Leakage Vortex Breakdown Research Articles

Diagnosis of unsteady disturbance characteristics induced by the tip leakage vortex in a compressor based on data-driven modal decomposition methods

The structural information about the tip leakage vortex at the design point remains largely unknown. Here, the dynamic mode decomposition method is utilized to visualize the main coherent structures corresponding to unsteady disturbance frequencies induced by the tip leakage vortex of an isolated rotor at the design point. The results show that the tip clearance size has a significant impact on unsteady disturbance characteristics at the blade tip region. The flow field within the blade tip region can be categorized into four distinct regions: the formation region of the main tip leakage vortex (MTLV), the formation region of the secondary tip leakage vortex (STLV), the merging zone where the MLTV and the STLV interact, and the vortex shedding zone induced by the leakage vortex breakdown. The disturbance peak in the frequency domain decreases from 121.3 RF to 70.96 RF as the tip clearance size increases from 1.5% blade height to 2%, resulting in a reduction of 41.36%. The increase in the tip clearance size amplifies unsteady disturbances caused by the MTLV and STLV. The STLV exhibits more pronounced oscillatory characteristics than the MTLV. The unsteady disturbance induced by the MLTV mainly occurs at around 0.5 blade passing frequency (BPF). In contrast, high-frequency unsteady disturbances (>1 BPF) in the flow field are caused by vortex shedding resulting from the interaction and collision between the STLV and the MTLV. A better understanding of the unsteady disturbance characteristics induced by leakage vortex benefits the study of stall warning technology.

Investigation of a rotating stall in a supercritical CO2 centrifugal compressor

Due to the nonlinear behavior of carbon dioxide properties at its critical point and the size effect of the supercritical carbon dioxide (S-CO2) centrifugal compressor, the stall causation mechanism differs between the S-CO2 centrifugal compressor and a conventional air compressor. The comprehension of the induced principle of the S-CO2 compressor rotating stall holds immense significance in enhancing stall margin and efficiency. This paper employs unsteady simulations to investigate the causes of the impeller rotating stall in the S-CO2 centrifugal compressor. The results show that the leading edge breakdown vortex (LEBV) formed by the tip leakage vortex (TLV) breakdown and the reverse flow in the passage are the reasons for blocking the passage and ultimately causing the rotating stall of the impeller. The migration motion of the LEBV not only induces the leading edge spillage phenomenon but also influences the intensity of the tip leakage flow (TLF) in adjacent passages, causing the propagation of the TLV breakdown phenomenon in the opposite direction to that of impeller rotation. The TLV undergoes intermittent breakdown in flow field, which is influenced by variations in TLF intensity. Additionally, there is a preceding process of breakdown-induced vortex formation and disappearance prior to TLV fragmentation.

Role of tip leakage flow in an ultra-highly loaded transonic rotor aerodynamics

Tip clearance size is a very critical design parameter for the modern transonic rotor. This investigation attempts to reveal the role of tip leakage flow in an ultra-highly loaded low reaction transonic rotor (loading coefficient equals to 0.782) aerodynamics. Flow fields in the rotor with varied tip clearance sizes, ranging from 0 to 3 times of the datum case, are discussed at length. Both performance variation and stall mechanisms involved are elaborated. As a result, the rotor performance presents a continuous decline with the increased tip clearance size as a result of the additional secondary flow loss induced by the tip leakage flow. While for stall margin, an optimal tip clearance size exists, and it happens to be the datum case. The tip leakage flow is found to have the ability to blow away the low-energy flow accumulated in the rotor tip suction side. Specially, the forward portion of tip leakage flow plays the dominate role in extending the rotor stall margin. However, as the tip clearance size exceeds the datum case, the shock-tip leakage vortex interaction causes the tip leakage vortex breakdown and the stall margin is hence narrowed.

