Stall Margin Improvement Research Articles

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.

Flow measurements near the open surfaces of single circumferential grooves in a low-speed axial compressor

Circumferential grooves (CG) is an effective method to enhance the compressor stall margin with minor efficiency disturbances. The interaction between the groove and the blade passage is found to be responsible for the stall margin improvement (SMI) by many studies. The momentum transport across open surface of an individual groove could be an indicator to assess its effectiveness. These findings are all derived by numerical means. Two single grooves are chosen as the examples to demonstrate the needs of the experimental validation. In experiments, one groove, CG5 at 27% axial chord, produced less SMI than the other one, CG7 at 43% axial chord. Yet, many CFD simulations predicted just the opposite. In order to validate the numerical simulations, detailed flow structures are needed near the open surfaces of the casing grooves. However, such experimental results are still missing due to the difficulties of measurement, especially for the flow measurement inside the grooves. Aiming at providing reasonable experimental results for these casing grooves, a measuring technique is proposed in this paper. A special algorithm is utilized to obtain the time-averaged magnitudes of velocity components by using three hot-wire probes. The groove–passage interaction is recorded using the fluctuations of each velocity component and the frequency spectrum. Intensive interaction is detected for CG7. The experimental results demonstrate that the flow interaction between the groove and blade tip is crucial to the stability enhancement.

Flow mechanism of affecting an axial flow compressor performance and stability with cross-blade slot casing treatments

The objective of this study is to evaluate the effect of cross-blade slot casing treatment on the stability and performance of an axial flow compressor rotor. The experimental and unsteady calculated results both show that cross-blade slot casing treatment can generate about 22% stall margin improvement, and the compressor peak efficiency is reduced by about 13%. The detailed flow-field analyses indicate that the sucked and injected flow caused by the slots of cross-blade slot casing treatment can restrain the rotor tip passage blockage, which is made by the low energy tip clearance leakage vortex. When cross-blade slot casing treatment is applied, not only the rotor wheel flange work becomes lower in most of the rotor blade span, but also the flow loss in the blade tip passage becomes fairly large due to the strong interaction between the mainstream and the injected flows made by the slots. As a result, the compressor total pressure ratio and efficiency for cross-blade slot casing treatment are reduced obviously. Three kinds of new cross-blade slot casing treatment were designed according to the previous successful experience and investigated in this paper. The numerical results show that the new three cross-blade slot casing treatments both generate about 54% stall margin improvement at the cost of minor peak efficiency. For one new cross-blade slot casing treatment (CSCT2), the compressor peak efficiency is reduced by about 0.3%. The low energy TLV, which is present for cross-blade slot casing treatment, is removed by the strong sucked flow made by CSCT2. Moreover, the interaction between the mainstream and the injected flows caused by CSCT2 becomes weak obviously, and the corresponding flow loss is reduced greatly. Hence, the compressor stability and performance with CSCT2 are higher than those with cross-blade slot casing treatment.

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 analysis of the circumferential grooves casing treatment in a counter-rotating axial flow compressor

The impact of the circumferential grooves casing treatment over the rear rotor (R2) in a counter-rotating axial flow compressor has been investigated based on numerical simulations. The main purpose is to understand the effects of grooved casing treatment on the unsteady flow behaviors and the corresponding mechanisms of the stability enhancement in the compressor. The results show that the interface between incoming main flow and tip leakage flow in R2 is pushed downstream obviously and the flow stability is enhanced with grooved casing treatment. The compressor performance at near stall condition is also improved remarkably. The blade loading below the grooves, the incidence angle near the blade tip and the backward axial momentum flux injected into main flow passage through the tip gap are all reduced, which is beneficial to the stall margin improvement. Frequency analysis near the blade tip of R2 indicates that the oscillations with lower frequency are suppressed by the casing grooves and the fluctuating intensity decreases, which also contributes for the enhancement of flow stability. Detailed observation of tip leakage flow structures illustrates that it is more effective to improve the flow stability by controlling the tip leakage flow released from near mid-chord.

A new approach of casing treatment design for high speed compressors running at partial speeds with low speed large scale test

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.

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.

