Experimental and numerical study of tip injection in a subsonic axial flow compressor

Wei WANG,Wuli CHU,Hogung ZHANG,Hiyng KUANG

aSchool of Power and Energy,Northwestern Polytechnical University,Xi’an 710072,China

bFaculty of Water Resources and Hydroelectric Engineering,Xi’an University of Technology,Xi’an 710048,China

cCollaborative Innovation Center of Advanced Aero-Engine,Beijing 100083,China

Experimental and numerical study of tip injection in a subsonic axial flow compressor

Wei WANGa,b,Wuli CHUa,c,*,Haoguang ZHANGa,Haiyang KUANGa

aSchool of Power and Energy,Northwestern Polytechnical University,Xi’an 710072,China

bFaculty of Water Resources and Hydroelectric Engineering,Xi’an University of Technology,Xi’an 710048,China

cCollaborative Innovation Center of Advanced Aero-Engine,Beijing 100083,China

Available online 7 May 2017

*Corresponding author at:School of Power and Energy,Northwestern Polytechnical University,Xi’an 710072,China.

E-mail address:wlchu@nwpu.edu.cn(W.CHU).

Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.cja.2017.04.004

1000-9361©2017 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Parametric study of tip injection was implemented experimentally on a subsonic axial flow compressor to understand the underlying flow mechanisms of stability improvement of the compressor with discrete tip injection.Injector throat height varied from 2 to 6 times the height of rotor tip clearance,and circumferential coverage percentage ranged from 8.3%to 25%of the annulus.Static pressure fluctuations over the rotor tip were measured with fast-response pressure transducers.Whole-passage time-accurate simulations were also carried out to help us understand the flow details.The combinations of tip injection with traditional casing treatments were experimentally studied to generate an engineering-acceptable method of compressor stall control.The results indicate that the maximum stability improvement is achieved when injectors are choked despite their different sizes.The effect of circumferential coverage percentage on compressor stability depends on the value of injector throat height for un-choked injectors,and vice versa.Tip blockage in the blade passage is greatly reduced by the choked injectors,which is the primary reason for stability enhancement.The accomplishment of blockage diminishment is maintained in the circumferential direction with the unsteady effect of tip injection,which manifests as a hysteresis between the recovery of tip blockage and the recovery of tip leakage vortex.The unsteady effect is primarily responsible for the effectiveness of tip injection with a partial circumferential coverage.Tip injection cannot enhance the stability of the rotor with axial slots significantly,but it can improve the stability of the rotor with circumferential grooves further.The combined structure of tip injection with circumferential grooves is an alternative for engineering application.

©2017 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.This is an opena ccess article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Axial flow compressor;

Casing treatments;

Compressor stability;

Tip blockage;

Tip injection;

Tip leakage vortex

1.Introduction

The safe operation of axial flow compressors in aero engines requires sufficient stall margin which is defined by the range between the aerodynamic design point and stall limit.However,increased stage loading tends to dampen the compressor stall margin,resulting in a higher risk of running into rotating stall and surge.Tip injection,which utilizes high-energy jets to energize the tip flow of compressors,is an effective method to postpone the occurrence of stall.Compared with traditional casing treatments with slots or grooves,tip injection has the potential to improve both compressor stability and efficiency.

Freeman et al.1experimentally confirmed the beneficial effect of tip injection on a Rolls-Royce Viper turbojet between 1989 and 1994.Much effort was then made to understand the underlying flow mechanisms of tip injection as well as the effects of various injection parameters on compressor stability to make the best use of injected jets.