The unsteadiness of tip leakage vortex breakdown and its role in rotating instability

The unsteadiness due to tip leakage vortex (TLV) breakdown was studied using a special experimental test campaign in parallel with numerical simulations. The back flow vortex (BFV), an isolated vortex caused by TLV spiral-type breakdown, was found to play a key role in rotating instability (RI). High-speed pressure transducers were used to measure the unsteady pressure field at the casing end wall of the blade in an isolated subsonic compressor rotor, which identified a low-frequency fluctuation at the near stall condition. A single-passage unsteady Reynolds-averaged Navier–Stokes simulation was used to study the evolution of unsteady flow structures, validated by the experimental measurements. Two distinct kinds of periodically unsteady flow were revealed by the simulations. A high-frequency fluctuation corresponding to 1.0 blade pass frequency (BPF) was caused by the spiral-type breakdown of the TLV. The other low-frequency fluctuation corresponding to 0.5BPF was caused by the feedback interaction between the BFV and the blade loading. The BFV was generated by the TLV breakdown, which was separated from the twisted vortex core of the TLV, and it moved downstream along the pressure side of the adjacent blade. A larger sized BFV reduced the local loading of the adjacent blade. The TLV was weakened as a consequence of the reduced loading, resulting in a smaller sized BFV. The blade tip loading was relatively less affected by the small sized BFV rather than the larger sized BFV. Therefore, the blade loading recovered and the size of the BFV increased, repeating the cycle. This feedback mechanism produced a pressure fluctuation with a frequency equal to 0.5BPF, which was closely related to RI.

Interaction mechanism between incoming vortex and tip leakage vortex breakdown of a compressor cascade

Tip leakage vortex breakdown (TVB) is an important flow character that affects the performance of axial compressors. In this study, the interaction mechanism between incoming vortex (IV) and TVB is investigated to improve the performance of compressor cascade. First, the pitchwise location effect, swirling direction effect, and intensity effect are investigated under steady condition; then, an unsteady investigation is conducted based on the steady results. Under steady condition, TVB can be completely removed by the incoming vortex near the suction surface with different swirling directions, and the normalized circulation at the edge of incoming vortex is 0.7. The loss coefficient caused by TVB is also reduced; however, a new loss is induced by IV. The flow mechanism is that the interaction between the incoming vortex and tip leakage vortex (TLV) can increase the chordwise momentum at the vortex core and the stability of the TLV. For TLV and IV, the two vortices have a tendency to combine downstream of the compressor cascade if they have the same swirling directions. However, when the swirling directions are different, TLV and IV propagate separately, and the intensity and stability of TLV are much higher. The loss of TLV is higher for the anticlockwise IV scheme because the intensity of the TLV is greater due to the interaction between TLV and IV. By utilizing the incoming vortex under unsteady condition, the TVB can also be reduced. The unsteadiness of the compressor cascade with the incoming vortex is significantly reduced by reducing the TVB.

Understanding of tip clearance flow structure in high speed mixed flow compressor

This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet total pressure ratio of 3.8 with 39,836 rpm rotor speed. The four different tip configurations namely the constant (λ = 0.016 and 0.019) and variable (λ = 0.011 (inlet)-0.019 (exit) and 0.019 (inlet)-0.022 (exit)) tip clearances were numerically analysed using available experimental data-set. The numerical investigation reveals that in contrast to the classic jet-wake pattern, two anomalous velocity profiles formed at the impeller exit which results in pressure losses in the vaneless diffuser. Near the impeller inlet, the tip leakage flow rolls up to discrete tip leakage vortex structure for each tip clearance configuration. This results in the formation of a region of momentum deficit, recirculation zone, which gets weakened as it moves downstream. The tip clearance configuration is observed to profoundly influence the extent and vorticity of the tip leakage vortex. In the splitter blade passage, the tip leakage flow and Coriolis flow interact with passage flow, resulting in the formation of two secondary passage vortices that move downstream along the pressure and suction surface of the splitter blade. The tip clearance configuration directly influences the impeller exit jet-wake pattern by modulating the secondary passage vortices trajectory and vorticity. Moreover, off-design analysis for tip clearances λ = 0.016 and λ = 0.019, depict distinctive tip leakage vortex characteristics. When operating near the stall conditions (80% of design mass flow rate), λ = 0.019 exhibits bubble shape tip leakage vortex breakdown occurring near the impeller inlet. This result in a substantial change in the tip leakage vortex nature; expansion of the recirculation zone and early weakening of the vorticity in the tip leakage vortex. It is observed that vortex breakdown plays a vital role in characteristics of the passage flow field structure and compressor performance near the stall conditions.