Investigation on axial effect of slot casing treatment in a transonic compressor

This paper reports the effect of the axial position of the slot casing treatment on the performance of transonic compressor NASA Rotor 67 by unsteady numerical simulation. The interaction of the recirculation in the slots and flow near the blade tip is analyzed to understand the flow mechanisms. The relative importance of the mechanisms of stall margin improvement due to the slot casing treatment is evaluated with the relative weight method. The results show that the bleeding and injecting effect caused by the recirculation is the most important factor that affects the blockage in the blade tip region, which determines the stall margin improvement. When the slots cover the initial position of the tip leakage vortex (TLV) and the boundary layer separation zone downstream the shock, the recirculation is stronger due to the larger pressure difference between the front and rear part of the slots. Consequently, the slot casing treatment can reduce the blockage near the casing more effectively, which results in a larger stall margin improvement. Shifting the slots upstream or downstream will reduce the driving force for the recirculation and the effect extent on the low energy fluid of the slots, which decreases the stall margin improvement.

Flow Mechanism for Stall Margin Improvement via Axial Slot Casing Treatment on a Transonic Axial Compressor

Flow Mechanism for Stall Margin Improvement via Axial Slot Casing Treatment on a Transonic Axial Compressor

Influence of SPS casing treatment on axial flow compressor subjected to radial pressure distortion

The present work is about the stall margin enhancement ability of a kind of stall precursor-suppressed (SPS) casing treatment when fan/compressor suffers from a radial total pressure inlet distortion. Experimental researches are conducted on a low-speed compressor with and without SPS casing treatment under radial distorted inlet flow of different levels as well as uniform inlet flow. The distorted flow fields of different levels are generated by annular distortion flow generators of different heights. The characteristic curves under these conditions are measured and analyzed. The results show that the radial inlet distortion could cause a stall margin loss from 2% to 30% under different distorted levels. The SPS casing treatment could remedy this stall margin loss under small distortion level and only partly make up the stall margin loss caused by distortion in large level without leading to perceptible additional efficiency loss and obvious change of characteristic curves. The pre-stall behavior of the compressor is investigated to reveal the mechanism of this stall margin improvement ability of the SPS casing treatment. The results do show that this casing treatment delays the occurrence of rotating stall by weakening the pressure perturbations and suppressing the nonlinear amplification of the stall precursor waves in the compression system.

The stall inceptions in an axial compressor with single circumferential groove casing treatment at different axial locations

Circumferential groove (CG) casing treatment is known to be a good method to improve the stall margin of axial compressor. Recent studies on single groove indicate the stall margin improvement (SMI) is significantly affected by groove's axial location, which means the effects of the groove on the passage flow should be further studied as the groove's axial location is changed. In this paper the features of stall inception with and without the groove treatment are experimentally researched in a low-speed axial compressor. The contour of static pressure on casing is obtained by fast-response pressure transducers concentrative mounted as a line in chordwise direction, and the data acquired by transducers in circumferential direction is processed with the method of wavelet analysis to demonstrate the development of stall inception. It is found the stall inception is the typically spike-type for smooth casing (without treatment), which is manifested as short-scaled disturbance of only 2–4 blade passages when it appears. Two single grooves at different axial locations, which can be ineffective and effective in SMI, are further tested. The results indicate the stall inception is still spike-type for both of them. However, for the ineffective groove, the stall inception is found to be relatively longer-scaled disturbance. For the effective groove, it is still the typical short-scaled disturbance similar to the smooth casing. In the further study on double grooves treatment in which the ineffective groove is included, the stall inception is also found to be relatively longer-scaled type. According to these experimental results and previous CFD simulation, the groove with almost no SMI is believed to take different effects on the compressor which modifies the stall inception into the relatively longer-scaled type, and the corresponding tip–region flow structure deserves more future study in this regard.

