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The internal losses in the tip clearance region strongly influence the compressor performance and its operational range. Previous research proved that passive wall treatments with circumferential grooves in axial compressors effectively increase the compressor stall margin. The vortex generated inside the circumferential grooves creates a resistance to the flow that leaks into the tip clearance region of the compressor. However, most works found in the literature on circumferential grooves in axial compressors deal only with high-performance single-stage axial compressors. Therefore, there is a need to investigate and analyze the behavior of circumferential grooves in a multi-stage environment. In the present work, a passive wall treatment with circumferential grooves was implemented in a multi-stage axial compressor. Different configurations of circumferential grooves were created at the casing of the first and second rotor rows used in a four-stage axial flow compressor. Numerical simulations were performed to evaluate the influence of the circumferential grooves on the performance of a multi-stage axial compressor. The results obtained after the simulations for the different circumferential groove configurations were compared with the results obtained for the compressor without casing treatment (smooth wall) for different rotational speeds. Furthermore, the complete compressor map characteristics were simulated for the different casing treatment configurations, and the results were compared with the compressor characteristics of the smooth wall case. The passive wall treatment with circumferential grooves produced changes in the multi-stage axial compressor flow field, especially in the tip clearance region, improving the compressor stability mainly for part load speeds. [ABSTRACT FROM AUTHOR]

Aimed at achieving stable operation of mixed flow pumps, circumferential grooves casing treatment is adopted and its effect on stability expansion is investigated. The design speed and flow rate of the studied mixed flow pump are 980 r/min and 1500 m2/h, respectively. Single groove and three grooves are applied in three different positions, and the comparative study is carried out through numerical simulation based on CFD software NUMECA. The results manifest that, through circumferential grooves casing treatment, the flow range of the mixed flow pump is effectively expanded. The treatment effect of three grooves at each position is better than that of single groove, and it is not linearly enhanced. There is an optimal treatment position above the top of the impeller, which is near the leading edge of the blade. Due to the improvement of the leading-edge overflow, the effect of stability expansion gradually deteriorates as the position moves back to the trailing edge. Heads are increased both with single groove and three-groove treatment near the stall point, while efficiency only rises at the leading edge. It is also found that, through the circumferential grooves casing treatment, the pressure difference between the two sides of the blade makes the tip leakage flow into the grooves, then converge into the main stream by circumferential transport, which alleviates the pump stall caused by the congestion of the tip leakage flow, also prevents the tip leakage from becoming a leakage vortex, which will affect the operating stability of the mixed flow pump. This work provides reference for the investigation on circumferential grooves casing treatment for stability expansion of mixed flow pumps. [ABSTRACT FROM AUTHOR]

A wire mesh casing treatment is proposed and numerically tested on the transonic axial compressor (Rotor 37). A theoretical stall inception prediction model was developed to account for the wire mesh casing treatment, which was modeled as an impedance boundary condition. This model was validated on a low-speed compressor, replicating the experimental trend with reliable precision. The impedance boundary condition effectively attenuates disturbances and confines the distribution range of perturbations compared to the solid wall boundary condition. Furthermore, the installation of the wire mesh casing treatment induced a downstream shift in the passage shock wave and enhanced the flow capacity in the blade tip region. The effects of the casing treatment on different installation locations were investigated, and the results indicated that the maximum stall margin improvement occurred when the casing treatment was positioned at the midchord location. The integrated design of the casing treatment and blade profile, along with the established prediction model, is also discussed. This model demonstrates reliable precision when compared to the experimental results and addresses the need for rapid design iteration during the initial design phase. [ABSTRACT FROM AUTHOR]

The internal losses in the tip clearance region strongly influence the compressor performance and its operational range. Previous research proved that passive wall treatments with circumferential grooves in axial compressors effectively increase the compressor stall margin. The vortex generated inside the circumferential grooves creates a resistance to the flow that leaks into the tip clearance region of the compressor. However, most works found in the literature on circumferential grooves in axial compressors deal only with high-performance single-stage axial compressors. Therefore, there is a need to investigate and analyze the behavior of circumferential grooves in a multi-stage environment. In the present work, a passive wall treatment with circumferential grooves was implemented in a multi-stage axial compressor. Different configurations of circumferential grooves were created at the casing of the first and second rotor rows used in a four-stage axial flow compressor. Numerical simulations were performed to evaluate the influence of the circumferential grooves on the performance of a multi-stage axial compressor. The results obtained after the simulations for the different circumferential groove configurations were compared with the results obtained for the compressor without casing treatment (smooth wall) for different rotational speeds. Furthermore, the complete compressor map characteristics were simulated for the different casing treatment configurations, and the results were compared with the compressor characteristics of the smooth wall case. The passive wall treatment with circumferential grooves produced changes in the multi-stage axial compressor flow field, especially in the tip clearance region, improving the compressor stability mainly for part load speeds.

Casing treatment and three-dimensional blade design are effective techniques to remedy the deficiency of compressor stability margin, yet their combined effects on compressor performance are seldom studied. With the help of URANS simulations, this paper explored the combined influence of aerodynamic sweep and casing treatment in a transonic compressor rotor. Compared with the configurations of single sweep and single casing treatment, the combined configuration presented an outstanding advantage for improving compressor performance. For compressor overall performance, the stall margin improvement of combined configuration was up to 11.8%, larger than single forward sweep and single casing treatment. Meanwhile, peak efficiency penalty of combined configuration (-1.02%) was significantly lower than that of single casing treatment. Under the effect of casing treatment, the results showed that the pulsating axial velocity could effectively delay the interface of the tip leakage flow and main flow, preventing the overflow of tip leakage flow. The distributions of blockage coefficient and loading coefficient indicated that combined configuration was more effective due to tip unload by casing treatment and blockage resistance ability by forward sweep. Furthermore, the spatial and temporal evolutions of axial velocity at slot opening surface were discussed in detail. Compared with the single casing treatment, higher positive axial velocity excitation and smaller range of negative axial velocity were introduced by combined configuration, which contributed to better stability enhancement. [ABSTRACT FROM AUTHOR]

