Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-011-0332-0

Injection profile effects on low speed axial compressor stability enhancement†

Hyung-Soo Lim1,*, Hyo-Jo Bae1, Young-Cheon Lim1, Seung-Jin Song1, Shin-Hyoung Kang1 and Soo-Seok Yang2

1Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul, 151-742, Korea 2Propulsion Division, Korea Aerospace Research Institute, Daejeon 305-333, Korea

(Manuscript Received October 17, 2010; Revised March 21, 2011; Accepted March 21, 2011)

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Abstract This study presents stability enhancement of a four-stage low speed axial compressor with different injection profiles. The injection

profiles include one-step injection, multi-step injection, and continuous injection. For the tip injection, eight Coanda-shaped nozzles have been installed at eight equally spaced circumferential locations upstream of the first stage rotor. Two external blowers injected air steadily through the Coanda-shaped nozzles. With tip injection, the compressor operation range has been extended. To analyze stall margin im-provement, spatial Fourier transform (SFT) has been performed. The coefficient of SFT (SFC) is a complex number, containing informa-tion about the magnitude and the phase of SFC. By analyzing the distribution of the magnitude and the phase of SFC, the stall onset point has been verified. Furthermore, the injection flow rate has been changed during injection process to examine the possibility of attaining an additional flow extension. Increasing the injection rate during stabilization can bring about additional operation range extension. These results suggest a new injection method to reduce the total amount of the injection air.

Keywords: Axial compressor; Operation range; Injection; Injection profile; Rotating stall; Mode ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Unsteady aerodynamic phenomena in axial compressors in-clude rotating stall and surge. Depending on the compressor characteristic and system properties, the compression system can go into either surge or rotating stall. The surge is a one- dimensional oscillation which occurs in compressor systems. The flow is axially pulsating, and thus, the compressor opera-tion becomes unstable.

The rotating stall is a two-dimensional phenomenon in which the incoming air is partially blocked at certain circum-ferential locations (called “stall cells”). Once rotating stall occurs, the compressor’s pressure rise is subsequently de-creased. Though the mechanism of rotating stall onset is still a research issue, the cause is generally known to be increased blockage in the blade passage. In stalled condition, the stall cells rotate in the same direction as the rotor at a fraction of the rotor speed.

To prevent a rotating stall or surge, numerous compressor stabilization methods have been investigated. These control methods can be classified into passive control and active con-trol. In passive control, a specially designed device is installed

in the compressor to enhance its stability near the surge line. The passive control device is, in general, simpler than an ac-tive control device. Therefore, the passive control technique is occasionally applied to the commercial gas turbine engines. However, the efficiency of a passively controlled compressor is generally lower than that without such device.

In active control, fast response sensors are installed to ob-serve the compressor stability in real time. If an undesirable signal is detected, a controller will be activated to stabilize the compressor. In active control, a sensor, controller, and actua-tor are required.

Active control of rotating stall is more complex than passive control. However, compressor efficiency and pressure rise are generally higher with active control.

One active control method is tip injection. External air with high axial momentum is injected upstream of a blade row to compensate for the low momentum flow in the endwall boundary layer. Also, the incidence angle of inlet flow can be reduced by injection. Consequently, the axial compressor can be operated at a lower flow rate with injection. Day [1], Weigl et al. [2] and Suder et al. [3], demonstrated enhancement of compressor stability with tip injection. In these investigations, the injection air mass flow rate varied between 1 and 6% of the design point or stall onset mass flow rate.

In many cases, highly pressurized air has been used for in-jection. Thus, the injection air is “expensive”, and reducing

† This paper was recommended for publication in revised form by Associate EditorByeong Rog Shin

*Corresponding author. Tel.: +82 2 880 8047, Fax.: +82 2 889 6205 E-mail address: limbo999@snu.ac.kr

© KSME & Springer 2011

1502 H.-S. Lim et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507

the amount of injection air is important. However, the possi-bility of reducing the amount of injection air in steady injec-tion has not been investigated. Therefore, this paper presents experimental research which shows the potential to reduce the amount of injection air.

2. Test facility and measurements

2.1 Low speed axial compressor

A schematic of the four-stage low speed axial compressor at Seoul National University (SNU compressor) is shown in Fig. 1. The compressor is set up vertically, drawing in ambient air from the top. A mesh screen has been installed at the intake. The volume flow rate is controlled by a throttle valve.

