Unsteady Tip Leakage Flow Research Articles

Analysis on stall mechanism of axial flow compressor with rotating inlet distortion by dynamic mode decomposition

To study the impact of rotating inlet distortion on stall mechanism of an axial compressor, the full-annulus unsteady simulation results are analysed using dynamic mode decomposition (DMD). Firstly, the frequency and energy ratio of each mode at near stall condition for the compressor with uniform inflow are obtained. A dominating frequency band 0.3–0.6 BPF regarded as the characteristic frequency of unsteady tip leakage flow is observed. The distribution of eigenvalues further confirms that there are a few critical unstable modes in the flow field. With further throttling, four large-scale disturbance structures are captured by the second-order mode, whose frequency is twice as the rotor frequency. The circumferential propagation velocity of the stall cell is thus deduced to be about 50% of the rotor speed. Further, when the compressor is subjected to rotating inlet distortion, the change of stalling process is related to the relative position of distortion plate and stall cell according to the DMD mode frequency and shape. The addition of rotating distortion does not change the stall type of the compressor, but the unstable modes indicate that the number and shape of the stall cells have been varied.

Simplified Numerical Models of the Unsteady Tip Leakage Flow in Compressor

Simplified Numerical Models of the Unsteady Tip Leakage Flow in Compressor

Role of unsteady tip leakage flow in acoustic resonance inception of a multistage compressor

In previous studies, a theoretical model was developed after Acoustic Resonance (AR) was experimentally detected in a four-stage compressor, and AR inception was proposed to be triggered by an unknown sound source, which is a pressure perturbation of a specific frequency with a suitable circumferential propagation speed. The present paper, which is not dedicated to the simulation of acoustic field, aims to identify the specific sound source generated by the unsteady tip leakage flow using the unsteady Computational Fluid Mechanics (CFD) approach. After a comprehensive analysis of an Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation, a pressure perturbation of non-integer multiple of rotor frequency is found at the blade tip. Since the essence of the tip leakage flow is a jet flow driven by the pressure difference between two sides of blade, a simplified tip leakage flow model is adopted using Large Eddy Simulation (LES) in order to simulate the jet flow through a tip clearance. It is found that the convection velocity of shedding vortices fits the expected propagation speed of the sound source, the frequency is also close to one of the dominating frequencies in the URANS simulation, and the resultant combination frequency coincides with the experimentally measured AR frequency. Since such a simplified model successfully captures the key physical mechanisms, it is concluded that this paper provides a piece of unambiguous evidence on the role of unsteady tip leakage vortex in triggering the AR inception of the multistage compressor.

Effects of moving endwall on the unsteadiness of tip leakage flow in compressor cascades

The influence of the moving endwall on the behavior of the tip leakage flow in compressors holds substantial significance. This work investigates the unsteadiness of the compressor's tip leakage flow under the impact of the moving endwall, employing both experimental methods and unsteady Reynolds-averaged Navier–Stokes (URANS) simulations. First, a numerical decomposition technique is proposed to isolate the tip leakage flow within a controlled diffusion airfoil (CDA) cascade. Subsequently, URANS simulations based on the CDA cascade are conducted at various incidence angles (−6° to +7°), tip clearances (clearance to chord: 0.83% to 3.33%), and flow coefficients (0.4 to 0.6), and additional URANS simulations based on the Virginia Tech cascade are also performed. The results indicate that the moving endwall induces a greater inclination of the tip leakage flow in the pitchwise direction, allowing it to impinge upon the pressure side of the adjacent blade. The impingement, if sufficiently intense, leads to the periodic generation of unsteady tip leakage flow by altering the blade loading. Moreover, the degree of unsteadiness diminishes significantly as the endwall speed and clearance height decrease. Additionally, it is observed that the breakdown of the tip leakage vortex does not serve as the root cause of the periodic unsteadiness. These findings underscore the critical role played by the moving endwall in the unsteadiness of the compressor's tip leakage flow.