The aim of the paper is to explore the influence of the reversed blade-angle slot casing treatment (RBSCT) and its axial deflected angle (ADA) on the compressor performance and stability, and to reveal the mechanism that the change in ADA of the RBSCT influences the effect to broaden the compressor stable working range. The NASA Rotor 35 is used as the object of the investigation, and four RBSCTs with ADA of −15°, −30°, −45° and −60° are designed and investigated by unsteady numerical simulation. The results show that as the absolute value of the axial deflected angle increases, the capacity to improve the compressor stability of the RBSCT increases and then decreases. The unsteadiness of the injection and suction flows formed by the reversed blade angle slot plays an important role in the removal of the low-velocity zone. When ADA is −30°, the unsteadiness amplitude of the injection and suction flows is significantly higher than those of the other three. Consequently, the RBSCT with −30° ADA obtains the maximum stall margin improvement of 17.41% and the maximum design point efficiency improvement of 1.06% among the four RBSCTs. [ABSTRACT FROM AUTHOR]

To explore the influence of circumferential grooved casing treatment on subsonic cascade performance, a numerical simulation of subsonic cascade was conducted. In contrast to traditional research on variable single parameters for casing treatment, this paper used the Latin hypercube sampling method to randomly sample multiple geometric parameters of casing treatment and compared many sample data with the total pressure loss of the cascade as a measurement standard. After selecting several typical cases of high and low total pressure loss cases for in-depth flow field analysis, it was found that casing treatment affects the strength and structure of the leakage vortex, thereby reducing the blockage of fluid in the passage of the cascade. Changes in the total pressure loss and in the margin of the cascade meant that casing treatment affected cascade performance. This prompted analysis of the correlation between the casing treatment parameters and total pressure loss of the cascade. The clearance height and groove depth had the greatest influence on the total pressure loss of the cascade. [ABSTRACT FROM AUTHOR]

In the present study, various groove casing treatments were evaluated under a high-speed subsonic axial flow compressor using experimental and numerical simulation methods. The aim of this study was to explore the effect of inclination of grooves on compressor stability and performance. The potential flow mechanisms were also evaluated. Three different inclination grooves were designed in this study: grooves with no inclination, grooves with 30 degrees upstream inclination and grooves with 30 degrees downstream inclination. Similar effect of the grooves on the compressor stability and efficiency was observed under experimental and numerical analyses. The grooves with no inclination, 30 degrees upstream inclination and 30 degrees downstream inclination enhanced stall margin by 6.08%, 8.74% and 3.03%, respectively. The peak efficiency losses of the three types of grooves were 1.62%, 0.94% and 2.33%, respectively. Tip flow field analyses demonstrated that the radial transport effect caused by grooves effectively reduced tip loads and alleviated tip blockage. This explains why the grooves enhanced the compressor stability. The radial transport effect was enhanced, and a larger stall margin improvement was obtained when grooves inclined upstream were applied. The tip flow loss was the dominant loss observed after grooves were applied on the compressor. The grooves with upstream inclination markedly reduced the tip flow loss, indicating that they exhibited the lowest effect on reducing compressor efficiency compared with the other types of grooves.

Foam-metal casing treatment (FMCT) has the potential to enhance the stall margin while attenuating the broadband noise of compressors. An optimized structure of FMCT backed with an air gap was developed in this study with the basis of a prediction model. The eigenvalue theory with a linear perturbation assumption was employed to judge the stability of the compression system, where the casing treatment was considered as an impedance boundary condition. The effect of this kind of casing treatment on the compression system is introduced through the equivalent surface source method. The transfer element method was applied to match the relations between perturbation parameters on the surface of each section. Prediction results showed that the double-layer configuration has better stall margin improving ability, with experimental results on the steady characteristics confirming a 39.5 % stall margin improvement. The mechanism of exerting damping on the system and thus suppressing stall precursors was proved. [ABSTRACT FROM AUTHOR]

The hybrid slot-groove (S-G) casing treatment (CT), which combines the advantages of slot and groove in consideration of stall margin enhancing and efficiency penalty, was experimentally investigated under circumferential distorted inflows. Previous experiments showed that the hybrid S-G CT can extend the stability by 19.79% with uniform inflow condition. To further estimate its stability enhancement ability with distorted inflow, three types of circumferential total pressure distortion inflow that the distorted intensities (DC(60)) are equal to 0.90%, 4.12%, and 24.75%, are selected to conduct a serial of experiments. Results demonstrated that the stability of the compressor were deteriorated by 7.87%, 9.19% and 39.08% respectively under three distorted inflows. It was founded that, under the above-mentioned distorted inflows, the hybrid S-G CT was able to extend the stability by 18.48%, 17.81%, and 13.80%, respectively, which proved the strong anti-distortion ability of the hybrid S-G CT. By using the dynamic pressure sensors fixed on the casing wall, the unsteady measurements demonstrated that the stall precursor with uniform and circumferential distorted inflows are always spiky-wave; thus the hybrid S-G CT can play an excellent stability enhancement capability. The analytical results with power spectral density proved that, when at the same flow point, the perturbation, with frequency being around the rotating stall frequency band, was distinctly suppressed by hybrid S-G CT, thus delaying the rotating stall. The stall precursor detected in the casing wall depicted that unlike the short length-scale of stall precursor (5–6 blade passage) under smooth casing; the hybrid S-G CT can create a stall precursor with long length-scale. Under the distortion intensities from 0 to 4.12%, the length scale of the stall precursors occupies approximately 10–12 blade passages. As the distortion intensity further increases to 24.75%, the length scale of stall precursor increases to occupy 16–17 blade passages. This phenomenon can guide the stall warning studies with compressors using CT in the future. [ABSTRACT FROM AUTHOR]