Suder et al.2implemented a comprehensive study on steady tip injection and demonstrated a correlation between stability improvements and the increases in the mass-averaged axial velocity at the rotor tip.Nie et al.3disclosed that micro air injection with a small amount of air could improve compressor stability by shifting the position of tip leakage vortex(TLV)toward downstream direction.The micro tip injection was also investigated by Lu4and Geng5et al.Roy et al.6pointed out that the effect of tip injection on straight blades was more pronounced than that on swept blades.Beheshti et al.7proposed a method of injection through a casing groove near the blade tip.Weigl8and Dobrzynski9et al.reported that reducing injector numbers to half had no obvious influence on stability enhancement.Parametric studies of tip injection on the parameters such as injected mass flow rate and injection angle were performed numerically by Cassina10and Khaleghi11,12et al.Wang et al.13,14numerically studied the effects of injector size and circumferential coverage on compressor stability and built a relationship between stability improvements and the decrements in tip blockage.Tip injection in multi-stage compressors was experimentally studied by Hiller15and Strazisar et al.16.The controlled pulsed injection,injection frequency and injection profile were also investigated by Hiller17,Zhou18and Lim19et al.,respectively.

All the above exhibited studies utilized external air sources to implement tip injection,which made it hard to evaluate the influence on compressor efficiency.Tip injection with internal air source,i.e.,recirculating casing treatment,was investigated by Hathaway20,Yang21Weichert22,Khaleghi23and Wang.24et al.The simultaneous improvements in the compressor efficiency and stability were observed in some studies,which confirmed the advantage of tip injection compared with the traditional casing treatments.

As far as the underlying flow mechanism is concerned,the stabilizing effects of tip injection can be summarized as three aspects as shown below.

(1)The unloading effect.The axial component of velocity is increased by high-velocity jets,resulting in the declines in the incidence angle and blade loading at the rotor tip.This effect is regarded as the main reason for stability enhancement in.2,10–12

(2)The action on tip leakage flow(TLF)or tip blockage.Tip injection generates high-velocity jets which can shift the TLV downstream or diminish the tip blockage related with the TLV.Stability enhancement is attributed to this effect in.3,4,7,9,13,14

(3)The unsteady effect.This view primarily refers to the unsteady response of passage flow to tip injection.Nie et al.3pointed out that tip injection could clear up the tip flow disturbances before they grew up.Matzgeller et al.25,26argued that the unsteady adaption of passage flow to the varying inflow conditions was the main reason for the beneficial effect of tip injection rather than a shift of TLV.The unsteady effect was also identified indirectly by studying the injection frequency18and the growth and propagation of compressor unsteadiness.19

More than one aspect may account for the stability improvement in a compressor with tip injection.Roy and Veraararapu27explained that the action on TLF was responsible for the stability gain at a low injection angle,and the unloading of rotor tip might also work at a high injection angle.Li et al.28determined that compressor stability was extended by acting on the TLF for micro-injection,while the unloading effect should also be considered for macro-injection with a large amount of air.The unsteady effect of tip injection is seldom investigated and has not been well understood yet.

In the study,all the three potential stabilizing effects of tip injection are discussed in detail to identify their different effects on the test rotor.Parametric studies of tip injection on injector throat height and circumferential coverage percentage are implemented experimentally.Whole-passage unsteady simulations are also carried out to help us understand the flow details.Another interest of the study is to find out whether the traditional casing treatments can collaborate with tip injection to generate an engineering-acceptable method of compressor stall control.

2.Experimental setup

2.1.Test facility

Tip injection was tested on an axial flow compressor at the national key laboratory in Northwestern Polytechnical University.The isolated rotor was used to implement tip injection.The compressor parameters at the design and off-design speeds are given in Table 1.The tested facility was described in detail in Refs.24,29,30.The nominal running tip clearance was 0.3 mm,which was equal to 0.52%of blade height at the test speed.The shaft speed was set to be 8130 r/min because the stall of the compressor was triggered by the TLV,i.e.,the compressor is tip-critical(spike-initiated)at the test speed.Slot-type31,groove-type29and recirculating casing treatments24were also tested on the compressor at this speed.The study on tip injection was supplementary to the knowledge of passive endwall technologies for the test compressor.

Table 1 Compressor parameters at design and off-design speeds.

