Stall Margin Enhancement Research Articles

Application of a novel stall margin improvement indicator in optimizing casing treatment for a transonic compressor

This paper presents a multi-objective design optimization of axial slot casing treatments in a 1.5 stage transonic compressor. To perform the optimization, an in-house optimization design platform is constructed using genetic algorithm and Kriging surrogate model. The optimization objectives are the peak efficiency at the design rotating speed and the stall margin improvements at both design and off-design rotating speeds. Instead of massive unsteady simulations that are required to accurately predict the stall margin, a stall margin improvement indicator is introduced based on the time-averaged axial momentum budget analysis at the rotor tip region. With this indicator, only one steady simulation is needed to evaluate the stall margin enhancement ability of a new casing treatment design at a given rotating speed. The axial slots are parameterized with a total of eight design parameters. The meridional profiles are described with two B-spline curves to define parameters such as the size, axial position, and shape of the slots. Circumferential parameters include the open area ratio and inclination angle. When the optimization finally converges, different optimal variants are selected from the database and their effects are investigated with both experiments and numerical simulations. The detailed flow fields are analyzed in depth with results from unsteady simulations. The application of the optimal casing treatments exerts a suction and re-injection effect, pushes the interface between tip leakage flow and incoming main flow downstream and reduced flow blockage in the blade tip region. Consequently, the stability of the compressor is significantly improved.

Investigation of Vaned-Recessed Casing Treatment in a Low-Speed Axial Flow Compressor, Part I: Time-Averaged Results

This paper investigates the effects of two modifications to a vaned recessed casing treatment. First, the shape of a circular curve was used in the top of the treated casing. Second, a fully curved guide vane was also applied. The goals of the modifications are to enhance the flow recirculation as well as to relieve the low-speed flow, which is normally accumulated within the corners of the vaned recessed casing treatment. The solid casing in addition to the two vaned recessed configurations with 23.2% and 53.5% rotor blade tip axial chord exposure have been studied numerically. The results indicated that two mechanisms are involved in the stall margin enhancement. First, the circumferential pressure gradient is reduced for both configurations. The reduction in pressure gradient largely reduces the development of tip leakage vortex and, thus, the generation of low-speed fluid is diminished. Second, the main flow/tip leakage interface moves toward downstream and the movement of interface toward the leading edge is delayed. The second configuration with a greater rotor blade tip exposure enables extra flow recirculation due to decreasing surface area and, therefore, could be superior to the application of the first casing treatment configuration. The major streamlines within the casing treatment are also discussed. The time-averaged results are presented in this paper, while the unsteady results including instantaneous flow fields, origins of the unsteadiness and frequency analysis are discussed in part II.

Effects of Inlet Swirl Distortion on a Multi-Stage Compressor with Inlet Guide Vanes and Stall Margin Enhancement Method

Inlet swirl distortion is generally considered as a type of velocity distortion, and inlet guide vanes (IGVs) are widely used in the multi-stage compressor of aero-engines to eliminate the tangential velocity of the swirl flow. However, few studies have explored whether there still exists some negative influence of inlet swirl distortion on the compressor, even after the installation of IGVs. Therefore, in this study, the influence of various types of inlet swirl distortions on a multi-stage compressor with the installation of IGVs is investigated. A swirl distortion generator installed in the inlet duct was designed to produce various types of swirl flow patterns. When the distortion intensity increased to some degree, there still existed a decrease in the compressive capability and an obvious additional efficiency loss. The inlet twin swirl distortion was accompanied by total pressure distortion, so even with the installation of IGVs, there was still a significantly negative influence on the performance of the multi-stage compressor, especially the stall margin. Subsequently, to improve the stall margin under inlet swirl distortion, the stall precursor-suppressed (SPS) casing treatment was installed in the first stage of the multi-stage compressor. It could enhance the stall margin of the compressor with no obvious change in the characteristic curves and no additional efficiency loss under various types of inlet swirl distortions, and its mechanism was verified by capturing the dynamic pressure characteristics.

Numerical study on the stability enhancement mechanism of self-recirculating casing treatment in a counter-rotating axial-flow compressor

Counter-rotating axial-flow compressor (CRAC) is a promising technology to increase the thrust-to-weight ratio of aero-engines. Self-recirculating casing treatment (SRCT) is selected to explore its stability enhancement capacity in the CRAC. In the present work, an effective SRCT is designed by the design of experiment method, and the stability improvement potential of the SRCT and its mechanism of stall margin enhancement are studied. Results show that the optimal SRCT scheme increases the stall margin by about 7.73% and without remarkable peak efficiency loss, and the overall performance of the compressor is enhanced at the near-stall condition. The SRCT improves the quality of the tip flow field by sucking out low-energy fluid and restrains the spillage of the tip leakage flow by the jet effect from the injection port. In addition, the SRCT also reduces the unsteady interference between the adjacent rotors. The effect of speed ratio on the effectiveness of SRCT is further investigated, and results indicate that the SRCT increases the stall margin of the CRAC by about 4.13–5.80% at some off-design speed ratio conditions. The speed ratio affects the first stall stage of the CRAC, thus affecting the effectiveness of the SRCT.