In the present study, various groove casing treatments were evaluated under a high-speed subsonic axial flow compressor using experimental and numerical simulation methods. The aim of this study was to explore the effect of inclination of grooves on compressor stability and performance. The potential flow mechanisms were also evaluated. Three different inclination grooves were designed in this study: grooves with no inclination, grooves with 30 degrees upstream inclination and grooves with 30 degrees downstream inclination. Similar effect of the grooves on the compressor stability and efficiency was observed under experimental and numerical analyses. The grooves with no inclination, 30 degrees upstream inclination and 30 degrees downstream inclination enhanced stall margin by 6.08%, 8.74% and 3.03%, respectively. The peak efficiency losses of the three types of grooves were 1.62%, 0.94% and 2.33%, respectively. Tip flow field analyses demonstrated that the radial transport effect caused by grooves effectively reduced tip loads and alleviated tip blockage. This explains why the grooves enhanced the compressor stability. The radial transport effect was enhanced, and a larger stall margin improvement was obtained when grooves inclined upstream were applied. The tip flow loss was the dominant loss observed after grooves were applied on the compressor. The grooves with upstream inclination markedly reduced the tip flow loss, indicating that they exhibited the lowest effect on reducing compressor efficiency compared with the other types of grooves. [ABSTRACT FROM AUTHOR]

• Enhanced stability for ultra-high-load transonic rotors via tailored casing treatment strategy. • Insights into stall mechanism shifts post-casing treatment. • Revealed unsteady mechanisms to suppress unstable flow structures. • Stall margin is extended by approximately 12 %. Low-reaction compressors are promising for achieving high loads but face severe flow instability challenges. This study investigates a low-reaction transonic compressor rotor using casing treatment technologies to control unstable flow dynamics precisely. First, a design integrating self-recirculating and circumferential groove casing treatments near the leading edge is implemented. This design enhances flow capacity at the tip passage inlet. However, it causes a "stall transposition" phenomenon. The unstable flow structures shift from shock-tip leakage vortex interactions at the front to corner separation at the rear. Consequently, the stall mechanism transitions from an end-wall stall to a blade stall. To address this issue, a new casing treatment layout is proposed. Grooves are added after the mid-chord of the initial front casing configuration. This adaptation suppresses emerging unstable flow structures and extends the stall margin by approximately 12.07 %. Detailed flow field analysis shows that the rear grooves effectively reduced the trailing edge separation vortex. They also limit downstream corner separation and mitigate disturbances from tip leakage flows in the rear of the passage. [ABSTRACT FROM AUTHOR]

Foam metal is a foam-like substance with a high porosity; it has been used in flow control, vibration abatement, and acoustic absorption, mainly based on its physical properties. The aim of the current paper is to investigate the effect of foam metal casing treatments on the stability and acoustic level of a low-speed axial flow compressor. The experimental results show that the casing treatment improves the stall margin by 14.9%, without any efficiency loss. In terms of noise, the SPL of the tonal noise at the third order of BPF is reduced by 3.2 dB, while the SPL of the broadband noise is reduced up to 2.4 dB. The comparison in evolutions of the tip structure in a smooth casing condition and with a casing treatment indicates that the casing treatment affects the origination and the development of the tip leakage vortex. The working mechanism is also discussed. [ABSTRACT FROM AUTHOR]

To explore the influence of circumferential grooved casing treatment on subsonic cascade performance, a numerical simulation of subsonic cascade was conducted. In contrast to traditional research on variable single parameters for casing treatment, this paper used the Latin hypercube sampling method to randomly sample multiple geometric parameters of casing treatment and compared many sample data with the total pressure loss of the cascade as a measurement standard. After selecting several typical cases of high and low total pressure loss cases for in-depth flow field analysis, it was found that casing treatment affects the strength and structure of the leakage vortex, thereby reducing the blockage of fluid in the passage of the cascade. Changes in the total pressure loss and in the margin of the cascade meant that casing treatment affected cascade performance. This prompted analysis of the correlation between the casing treatment parameters and total pressure loss of the cascade. The clearance height and groove depth had the greatest influence on the total pressure loss of the cascade.

The research on geometric parameters of casing treatment (CT) has always been a hot topic, yet the multi-parameter interaction is rarely studied. In order to gain better knowledge of the interaction mechanism of geometric parameters of CT, the experimental and numerical study based on the response surface method has been carried out in an axial compressor rotor. First of all, the statistical analysis based on the database of experimental and numerical results is presented to summarize the influence law of varied parameters on the stability enhancement. It was found that axial overlap, open area ratio, and their interaction had the most significant influence on the stability enhancement of CT. Subsequently, the interaction between axial overlap and open area ratio was analyzed by visualization flow field in details, which provided a deeper insight into stability enhancement mechanism of CT. It indicated that the mass flow and momentum dominated by injection and the suction effect played a key role for extending stability. With smaller open area ratio, it was difficult for the slots to manipulate and control the tip leakage flow or secondary tip leakage flow, resulting in the weak effect of CT on compressor performance. Finally, the underlying flow physics in the tip region and dominant region of CT has also been discussed to penetrate the essential reason of multi-parameter interaction. [ABSTRACT FROM AUTHOR]