Investigation of Tip Leakage Vortex Breakdown in a High-Speed Multistage Axial Compressor

AbstractIn this article, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near-surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in the static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge were observed. It was found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship among this unsteady flow phenomenon, tip clearance size, and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference was proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the aforementioned results, a high-speed multistage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted, and an increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade, which suppressed the unsteady tip leakage flow instability, was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

A stall diagnosis method based on entropy feature identification in axial compressors

AbstractA stall diagnosis method based on the entropy feature extraction algorithm is developed in axial compressors. The reliability of the proposed method is determined and a parametric sensitivity analysis is experimentally conducted for two different types of compressor stall diagnoses. A collection of time‐resolved pressure sensors is mounted circumferentially and along the chord direction to measure the dynamic pressure on the casing. Results show that the stall and prestall precursor embedded in the dynamic pressures are identified through nonlinear feature perturbation extraction using the entropy feature extraction algorithm. Further analysis demonstrates that the prestall precursor with the peak entropy value is related to the unsteady tip leakage flow for the spike‐type stall diagnosis. The modal wave inception with increasing amplitude is identified by the considerable increase of the entropy value. The flow field in the tip region indicates that the modal wave corresponds to the flow separation in the suction side of the rotor blade. The warning time is 100–300 rotor revolutions for both types of stall diagnoses, which is beneficial for stall control in different axial compressors. Moreover, a parametric study of the embedding dimension m, similar tolerance n, similar radius r, and data length N in the fuzzy entropy method is conducted to determine the optimal parameter setting for stall diagnosis. The stall warning based on the entropy feature extraction algorithm provides a new stall diagnosis approach in the axial compressor with different stall types. This stall warning can also be adopted as an online stability monitoring index when using the concept of active stall control.

Impact of complex flow structures on the turbine blade tip region mixing

Mixing of the turbine blade tip leakage and mainstream flows causes considerable aerodynamic loss. Its understanding is crucial to raise a consequential improvement of the turbine performance. In the present paper, a typical high pressure turbine rotor flow is simulated by detached eddy simulation. The complex mixing in the blade tip region is assessed by the dilution index algorithm in the streamwise direction. Influences of typical parameters on mixing are consecutively identified and analyzed. Relating the influences to flow structures, the mixing mechanism is obtained. It is revealed that the normal effective diffusion coefficient is key in mixing and is correlated with the unsteady tip leakage flow stick vortices. The latter induced by the Kelvin–Helmholtz instability can significantly enhance the local mass and energy transfer and hence diffusion. As a result, mixing is strengthened. Furthermore, the tip region is knowingly divided into the juxtaposing near and far fields. The former contains the tip leakage vortex, leakage jet, mainstream flow, and two entrainment zones. Meanwhile, the latter contains the leakage jet, mainstream flow, and entrainment zone. It was found that the entrainment zone is mixed to a high-degree, whereas the leakage jet is barely mixed.

Vortex and cavity dynamics for the tip-leakage cavitation over a hydrofoil

Large eddy simulations were used to investigate unsteady tip-leakage cavitating flow over a National Advisory Committee for Aeronautics 0009 hydrofoil. The proper orthogonal decomposition (POD) was combined with fast Fourier transforms to help better understand the deformation dynamics of the vortex and cavity. The results show three typical vortices, i.e., the tip-leakage vortex (TLV), tip-separation vortex (TSV), and shedding–trailing vortex (STV) with their own helical core lines in the averaged cavitating flows. Upstream of x/C = 1.8, the mean TLV core carries the breathing mode waves through the vortex dilatation effect, while the TSV core generates the dilatation, shrinkage, and bending distortions near the cavity interface by vortex stretching and deflection effects. Further downstream, the TLV starts to intertwine with the TSV, and the STV gradually diffuses. The TLV wandering encourages large cavity deformations dominated by the breathing and bending modes. However, in the lower-order POD modes, the contribution of bending modes to the cavity fluctuation energy is greatly weakened due to the variation of the relative cavity radius, r*, which indicates that the breathing mode and the double helix mode play major roles in the cavity interfacial waves. The results also show that the cavity interfacial oscillations are highly correlated with the surrounding velocity fluctuations. Moreover, a modified theoretical dispersion equation can well reproduce the quantitative relation between the vibration frequencies and the axial wavenumbers of the TLV cavity, especially for the wave dynamics of the breathing modes and the double helix modes.

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.