Numerical Investigation of Circumferential Groove Casing Treatment in a Highly-Loaded Low-Reaction Transonic Compressor Rotor

In this paper, a computational investigation of circumferential groove casing treatment in a highly-loaded low-reaction transonic compressor rotor is conducted, in which the stage reaction is significantly reduced due to a larger meridional contraction with respect to conventional transonic compressors. Steady computation at near-stall point is performed first to capture the stall inception of the rotor with smooth casing. Detailed observations, which mainly focus on the tip leakage flow behavior, obstruction and vortical structures in the tip region, determine the reason for the compressor stall. There is tip leakage vortex breakdown in the tip region. Moreover, it yields passage obstruction, and finally leads to the compressor stall. Then, attempts are made to investigate how the circumferential grooves can be applied for the compressor’s stall margin enhancement without compromising efficiency. Three configurations are obtained and analyzed by changing axial position and the number of the circumferential grooves. The results of computational parametric study indicate the optimal location of the groove is near the leading edge and the downstream grooves combine their influence on the compressor’s stabilization and performance in a cumulative manner. The optimal circumferential groove configuration produces an increase of 1% in total pressure ratio at the near-stall point and a gain of 3.7% in stall margin, without any penalty in efficiency. Furthermore, the impact the grooves will exert on the flow mechanisms between the grooves and the main flow is also considered.

Impact of hub gap leakage on stator endwall flow in an axial compressor stage with casing treatment

Abstract In an axial compressor stage equipped with slot-type casing treatment, simulation without hub gap between rotors and stators achieves much lower stall margin enhancement than experiment, which results from the over-predicted corner stall in stators. Aiming at clarifying the combined effect of hub leakage flow and casing treatment on operating range and the flow mechanism of leakage flow interacting with stator flow, a detailed numerical research is performed in this compressor stage. The effects of hub leakage mass flow are elaborated in terms of compressor performance and flow fields. Unsteady simulations with appropriate leakage mass flow condition are able to predict the stall margin improvement and total pressure characteristics preferably. Casing treatment slots tend to re-distribute the loading of downstream stator by decreasing loading above 70% span and increasing that below 70% span, which result in the exaggerated corner stall in stators. Hub leakage flow from cavity resists the tendency brought by casing treatment and succeeds in relieving the heavy loading near the hub. The flow mechanism for the effect of cavity leakage flow on stall margin is elucidated. Cavity hub leakage gives rise to a thickened boundary layer upstream stator inlet by producing a separation bubble and aggravating the boundary layer skewing. The thickened boundary layer produces a horseshoe vortex in the stator passage, which induces main flow to rolling up as the pressure-side branch and avoids the emergence of corner stall.

Calculation of Stall Margin Enhancement With Micro-Tip Injection in an Axial Compressor

Although steady micro-injection is experimentally validated as an attractive method in improving the stall margin of axial compressors, up to now a fast prediction of stall boundary remains some way off. This investigation is to propose such a prediction model. A flow stability model is developed to further consider the effect of high-speed micro-injection. After the base flow field is calculated by steady computational fluid dynamics simulation, a body force model is applied to reproduce the effect of blade on the flow turning and loss. A group of homogeneous equations are obtained based on linearized Navier–Stokes equations and harmonic decomposition of small flow disturbance. The stall onset point can be judged by the imaginary part of the resultant eigenvalue. After the existing experimental results are summarized, an unsteady numerical simulation reveals that the computed characteristics and radial profile of pressure rise coefficient are almost unchanged. The unsteady response of compressor to the micro-injection is preliminarily verified based on the observation of the disturbed spillage of tip leakage flow. It is verified that this approach can provide a qualitative assessment of stall point with acceptable computational cost. Both high injection velocity and short axial gap between injector and rotor leading edge are beneficial for the stall margin extension. These theoretical findings agree well with experimental measurements. It is inferred that the spillage of tip clearance flow, which is inward pushed by higher speed injection with shortened distance away from rotor, could lead to further stable flow field.

Stall margin enhancement of a novel casing treatment subjected to circumferential pressure distortion

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.