• Eccentric swirl inlet leads to additional efficiency and stall margin losses, resulting in a decrease in the compression capacity of the system. • Eccentric swirls generate localized over-loading, where abnormal prestall disturbances in the frequency range of 0.3-0.5 BPF amplify. • A novel casing treatment is proven to address the circumferential non-uniformity problem in the fan. • IBC CT enables the fan under an eccentric inlet to reach the stability boundary of the concentric inlet which has the same average loading. An eccentric inlet swirl, a source of circumferential non-uniformity in aero-engines, can compromise the stable operational range of the compression system and potentially jeopardize the safety of the entire flight vehicle. This study experimentally examined the additional effects of an eccentric inlet swirl on an axial fan in comparison with a concentric inlet condition. Then, the effectiveness of an impedance-boundary-controlled (IBC) casing treatment (CT) in extending the stable operating range of the fan under eccentric inlets is evaluated. Steady-state loading and prestall disturbance analyses were conducted using a five-hole probe and high-frequency response pressure transducers to elucidate the instability mechanisms of fans exposed to eccentric inlets. The findings indicate that the eccentric swirl generates localized over-loading regions around the circumference, where abnormal prestall disturbances amplify in amplitude across a frequency range of 0.3 to 0.5 times the blade passing frequency. These characteristics were mitigated when IBC CT was applied over the rotor tip, allowing the fan to operate under concentric inlet conditions. The IBC CT enhances the stall margin of the fan by 9.3–19.6% in response to a range of swirl inlet conditions, suggesting its potential to address the irregularity problems in fans/compressors. The mechanisms by which IBC CT extends the stall margin are discussed from the unique perspective of evaluating steady loading and system damping. [ABSTRACT FROM AUTHOR]

This paper will first carry out experimental investigations on a test compressor which is a large-scale, two-stage and low-speed axial one to evaluate the participation of a novel casing treatment (stall precursor-suppressed casing treatment, SPS CT). The compressor performance for its pressure rise characteristics and efficiency are tested and illustrated. Besides, the time-averaged steady and the time-resolved unsteady measurement in this paper finally lead to some further analyses about the stall evolution of this compressor tending to uncover the process of the stall evolution and the mechanism of the SPS CT. The overall performance results indicate that the SPS CT can extend the compressor stall margin by 3%–11% with very limited efficiency loss. And the mechanism for such casing treatment has also been revealed as stall-precursor suppression by observing the evolution of the inception waves. In addition, modeling work have been conducted subject to the effects of SPS CT on the test two-stage axial compressor. The compressor stability model is able to investigate the effects of different geometrical parameters on the compressor stability and then make an optimal design for the SPS CT when it is applied on the test compressor.

In this paper, the influence of a shallow reversed slot-type casing treatment on the performance of a tip-critical transonic compressor has been numerically investigated. Firstly, the complex flow fields in the rotor tip region are studied in details. It shows the severe blockage induced by suction surface boundary separation triggers compressor stall at 100% design speed, while the blockage due to tip leakage vortex dominates at 80% design speed. Secondly, the mechanism of stability extension is presented at different rotating speeds. The casing treatment alleviates greatly the tip blockage by manipulating the tip leakage flow, accompanied by the redistribution of aerodynamic loading and mass flux. As a result, the casing treatment is more efficient for the blockage induced by tip leakage vortex (at 80% design speed). Further analysis of the pressure field and passage shock distribution demonstrates that the passage shock intensity and its location will affect the effectiveness of casing treatment. Finally, the instability characteristics of compressor with casing treatment are revealed. The numerical results reflect when the mass flow approaching the instability boundary, the stator passage blockage presumably is dominant for triggering the compressor stall.

Casing treatment is an effective technique in extending stall margin of axial and centrifugal compressor. However, its impacts on the stall behaviour of mixed-flow compressor are still not completely understood until now. To conquer this issue, unsteady full-annulus simulations were conducted to investigate the stall mechanism of a mixed-flow compressor with and without axial slot casing treatment (ASCT). The circumferential propagating speed of spike inception resolved by the numerical approach is 87.1% of the shaft speed, which is identical to the test data. The numerical results confirmed that the mixed-flow compressor fell into rotating stall via spike-type with and without ASCT. The flow structure of the spike inception was investigated at 50% design rotational speed. Instantaneous static pressure traces extracted upstream of the leading edge had shown a classic spiky wave. Furthermore, it was found that with and without ASCT, the mixed-flow compressor stalled through spike with the characteristic of tip leakage spillage at leading edge and tip leakage backflow from trailing edge, which is different from a fraction of the centrifugal compressor. The resultant phenomenon provides corroborating evidence for that unlike in axial-flow compressor, the addition of ASCT does not change the stall characteristics of the mixed-flow compressor. The flow structure that induced spike inception with ASCT is similar to the case with smooth casing. In the throttling process, tip leakage flow vortex had been involved in the formation of tornado vortices, with one end at the suction side, and the other end at the casing-side. The low-pressure region relevant to the downward spike is caused by leading-edge separation vortex or tornado vortex. The high-pressure region relevant to the upward spike is induced by blockage from the passage vortex. These results not only can provide guidance for the design of casing treatment in mixed-flow compressor, but also can pave the way for the stall waring in the highly-loaded compressors of next-generation aeroengines. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation was conducted on a subsonic compressor rotor with blade angle slot casing treatments. The purpose of the investigation is to reveal the influence of axial overlap of blade angle slot on the compressor stability. Six kinds of blade angle slot casing treatments with different axial overlap rates were investigated in this paper. The results show that with the increasing of axial overlap rates, the stall margin improvement firstly increased and then decreased. And the optimal blade angle slot can obtain 62.51% improvement of stall margin. The flow field analyses show that the bleeding flows formed inside slots can restrain the adverse tip leakage flow, which is the critical factor making the onset of the rotor stall. With the increasing of axial overlap rate of the slots, the relative position between bleeding flows in the slots and tip leakage flow plays an important role in the stall margin improvement.