2.2.Tip injection system

The Coanda injector used in the study made references to the design of Strazisar et al.16The flow channel was optimized to make injected jets attached to the flow path wall as close as possible.The various configurations of tip injection were constructed with a series of modules(Fig.1).Each module covered 8.3%of the annulus.Three modules at most could be assembled together,resulting in a circumferential coverage percentage(ccp)of 25%.Injector throat height(h)varies from 2 to 6 times the height of rotor tip clearance τ.The injection location was set as 11%ca(ca=axial chord length of rotor tip)upstream of the rotor.Six identical injectors were uniformly mounted around the annulus for all test configurations.Solid casing(SC)treated as the reference was achieved by changing the modules.In this way,the rotor tip clearance size was kept constant strictly for the casings with and without injection.Parametric studies were implemented experimentally to evaluate the effects of ccp andhon compressor stability.A total of 9 injection configurations were fabricated.

The injectors were supplied with air at ambient temperature from an external source(Fig.2(a)).The required injection mass flow rates were obtained by varying the pressure at the outlet of air tank.The pressure in the manifold was monitored using a pressure transducer to guarantee a constant pressure supply and therefore a constant injection mass flow rate for each case.Six bypass pipes with equal length were linked to plenum chambers.The plenum chambers were used to disperse the air originating from the bypass pipes to make the air uniformly distributed in the flow channel of injectors.The whole air supply system was simulated with a CFD method.Fig.2(b)demonstrates the distributions of Mach number and streamlines in the plenum chamber and injector.The predicted flow field features the jets with high axial velocity and nearly uniform flow distributions in the circumferential direction at the injector outlet.

Fig.3 compares the variations of injected mass flow rate(normalized by the stalling mass flow of SC)with pressure in the manifold between experiment and simulation.The differences between experiment and simulation at lower and higher pressures are relatively high with a maximum error of 10.45%.The differences are probably caused by the complexity of the whole air supply system and the incapability of the turbulence model under off-design conditions.However,the calculated result matches the experimental result well in the domain of medium pressures in the manifold.The pressure of 180632 Pa in the manifold results in a pressure of 146795 Pa at the outlet of bypass pipes,which is used as the inlet boundary conditions in whole-passage unsteady simulations for the rotor with tip injection.The difference between experiment and simulation is less than 0.5%when the pressure is equal to 180632 Pa in the manifold.

2.3.Tip injection with casing treatments

Figs.4 and 5 depict the photographs of tip injection(h=2τ,ccp=25%)coupled with circumferential grooves and axial slots,respectively.The geometric parameters of grooves and slots are also presented in the figures.More information can be found in29for the circumferential grooves and in31for the axial slots.The inner casing with injectors was pushed upstream to install the inner casing with grooves or slots.The stall limits of the two casing treatments were tested with and without tip injection.The stall limit of tip injection with the SC over the rotor tip was also measured.The purpose was to find out whether the stall margin of casing treatments can be further improved by implementing tip injection.

2.4.Instrumentation

In the experiment,the compressor’s total pressure ratio was measured with static and total pressure-combined probes.The probes were installed 250%caupstream and 200%cadownstream of the rotor,respectively.The probes were traversed radially to measure the absolute flow angle and total pressure.There were two measurement locations in the circumferential direction at the rotor outlet because the exit flow was nonaxisymmetric under the effect of discrete distributions of injectors.One probe faced the flow under an injector,and the other faced the un-disturbed flow.The casing ring equipped with injectors was rotated in the circumferential direction to make the measurement.The flow parameters measured at the two locations were mass-averaged to generate the rotor outlet flow parameters.The compressor input torque was monitored with the torque meter device to calculate the compressor efficiency.Mass flow rate through the compressor was measured with the calibrated orifice plate.The stall limit was tested at least thrice for each case to obtain the mean value of stalling mass flow rates in several individual tests,thereby resulting in an accuracy of±0.6%maximum for the mass flow.The measurement uncertainties of total pressure were 100 and 300 N/m2for the rotor inlet and outlet,respectively.The errors in calculated efficiency were approximated at 0.5%maximum by comparing the results of the same case.