Tip Leakage Flow Instability in a Centrifugal Compressor

Abstract In this paper, an unsteady tip leakage flow phenomenon is identified and investigated in a centrifugal compressor with a vaneless diffuser at near-stall conditions. This phenomenon is associated with the inception of a rotating instability in the compressor. The study is based on numerical simulations that are supported by experimental measurements. The study confirms that the unstable flow is governed by a Kelvin–Helmholtz type instability of the shear layer formed between the main-stream flow and the tip leakage flow. The shear layer instability induces large-scale vortex roll-up and forms vortex tubes, which propagate circumferentially, resulting in measured pressure fluctuations with short wavelength and high amplitude which rotate at about half of the blade speed. The 3D vortex tube is also found to interact with the main blade leading edge, causing the reduction of the blade loading identified in the experiment. The paper also reveals that the downstream volute imposes a once-per-rev circumferential nonuniform back pressure at the impeller exit, inducing circumferential loading variation at the impeller inducer, and causing circumferential variation in the unsteady tip leakage flow.

Experimental investigation of the tip leakage flow in a low-speed multistage axial compressor.

Casing pressure measurements and Stereoscopic Particle-Image Velocimetry (SPIV) measurements are used together to characterize the behavior of the rotor tip leakage flow at both the design and near-stall conditions in a low-speed multistage axial compressor. A three-dimensional Navier-Stokes solver is also performed for the multistage compressor and the prediction of tip leakage flow is compared with SPIV data and casing dynamic static pressure data. During the experiment 10 high-frequency Kulite transducers are mounted in the outer casing of the rotor 3 to investigate the complex flow near the compressor casing and Fourier analyses of the dynamic static pressure on the casing of the rotor 3 are carried out to investigate the tip leakage flow characteristics. At the same time, the two CCD cameras are arranged at the same side of the laser light sheet, which is suitable for investigating unsteady tip leakage flow in the multistage axial compressor. The SPIV measurements identify that the tip leakage flow exists in the rotor passage. The influence of tip leakage flow leads to the existence of low axial velocity region in the rotor passage and the alternating regions of positive and negative radial velocity indicates the emergence of tip leakage vortex (TLV). The trajectory of the tip leakage vortex moves from the suction surface toward the pressure surface of adjacent blade, which is aligned close to the rotor at the design point (DP). However, the tip leakage vortex becomes unstable and breaks down at the near-stall point (NS), making the vortex trajectory move upstream in the rotor passage and causing a large blockage in the middle of the passage.

Effect of inlet radial distortion on aerodynamic stability in a multi-stage axial flow compressor

Inlet radial distortion caused by boundary layer separation is experimentally studied in a three-stage axial flow compressor. Two lattice rings were designed to generate tip and hub radial distortion, respectively, and distortion intensity was measured using a five-hole probe installed upstream of the first rotor blade row. The unsteady flow field is captured by using a collection of pressure transducers installed on the casing wall with circumferential and chordwise spatial resolution. The stall route measured on the casing wall indicates that the first stage stalls under all inflow conditions, tip radial distortion can modify the scale of stall inception to long-length scale (6–8 blade passages), unlike the uniform inflow and hub radial distortion with typical short-length-scaled inception (3–4 blade passages). Detailed measurements indicate that tip radial distortion can strengthen the tip leakage vortex and increase the tip separation to induce the compressor to stall earlier than uniform inflow. However, hub radial distortion redistributes the mainstream flow toward the blade tip region, thereby weakening the unsteadiness of tip leakage flow, reducing the outlet flow angle and increasing the outlet axial velocity near the blade tip to remove the separation or blockage, and finally extending the stall margin. The aerodynamic measurements downstream different rotor blade rows also indicate that the inlet radial distortion mainly affects the flow field at the first stage, which is obviously weakened at the second stage and completely unaffected at the third stage.