This study concerns a multi-objective optimization of circumferential single grooved casing treatment for a low-reaction transonic rotor with ultra-high loading. The axial location, width and depth of the groove are investigated as design variables. The optimization problem seeks to fully extend the operation range of the rotor while minimizing efficiency degradation. Artificial neural network of radial basis function is applied to construct the surrogate model. The optimal groove configuration was determined using non-dominated sorting genetic algorithm II (NSGA II) in conjunction with technique for order preference by similarity to ideal solution (TOPSIS). Detailed analysis of flow field reveals that two flow features involving stability enhancement for the low-reaction rotor are the inhibition of shock/vortex interaction in the rotor tip region and the reduction or elimination of double-leakage tip gap flow. The blocking region located right downstream of the interface between the tip leakage flow and the main flow is decreased due to the tip unloading effect and recirculation flow induced by the groove. Additionally, the efficiency improvement can be observed as the intensity of tip leakage vortex decreases. Based on the single groove optimization, the prospect of a particular multiple groove casing configuration consisting of component grooves with varied geometrical dimensions is also discussed in the paper. The simulation results indicate that the new-type multiple groove configuration is more advantageous to the rotor’s performance.

In this paper, the influence of a shallow reversed slot-type casing treatment on the performance of a tip-critical transonic compressor has been numerically investigated. Firstly, the complex flow fields in the rotor tip region are studied in details. It shows the severe blockage induced by suction surface boundary separation triggers compressor stall at 100% design speed, while the blockage due to tip leakage vortex dominates at 80% design speed. Secondly, the mechanism of stability extension is presented at different rotating speeds. The casing treatment alleviates greatly the tip blockage by manipulating the tip leakage flow, accompanied by the redistribution of aerodynamic loading and mass flux. As a result, the casing treatment is more efficient for the blockage induced by tip leakage vortex (at 80% design speed). Further analysis of the pressure field and passage shock distribution demonstrates that the passage shock intensity and its location will affect the effectiveness of casing treatment. Finally, the instability characteristics of compressor with casing treatment are revealed. The numerical results reflect when the mass flow approaching the instability boundary, the stator passage blockage presumably is dominant for triggering the compressor stall. [ABSTRACT FROM AUTHOR]

Casing treatment is an efficient technique that is used to increase the compressor stall margin with a minor reduction in efficiency. The interaction between the blade passage and the groove is the main cause of the stall margin improvement by various researches. A numerical study is conducted on a single circumferential casing groove using NASA rotor 37 in the current study. The performance of the two circumferential groove models is studied by discretizing RANS 3D equations using a finite volume technique. The inception of the stall is predicted according to the criteria of convergence. Two models of the circumferential groove have been suggested and numerically tested. A single passage simulation is selected for the two models. The performance of the smooth casing and the two models are analyzed. Moreover, the stall margin, total pressure ratio, and peak adiabatic efficiency of the normal casing, and the two models are analyzed to determine the influence of the groove on the axial compressor performance and stability. Models 1 and 2 stall margins are enhanced by 6.62 % and 4.45% respectively. The adiabatic efficiency of model 1 and model 2 are decreased by 0.79 % and 1.08 % respectively. [ABSTRACT FROM AUTHOR]

Foam metal is a foam-like substance with a high porosity; it has been used in flow control, vibration abatement, and acoustic absorption, mainly based on its physical properties. The aim of the current paper is to investigate the effect of foam metal casing treatments on the stability and acoustic level of a low-speed axial flow compressor. The experimental results show that the casing treatment improves the stall margin by 14.9%, without any efficiency loss. In terms of noise, the SPL of the tonal noise at the third order of BPF is reduced by 3.2 dB, while the SPL of the broadband noise is reduced up to 2.4 dB. The comparison in evolutions of the tip structure in a smooth casing condition and with a casing treatment indicates that the casing treatment affects the origination and the development of the tip leakage vortex. The working mechanism is also discussed.

Centrifugal compressors have been widely used in light-duty turboshaft engines and automotive turbochargers. One of the essential requirements for centrifugal compressors in these applications is to have adequate stable flow range. As the pressure ratio increases, the stable flow range drops dramatically, and this highlights the importance of stability improvement for centrifugal compressors. This paper first gives a brief review on the measures available for extending the stable flow range, and then focuses on the realization of self-recirculating casing treatment for a centrifugal compressor. The design principles of the casing treatment were introduced. Then the simulations were conducted to design and optimize the geometry of the casing treatment, and finally an optimum case was selected for experimental validation. The experiment results indicate that the designed casing treatment improve the stable flow range by about 5 percent at design speed, while keeping the efficiency almost identical. [ABSTRACT FROM AUTHOR]

The existence of pressure pulsations greatly increases the vibration and noise of pumps and harms their service life. In this paper, a casing treatment was employed to explore its impact on the pressure pulsations. A U-tube type groove was created at the inlet end-wall of a centrifugal pump and front cover of the impeller to connect the impeller with the inlet pipe by passing impeller leading edge. An unsteady numerical investigation was launched of the pump with and without this casing treatment, to study its influence on the pressure pulsations inside the pump and the mechanisms behind. The numerical results of the pump without casing treatment was first compared with the test performance of the pump to validate the numerical method, and gave excellent agreements with the test results. The CFD results also showed that the casing treatment increases the head coefficient and efficiency of the pump. Pressure pulsations at a reduced mass flow condition were studied by monitoring unsteady pressure signals generated by the CFD at various locations inside the pump. A Fast Fourier transform (FFT) was performed on the signals. The pump employs a double tongues volute with each tongue covering 180 ∘ circumference. However, the two tongues are not identical with regard to the discharge of the pump. These geometric features of the volute and the pump's operating condition generate several pressure pulsations in the frequencies of f n , 2 f n , 3 f n in the original pump. Due to the circumferential unifying capability of the casing treatment and its improvement to the impeller flow, these pulsations at impeller inlet are weakened or disappear when the U-tube is present. The pressure pulsation inside the impeller is less affected by the treatment. The 3 f n pulsation at volute tongues also decreases or disappears for the same reasons, but f n pulsation increases slightly and this is due to the improved pressure recovery in the volute by the treatment which increases the pressure difference across one of the volute tongues. The unsteady radial force of the impeller exerting on journal bearings becomes more uniform and smaller when the casing treatment is employed. [ABSTRACT FROM AUTHOR]

