Tip Leakage Flow Research Articles

Flow behavior in a contra-rotating transonic axial compressor

This study investigated a three-dimensional flow analysis on a two-stage contra-rotating axial compressor using the Navier–Stokes, continuity, and energy equations with Ansys CFX commercial software. In order to validate the obtained results, the absolute and relative flow angles curves for each rotor in radial direction were extracted and compared with the other investigation results, indicating good agreement. The compressor efficiency curve also was extracted by varying the compressor pressure ratio and compressor efficiency against mass flow rate. The flow results revealed that further distortion of the flow structure in the second rotor imposed a greater increase in the amount of entropy, especially at near-stall conditions. The increase of entropy in the second rotor is due to the interference of the tip leakage flow with the main flow which consequently caused more drops in the second rotor, suggesting that more efficacy of flow control methods occurred in the second rotor than in the first rotor.

Influence of different blade numbers on the performance of “saddle zone” in a mixed flow pump

In order to study the effect of different numbers of impeller blades on the performance of mixed-flow pump “saddle zone”, the external characteristic test and numerical simulation of mixed-flow pumps with three different impeller blade numbers were carried out. Based on high-precision numerical prediction, the internal flow field and tip leakage flow field of mixed flow pump under design conditions and stall conditions are investigated. By studying the vorticity transport in the stall flow field, the specific location of the high loss area inside the mixed flow pump impeller with different numbers of blades is located. The research results show that the increase in the number of impeller blades improve the pump head and efficiency under design conditions. Compared to the 4-blade impeller, the head and efficiency of the 5-blade impeller are increased by 5.4% and 21.9% respectively. However, the increase in the number of blades also leads to the widening of the “saddle area” of the mixed-flow pump, which leads to the early occurrence of stall and increases the instability of the mixed-flow pump. As the mixed-flow pump enters the stall condition, the inlet of the mixed-flow pump has a spiral swirl structure near the end wall for different blade numbers, but the depth and range of the swirling flow are different due to the change in the number of blades. At the same time, the change in the number of blades also makes the flow angle at 75% span change significantly, but the flow angle at 95% span is not much different because the tip leakage flow recirculates at the leading edge. Through the analysis of the vorticity transport results in the impeller with different numbers of blades, it is found that the reasons for the increase in the values of the vorticity transport in the stall condition are mainly impacted by the swirl flow at the impeller inlet, the tip leakage flow at the leading edge and the increased unsteady flow structures.

Effect of Tip Clearance on Helico-Axial Flow Pump Performance at Off-Design Case

To reveal the effect of tip clearance on the flow behaviors and pressurization performance of a helico-axial flow pump, the standard k-ε turbulence model is employed to simulate the flow characteristics in the self-developed helico-axial flow pump. The pressure, streamlines and turbulent kinetic energy in a helico-axial flow pump are analyzed. Results show that the tip leakage flow (TLF) forms a tip-separation vortex (TSV) when it enters the tip clearance and forms a tip-leakage vortex (TLV) when it leaves the tip clearance. As the blade tip clearance increases, the TLV moves along the blade from the leading edge (LE) to trailing edge (TE). At the same time, the entrainment between the TLV and the main flow deteriorates the flow pattern in the pump and causes great hydraulic loss. In addition, the existence of tip clearance also increases the possibility of TLV cavitation and has a great effect on the pressurization performance of the helico-axial flow pump. The research results provide the theoretical basis for the structural optimization design of the helico-axial flow pump.

Large eddy simulation of tip-leakage cavitating flow using a multiscale cavitation model and investigation on model parameters

For understanding tip-leakage cavitating flow features, the present work aims to implement a multiscale model to comprehensively reproduce the complicated phase structure. The volume of fluid (VOF) interface capturing method is applied to simulate macroscale cavities, while a discrete bubble model using the Lagrangian formulation is newly developed to take the microscale bubbles into account. The Schnerr–Sauer cavitation model is incorporated into the VOF model to calculate the mass transfer rate between phases from the macroscale point of view. For microscale bubbles, the simplified Rayleigh–Plesset equation is adopted to simulate the bubble growing and collapsing stages. An algorithm for coupling the approaches simulating macroscale cavities and microscale bubbles is also implemented to achieve multiscale simulation. Unsteady flow features are simulated using the large eddy simulation approach. The results show that an anti-diffusive compression scheme for the spatial discretization of volume fraction equation is relatively accurate for simulating the tip-leakage cavitating flow. Applying the multiscale model, the tip-leakage cavitating flow features with multiple time and space scales including the formation of glass cavity tube and the transport of bubble clouds can be revealed. Suitable model parameters including the coefficient of saturated pressure, and the bubble evaporation and condensation coefficients are studied.