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.

Investigation of the unsteady flow in a transonic axial compressor adopted in the compressed air energy storage system

During operation, compressed air energy storage systems should respond rapidly to variations in power network demand, requiring that the compression system should always be in changeable off-design conditions. Compression systems with low flow rates confront difficulties such as diminished aerodynamic performance and increased flow losses. Given that the emergence of compressor instability is a process of the accumulation and intensification of flow unsteadiness. In this study, a numerical simulation was conducted for a transonic compressor to investigate the unsteady evolution characteristics of the tip leakage flow field, and it focused on the flow field of a condition where the flow unsteadiness was stimulated and further investigated the origin of flow unsteadiness, which was supposed to provide insights in developing flow control strategies and enable the application in a compressed air energy storage system under low mass flow rate conditions. The Fourier fast transformation (FFT) signal analysis showed that characteristic frequency of the unsteady tip flow field was 1863.3 Hz. Unsteady disturbances were predominantly distributed in the following three regions: the leakage vortex region of the blade suction surface, the location of the epitaxially detached shock wave at the entrance of passage, and the 5 %–30 % chord of the blade pressure surface. The high disturbance region of leakage vortex was generated by the periodic shift in leakage vortex intensity over time, the interaction between the main flow and various types of secondary flow due to leakage vortex breaking processes was the primary cause for the disturbance region of the epitaxially detached shock wave at the entrance of passage, and the high disturbance region of blade pressure surface was caused by the adverse pressure gradient effect and flow direction deflection of the airflow in the passage, as well as the impacted on the pressure surface of the adjacent blade. It was speculated that leakage vortex breakdown due to vortex wave interference was the key factor to induce the unsteadiness of the flow field. A new vortex structure was formed at the leading edge of adjacent blades after leakage vortex breakdown, which was defined as a “leading edge vortex”, and its series of evolutionary processes occurring timely were the main reasons for the unsteady flow in the tip flow field. This study laid a theoretical foundation for the research of the flow field control strategy of axial compressor operating in off-design conditions in a large-scale compressed-air energy storage system.

Pitchwise casing treatment for effects of grooves on dynamics of vortices and loss of blade tip region of a compressor cascade

In order to find the controlling mechanism of the pitchwise casing treatment (CT) on a compressor tip leakage vortex (TLV) breakdown and the complex flow on the blade tip, the pitchwise casing treatment of a compressor cascade with a gap is studied using a numerical simulation method. Firstly, the cascade is verified with some existing experimental data. Then, the controlling effect of single pitchwise CT grooves with different axial positions on the TLV breakdown and the tip flow field is studied, and the impact of pitchwise CT grooves about the tip loss is evaluated. Next, the effect of a single pitchwise CT groove on the tip loss is evaluated in cases of different incidences. Then, the controlling mechanism of the blade tip is studied on double pitchwise CT grooves. The obtained results show that the CT grooves at different axial positions can effectively reduce the range of the TLV breakdown, but they have different effects on the tip flow field. Based upon their influence on the tip loss, they can be divided into “Narrow region” CT grooves and “Wide region” CT grooves. When a “Wide region” CT groove is located at 48.7% of the axial chord, the range of the TLV breakdown becomes the smallest and the tip loss is reduced by 24.21%. A “Narrow region” CT groove mainly inhibits the TLV breakdown by delaying the formation of TLV, but the suction effect of the CT groove enhances other backflow structures in the blade tip region. A “Wide region” CT groove mainly inhibits the TLV breakdown by the injection acceleration of the CT groove, and it also reduces other backflow structures in the blade tip region. The “Wide region” CT grooves have better treatment effects than the “Narrow region” CT grooves. The “Wide region” CT groove significantly increases the incidence range of low loss, and the loss is lower than that of a solid wall in large incidence conditions. To a certain extent, it indicates the potential for the compressor that the “Wide region” CT groove to improve the stability margin. When the “Wide region” CT groove is combined with the “Wide region” CT groove, the double groove significantly reduces the tip loss.