Self-Adaptive Stability-Enhancing Technology With Tip Air Injection in an Axial Flow Compressor

Self-adaptive stability control with discrete tip air injection and online detection of prestall inception is experimentally studied in a low-speed axial flow compressor. The control strategy is to sense the cross-correlation coefficient of the wall static pressure patterns and to feed back the signal to an annular array of eight separately proportional injecting valves. The real-time detecting algorithm based on cross-correlation theory is proposed and experimentally conducted using the axisymmetric arrangement of time-resolved sensors. Subsequently, the sensitivity of the cross-correlation coefficient to the discrete tip air injection is investigated. Thus, the control law is formed on the basis of the cross-correlation as a function of the injected momentum ratios. The steady injection and the on–off pulsating injection are simultaneously selected for comparison. Results show that the proposed self-adaptive stability control using digital signal processing (DSP) controller can save energy when the compressor is stable. This control also provides protection when needed. With nearly the same stall margin improvement (SMI) as the steady injection (maximum SMI is 44.2%), the energy of the injected air is roughly a quarter of the steady injection. Unlike the on–off pulsating jet, the new actuating scheme can reduce the unsteady force impinging onto the compressor blades caused by the pulsating jets in addition to achieve the much larger stability range extension.

Stability management of high speed axial flow compressor stage through axial extensions of bend skewed casing treatment

This paper presents the experimental results to understand the performance of moderately loaded high speed single stage transonic axial flow compressor subjected to various configurations of axial extensions of bend skewed casing treatment with moderate porosity. The bend skewed casing treatment of 33% porosity was coupled with rectangular plenum chamber of depth equal to the slots depth. The five axial extensions of 20%, 40%, 60%, 80% and 100% were used for the experimental evaluations of compressor performance. The main objective was to identify the optimum extension of the casing treatment with reference to rotor leading edge which results in maximum stall margin improvements with minimum loss in the stage efficiency. At each axial extension the compressor performance is distinctive. The improvement in the stall margin was very significant at some axial extensions with 4%–5% penalty in the stage efficiency. The compressors stage shows recovery in terms of efficiency at lower axial extensions of 20% and 40% with increase in the peak stage efficiency. Measurements of flow parameters showed the typical behaviors at near stall flow conditions. Hot wire sensor was placed at the rotor upstream in the tip region to capture the oscillations in the inlet axial and tangential velocities at stall conditions. In the absence of casing treatment the compressor exhibit abrupt stall with very high oscillations in the inlet axial and tangential velocity of the flow. The extents of oscillations reduce with bend skewed casing treatment. Few measurements were also performed in the plenum chamber and salient results are presented in this paper.

Investigation of groove casing treatment in a transonic compressor at different speeds with control volume method

This article uses a control volume method to analyze the circumferential groove casing treatment in a transonic compressor. To analyze the axial momentum transport through the tip gap, the control volume near the casing is divided into two parts: the control volumes inside and outside the tip gap. Besides, the association between the forces acting on the control volume and flow structures is studied by analyzing the distributions of axial momentum flux and axial shear stress. With this method, the flow mechanisms of stall margin improvement due to casing grooves in Rotor 35 are quantitatively analyzed. The analysis is conducted at 100% and 60% design speed with supersonic and subsonic tip speed, respectively. At design speed, the casing grooves decrease the axial shear force and the axial force due to the transport of axial momentum induced by the tip leakage flow. Meanwhile, the bleeding and injecting effect of grooves contribute much to the axial force due to the transport of axial momentum. Based on the axial distribution of the axial forces, the contribution of each groove to the stall margin improvement is assessed. And the grooves that play a major role in stall margin improvement are ascertained. At 60% design speed, because the blade loading is reduced, the axial momentum transport caused by the grooves cannot suppress the boundary layer separation effectively. Consequently, the stall margin of the compressor is not significantly improved by the casing grooves.

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.

Initial selection of groove location combination for multi-groove casing treatments

Stall margin improvement (SMI) curve is a function of a single groove location for a low-speed axial compressor and is investigated numerically and experimentally in this paper. SMI curve illustrates that good grooves are located in a fairly wide region approximately from 30% Cax to 80% Cax aft of the rotor leading edge. The narrow region that contains bad grooves is near 20% Cax aft of the rotor leading edge. The initial selection of groove location combination for multi-groove casing treatments is proposed based on the SMI and efficiency improvement data generated by a series of single-groove and multi-groove casing treatments. The grooves should be set in the “wide region” of the SMI curve. The grooves located near the “narrow region” and the rotor trailing edge should be avoided. The “narrow region” can be identified by the axial momentum distribution of the tip leakage flow or the circumferentially averaged axial shear stress on the casing using the simulation results of the smooth casing at the near-stall point.