Numerical simulation on hydraulic performance and tip leakage vortex of a slanted axial-flow pump with different blade angles

The blade angle has a great effect on hydraulic performance and internal flow field for axial-flow pumps. This research investigated the effect of the blade angle on hydraulic performance and tip leakage vortex (TLV) of a slanted axial-flow pump. The hydraulic performance and the TLV are compared with different setting angles. The dimensionless turbulence kinetic energy (TKE) is used to investigate the TLV. A novel variable fv is utilized to analyze the relation among the TLV, strain tensor and vorticity tensor. The proper orthogonal decomposition (POD) method is used to analyze TLV structure. The results show that with the increase of the blade angle, the pump head is getting larger, the flow rate of the best efficiency moves to be larger, and both the primary TLV (P-TLV) and the secondary TLV (S-TLV) are getting stronger. The P-TLV often exists in the outer edge of TKE distribution and S-TLVs often exist in the largest value area of TKE. This phenomenon is more evident with blade angle increasing. Through POD method, it shows that the first six modes contain more than 90% of TKE. The reason why the TKE value near the region of S-TLV is high is that the tip leakage flow is a kind of jet-like flow with high kinetic energy. The main structure of the P-TLV is shown in modes 4−6, resulting in a reflux zone but not with the highest TKE.

Effect of Tip Clearance Size on Tubular Turbine Leakage Characteristics

To study the effect of tip clearance on unsteady flow in a tubular turbine, a full-channel numerical calculation was carried out based on the SST k–ω turbulence model using a power-plant prototype as the research object. Tip leakage flow characteristics of three clearance δ schemes were compared. The results show that the clearance value is directly proportional to the axial velocity, momentum, and flow sum of the leakage flow but inversely proportional to turbulent kinetic energy. At approximately 35–50% of the flow direction, velocity and turbulent kinetic energy of the leakage flow show the trough and peak variation law, respectively. The leakage vortex includes a primary tip leakage vortex (PTLV) and a secondary tip leakage vortex (STLV). Increasing clearance increases the vortex strength of both parts, as the STLV vortex core overlaps Core A of PTLV, and Core B of PTLV becomes the main part of the tip leakage vortex. A “right angle effect” causes flow separation on the pressure side of the tip, and a local low-pressure area subsequently generates a separation vortex. Increasing the gap strengthens the separation vortex, intensifying the flow instability. Tip clearance should therefore be maximally reduced in tubular turbines, barring other considerations.

Vortex suppression of the tip leakage flow over a NACA0009 hydrofoil via a passive jet induced by the double-control-hole

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.

Near Tip Loss Control With a Winglet Baffle Cavity Tip in a Turbine Cascade

Abstract The aerodynamic performance of a winglet baffle cavity tip is investigated at different inlet incidences from −12.5 to +12.5 deg. This blade tip shows a geometry feature with a pressure side winglet and a baffle within the tip cavity. The experimental studies were carried out in a large-scale linear cascade. Numerical methods were also used to obtain the detail flow physics. The baffle on the tip divides the cavity vortex into two main parts, and enhances flow mixing over the tip. As the tip leakage flow exits the tip near the baffle and the cavity corner, flow separation occurs over the suction side and its blockage effect reduces the local tip leakage mass flow rate. The additional pressure side winglet reduces the contraction coefficient on the pressure side squealer. Compared with the squealer tip, the winglet baffle cavity tip reduces the tip leakage mass flow by 12.1% and the near tip loss by 4.2%. Removing the baffle on the tip also results in an increase of the loss. As the incidence of the incoming flow decreases, the loss near the tip reduces mainly due to a reduction of the strength of the passage vortex, which develops from the casing endwall. At all incidences studied, the winglet baffle cavity tip shows better aerodynamic performance than the squealer tip.