Fig.6 exhibits the arrangement of fast-response pressure transducers(Kulite XCQ-080).In the axial direction(Z),seven transducers were arranged in a line to depict static pressure fields over the rotor tip.Two rows of transducers were necessary.Row 1 was placed next to an injector in the direction of blade rotation to detect the immediate influence of injection on the tip flow.Row 2 was separated from the former row with 30°around the annulus to achieve a comparable flow field that was relatively free from the influence of injection.The phaselocked patterns of casing wall pressure were obtained at the two detection rows.

3.Numerical methodology

A commercial CFD package NUMECA FINE/Turbo was utilized to carry out whole-passage unsteady simulations of tip injection(h=2τ,ccp=25%).Each blade passage was modeled with H-blocks for the inlet and outlet and an O-block surrounding the rotor blade(Fig.7(a)).The H-blocks were kept stationary to connect with the blocks of injectors,whereas the O-block rotated with the blade.The grids used for each blade passage consisted of 41 pitchwise,73 spanwise and 69 streamwise points.The tip clearance was meshed with butterfly topology.The radial-axial inlet was also modeled according to its full size to capture the inlet boundary layer development sufficiently(Fig.7(b)).The mesh was stretched toward all solid boundaries to meet the resolution requirement ofy+≤2.The mesh for the whole blade passage consisted of approximately 13 million grid points.Grid-independent results were achieved when the grid number was more than 6.6 million for the whole blade passage in steady simulations,which was also found in the study of Wu et al.30The injectors were meshed in H-blocks that were connected directly to the passage inlet H-blocks with the method of full non-matching boundary.The cell numbers for all the injectors were approximately 1.5 million.

The equations were discretized in space with a cell-centered finite volume formulation.The turbulence closure problem was satisfied with a Spalart-Allmaras model.A central differencing manner with Gauss’s theorem was used to evaluate the viscous fluxes.The inviscid fluxes were calculated with a second-order upwind scheme based on a flux difference splitting formula.The integration of time was implemented in the implicit pseudo-time scheme.There were 750 physical time steps for the whole passage,which indicated that it took a blade 25 time steps to pass through one blade pitch(one blade pitch is the spacing between two adjacent blades in a blade row).The numbers of pseudo-time interactions were 20 with a CFL(Courant Friedrichs Lewy)number of 3 in each physical time step.

The inlet boundary conditions of blade passages were constant total pressure of 101325 Pa and constant total temperature of 288.2 K with a radial inlet flow direction.The averaged static pressure was varied at the outlet boundary to obtain different operating points.Adiabatic and nonslip conditions were imposed on solid walls.The fluid was defined as a perfect gas with constant isentropic coefficients and constant heat capacity.The inlet boundary conditions of bypass pipes were constant total pressure of 146795 Pa and constant total temperature of 305.3 K,which were derived from the simulated result of air supply system.

The convergence history of inlet and outlet mass flow rates under the near stall operating condition is plotted in Fig.8 for the tip injection in the unsteady simulation.The convergence criteria of simulation are the periodic fluctuations of mass flow rate,total pressure ratio and adiabatic efficiency,which were achieved at the sixth revolution with a mass flow rate of 2.69 kg/s.When the static pressure at the outlet boundary was improved by 100 Pa,the computation showed completely divergence during the fifth revolution,which resulted in a sharp decrease of mass flow rate.

Fig.9 demonstrates the compressor maps of the SC and tip injection(h=2τ,ccp=25%).The time-averaged performance at the near-stall point derived from the full-annulus unsteady simulations is shown for both configurations.The calculation of the compressor’s overall performance removes the additional energy supplied with external air source using the method described in.13The experimental results show that the total pressure ratio and stall margin are obviously improved with the tip injection compared with the SC,and the compressor efficiency is enhanced at lower mass flow rates.The calculated stall limits agree fairly well with the experimental results for both configurations.The calculated total pressure ratio and efficiency are slightly higher than the experimental result for both configurations probably because of the non-uniformity of the rotor tip clearance and the incapability of the turbulence model.Although only one converged result for each configuration is achieved in the unsteady simulations,the plots indicate that the calculated performance of tip injection is better than that of the SC,which is consistent with the experiment.