The performance curve of the compressor is limited by the surge boundary, so it is of great significance to increase the stable working range of the compressor. The self-circulating casing treatment is an effective way to improve the stable working range of the compressor. In this paper, the study of the influence of the injector position of the self-circulating casing treatment on the transonic axial flow compressor rotor performance is carried out by using the numerical simulation. The influence mechanism of the injector position on the enhancing stability effect of the self-circulating casing treatment is revealed. It is found that the self-circulating casing treatment can reduce the blade tip blockage by restraining the blade tip clearance leakage flow and changing the trajectory of the tip clearance leakage vortex, thus delaying the deterioration of the rotor tip flow field and improving the rotor stability. When the injector position of the self-circulating casing treatment moves from the upstream of the leading edge of the blade tip to the trailing edge of the blade tip, the enhancing stability effect of the self-circulating casing treatment increases first and then decreases. But the high-velocity jet from the injector of the self-circulating casing treatment aggravates the mixing loss of the rotor tip flow field, so that the rotor efficiency slightly decreases after using the self-circulating casing treatment. [ABSTRACT FROM AUTHOR]

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Experimental and numerical analyses were performed on a low-speed axial compressor rotor to investigate the aerodynamic and acoustic effects of axial skewed slots casing treatment on the rotating instability. The experimental results showed that the stall margin could be improved by 8.0% and the frequency broadband hump owing to the rotating instability was suppressed effectively. In the noise spectra, the two dominant broadband humps on both sides of the blade-passing frequency also reduced in amplitude. Full-annulus unsteady computational fluid dynamics simulations were performed near the design condition. Time- and frequency-domain analyses as well as a proper orthogonal decomposition method were applied to obtain the oscillation, frequency, energy and flow characteristics of the rotating instability. Axial skewed slots casing treatment causes a distinct reduction in the amplitude of the pressure fluctuations and frequency spectra with a decrease in the energy of the rotating instability modes. The slots alleviated the tip flow blockage by the periodic injection and removal of the fluid from the passage, which enabled a high tip clearance flow downstream with little impingement on the neighbouring blade tip. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation was conducted on a subsonic compressor rotor with blade angle slot casing treatments. The purpose of the investigation is to reveal the influence of axial overlap of blade angle slot on the compressor stability. Six kinds of blade angle slot casing treatments with different axial overlap rates were investigated in this paper. The results show that with the increasing of axial overlap rates, the stall margin improvement firstly increased and then decreased. And the optimal blade angle slot can obtain 62.51% improvement of stall margin. The flow field analyses show that the bleeding flows formed inside slots can restrain the adverse tip leakage flow, which is the critical factor making the onset of the rotor stall. With the increasing of axial overlap rate of the slots, the relative position between bleeding flows in the slots and tip leakage flow plays an important role in the stall margin improvement. [ABSTRACT FROM AUTHOR]

Casing treatment is an effective way to improve the stable working margin of the compressor and plays an important role in ensuring the stable and safe work of the compressor. On the premise that the numerical results are in agreement with the experimental values, the paper study on a new type of the self-circulating casing treatment with double-bleed ports structure was carried out on the NASA rotor 37 by numerical simulation. It is found that compared with the self-circulating casing treatment with single-bleed port structure, the self-circulating casing treatment with double-bleed ports structure is more favorable for improving the stall margin of the rotor, and the improvement of the stall margin obtained at the rotor design speed is as high as 9.57%. Compared with the other two self-circulating casing treatments with single-bleed port structure, the improvement of the stall margin is increased by 37.90 and 49.53%, respectively. The self-circulating casing treatment with double-bleed ports structure has a significant effect on the enhancing stability, but at the same time, it causes a greater efficiency loss, which reduces the design point efficiency of the rotor by 0.34%. Through detailed analysis of the flow field, it is found that the self-circulating casing treatment improves the flow capacity of the blade tip region, weakens the blockage degree of the blade tip region, and reduces the blade tip load, thereby improving the stall margin of rotor. Compared with the self-circulating casing treatment with single-bleed port structure, the self-circulating casing treatment with double-bleed ports structure is more advantageous for delaying the flow separation of the suction surface, delaying the occurrence of the blade tip stall to make the stability of rotor more favorable, and obtained stall margin improvement is the greatest. [ABSTRACT FROM AUTHOR]

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

Axial slot CTs were designed and applied on Rotor 67 to understand the physical mechanisms responsible for the improvement of the stall margin. Unsteady Reynolds-averaged Navier-Stokes was applied in addition to steady Reynolds-averaged Navier-Stokes to simulate the flow field of the rotor. The results show that aerodynamic performance and the rotor stability were improved. Stall margin improvement (SMI) improved by 26.85% after the CT covering 50% of the axial tip chord was applied, whereas peak efficiency (PE) decreased the least. The main reason for the rotor stall in the solid casing is the blockage caused by tip leakage flow. After axial slot CTs were applied, the tip leakage flow in the front part of the chord was obviously reduced, and the majority of the blockages in the tip region were removed. The absolute value of the axial momentum before 45% axial chord in CT_50 was reduced by 50%, whereas the maximum tangential momentum value of CT_50 was decreased by 70% relative to the solid casing. CT_50 configuration was located across the shock wave; thus, it can fully utilize the pressure gradient to bleed and remove the blockage region, and the across flow is considerably depressed.

