no fluctuation, even when the downstream rotor entered into stall state. Such a phenomenon indicates that the stall point of the contra-rotating compressor is determined by the downstream rotor. Moreover, the maximum fluctuation position is mainly concentrated on the interface between the mainstream and the tip leakage vortex of the downstream rotor. By throttling the compressor, the angle between the main leakage vortex and the circumferential direction decreases gradually. When the main leakage vortex touches and continuously impacts on the leading edge of the adjacent blade, the unsteady disturbance, which is different from that of BPF, appears firstly.

Experimental investigation of tip-leakage flow in an axial flow fan at various flow rates

We investigate the evolution of tip-leakage vortex (TLV) in a low-pressure axial flow fan using digital particle image velocimetry. The blade rotational speed is fixed at 1000 rpm, and the Reynolds number is 547000 based on the blade tip radius and tip velocity. The evolution of TLV in downstream is highly influenced by the incoming flow rate. The TLV breakup occurs for the peak efficiency and stall conditions and sometimes for a higher flow rate, but does not occur for the highest flow rate considered. Thus, the migration speed of the TLV for the peak efficiency condition is faster due to the TLV breakup than those for higher flow rates. The scatter plot of the TLV center location indicates that the TLV wanders around its mean location and the region swept by the wandering motion increases as the TLV migrates downstream. High turbulent kinetic energy exists at the phase-averaged TLV center and its upstream location, respectively, owing to the wandering motion and the interaction between the TLV and main axial flow.

A study on tip leakage vortex dynamics and cavitation in axial-flow pump

The tip leakage flows and related cavitation in the tip region of an axial-flow pump were investigated in detail using the numerical and experimental methods. The numerical results of the pump model performance were in good agreement with experimental data. The flow structures in the tip clearance were clarified clearly with detailed data involving the axial velocity and turbulent kinetic energy. When depicting the feature of vortex core, the advanced vortex identification method λ2-criterion was used. Simultaneously, the minimum tension criterion was also applied to predict the cavitation inception for different flow rates and it is consistent with the distributions of vorticity and pressure in the vortex core. The roll-up process of TLV is highly three-dimensional and the entrainment would follow different paths. Then, both the numerical and experimental approaches show the cavitation patterns for different cavitation conditions, and it also finds that slight cavitation would promote the development of tip leakage vortex (TLV) while the TLV seems to be eliminated for a low cavitation number, especially before a specific location of blade tip due to the blade loading change induced by cavitation possibly.