Casing Treatment Research Articles

Effects of impedance-boundary-controlled casing treatment on the fan performance with eccentric inlet swirl

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.

Effect of impedance boundary-controlled casing treatment on performance of a fan subjected to inlet swirls

An experimental investigation is conducted to evaluate the performance and the stalling process of a fan subjected to inlet swirls, as well as the effectiveness of an Impedance Boundary-Controlled (IBC) Casing Treatment (CT) on the stall margin recovery. An operating cycle is proposed based on the hysteresis effect of harmonic flap oscillation of airfoils and parallel compressor theory to explain the pressure characteristic of the fan under twin swirl inlets. Twin swirls are observed to reduce the stall margin of the fan, and the circumferential location where the spike is detected turns to the intersection area of the twin swirl. The IBC CT is proven to extend the stall margin of the fan for 12.7%–22.3% when subjected to inlet swirls with an efficiency loss of around 1%. The IBC CT helps to reduce the size of the operating cycle of the fan by redistributing the blade loading and adding the system damping to dissipate the perturbation energy.

Stall and stability enhancement mechanisms of transonic compressor with inlet total temperature distortion

Inlet total temperature distortion refers to the nonuniform distribution of the total temperature at the inlet of the aero-engine, which is one of the external destabilizing factors that have the most remarkable impact on the stability of an aero-engine. In this study, the stalling process of the Darmstadt transonic compressor is investigated under the total temperature distortion of 180 deg circumferential extent and 500 K intensity by full-annulus unsteady numerical simulation. The analysis shows the addition of inlet total temperature distortion deteriorates the compressor stall margin from 22.7% to 17.0% with a considerable decrease in the pressure ratio. The type of compressor stall inception under temperature distortion conditions remains spike. Pressure perturbations and radial vortex formation are first clearly detected when the rotor rotates into the high temperature distortion region. The circumferential propagation of the stall cells under the total temperature distortion is 66% of the rotor speed, which is faster than that under uniform condition whose value is 44% of the rotor speed. The optimized casing treatment (CT) has extended the stall margin of the rotor without producing efficiency loss under uniform condition. The anti-distortion ability of CT is first verified despite its negative impact on efficiency under total temperature distortion. The adoption of CT could obviously push the shock wave into the blade passage under total temperature distortion condition. In addition, it can reduce the tip blade loading, thus removing the low Mach number area in the tip region, while increasing the blade loading below 80% span.

Experimental and Computational Investigation of an Advanced Casing Treatment in a Single-Stage High-Speed Compressor

Abstract An advanced casing treatment (CT) design was tested in a single-stage, high-speed axial compressor. The geometry of the casing included bent-axial slots applied near the rotor leading edge. Compressor performance and stall margin (SM) measurements were acquired for both casing treatment and for the smooth wall case at four different corrected speeds. The measurements also included rotor-exit radial traverses of unsteady total pressure. The results indicated an increase in rotor pressure ratio at all operating conditions with the application of the casing treatment. The stall margin also increased at all rotor speeds. Efficiency increases were observed with the casing treatment at design conditions and some off-design conditions. Numerical simulations of the IGV, rotor, and stator were conducted with and without the casing treatment. These results served to validate the efficacy of the part-wheel unsteady Reynolds-averaged Navier–Stokes (URANS) simulations to predict the effects of the casing treatment. The simulations also allowed a detailed study of the flow physics related to the interactions between the rotor and casing.

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.

Research on the stability enhancement mechanism of multi-parameter interaction of casing treatment in an axial compressor rotor

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.

Effect of Circumferential Single Casing Groove Location on the Flow Stability under Tip-Clearance Effect in a Transonic Axial Flow Compressor Rotor

Numerical simulations have been performed to study the effect of the circumferential single-grooved casing treatment (CT) at multiple locations on the tip-flow stability and the corresponding control mechanism at three tip-clearance-size (TCS) schemes in a transonic axial flow compressor rotor. The results show that the CT is more efficient when its groove is located from 10% to 40% tip axial chord, and G2 (located at near 20% tip axial chord) is the best CT scheme in terms of stall-margin improvement for the three TCS schemes. For effective CTs, the tip-leakage-flow (TLF) intensity, entropy generation and tip-flow blockage are reduced, which makes the interface between TLF and mainstream move downstream. A quantitative analysis of the relative inlet flow angle indicates that the reduction of flow incidence angle is not necessary to improve the flow stability for this transonic rotor. The control mechanism may be different for different TCS schemes due to the distinction of the stall inception process. For a better application of CT, the blade tip profile should be further modified by using an optimization method to adjust the shock position and strength during the design of a more efficient CT.