Effects of tip clearance size on vortical structures and turbulence statistics in tip-leakage flows: A direct numerical simulation study

Direct numerical simulations of the tip-leakage flow, generated by a gap between a straight National Advisory Committee for Aeronautics 0012 hydrofoil and the end wall of a channel, have been performed to investigate the effects of tip clearance size on vortical structures and turbulence statistics. The tip-leakage vortex, tip separation vortex, and induced vortex are the predominant vortical structures in tip-leakage flow for a relatively large gap (3.33%Ca), while the reverse flow vortex dominates the tip clearance region for smaller gaps (1.67%Ca), where Ca is the truncated chord length of the hydrofoil. Detailed analysis of turbulence statistics reveals that the tip-leakage vortex is caused by the rollup of the tip-leakage jet, while the spanwise inflow interacting with the sidewall of the hydrofoil leads to the formation of a reverse flow vortex. The turbulent kinetic energy contours show an arc-shaped distribution on the suction side of the hydrofoil, but their locations are significantly affected by the tip clearance size. In addition, the investigation of tip-leakage loss indicates that increasing the size of the tip clearance can reduce the tip-leakage loss across the hydrofoil. This can be attributed to the dominant vortical structures in the tip-leakage flow.

Numerical Prediction of the Aerodynamics and Acoustics of a Tip Leakage Flow Using Large-Eddy Simulation

A Large-Eddy Simulation of the tip leakage flow of a single airfoil is carried out. The configuration consists of a non-rotating, isolated airfoil between two horizontal plates with a gap of 10 mm between the tip of the airfoil and the lower plate. The Mach number of the incoming flow is 0.2, and the Reynolds number based on the chord is 9.3 × 105. The objective of the present study is to investigate the best way to compute both the aerodynamics and acoustics of the tip leakage flow. In particular, the importance of the inflow conditions on the prediction of the tip leakage vortex and the airfoil loading is underlined. On the other hand, the complex structure of the tip leakage vortex and its convection along the airfoil was recovered due to the use of a mesh adaptation based on the dissipation of the kinetic energy. Finally, the ability of the wall law to model the flow in the tip leakage flow region was proven in terms of wall pressure fluctuations and acoustics in the far-field.

Effects of Tip Leakage Flow on the Aerodynamic Performance and Stability of an Axial-Flow Transonic Compressor Stage

To explain the effect of tip leakage flow on the performance of an axial-flow transonic compressor, the compressors with different rotor tip clearances were studied numerically. The results show that as the rotor tip clearance increases, the leakage flow intensity is increased, the shock wave position is moved backward, and the interaction between the tip leakage vortex and shock wave is intensified, while that between the boundary layer and shock wave is weakened. Most of all, the stall mechanisms of the compressors with varying rotor tip clearances are different. The clearance leakage flow is the main cause of the rotating stall under large rotor tip clearance. However, the stall form for the compressor with half of the designed tip clearance is caused by the joint action of the rotor tip stall caused by the leakage flow spillage at the blade leading edge and the whole blade span stall caused by the separation of the boundary layer of the rotor and the stator passage. Within the investigated varied range, when the rotor tip clearance size is half of the design, the compressor performance is improved best, and the peak efficiency and stall margin are increased by 0.2% and 3.5%, respectively.

Analysis of the relationship between turbulence characteristics and loss mechanism in the tip leakage flow of turbine blade

The tip leakage flow passed over the tip clearance makes the flow very complicated near the tip gap, and the interaction of the tip leakage vortex and endwall vortex enhances the instability of the flow. Accurately capturing detailed flow structures and investigating the relationship between the flow structures and loss are beneficial for understanding the flow physics and providing guidance on reducing the loss. Due to the conventional Reynolds Averaged Navier-Stokes (RANS) methods is limited to predict the complex turbulence structures of the tip clearance flow, high fidelity simulation approaches are needed. In this work, the hybrid RANS/Large Eddy Simulation (LES) is adopted to simulate the tip leakage flow in linear cascade and demonstrates its ability to capture the small-scale flow structures. With the POD method, the time-averaged flow field and the dominating modes are obtained. Based on the analysis of the POD modes, it is found that the induced vortex generated by the interaction between the leakage vortex and the endwall vortex has strong turbulence characteristics. Based on the entropy generation rates, viscous loss mechanism is further analyzed. It is found that the shear strain rates dominate the viscous dissipation losses, and the fluctuation dissipation has a strong local enhancement effect.