Numerical analysis of flow in a centrifugal compressor with circumferential grooves: influence of groove location and number on flow instability

As an effective and economic method for flow range enhancement, circumferential groove casing treatment (CGCT) is widely used to increase the stall margin of compressors. Different from traditional grooved casing treatments, in which the grooves are always located over the rotor in both axial and radial compressors, one or several circumferential grooves are located along the shroud side of the diffuser passage in this paper. Numerical investigations were conducted to predict the performance of a low flow rate centrifugal compressor with CGCT in diffuser. Computational fluid dynamics (CFD) analysis is performed under stage environment in order to find the optimum location of the circumferential casing groove in consideration of stall margin enhancement and efficiency gain at design point, and the impact of groove number to the effect of this grooved casing treatment configuration in enhancing the stall margin of the compressor stage is studied. The results indicate that the centrifugal compressor with circumferential groove in vaned diffuser can obtain obvious improvement in the stall margin with sacrificing design efficiency a little. Efforts were made to study blade level flow mechanisms to determine how the CGCT impacts the compressor’s stall margin (SM) and performance. The flow structures in the passage, the tip gap, and the grooves as well as their mutual interactions were plotted and analysed.

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.

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.

Numerical investigation on the effect of a plenum chamber with slot-type casing treatment on the performance of an axial transonic compressor

Casing treatment is an effective approach for improving compressor stall margin in axial compressors. A plenum chamber, which connects slots in the circumferential direction, can occasionally be found in slot-type casing treatments. This study investigates the effect of a plenum chamber on the compressor overall performance and stall margin via time-accurate three-dimensional numerical simulations in a transonic compressor NASA Rotor 37 with a slot-type casing treatment. Four configurations with variable height of plenum chamber are numerically investigated. The unsteady influence of the casing treatment on compressor flow fields is studied using fast Fourier transforms to gain a deeper insight into the function of the plenum chamber. The results indicate that the plenum chamber can reduce the penalty on the compressor’s total pressure ratio and adiabatic efficiency induced by the casing treatment effect at the design operating condition. A detuning effect of the plenum chamber is found and proven to be responsible for the decreased compressor performance penalty through a detailed analysis in both the blade passage and casing treatments. The detuning effect dampens the unsteady influence of the casing treatment on the compressor passage flow in terms of shock oscillations and mass flow fluctuations. Thus, the radial scope and degree of the influence induced by the casing treatment is distinctly restrained by the plenum chamber. Slot circulation, which is composed of a bleed flow from the passage into the slots and injection flow from the slots into the passage, is closely related to the stall margin improvement as the casing treatment is implemented. The plenum chamber has no significant impact on the slot circulation and thus on compressor stall margin. However, shallow slots decrease the slot circulation efficiency, which leads to a much lower stall margin enhancement.

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.

Theory of Compressor Stability Enhancement Using Novel Casing Treatment, Part I: Methodology

This paper proposes a novel casing treatment, designated a stall precursor-suppressed casing treatment. It is designed to suppress stall precursors to realize stall margin enhancement in turbomachinery. This paper first presents a physical and mathematical model to describe the casing treatment configuration as a wall boundary condition and then establishes the stability equation with its effect. The sensitivity of stall inception to different boundary conditions is considered numerically for both subsonic and transonic axial flow fan/compressors. One important finding from this numerical prediction and analysis is that such a casing treatment can be used to change stall inception even when using a casing treatment configuration with low open area ratio. Thus, a reduction of momentum loss due to recirculating flow can be realized, in comparison with a conventional recessed casing treatment with more than 50% open area ratio. It is thus theoretically possible to design an advanced casing treatment to affect the evolution of stall precursors and finally change stall inception without significantly changing the efficiency of the fan and compressors.

Exploration of a new-type grooved casing treatment configuration for a high-speed small-size centrifugal compressor

The main design target for a casing treatment device is to increase the stall margin of a given compressor without sacrificing efficiency. In this article, numerical and experimental investigations about a new-type grooved casing treatment configuration are presented for a high-speed small-size centrifugal compressor. Different from traditional grooved casing treatments, in which the grooves are always placed over the rotor (for both axial and radial compressors), one or several circumferential casing grooves are placed along the shroud side of the diffuser passage in this configuration, to enhance the stall margin of the compressor. Computational fluid dynamics analysis is performed under stage environment in order to find the optimum location of the circumferential casing groove for the subsequent experiments in consideration of stall margin enhancement and peak efficiency gain, and the impact of groove number to the effect of this new-type grooved casing treatment configuration in enhancing the stall margin of the compressor stage is studied. It is found that stall margin gain due to the existence of circumferential casing grooves arises from the suction–reinjection effect of the low momentum fluid and its transportation along the circumferential and streamwise directions in the grooves and the grooves located at the middle and rear part of the diffuser casing wall have more pronounced effect in expanding the stall margin of the centrifugal compressor. In order to verify the effectiveness of this new-type grooved casing treatment configuration, the solid casing compressor and the compressors with circumferential casing grooves are tested in our test rig. The results indicate that this new-type grooved casing treatment configuration can obtain obvious improvement in the stall margin without sacrificing peak efficiency. Also, the numerical investigation is conducted for the entire compressor with circumferential casing grooves and the result is compared with the experimental data.