Design of a rear-stage subsonic axial compressor with casing treatments

The aerodynamic design of a 3.5-stage high-pressure axial compressor, representing a modern rear stage design with low aspect ratio blades and large relative tip gaps is presented. The subsonic rotor and stator airfoils feature a controlled-diffusion (CDA) type of sectional design, obtained from 2D viscous-inviscid simulations using the MISES flow solver. Improvements of the main gas path, as well as the 3D blade design, are based on 3D steady Reynolds-averaged Navier–Stokes simulations (ANSYS CFX). Sweep and dihedral were applied to improve the design and off-design performance of the compressor. The matured set of blading, with smooth casing, still exhibits large rotor tip vortices and double leakage, limiting the compressors operability towards stall point. Consequently, the application and development of axial slot casing treatments (CT) into the smooth casing is performed, aimed at improving local stage and overall stall margin with particular emphasis on avoiding efficiency penalties within stages. In order to investigate and optimise the geometric properties of the CTs with respect to efficiency a DoE (Design of Experiments) is executed. Based on the DoE a CT-geometry is deduced and applied to the critical rotor. Due to the fact, that the stage operability with casing treatment is limited by the downstream stator, a first adaption of the existing blade design is considered. In a first step, the downstream stator’s incidence is adjusted, resulting in a slightly increased stall margin. The second step incorporates the modification of rotor blade count, following the idea of trading rotor operability for an efficiency increase, due to reduced friction. However, no notable efficiency benefit was achieved, as the reduced friction loss is counterbalanced by increased secondary flow loss resulting from higher blade loadings.

Investigation of the casing groove location effect for a large tip clearance in a counter-rotating axial flow compressor

In the present work, the location effect of single grooved casing treatment (CT) on the compressor performance and flow stability are numerically studied at a large tip clearance in a two-stage counter-rotating axial flow compressor. The results show that the first stall stage is changed from the rear rotor (R2) to the front rotor (R1) as the tip clearance is increased from the normal design tip clearance (τ) to a large tip clearance of 2τ. The compressor stability can only be improved by the CT schemes in R1 without obvious change of compressor efficiency, while the compressor efficiency at near stall point can be increased remarkably for the effective CT configurations in R2 without stall margin improvement (SMI). The stall inception type may be changed for the least effective CT configuration in R1 due to the drastic change of flow structure of the tip leakage flow (TLF) released from near the blade leading edge. Detailed analysis of TLF structure shows that it is advisable to improve the compressor flow stability by controlling the TLF part which is released from the blade chord range away from the blade leading edge. After the application of effective CT schemes, the tip flow field is improved in the corresponding rotor and the interface is pushed further downwards to a different extent resulting in the reduction of the TLF intensity and loss generation.

Entropy Generation Analysis in a Mixed-Flow Compressor with Casing Treatment

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.

A Casing Treatment with Axial Grooves for Centrifugal Compressors

This paper provides an investigation of a casing treatment (CT) approach for pressure ratio improvements of centrifugal compressors between peak efficiency and surge. Results were experimentally verified for a variety of automotive turbocharger compressors and analyzed with 3D CFD. The CT design is an adaptation from an axial high-pressure compressor, which was successfully applied and intensively investigated in recent years. The aerodynamic working principle of the applied CT design and the achievable improvements are shown and described. The demand of operating range for automotive applications typically dictates high inlet shroud to outlet radius ratio (high trim) and past experiences indicate that a recirculation zone is formed in the inducer for those centrifugal compressors. This recirculation at the inlet shroud causes losses, a massive blockage and induces a co-rotating swirl at the inlet of the impeller. The result is a reduced pressure ratio, often leading to flat speed lines between the onset of recirculation and surge. This paper provides an understanding of inducer recirculation, its impacts and suggests countermeasures. The CT design for centrifugal compressors only influences flow locally at the inducer and prevents recirculation. It differs substantially in design and functionality from the classical bleed slot system commonly used to increase operating range. An experimental and CFD comparison between these designs is presented. While the classical bleed slot system often provides a massive increase in operating range, it often fails to increase the pressure ratio between onset of inducer recirculation and surge. In contrast, the CT design achieves a gain in pressure ratio near surge.