4.Results and discussion

This section is organized as follows.The possible stabilizing effects of tip injection that include the unloading effect,the action on TLV/tip blockage,and the unsteady effect are analyzed first.The effects of injector throat height,circumferential coverage,and injected mass flow rate on compressor stability are then discussed to have a better understanding of the flow mechanisms.Finally,the effects of the combinations of tip injection with casing treatments on compressor stability are presented.

4.1.Stabilizing effect of tip injection

Fig.10 demonstrates the phase-locked contour maps of static pressure on the casing wall for the two test locations(Fig.6)under the operating condition of near stall.The injector(h=2τ,ccp=25%)is choked with an injection mass flow rate of 0.64%of the stalling mass flow of SC.A pressure trough that is connected with a bold line can be detected on the upstream side of suction surface.The pressure trough is the characteristic of TLV,and represents the core of TLV.32The flow measured at test 1 is under the immediate effect of injection compared with that measured at test 2.The locations where the TLV originates are not much different from each other.However,the core of TLV appears inclined to the axial direction under the effect of injection,which indicates that the resistance for the incoming main flow to pass through the blade passage is lower.This observation concludes that tip injection does not shift the position of TLV but makes it easier for the incoming main flow to pass through the blade passage as far as the flow at test 1 is concerned.

The calculated static pressure contours along 99%span at an arbitrary time step are shown in Fig.11 for the same operating condition as that in Fig.10.The zones where the axial component of velocity(Vz)is negative are also presented with brown iso-surface.The design parameters of injector in the simulation are the same as those in the experiment.Passages 2 and 5 approximately correspond to the measurement locations of test 1and test 2 respectively(Fig.10).In passage 2,the rear part of TLV core is inclined to the axial direction whereas the TLV origin is quite the same as that in passage 5,which is in agreement with the experimental result.However,the shape of TLV core is slightly different from that in the experiment.This is because the pressure contours in the experiment are achieved by spreading the time-dependent pressure measured along a line in the circumferential direction,namely,it depends on time and space,whereas the calculated pressure contours are only space-dependent because they are derived from an instant of time in the full-annulus unsteady simulation.The observation above indicates that the simulation captures the main influence of tip injection on the TLV.

The origin of TLV in passage 1 is obviously shifted downstream by the injection compared with those in other passages.Fig.12 depicts the tip leakage streamlines in passages 1 and 2,Vxyzis relative velocity.The front part of tip leakage flow in passage 1 is pushed extremely close to the blade suction surface,whereas the rear part still shows an upstream trend with a large zone of negative axial velocity because the injected jets have not reached the place.As the passage recedes from the injector,the front part of tip leakage flow in passage 2 has recovered,but most of the tip leakage flow can pass through the passage with the minimum blockage(The flow in this passage is treated as an ideal flow condition with a completely removed blockage although small zones of negative axial velocity still exist).As the passage travels further,the TLV in passage 3 is completely recovered(Fig.11).However,tip blockage does not obtain its maximum in passage 3 but increases gradually until meeting a next injector.The observation concludes that the recovery of tip blockage lags behind the recovery of TLV,which is named as hysteresis effect in this paper.The hysteresis effect is primarily responsible for the stability enhancement of tip injection with a partial coverage in the circumferential direction.The hysteresis phenomenon was also pointed out to some extent by Matzgeller et al.25They depicted the velocity flow fields over the rotor tip with particle image velocimetry(PIV)measurements and disclosed that the duration of the effect of injected jets on the whole passage,which corresponded to the blockage diminishment,was much longer than that on the leading edge,which was related with the TLV.