The Effects of Wet Compression and Blade Tip Water Injection on the Stability of a Transonic Compressor Rotor

The rotor blade tip leakage flow and associated formation of the tip leakage vortex and interaction of the tip leakage vortex with the shockwave, particularly in the case of a transonic compressor rotor have significant impact on the compressor performance and its stability. Air injection upstream of the compressor rotor tip has been shown to improve compressor performance and enhance its stability. The air required for rotor blade tip injection is generally taken from the later stages of the compressor thus causing penalty on the gas turbine performance. In this study, effects of water injection at the rotor tip with and without the wet compression on the compressor performance and its stability have been examined. To achieve the stated objectives, the well tested transonic compressor rotor stage, NASA rotor stage 37, has been numerically simulated. The evaluation of results on various performance parameters, such as total pressure ratio, inlet flow capacity, and adiabatic efficiency combined with contours of total pressure losses, entropy, Mach number, and temperature including limiting streamlines, shows that the blade tip water injection could help in reducing low energy region downstream of the shockwave and strength of the tip leakage vortex with the compressor operating at its rotating stall boundary condition. The extent of reduction depends on the droplet size, injection flow rate, and its velocity. Furthermore, results show that combined case of the blade tip water injection and the wet compression could provide better stall margin enhancement than the blade tip water injection case.

Control of Tip-Clearance Flow in a Low Speed Axial Compressor Rotor With Plasma Actuation

This research investigates different dielectric barrier discharge (DBD) actuator configurations for affecting tip leakage flow and suppressing stall inception. Computational investigations were performed on a low speed rotor with a highly loaded tip region that was responsible for stall-onset. The actuator was mounted on the casing upstream of the rotor leading edge. Plasma injection had a significant impact on the predicted tip-gap flow and improved stall margin. The effect of changing the actuator forcing direction on stall margin was also studied. The reduction in stalling flow was closely correlated with a reduction in loading parameter that quantifies mechanisms responsible for end-wall blockage generation. The actuation reduced end-wall losses by increasing the static pressure of tip-gap flow emerging from blade suction-side. Lastly, an approximate speed scaling developed for the DBD force helped estimate force requirements for stall margin enhancement of transonic rotors.

Numerical investigations of the flow mechanisms in a subsonic compressor rotor with axial-skewed slot

By applying a concept similar to ‘domain scaling’ treatment often used in multi-stage turbomachinery flow field to the interface between the rotor blade passage and end-wall treatments, a series of computational studies were carried out to understand the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the axial-skewed slot casing treatment. Particular attention was given to examining the interaction between the rotor tip flow and the axial-skewed slot. In order to validate the multi-block model applied in the rotor blade end-wall region, the computational results for the modern subsonic compressor rotor both with and without axial-skewed slot casing treatment were correlated with available experimental test data. The computed results agree fairly well with the experimental data, and the computations are then used to describe the interaction between the rotor tip flow and axial-skewed slot. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases with axial-skewed slot and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by the axial-skewed slot casing treatment is a result of the tip flow manipulation. The repositioning of the tip vortex trajectory further towards the trailing edge of the blade passage and delaying the movement of incoming/tip clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.

Numerical and Experimental Investigations of Steady Micro-Tip Injection on a Subsonic Axial-Flow Compressor Rotor

Steady tip injection has been demonstrated to be an effective means of extending the stable operating range of a tip-critical compressor. This study presents a state-of-the-art design for the tip injection through the casing with flush-mounted inclined holes and the effectiveness of steady micro-air injection to enhance stability in a subsonic axial-flow compressor rotor using an external-air supply. For the tested rotor, experimental results demonstrate that at 53% design speed, the stalling mass flow can be reduced by 7.69% using an injected mass flow equivalent to 0.064% of the annulus flow. Time-dependent CFD simulations were conducted to identify the physical mechanic that accounts for the beneficial effects of the steady micro-air injection on the performance and stability of the compressor. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases without tip injection and with tip injection. It was found that the primary stall margin enhancement afforded by the steady micro-air injection is a result of the tip-clearance flow manipulation. The repositioning of the tip-clearance vortex further towards the trailing edge of the blade passage and delaying the movement of incoming/tip-clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.