Further Investigation on Acoustic Stall-Warning Approach in Compressors

A further investigation of an acoustic theory-based stall-warning approach is presented in this paper, which contains the basis of this approach and an application on a low-speed compressor (LSC) with a stabilization system. In the present work, this stall-warning approach is first explained through a numerical simulation in which the periodicity of pressure signals is analyzed, and then application experiments of this approach are actualized on a LSC with a stall precursor-suppressed (SPS) casing treatment (CT) as a stabilization system. For this stall-warning approach, a parameter named Rc is calculated through pressure signals of compressor to evaluate the periodicity of pressure signal, and statistical estimates are implemented on Rc so that the probabilities for Rc less than a threshold Rcth can be used as a criterion for stall warning. The numerical and experimental results both show that the signal resolution is determined by the sensor position, the prestall signal amplitude decreases rapidly with the increase of the distance between sensor and blades. Results also show that the probability increases significantly when the operating point is nearing the stall boundary. And at the same operating point, the probability value of Rc will decrease when the SPS CT is engaged. Through this stall-warning approach, a stabilization system based on SPS CT can be activated when the stall margin needs to be extended.

Stability enhancement using a new hybrid casing treatment in an axial flow compressor

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.

Effects of Stall Precursor-Suppressed Casing Treatment on a Low-Speed Compressor With Swirl Distortion

Swirl inlet distortion is usually encountered in modern flight vehicles since their inlet ducts usually consist of one or two bends, such as S-inlet duct. An experimental device is first designed to simulate the swirl inlet distortion and then used to test the effectiveness of a novel casing treatment (CT) on a low-speed compressor under the swirl distortions of various intensities. The influences of co- and counter-rotating swirl inlet distortion on the test compressor and the stabilization ability of this novel CT are well demonstrated by the illustrations of static pressure rise curves and efficiency curves. The dynamic prestall pressure signals are also captured to reflect the perturbation energy in the flow field through which the mechanism of the novel CT will be indicated. The relevant results show that counter-rotating swirl distortion in small intensity could increase the compressive ability of compressor with small efficiency loss, and the co-rotating swirl distortion always brings about detrimental effects on compressor performance. At the same time, the distortion of twin swirls can cause nonuniform total pressure profile which can seriously damage the compressor performance. Besides, the stall precursor-suppressed (SPS) CT shows a good capability of stall margin (SM) enhancement no matter what swirl inlet distortions are encountered in the test compressor.

The impact of casing groove location on the flow instability in a counter-rotating axial flow compressor

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.

Numerical Investigation for the Impact of Single Groove on the Stall Margin Improvement and the Unsteadiness of Tip Leakage Flow in a Counter-Rotating Axial Flow Compressor

A low-speed counter-rotating axial flow compressor (CRAC) with single circumferential grooved casing treatment (CT) was investigated numerically. Both steady and time-accurate numerical calculations were performed to study the effects of the single grooved CTs over the rear rotor on the stability enhancement and the unsteadiness of tip leakage flow (TLF) in the CRAC. Parametric studies indicate that the best position of the single groove should be located near about 20% axial tip chord in terms of the stall margin improvement (SMI). The coincidence of the effective CT locations and the high fluctuating region on blade pressure surface in the smooth wall case shows that the unsteadiness of TLF plays an important role in the stall inception process. Frequency analysis for the static pressure signals near the blade tip shows that both the disappearance of the low frequency components and the suppression of unsteady TLF are beneficial to the SMI. Detailed observation of the flow structures illustrates that the action of the grooves on the different parts of TLF is responsible for the difference of SMI in the CTs. It is more effective to improve the flow stability by controlling the critical TLF released from near the mid-chord.