Numerical Investigation on the Influences of Boundary Layer Ingestion on Tip Leakage Flow Structures and Losses in a Transonic Axial-Flow Fan

Abstract In a boundary layer ingesting (BLI) propulsion system, the fan is continuously exposed to inflow distortions. The distorted inflows lead to nonuniform loss distributions along the radial and circumferential directions. Since the rotor tip suffers from higher intensive distortion, the local loss increment is a major contributor to the BLI fan performance penalty. To explore the effects of distorted inflows on tip leakage flow evolutions and associated mechanisms for increased loss in a BLI fan, three-dimensional full-annulus unsteady simulations are conducted. Results show that about 54% of total additional losses due to distortion are formed in the tip region and more than 80% of tip region entropy generation is related to the tip leakage flow. The intensities of leakage vortex–shock interactions vary at different annulus locations. When the rotor moves into the distorted region, the vortex–shock interaction is weaker than the undistorted locations due to attenuated leakage flow. At the locations where the rotor is moving out from the distorted region, the vortex–shock interaction is notably enhanced because the front part of the blade tip airfoil suffers a higher load, resulting in a rapid vortex core expansion and eventually vortex breakdown. The increase of flow blockage in the front section of blade tip passages at local circumferential positions leads to a corresponding rise of flow loss. The findings in this study highlight the impacts of tip leakage flow on aerodynamic loss of fan working under BLI inflow distortion and provide improved understandings of loss mechanisms in a BLI fan.

Blade Lean and Tip Leakage Flows in Highly Loaded Compressor Cascades

Blade Lean and Tip Leakage Flows in Highly Loaded Compressor Cascades

Effects of Squealer Geometry of Turbine Blade Tip on the Tip-Leakage Flow and Loss

Effects of Squealer Geometry of Turbine Blade Tip on the Tip-Leakage Flow and Loss

Optimization of Turbine Cascade Squealer Tip Cooling Design by Combining Shaping and Flow Injection

Abstract In this study, a turbine cascade squealer tip is optimized by a multi-objective genetic algorithm (MOGA) with varying squealer heights and tip cooling configurations. The three objectives selected are the aerodynamic efficiency, the film cooling effectiveness, and the surface fluid temperature variance. The multi-scale methodology is implemented to reduce the computational cost and to skip the meshing of cooling holes. Two optimization approaches are compared: (a) a conventional method that optimizes an uncooled shape and the cooling configuration sequentially and (b) a method that optimizes shaping and cooling concurrently. The concurrent method is found to obtain better aerodynamic efficiency and heat transfer performance than the conventional optimization. Moreover, the aerodynamic efficiency ranking is changed by adding cooling to the uncooled blades. These observations are due to the strong interaction between the coolant and the tip leakage flow. They indicate that the coolant injected at the tip is not passive as expected in the conventional film cooling designs. By blocking the over tip leakage flow or forming a layer of air to level up the equivalent squealer cavity floor, the coolant can reduce the tip leakage loss, which contradicts the conventional wisdom that the added coolant should always lead to extra losses due to the extra mixing. The detailed observations of the flow field indicate that the influence of the squealer height towards the aerodynamic efficiency is caused by two competing effects: the blockage effect to reduce the tip leakage mass flowrate and the sudden expansion/contraction loss effect to generate additional loss.