Tip air injection to extend stall margin of multi-stage axial flow compressor with inlet radial distortion

To prevent the deterioration of the inlet tip and hub radial distortion on stall margin, tip air injection was experimentally studied in a three-stage axial flow compressor. By generating a total pressure distortion intensity of 16.8% and 8.6%, the inlet tip and hub radial distortion can obtain stall margin variation of -5.1% and 10.5%, respectively. With the help of time-resolved pressure transducers installed on the casing wall and five-hole probes installed downstream of each rotor blade row, the unsteady measurement results demonstrate that the first stage stalls first under different inflow conditions and only the unsteady tip leakage flow at the first stage becomes stronger near the stall limit. Therefore, tip air injection is applied at the first stage under the distorted inflow to ensure the stable operation. Under inlet tip radial distortion, which strengths the unsteady tip leakage flow and blade tip separation reflected by the outlet axial velocity and flow angle, tip air injection can weaken the unsteady tip leakage flow, and remove the tip separation at the first stage, thereby making the second stage stall first such that only stall margin improvement of 19.2% can be obtained compared with uniform inflow. Although hub radial distortion can delay the occurrence of stall by weakening unsteady tip leakage flow and increasing the tip flow capability, overlarge injection can still improve the stall margin by 20.5% through further weakening the unsteady tip leakage flow and reducing the blade tip separation.

Comparison of DDES and URANS for Unsteady Tip Leakage Flow in an Axial Compressor Rotor

Tip leakage vortex (TLV) has a large impact on compressor performance and should be accurately predicted by computational fluid dynamics (CFD) methods. New approaches of turbulence modeling, such as delayed detached eddy simulation (DDES), have been proposed, the computational resources of which can be reduced much more than for large eddy simulation (LES). In this paper, the numerical simulations of the rotor in a low-speed large-scale axial compressor based on DDES and unsteady Reynolds-averaged Navier–Stokes (URANS) are performed, thus improving our understanding of the TLV dynamic mechanisms and discrepancy of these two methods. We compared the influence of different time steps in the URANS simulation. The widely used large time-step makes the unsteadiness extremely weak. The small time-step shows a better result close to DDES. The time-step scale is related to the URANS unsteadiness and should be carefully selected. In the time-averaged flow, the TLV in DDES dissipates faster, which has a more similar structure to the experiment. Then, the time-averaged and instantaneous results are compared to divide the TLV into three parts. URANS cannot give the loss of stability and evolution details of TLV. The fluctuation velocity spectra show that the amplitude of high frequencies becomes obvious downstream from the TLV, where it becomes unstable. Last, the anisotropy of the Reynolds stress of these two methods is analyzed through the Lumley triangle to see the distinction between the methods and obtain the Reynolds stress. The results indicate that the TLV latter part in DDES is anisotropic, while in URANS it is isotropic.

Unsteadiness of Tip Leakage Flow in the Detached-Eddy Simulation on a Transonic Rotor with Vortex Breakdown Phenomenon

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% span. The unsteadiness at near choke point and peak efficiency point is not obvious. DES method can resolve more detailed flow patterns than RANS (Reynolds-averaged Navier–Stokes) results, and detailed structures of the tip leakage flow were captured. A spiral-type breakdown structure of the TLV was successfully observed at the near stall point when the TLV passed through the bow shock. The breakdown of TLV contributed to the unsteadiness and the blockage effect at the tip region.

Circumferentially propagating characteristic dominated by unsteady tip leakage flow in axial flow compressors

The unsteady tip leakage flow (UTLF) is experimentally studied to observe the circumferential propagation in axial flow compressors. The evolutionary process of the circumferential propagation is captured by utilizing a collection of time-resolved pressure transducers on the casing with circumferential and chord-wise spatial resolution. Results show that the circumferential propagation dominated by the UTLF exists and occurs only after the emergence of the UTLF in the throttling process. Since then, the propagating speed and the scale of disturbance gradually augment, until the transition to the propagating speed and scale of stall inception. The tip air injection is applied to further verify the aforementioned circumferential propagation. Two kinds of injected momentum ratios, namely, micro and macro injection adopted in single-rotor compressor, and two types of injected methods, i.e., external air source and self-recirculation applied in single-stage and three-stage compressor demonstrate that either micro injection with a small stall margin improvement (SMI) or macro injection with a large SMI, or the different injected types in single- and three-stage compressor, with the increasing stability-enhancing capability of tip air injection, the UTLF is effectively weakened, and the circumferentially propagating speed and the scale of the disturbance dominated by UTLF accordingly shows a decreasing trend. The above results provide guidance for understanding the relationship between the UTLF and stall inception in terms of circumferential propagation that can be used to predict the compressor stability.