Influencing Parameters of Tip Blowing Interacting With Rotor Tip Flow

The design space of axial-flow compressors is restricted by stability issues. Different axial-type casing treatments (CTs) have shown their ability to enhance compressor stability and to influence efficiency. Casing treatments have proven to be effective, but there still is need for more detailed investigations and gain of understanding for the underlying flow mechanism. Casing treatments are known to have a multitude of effects on the near-casing 3D flow field. For transonic compressor rotors, these are more complex, as super- and subsonic flow regions alternate while interacting with the casing treatment. To derive design rules, it is important to quantify the influence of the casing treatment on the different tip flow phenomena. Designing a casing treatment in a way that it antagonizes only the deteriorating secondary flow effects can be seen as a method to enhance stability while increasing efficiency. The numerical studies are carried out on a tip-critical rotor of a 1.5-stage transonic axial compressor. The examined recirculating tip blowing casing treatment (TBCT) consists of a recirculating channel with an air off-take above the rotor and an injection nozzle in front of the rotor. The design and functioning of the casing treatment are influenced by various parameters. A variation of the geometry of the tip blowing, more specifically the nozzle aspect ratio, the axial position, or the tangential orientation of the injection port, was carried out to identify key levers. The tip blowing casing treatment is defined as a parameterized geometric model and is automatically meshed. A sensitivity analysis of the respective design parameters of the tip blowing is carried out on a single rotor row. Their impact on overall efficiency and their ability to improve stall margin are evaluated. The study is carried out using unsteady Reynolds-averaged Navier–Stokes (URANS) simulations.

The Impact of Casing Groove Location on Stall Margin and Tip Clearance Flow in a Low-Speed Axial Compressor

In this study, the impact of single grooves at different locations on compressor stability and tip clearance flow are numerically and experimentally investigated. Initially, the numerical stall margin improvement (SMI) curve is examined using experimental data. Then, the evolution of the interface between the tip leakage flow (TLF) and the incoming main flow (MF) in the prestall and stall inception processes for two typical grooves, i.e., the worst and the optimal grooves in terms of their SMI, are compared with the smooth casing. The results show two different interface behaviors throughout the throttling process. The compressor with the worst single groove casing first experiences a long-length-scale disturbance after the interface near the blade suction side spills in front of the rotor leading-edge plane, and then goes through spikes after the whole interface spills. With the smooth casing and the optimal single groove near midchord, the interface reaches the rotor leading edge at the last stable operating point and spikes appear once the whole interface spills over the rotor leading edge. A model that illustrates the spillage patterns of the interface for the two stall precursors is thus proposed accordingly and used to explain their effectiveness in terms of the SMI. At last, the relevance of these results to the preliminary selection of groove locations for multigroove casing treatments (CTs) is verified by test data and discussed.

Unsteady Measurements of Periodic Effects in a Transonic Compressor With Casing Treatments

Application of nonaxisymmetric casing treatments (CTs) can extend the operating range of a transonic compressor significantly. Recent CT designs have proven successful at achieving operating range extension without efficiency loss under design conditions. Two different CT designs were investigated on a high-speed one and a half stage test rig using extensive instrumentation. The stage setup is representative of the front stage of a modern high-pressure compressor. Results of particle image velocimetry (PIV) measurements taken in the blade tip region underneath the CT show a significantly modified flow structure compared to the smooth casing reference case. Blockage zone, secondary flow, and shock structures are affected by the CT, especially in highly throttled operating conditions. The stall inception process of the system with axial slots shows unexpected behavior, with modal activities that are not observed without CT. These activities are resolved using unsteady wall pressure (WP) and hot wire measurements.

Experimental investigation on SPS casing treatment with bias flow

Generally, casing treatment (CT) is a passivity method to enhance the stall margin of fan/compressor. A novel casing treatment based on the small disturbance theory and vortex and wave interaction suggestion is a method combining passive control and active control, which has been proved effective at enhancing the stall margin of fan/compressor in experiment. In order to investigate the mechanism of this kind of casing treatment, an experimental investigation of a stall precursor-suppressed (SPS) casing treatment with air suction or blowing air is conducted in the present paper. The SPS casing treatment is designed to suppressing stall precursors to realize stall margin enhancement in turbomachinery. The experimental results show that the casing treatment with blowing air of small quantity can improve the stall margin by about 8% with about 1% efficiency loss. By contrast, the SPS casing treatment with micro-bias flow does not improve the stall margin much more than that without bias flow, even worse. Meanwhile, the present investigation has also attempted to reveal the mechanism of stall margin improvement with the casing treatment. It is found that the stall margin improvements vary with the modification of the unsteady shedding flow and the unsteady wall boundary impedance. The experimental results agree fairly well with the theoretical prediction using a flow stability model of rotating stall.