Flow mechanism of self-recirculating casing treatment in a low-reaction transonic compressor rotor

Casing treatment is used to ensure the stability of an aeroengine compressor when it is operated away from its design point. However, currently, there is little published data about the effect of casing treatment on highly loaded axial compressors. In this study, unsteady numerical simulations are applied to explore the flow mechanism responsible for the stability enhancement of a low-reaction (LR) transonic compressor rotor with ultra-high loading owing to self-recirculating casing treatment (SRCT). A special SRCT configuration is designed based on the suction effect, which can simultaneously improve the stall margin and aerodynamic performance. Radial skew in the direction of blade rotation is employed to enhance the control outcomes further. The results show that the best skew angle for stall margin and performance improvement is 45 degrees. Further analysis of the instantaneous flow structures reveals that for the smooth casing, the mechanism by which tip leakage vortex (TLV) breakdown, TSV, and blade loading interact induces periodic oscillations of the tip flow field. As the unsteady suction effect introduced by the SRCT dominates the evolution processes of vortical structures, the original oscillation frequency is converted into the SRCT-induced excitation frequency. Furthermore, a substantial amount of double-leakage flow is inhaled into the recirculation channel without rolling up into the adjacent TLV, which dampens the shock/TLV interaction and the formation of downstream TSVs. The blockage along the pressure side caused by the aforementioned large-scale vortices is effectively eliminated.

Assessment of a novel k–ω turbulence model for transonic centrifugal impeller simulations

Numerical simulation of high pressure ratio transonic centrifugal compressors is challenging for the existing turbulence models. A lagged k–ω model proposed by Olsen and Coakley for nonequilibrium effects was first applied to simulate the transonic centrifugal impeller SRV2-O. As comparative case studies, four other turbulence models ( k–ω model, RNG k–ε model, SST-CC model, and EARSM model) were also computed. The comparison showed that ( i) the selection of the turbulence model had a great influence on SRV2-O impeller simulations; ( ii) the lagged k–ω model had an advantage over other models in terms of overall pressure ratio and internal flow characteristics; and ( iii) the lagged model predicted a smaller blockage area caused by leakage vortex breakdown than other models, closer to the experimental result. The detailed parameter examination indicated that the nonequilibrium parameter a0 in the lagged model had little influence on the Mach number distribution and choking mass flow rate but a significant influence on the static pressure on the shroud casing. For a higher Mach number compressor, a smaller a0 is recommended for bettering the simulation accuracy.

Combined control of variant clearance and casing aspiration on tip flow field and instability of a transonic compressor

To enhance the transonic compressor performance, the axially non-uniform variant tip clearance schemes and combined schemes with casing aspiration were applied and their effects on the tip flow field and flow instability were studied numerically. The results show that the expanding tip clearance can increase the stall margin and peak efficiency by reducing the tip leakage flow intensity within the first half chord, and cause the interface between the leakage flow and mainstream to move downstream. The effect of the shrinking clearance is on the contrary. The compressor performance can be further improved by the combination of expanding clearance and casing aspiration. When the circumferential suction groove is located at 35% of the axial chord length corresponding to the low-velocity region due to the leakage vortex breakdown, the stability enhancement is the largest, which is about 7.2% higher than that under the uniform clearance scheme. Casing aspiration decreases the blade tip loading and attenuates the tip leakage vortex, tip secondary vortex, and their interaction, which results in the suppression of vortex breakdown and tip flow blockage. Moreover, the oscillations and instability of the flow field are weakened. Therefore, the tip flow field is modified and the compressor performance is 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.

Influence of Blade Lean on Performance and Shock Wave/Tip Leakage Flow Interaction in a Transonic Compressor Rotor