Blade tip loading defined by the pressure difference between suction surface and pressure surface along the axial direction is displayed in Fig.13 as a function of blade passing period(T).Tis equivalent to the time for a blade to pass through one blade pitch.The initial relative locations of the blade and injectors are also shown in the figure.For a better understanding of the figure,the injectors can be assumed to be rotating in an opposite direction of blade rotation,whereas the blade is treated as being stationary.The trace of injected jets can be clearly indentified,which is represented by a long arrow in the figure.The trace indicates that the time needed for the jets to travel all along the chord is approximately 2T.This confirms that tip blockage is not minimized until the injected jets pass through the whole passage as illustrated for passage 2 in Fig.11.The impingement of injected jets on the blade tip induces a loading jump which lasts a very short time.The blade tip is then unloaded under the effect of injected jets,during which the TLV in passage 1 is pushed downstream(Fig.12).The unloading distance along the axial direction is approximately 10%ca,after which the blade tip is reloaded.The reloading distance is much longer than the unloading one in the axial direction.The rear part of blade tip is not much influenced by the injection.In the time direction,the unloading duration is equivalent to the time for a blade passage to pass through an injector.Consequently,the unloading effect of injection is very limited for the test rotor.

The possible stabilizing effects of tip injection have been discussed and can be summarized as follows.Tip blockage is greatly decreased with injected jets after the jets pass through the passage all along the chord,whereas the position of TLV is only shifted downstream locally.The unsteady effect of tip injection manifests as a hysteresis between the recovery of tip blockage and the recovery of TLV,which maintains the accomplishment of blockage diminishment in the circumferential direction.The unloading effect is not very remarkable for the test rotor.

4.2.Test results of tip injection

Stall margin improvement(SMI)is evaluated with the changes of stalling mass flow rate

wheremSC,stallandminjection,stallare the stalling mass flow rates of the SC and tip injection,respectively.Fig.14 demonstrates the SMIs of various injectors as a function of injected mass flow rate(normalized by the stalling mass flow of SC).In each column of the figure,his kept constant whereas ccp varies.The choked injectors are marked with ellipse,while other injectors cannot be choked because of the limited size of bypass pipes which are determined by the space between the inner casing and outer casing in the experiment.For the injectors that are not choked,the injection velocity will diminish at the same injected mass flow rate when the injector size becomes larger.

The test SMIs increase with the improvement of injected mass flow for all the injections.The influence of ccp on SMI is unremarkable whenh=2τ,and is pronounced whilehbecomes larger,i.e.,the effect of ccp depends on the choice ofh.Similarly,the effect ofhalso depends on the choice of ccp,which can be observed with dashed lines in the figure.Similar trends can also be found at other injected mass flow rates.Thus,the interaction effects exist betweenhand ccp.The interaction effects may also exist between other injection parameters,which is probably a reason for the different effects of the same injection parameter on compressor stability in the open literature.

The maximum SMIs are obtained when injectors are choked.All the choked injectors provide almost equivalent SMIs despite their different geometric parameters.The enlargement ofhincreases the influencing domain of injection in the radial direction,and the expansion of ccp increases the influencing time in the circumferential direction.However,both of them cannot improve the SMI further compared with the case with the minimum injector size(h=2τ,ccp=8.3%).The observation indicates that the flow conditions at the rotor tip are similar among all the choked injectors,and therefore tip blockage in a blade passage is also eliminated completely by injected jets for the choked injector with the minimum size(Tip blockage in passage 2 is eliminated completely for the case withh=2τ and ccp=25%(Fig.11)).Thus,the maximum SMI primarily depends on whether the tip blockage in a blade passage is completely removed when injected jets pass through the blade passage all long the chord.Once the tip blockage is completely eliminated,enlarging the injector size for choked injectors will not increase the SMI notably,and the ultimate effect of injection on compressor stability only depends on the hysteresis effect described in the former section.The effectiveness of hysteresis effect is probably determined by injector numbers,which needs further studies in the future.For the injectors that cannot be choked and the injections with a small amount of injected mass flow,the injected jets do not have enough power to eliminate completely the tip blockage in a blade passage,and therefore a larger blockage can be expected when the blade passage leaves the injector further,thereby resulting in a less SMI.