Axial slot CTs were designed and applied on Rotor 67 to understand the physical mechanisms responsible for the improvement of the stall margin. Unsteady Reynolds-averaged Navier-Stokes was applied in addition to steady Reynolds-averaged Navier-Stokes to simulate the flow field of the rotor. The results show that aerodynamic performance and the rotor stability were improved. Stall margin improvement (SMI) improved by 26.85% after the CT covering 50% of the axial tip chord was applied, whereas peak efficiency (PE) decreased the least. The main reason for the rotor stall in the solid casing is the blockage caused by tip leakage flow. After axial slot CTs were applied, the tip leakage flow in the front part of the chord was obviously reduced, and the majority of the blockages in the tip region were removed. The absolute value of the axial momentum before 45% axial chord in CT_50 was reduced by 50%, whereas the maximum tangential momentum value of CT_50 was decreased by 70% relative to the solid casing. CT_50 configuration was located across the shock wave; thus, it can fully utilize the pressure gradient to bleed and remove the blockage region, and the across flow is considerably depressed.

Casing treatments (CT) can effectively extend compressors flow ranges with the expense of efficiency penalty. Compressor efficiency is closely linked to loss. Only revealing the mechanisms of loss generation can design a CT with high aerodynamic performance. In the paper, a highly-loaded mixed-flow compressor with tip clearance of 0.4 mm was numerically studied at a rotational speed of 30,000 r/min to reveal the effects of axial slot casing treatment (ASCT) on the loss mechanisms in the compressor. The results showed that both isentropic efficiency and stall margin were improved significantly by the ASCT. The local entropy generation method was used to analyze the loss mechanisms and to quantify the loss distributions in the blade passage. Based on the axial distributions of entropy generation rate, for both the cases with and without ASCT, the peak entropy generation rate increased in the rotor domain and decreased in the stator domain during throttling the compressor. The peak entropy generation in rotor was mainly caused by the tip leakage flow and flow separations near the rotor leading edge for the mixed-flow compressor no matter which casing was applied. The radial distributions of entropy generation rate showed that the reduction of loss in the rotor domain from 0.4 span to the rotor casing was the major reason for the efficiency improved by ASCT. The addition of ASCT exerted two opposite effects on the losses generated in the compressor. On the one hand, the intensity of tip leakage flow was weakened by the suction effect of slots, which alleviated the mixing effect between the tip leakage flow and main flow, and thus reduced the flow losses; On the other hand, the extra losses upstream the rotor leading edge were produced due to the shear effect and to the heat transfer. The aforementioned shear effect was caused by the different velocity magnitudes and directions, and the heat transfer was caused by temperature gradient between the injected flow and the incoming flow. For case with smooth casing (SC), 61.61% of the overall loss arose from tip leakage flow and casing boundary layer. When the ASCT was applied, that decreased to 55.34%. The loss generated by tip leakage flow and casing boundary layer decreased 20.54%) relatively by ASCT. [ABSTRACT FROM AUTHOR]

A stall warning approach based on aero-acoustic theory is studied in this paper. For this stall warning approach, a parameter Rc is defined to measure the periodicity of the blade-passing signal. Signal simulation is used to investigate the mechanism of the stall warning approach. The results suggest that the value of Rc is influenced by the power of the perturbations. The experiments on a two-stage compressor indicate this stall warning approach can generate a warning signal several seconds before the stall. It is demonstrated in this paper that the stall warning approach can detect the distribution and evolution of stall precursors. According to the distribution of the stall precursors, the partial stall precursor-suppressed casing treatment is applied and realized a stabilization of compressor. [ABSTRACT FROM AUTHOR]

The tip leakage flow over the blades of an axial compressor rotor adversely affects the axial rotor efficiency and can determine the onset of tip leakage stall. The performance of a new casing treatment concept in the shape of an axisymmetric recirculation channel is explored by steady Reynolds-Averaged Navier-Stokes (RANS) realizable k-# modelling on the NASA Rotor 37 test case. The modelling exposed a number of attractive features. The casing treatment increased the stall margin at no penalty to the rotor isentropic efficiency over the rotor operating line. A recirculation in the casing channel self-activated and self-adjusted with the rotor loading to provide more passive flow control at higher rotor loading conditions. The nozzle-shaped recirculation channel outflow opposed the tip leakage jet, re-located the casing surface flow interface further downstream, and reduced the rotor blade tip incidence angle. This combination of features makes the new casing treatment particularly attractive for applications to high thrust-to-weight ratio engines, typical of high-performance jet aircraft. [ABSTRACT FROM AUTHOR]

Abstract A hybrid slot–groove casing treatment (CT) composed of front axial slots and rear circumferential grooves is experimentally studied in a low-speed axial compressor in consideration of stability and efficiency. Results show that the hybrid slot–groove CT can improve the stall margin by 19.79% with an efficiency penalty of 1.5%. Compared to full-groove and full-slot CTs, the hybrid slot–groove CT's stall margin improvement and minimization of efficiency loss are excellent. Detailed measurements indicate that, unlike smooth casing (SC), the hybrid slot–groove CT can unload the blade tip, weaken the tip leakage vortex, and improve the disturbance observed downstream to postpone stall. Given the influence of serious passage blockage derived from the hub region after improving tip flow capability under the hybrid slot–groove CT, the stall route and inception captured in the casing wall and different downstream radial positions demonstrate that, unlike the SC with a typical spike-type inception (short length-scale with 2–3 blade passages), the hybrid slot–groove CT creates a long length-scale stall inception (6–8 blade passages). In addition, a new type of instability inception with a different frequency band from the stall cell is found in the hub region under the hybrid slot–groove CT. This inception appears more than hundreds of revolutions before the fast trigger of spike-type inception and can be used as an early stall warning signal under high hub loading. [ABSTRACT FROM AUTHOR]