Blade lean has been extensively used in axial compressor stators to control flow separations, but its influence mechanism on transonic compressor rotors remains to be revealed. The aim of this study is to numerically explore the influence of blade lean on the performance and shock wave/tip leakage flow interaction in a transonic compressor rotor. The effects of leaned pattern (positively lean and negatively lean), leaned angle and leaned height were studied. Results showed that, compared with baseline configuration, the efficiency and total pressure ratio of the entire constant rotating speed line of positively leaned rotor were both decreased. The absolute value of peak efficiency was reduced by as much as 4.34% at 20° lean angle, whereas the maximum reduction of peak total pressure ratio was 0.1 at 20° lean angle. The tip leakage flow streamlines of baseline transonic rotor can be divided into two parts, i.e., the primary vortex and secondary vortex which arises after the shock. Due to shock/tip leakage vortex interaction, the primary vortex enlarged and low-momentum region showed up after the shock; under near stall (NS) condition, tip leakage vortex breakdown occurred after interacting with shock. As positively leaned angle increased, the shock and the shock/tip leakage vortex interaction point moved upstream. In addition, the phenomenon of tip leakage vortex breakdown was enhanced. For negatively leaned rotors, as negatively leaned angle increased, the peak efficiency and total pressure ratio showed a tendency of first increasing and then decreasing. At 5° leaned angle, the peak efficiency was increased by 0.8% at most, and the maximum increment of total pressure ratio was 0.05 at 5° leaned angle. Besides, the loading of blade tip reduced and the loading moved toward trailing edge, resulting in the downstream movements of primary vortex, shock front and shock/tip leakage vortex interaction location. The results may help to improve the near tip flow field of transonic compressor rotor with leaned blade technology.

Influence of Shock Wave on Loss and Breakdown of Tip-Leakage Vortex in Turbine Rotor with Varying Backpressure

In modern turbine rotors, tip-leakage flow is a common phenomenon that accounts for about 1/3 of the stage loss. Studies show that as the imposed load increases, a shock wave appears in the tip region, which causes a significant interference on the leakage vortex. In the present study, numerical simulations are carried out to investigate the influence of the shock wave on the loss and breakdown of the tip-leakage vortex. The obtained results indicate that with no effective control on the flow, the loss of the leakage vortex has an approximate exponential growth up to about 10 times as the outlet Mach number increases from 0.67 to 1.15 and the corresponding proportion in the total loss increases sharply to 30.2%. It is found that the stagnation position of the breakdown changes with the backpressure and the amplitude of variation along the axial direction is up to 0.13 Cx. It is inferred that the breakdown of the leakage vortex core may be affected by the periodical passing of downstream blade and the induced pressure fluctuation may result in additional vibration in this rotor blade. The leakage vortex is unstable in supersonic flow with a shock wave and it may transfer to a flow with a low-velocity bubble in its core region. It is concluded that the leakage vortex breakdown mainly originates from interferences of the shock wave, while the internal cause of such breakdown is the centrifugal instability of the vortex.

Design Concept with Tip Leakage Vortex Control for Centrifugal Compressor Flow Stabilization

Centrifugal compressor applied to turbochargers is required to operate stably in wide range from choking to surging. In our past research, authors suggested a design policy called STB (flow Stabilization by inducing Tip leakage vortex Breakdown) concept, which indicates “Operating range can be enhanced with uniform blockage region generated by tip leakage vortex breakdown”. In this study, authors tried to validate the STB concept by redesigning a given impeller into a wide range compressor. In this research, detailed analyses showed that non-uniform and unstable stall occurred at small flow rate in the conventional impeller. On the other hand, in the redesigned new impeller having high inducer loading, unsteady numerical calculation showed that the strengthened blade tip leakage flow generated a circumferential uniform blockage region, which could stabilize its internal flow. As a result of performance test of the new impeller, considering that the surging flow rate at the same shaft speed was reduced by 3% and the pressure ratio at the surging point had been improved from 2.8 to 2.9, the surging flow rate at the pressure ratio of 2.8 could be reduced by 8%. In this way, it was found that the tip leakage flow was dominant with the stall phenomenon of the centrifugal compressor, and it was also confirmed that the STB concept was one of the effective means for the operation range enhancement.

Tip leakage vortex and its breakdown mechanism in aspirated compressor cascades designed with conventional method and curvature induced pressure recovery concept