It was found by the authors that the averaged blockage decrement in the injection domain,which was defined as injection efficiency,was an effective guideline to estimate the SMIs of NASA Rotor 37 with discrete tip injection.13Although the blockage in the transonic compressor is generated not only by the TLV but also by shock-induced boundary layer separation,the stability enhancement of compressor also depends on the blockage decrement that is averaged in the injection domain.Consequently,the flow mechanism of stability enhancement in the subsonic compressor can be the same as that in the transonic compressor if the compressor blockage is evaluated in an appropriate way.This observation concludes that the experimental study of tip injection in the subsonic compressor is valuable even for transonic compressors.

4.3.Combination of tip injection and casing treatments

Fig.15 demonstrates the SMIs of tip injection,casing treatments with circumferential grooves or axial slots,and their combined structures(Injection+Grooves,Injection+Slots).The virtual summation of the individual effect of tip injection and each casing treatment is also presented in each figure.Compressor stability is improved further by tip injection for the circumferential grooves.The SMI of the combined structure is nearly equivalent to the virtual summation when the injected mass flow rate is relatively low,whereas the SMI of the combined structure is lower than the virtual summation at higher injected mass flow rates.The axial slots improve compressor stability significantly,and the implementation of tip injection contributes little to the SMI of axial slots because the stall is triggered by lower spans with the axial slots equipped.31Although the axial slots provide a much higher SMI than the circumferential grooves,the penalty on the compressor efficiency is much higher for the axial slots compared with the circumferential grooves.29,31This makes the engineering application of axial slots unacceptable.An alternative is to implement the combined structure of tip injection with circumferential grooves.The circumferential grooves provide a moderate SMI during the whole operation process of a compressor,and the control valves can be opened to implement tip injection under emergencies,such as inlet distortions.

5.Conclusions

(1)The test stall margins increase with the raising injected mass flow rate.The maximum stability improvements are obtained when injectors are choked.Enlarging the injector size has no obvious influence on compressor stability for choked injectors,whereas the increase of circumferential coverage results in the decrease of stability improvement while injector throat height is relatively large for un-choked injectors.The interaction effects exist between the injector throat height and circumferential coverage percentage as far as the influence on compressor stability is concerned.

(2)The position of TLV is only shifted downstream locally by injected jets,whereas tip blockage is greatly reduced after injected jets pass through the passage all along the chord.The accomplishment of blockage diminishment is maintained by the unsteady effect of injection which manifests as a hysteresis between the recovery of tip blockage and the recovery of tip leakage vortex.The maximum effectiveness of tip injection depends on whether the tip blockage in a blade passage can be completely removed when the passage passes through an injector.The tip blockage is completely eliminated by choked injectors regardless of their different sizes,thereby resulting in nearly equivalent stability improvements for all the choked injectors.The rotor tip is only unloaded locally with tip injection,which makes the unloading effect of tip injection unremarkable for the stability enhancement of the test rotor.

(3)Tip injection cannot enhance the stability of the rotor with axial slots significantly,but it can further improve the stability of the rotor with circumferential grooves.The stability improvement of the combined structure of tip injection with circumferential grooves or axial slots is lower than the virtual summation of their individual effect.The combined structure of tip injection with circumferential grooves is an alternative for engineering application.

Acknowledgements

The authors would like to thank the support of the National Natural Science Foundation of China(Nos.51576162 and 51236006).The Doctorate Foundation of Northwestern Polytechnical University(No.CX201422)is also appreciatively acknowledged.

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18 April 2016;revised 22 July 2016;accepted 12 January 2017