Investigation of tip leakage flow unsteadiness and rotating instability in a centrifugal compressor impeller

A phenomenon called rotating instability was rarely reported in a centrifugal compressor, although some associated discussions can be found in axial compressors. This paper presents a numerical investigation on the unsteadiness of the tip leakage flow and rotating instability in an isolated centrifugal compressor impeller. A three-dimensional, unsteady full annuls simulation is performed under three different operating conditions. The tip leakage flow unsteadiness at the impeller inlet and inside the impeller passage is detected by the pressure fluctuation. During the throttling process, the temporal and spatial propagation characteristic of the tip leakage flow unsteadiness under the near stall operating condition is revealed based on the wavelet analysis, Fourier transform, and dynamic mode decomposition. Furthermore, the relationship between the tip leakage flow unsteadiness and the rotating instability is also discussed. The results show that the pressure fluctuation mainly concentrates in the tip region at the near stall operating condition, which indicates the occurrence and circumferential propagation of tip leakage flow unsteadiness. The circumferential propagation of the tip leakage flow unsteadiness induces a significant circumferential rotating pressure wave with a wave number of 9 and approximately 48.2% of the rotational speed of the impeller. The characteristic frequency of the tip leakage flow unsteadiness agrees well with the dominant mode frequency of rotating instability. Therefore, the tip leakage flow unsteadiness is assumed to be responsible for the rotating instability. During the throttling process, the formation and development of the rotating instability at near stall operating condition can be divided into three stages based on signal analysis: prior to rotating instability, generation and development, and rotating instability.

Analytical grid generation and numerical assessment of tip leakage flows in sliding vane rotary machines

The research presents a new analytical grid generation methodology for computational fluid dynamics studies in positive displacement sliding vane rotary machines based on the user defined nodal displacement approach. This method is more inclusive than state of the art ones since it enables the investigation of a broader range of design configurations, such as single, double and multiple-acting vane machines with non-circular housing, slanted blade and asymmetric blade tip profiles. Node number and radial divisions of blade tip are the parameters that affect most the mesh quality. The method was validated against indicated pressure measurements on a rotary vane expander resulting in a confidence interval within 4.31%. The benchmark analysis showed that the proposed method is as accurate as the manual ANSYS ICEM one but more than 1500 times faster (111s instead of 48h to generate 360 grids). The paper further proposes a novel method to track the leakage flows at the blade tip gaps of vane machines through a post-processing routine in ANSYS CFD-Post based on rotating monitoring planes. The leakage assessment on the vane expander case study showed that a 10 μm gap between blade tip and the 76 mm stator led to a 0.06 unit increase of the expander filling factor.

Multiscale modeling of tip-leakage cavitating flows by a combined volume of fluid and discrete bubble model

For the tip-leakage cavitating flow, the existence of both interface and microbubbles at scales separated by orders of magnitude makes it difficult to be comprehensively reproduced by numerical modeling. This work aims to develop a multiscale model that directly resolves the large-scale cavities and models small discrete bubbles. The volume of fluid (VOF) method and the Schnerr–Sauer cavitation model are first adopted to represent the phase field of water and vapor from a macroscale point of view. A discrete bubble model (DBM) based on the Lagrangian formulation is then developed to simulate the microscale cavitation bubbles that are smoothed in the macroscale model due to the limited mesh resolution. The transition between DBM and VOF is also implemented to achieve the multiscale simulation. For modeling of the turbulence, the scale adaptive simulation approach is used. The tip-leakage cavitating flows induced by the NACA0009 hydrofoil under the conditions with different gaps according to the reported experiment are simulated. Results show that, using the proposed multiscale model, better agreements can be obtained compared with the macroscale model, and specific phenomena can be well revealed including the bubble nucleation, growth and collapse, the interaction between discrete bubbles and large-scale cavities.

Research on tip clearance leakage flow field characteristics of tidal energy hydroelectric unit based on NACA0009 hydrofoil

In the rotor passage of tidal energy hydroelectric unit, there is a tip leakage problem between rotor and runner chamber. In order to research the flow characteristics of tidal energy hydroelectric units and explore the control method of tip leakage, based on the experimental data of NACA0009 hydrofoil, the SST-CC turbulence model was used for numerical simulation and compared with the experimental results. The characteristics of tip leakage flow and vortex were analysed under different hydrofoil shapes and gap widths. The results show that the rounded hydrofoil can effectively control the generation of tip separation vortex, but the rectangular hydrofoil has higher vortices coefficient and lower pressure coefficient in the downstream section; the tip leakage amount will increase with the increase of gap width; the tip vortex intensity will increase first and then decrease, and cavitation may occur when the vortices is maximum. The results can provide reference for studying the control method of tidal energy hydraulic machinery clearance leakage.