Investigation into the flow mechanism of rotating instability in a subsonic axial flow compressor rotor

Experimental and numerical investigations on a subsonic axial flow compressor rotor are carried out to clarify the flow mechanism of rotating instability (RI). RI is observed at the narrow stable operating range near the stability limit of the test rotor for the experiments. The simulation results show that RI appears near stall condition, which is similar to that in the experiments. An unsteadiness of tip leakage flow (TLF) is detected in a passage for the numerical results. Its characteristic frequency agrees well with the dominant mode frequency of RI in experiment. Details of the simulated flow fields indicate that the unsteadiness of TLF is induced by the interaction between TLF and the loading of blade tip in a single passage. This process propagates circumferentially against the rotor and forms a traveling wave in the circumferential annulus, which is responsible for RI. Since the blockage effect increases with decreasing mass flow rate, the fluctuation region of TLF gets larger, the period of flow unsteadiness increases and the parameters of RI vary.

Oscillatory Tip Leakage Flows and Stability Enhancement in Axial Compressors

Rotating stall axial compressor is a difficult research field full of controversy. Over the recent decades, the unsteady tip leakage flows had been discovered and confirmed by several research groups independently. This paper summarizes the research experience on unsteady tip leakage flows and stability enhancement in axial flow compressors. The goal is to provide theoretical bases to design casing treatments and tip air injection for stall margin extension of axial compressor. The research efforts cover (1) the tip flow structure at near stall that can explain why the tip leakage flows go unsteady and (2) the computational and experimental evidences that demonstrate the axial momentum playing an important role in unsteady tip leakage flow. It was found that one of the necessary conditions for tip leakage flow to become unsteady is that a portion of the leakage flow impinges onto the pressure side of the neighboring blade near the leading edge. The impediment of the tip leakage flow against the main incoming flow can be measured by the axial momentum balance within the tip range. With the help of the theoretical progress, the applications are extended to various casing treatments and tip air recirculation.

Stability Enhancement With Self-Recirculating Injection in Axial Flow Compressor

Self-recirculating injection, which bleeds air from the downstream duct of the last blade row and injects air as a wall jet upstream of the first rotor blade row, is experimentally investigated after the design of its structure in single- and three-stage axial flow compressors. External injection and outlet bleed air are selected for comparison. Results show that self-recirculating injection can improve the stall margin by 13.67% and 13% on the premise of no efficiency penalty in single- and three-stage axial flow compressors with only 0.7% and 4.2% of the total injected momentum ratio recirculated near stall, respectively. The self-recirculating injection is the best among all the three cases if the influence on pressure rise coefficient and efficiency is comprehensively considered. Moreover, findings indicate that the upstream injection plays an important role in terms of stability-enhancement. The details of the flow field are captured using a collection of pressure transducers on the casing with circumferential and chordwise spatial resolution. A detailed comparative analysis of the endwall flow indicates that the self-recirculating injection can postpone the occurrence of stalling in the proposed compressor by delaying the forward movement of the interface between the tip leakage flow (TLF) and main stream flow (MF), weakening the unsteadiness of TLF (UTLF), and sharply decreasing the circumferentially propagating speed dominated by the UTLF that triggers the spike-type stall inception. Finally, the stall control concept on the stage that first generates stall inception using self-recirculating injection is proposed. This study helps to guide the design of self-recirculating injection in actual application.

URANS simulations of the tip-leakage cavitating flow with verification and validation procedures

In the present paper, the Vortex Identified Zwart-Gerber-Belamri (VIZGB) cavitation model coupled with the SST-CC turbulence model is used to investigate the unsteady tip-leakage cavitating flow induced by a NACA0009 hydrofoil. A qualitative comparison between the numerical and experimental results is made. In order to quantitatively evaluate the reliability of the numerical data, the verification and validation (V&V) procedures are used in the present paper. Errors of numerical results are estimated with seven error estimators based on the Richardson extrapolation method. It is shown that though a strict validation cannot be achieved, a reasonable prediction of the gross characteristics of the tip-leakage cavitating flow can be obtained. Based on the numerical results, the influence of the cavitation on the tip-leakage vortex (TLV) is discussed, which indicates that the cavitation accelerates the fusion of the TLV and the tip-separation vortex (TSV). Moreover, the trajectory of the TLV, when the cavitation occurs, is close to the side wall.