Abstract The development of single-stage high pressure ratio centrifugal compressors for small gas turbine engines is hindered by compressor flow instabilities with various temporal and spatial scales. In this paper, the effect of a self-recirculation casing treatment device for suppressing various scales of flow instability has been discussed. Rig tests of a high pressure ratio centrifugal compressor with and without a casing treatment device are performed, and transient pressure signals are measured at endwalls by fast response pressure transducers. By analyzing measured pressure signals in both time and frequency domain, the casing treatment device is found effective in eliminating rotating instability and stall at the impeller inlet at low speeds, as well as suppressing surge of the compression system at middle and high speeds. The flow recirculation process generated inside the casing treatment device removes the vortical structures at the impeller inlet tip section, therefore eliminating the regional rotating instability and stall. Besides, the casing treatment device turns the pressure rise characteristics of the impeller more negative by enhancing the impeller work input, but turns that of the vaned diffuser more positive by increasing the incidence at the vaned diffuser inlet. Combining both effects, mild surge and the deep surge are suppressed. [ABSTRACT FROM AUTHOR]

To make up the lack of the experimental and numerical investigations about the grooved casing treatment (CT) applied in counter-rotating axial flow compressors (CRAC), the effects of circumferential single grooved CT on the stability enhancement were investigated in the rear rotor of a low-speed CRAC with numerical simulations. The main purpose was to gain a better understanding of the application principle of casing groove and the associated control mechanisms in the CRAC. Parametric studies show that the optimal position of the groove should be located near the blade leading edge in terms of stall margin improvement (SMI) and efficiency enhancement at the near stall condition. Due to the impact of casing groove on tip leakage flow (TLF), the blade tip unloading effect, redirection effect and the compound effect of suction–injection are all beneficial to the SMI. Detailed observation of flow structures illustrates that it is more effective to improve flow stability by controlling the critical TLF released from about mid-chord and the stall inception process may be probably changed due to the direct effect of groove on the main TLF released near the blade leading edge. Additionally, the effects that the groove has on rotor outflow blockage, backward axial momentum flux and TLF momentum perpendicular to the blade tip camber inside the tip gap are also studied in the paper. [ABSTRACT FROM AUTHOR]

Highly three-dimensional and complex flow structure in the tip gap between a blade tip and the casing leads to significant inefficiency in the aerodynamic performance of a turbine. The interaction between the tip leakage vortex and the main passage flow is a substantial source of aerodynamic loss. The present research deals with the effect of groove type casing treatment on the aerodynamic performance of a linear turbine cascade. Grooved casings are widely used in compressors in order to improve the stall margin whereas limited studies are available on turbines. In this study, various circumferential grooves are investigated using the computational approach for a single stage axial turbine blade. The specific HP turbine airfoil under numerical investigation is identical to the rotor tip profile of the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. The carefully measured aerodynamic flow quantities in the AFTRF are used for initial computational quality assessment purposes. Numerical calculations are obtained by solving the three-dimensional, incompressible, steady and turbulent form of the Reynolds-Averaged Navier–Stokes (RANS) equations. A two-equation turbulence model, Shear Stress Transport (SST) k- ω is used in the present set of calculations. Current results indicate that groove type casing treatment can be used effectively in axial turbines in order to improve the aerodynamic performance. Detailed flow visualizations within the passage and numerical calculations reveal that a measurable improvement in the aerodynamic performance is possible using the specific circumferential grooves presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents the stall margin enhancement of a novel casing treatment subjected to circumferential pressure inlet distortion. Experimental results are carried out on a low-speed compressor. Experimental results show that the stall margin of the test compressor is gravely narrowed by circumferential pressure inlet distortion. The stabilization ability of this novel casing treatment and the pre-stall behavior of the compression system under the impacts of circumferential pressure distortion are shown in the present work. Although subjected to the inlet distortion, the stall margin can also be made up by 6%–8% due to the application of this novel casing treatment. Furthermore, the pre-stall dynamic results can indicate that the mechanism of this stabilization method is associated with two main observations, one is the weakening of the unsteady flow perturbations in the compressor, and the other is the delay in of the nonlinear amplification of the stall precursor waves. [ABSTRACT FROM AUTHOR]

The instability problems tend to be more severe when high speed compressors operate at partial speeds. This paper proposes an economic approach for casing treatment design that suitable to this situation. Aiming at reducing the expensive and time-consuming high-speed casing treatment experiments, the idea of low-speed similitude of high-speed compressors, which was originally practiced in mid-1980 with the purpose of loss reduction, is now extended to simulate the stability enhancement with casing treatment in this paper. The core idea of this approach is to replace a large portion of design processes for the high-speed compressors (the Prototype) with their equivalent large scale model compressors (the Model). Two different transonic rotors with skewed slots and circumferential grooves casing treatments are conducted as examples to demonstrate this approach. Following the selected similarity rules, the Model is firstly acquired by modeling the near stall point of the Prototype. A variety of casing treatments are designed and assessed on the Model. Then a few more promising configurations can thus be selected via low speed experiments. They are believed to have similar tendency on stall margin improvement on the Prototype. Finally, the selected configurations are converted back to the Prototype with based on the rule of similarity and validated by experimental data. In this paper, principles that guarantee the similitude of the flow field at near stall condition and the effectiveness of the casing treatment are discussed. [ABSTRACT FROM AUTHOR]