Stall Margin Improvement in a Centrifugal Compressor through Inducer Casing Treatment

The increasing trend of high stage pressure ratio with increased aerodynamic loading has led to reduction in stable operating range of centrifugal compressors with stall and surge initiating at relatively higher mass flow rates. The casing treatment technique of stall control is found to be effective in axial compressors, but very limited research work is published on the application of this technique in centrifugal compressors. Present research was aimed to investigate the effect of casing treatment on the performance and stall margin of a high speed, 4 : 1 pressure ratio centrifugal compressor through numerical simulations using ANSYS CFX software. Three casing treatment configurations were developed and incorporated in the shroud over the inducer of the impeller. The predicted performance of baseline compressor (without casing treatment) was in good agreement with published experimental data. The compressor with different inducer casing treatment geometries showed varying levels of stall margin improvement, up to a maximum of 18%. While the peak efficiency of the compressor with casing treatment dropped by 0.8%–1% compared to the baseline compressor, the choke mass flow rate was improved by 9.5%, thus enhancing the total stable operating range. The inlet configuration of the casing treatment was found to play an important role in stall margin improvement.

Effects of Rotating Inlet Distortion on Compressor Stability With Stall Precursor-Suppressed Casing Treatment

This paper conducts an experimental research of rotating inlet distortion on a low-speed large size test compressor with emphasis on the stability problem of axial fan/compressors, and the stall margin enhancement with a kind of stall precursor-suppressed (SPS) casing treatment. Some results on compressor stall margin and prestall behavior under the restriction of rotating inlet distortion are presented. The experimental results show that whether the inlet distortion is co-rotating or counter-rotating, the SPS casing treatment can still improve the stall margin without leading to additional efficiency loss caused by such configuration. The experiment results also show that the mechanism of the stall margin improvement with such casing treatment is associated with delaying the nonlinear development of the stall precursor waves and weakening the unsteady flow disturbances in a compression system.

Further Investigation on Transonic Compressor Stall Margin Enhancement With Stall Precursor-Suppressed Casing Treatment

A kind of casing treatment, named as stall precursor-suppressed (SPS), has been developed recently, which was proved to be able to effectively improve stall margin (SM) without significant efficiency loss in low-speed axial flow compressors and a transonic compressor rotor. In this paper, the effectiveness of the SPS casing treatment is investigated in a single-stage transonic compressor. Based on an extended stall inception model, the quantitative evaluation of the SM enhancement by the SPS casing treatment is presented for the transonic compressor stage. The model predicts that a 2.5–6.8% of stall margin improvement (SMI), which is defined in terms of mass flow rate at stall inception, can be achieved at the design rotational speed. The experimental results show that the SPS casing treatment can achieve 3.5–9.3% of the SMI at 95% design rotational speed. Due to the fact that the distributions of the total pressure ratio along the spanwise direction are kept the same as those of the solid wall casing at the same mass flow rate, the SPS casing treatments with a small open area ratio and large backchamber enhance the SM without a recognizable efficiency loss and a migration of the pressure-rise characteristics. Furthermore, the mechanism of SMI with the SPS casing treatment is investigated in the experiments. In comparison with the solid wall casing, the emergence and the evolution of the stall inception waves are suppressed and the nonlinear development of the stall process is delayed with the SPS casing treatment.

Numerical investigation of a centrifugal compressor with circumferential grooves in vane diffuser

Enhancing stall and surge margin has a great importance for the development of turbo compressors. The application of casing treatment is an effective measure to expand the stall margin and stable operation range. Numerical investigations were conducted to predict the performance of a low flow rate centrifugal compressor with circumferential groove casing treatment in vane diffuser. Numerical cases with different radial location, radial width and axial depth of a circumferential single groove and different numbers of circumferential grooves were carried out to compare the results. The CFD analyses results show that the centrifugal compressor with circumferential grooves in diffuser can extend stable range by about 9% while the efficiency over the whole operating range decreases by 0.2 to 1.7%. The evaluation based on stall margin improvement showed the optimal position for the groove to be located was indicated to exist near the leading edge of the diffuser, and a combination of position, width, depth and numbers of circumferential grooves that will maximize both surge margin range and efficiency.