Boundary layer suction (BLS), or aspiration, is an effective methodology to eliminate flow separation and improve blade loading in axial compressors. The objective of this study is to reveal the flow physics and loss mechanism of tip leakage vortex in aspirated compressor cascades with multiple loading levels under diffusion factor ranging from 0.42 to 0.71. The effect of blade pressure distribution on tip leakage flows was investigated numerically based on three aspirated blades, including the one designed with conventional method and the other two designed with curvature induced pressure recovery. The influences of tip clearance dimension and BLS schemes (BLS location, BLS height and BLS mass flow rate) on tip leakage vortex as well as its breakdown mechanism were investigated. Results show that a pseudo-shock was found in aspirated compressor cascades designed with conventional method and curvature induce pressure recovery concept. Compared with conventional compressor blades, the main flow phenomenon of tip leakage flow of aspirated compressor blades is the tip leakage vortex breakdown (TLVB) caused by strong adverse pressure gradient introduced by BLS. The TLVB phenomenon occurred both in conventionally loaded compressor cascade and highly loaded compressor cascade. For conventionally loaded blade, TLVB phenomenon occurred under low tip clearance condition but disappeared under high tip clearance condition due to increased strength of tip leakage vortex. Lower BLS mass flow resulted in weaker pseudo-shock, which reduced the swirl number of tip leakage vortex and improved the vortex breakdown phenomenon as a consequence. The vortex breakdown of highly loaded cascades was resulted from the joint action of high blade loading and pseudo-shock. Pseudo-shock designed near blade trailing edge resulted in higher leakage mass flow but promised superior tip leakage flow field. For all the aspirated cascades studied, the tip leakage loss decreased as tip clearance dimension increased, which was opposite to the results of conventional compressor cascade.

Analysis of characteristics and mechanism of flow unsteadiness in a transonic compressor

This paper reveals the flow mechanism of unsteady flow oscillations at small mass flow rate conditions in a transonic compressor rotor. A series of unsteady RANS simulations were performed to achieve time-accurate and time-averaged flow fields. The predicted results were validated by experimental data. The present analysis indicates that the tip leakage vortex (TLV) breakdown occurs with the operating condition approaching the stability limit due to an interaction between the TLV and the shock wave. A bubble-type vortex breakdown is first observed. However, the TLV breakdown does not directly cause flow oscillations. Only if the breakdown region develops beyond a threshold volume value (9.15e−9 m3 in current case), it will influence the static pressure distribution at the pressure side of blade tip region. After that the flux of tip leakage flow, the strength of TLV, the shock/TLV interaction are varied, which in return changed the size of the breakdown region in the adjacent passage. In such a way, a self-sustained system which causes periodical flow oscillations is established. When the compressor is further throttled, the bubble- and spiral-type TLV breakdown occur alternatively, which induces a change in the frequency of the periodical flow oscillation. Although the characteristic frequency of flow oscillation changes with the transition of operating conditions, the trigger for flow unsteadiness is the same as that in the bubble-type breakdown case, namely the changes of static pressure distribution at the pressure side of blade tip. Moreover, during the transition from bubble- to spiral-type breakdown, the breakdown region significantly increases, resulting in a more severe pressure fluctuation in the flow field.

Efficient Passage-Spectral Method For Unsteady Flows Under Stall Conditions

AbstractThis paper presents an efficient method of approximating unsteady flows using a block-wise discrete spatial Fourier series for the modeling of three-dimensional non-axisymmetric flows without making any hypothesis about its temporal periodicity. The method aims at capturing the long-wavelength flow patterns that are present in many unsteady problems of industrial interest, such as compressor stability, with a drastic reduction in computational resources. The method is intended to be used to compute flows exhibiting large-scale instabilities and where the fundamental frequency of the problem is not known beforehand. The approach discretizes the domain using a finite number of blocks or passages, where the flow variables at the supposedly periodic boundaries are continuously updated using the spatial Fourier coefficients of a uniformly spaced set of reduced-passage domains. The NASA rotor 67 under the effect of distorted inflow conditions has been used as verification case to demonstrate the effectiveness and viability of the method. The comparison between the passage-spectral method and the full-annulus solution shows that sound solutions can be obtained with a low number of harmonics. The new method has also been applied to investigate the rotating stall inception of the NASA rotor 67 for distorted inlet flows near stall operating conditions. The method is shown to accurately reproduce the full-annulus solution with a few spatial harmonics, capturing the characteristic features of the complex flow induced by the tip leakage vortex breakdown. The computational cost in this application has been reduced by a factor of between three and seven. This number heavily depends on the ratio between the number of retained harmonics to the number of blades.

Numerical investigation into the mechanism regarding the inception and evolution of flow unsteadiness induced by the tip leakage flow in a transonic 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.