The compressor consists of 53 inlet guide vanes (IGV), 54 rotor blades, 74 stator blades. The compressor can be rotated up to 1200 rpm by a 55-kW DC motor, and has a design speed of 800 rpm. Details of the axial compressor are summarized in Table 1.

2.2 Injection through Coanda nozzles

To minimize mainstream flow disturbance by the nozzle

body, the so called Coanda nozzle has been designed (Fig. 2 (a)). The benefits of the Coanda nozzle are as follows. First, minimum loss is required to change the flow direction. Second, the position of the nozzle exit is the same as the casing inner surface, so the nozzle body does not disturb the incoming flow. Strazisar et al. [4] and Weigl et al. [2] first introduced the Coanda nozzle. In the present setup, eight Coanda nozzles are circumferentially spaced in 45° increments and positioned at 6% true chord upstream from the leading edge of the first stage rotor. The injectors are connected to two blowers which supply injection air as shown in Fig. 2(b).

2.3 Unsteady pressure measurement and tip injection control

In the SNU compressor, a long length scale disturbance was detected before stall onset. Thus, three fast response pressure transducers (Kulite XCQ-062) were installed per stage to measure the progress of rotating stall. For data acquisition, a 16-bit resolution A/D board (NI 6251) was used. To measure the nozzle flow rate, a differential pressure sensor (MKS) and a 16-channel pressure scanner (PSI) were used. The measure-ment uncertainties of Φ and Ψ are ± 0.35 percent and ± 0.69 percent of design point, respectively, and the uncertainty of nozzle flow coefficient are ± 2.8 percent for 95% confidence interval.

The rotation speed of the blower is controlled with an in-verter. To balance the rate of the injection flow in each valve, a gate valve was installed for each nozzle. By controlling the eight gate valves individually, flow rates could be balanced to

Table 1. Parameters of the SNU compressor.

Number of stage 4

Number of inlet guide vane/rotor/stator blades 53 / 54 / 74

System height (m) 3.89

Tip radius (m) 0.5

Hub/Tip ratio 0.85

Aspect ratio 1.2

Chord_rotor (mm) 62.5

Stagger_rotor (°) 51

DC motor (kW) 55

Number of nozzle 8

External blower (kW) 7.5

Fig. 1. Schematic of SNU compressor.

(a)

(b)

Fig. 2. Schematic of injection system (a) Coanda nozzle; (b) Injection pipe layout.

H.-S. Lim et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507 1503

within +/- 3% about the mean injection flow rate. The eight Coanda nozzles were calibrated by the orifice

flow meter. Each nozzle has two pressure ports on the nozzle throat and pipe; thus, the relation between the flow rate and the differential pressure between the nozzle throat and pipe could be acquired. From this relation, the nozzle discharge coefficient was calculated for each nozzle.

3. Results

3.1 Definition of stall onset

To verify the stall onset point, the SFT was performed, and the variation of the SFC magnitude and the SFC phase was investigated; an example is shown in Fig. 3. In the case of the phase, its slope becomes constant and phase increases linearly after ①. In this figure, the inclination of the phase represents the mode velocity ( emodω ) at about 34% of the rotor speed. Moreover, the inclination further increases from ② and this increase represents the velocity of the rotating stall. In the SNU compressor, the velocity of the rotating stall is 45~50% of the rotor speed.

The magnitude does not change until ①. However, beyond ①, the magnitude increases because of the occurrence of the modal instability. Moreover, the magnitude increases abruptly from ② due to rotating stall. In this research, the stall onset is defined as the dimensionless performance point (Φ and Ψ) at the moment of mode occurrence. Whenever the SNU com-pressor is run near the surge line, the mode (a long length scale disturbance) is first detected before going into rotating stall and stall onset point can be acquired.

3.2 Selection of injection stage

To select the injection stage, spatial Fourier transform (SFT) has been performed for each stage. The coefficient of SFT (SFC) may be expressed as,

∑=

⋅=M

nnnk iktP

Mta

1

)exp(),(1)( ξξδ . (1)

The SFC has magnitude and phase, and it is a function of time. In Fig. 4, the variation of the static pressure signal, the SFC magnitude and the SFC phase are illustrated when the compressor is operated near stall. When the SNU compressor stalls, one rotating stall cell is detected. Therefore, three fast response pressure transducers have been applied at each stage. In Fig. 4(a), a long length scale (on the order of the compres-sor radius) disturbance is observed from ① at the 1st stage. However, it is difficult to define the stall onset stage from this figure.

Figs. 4(b) and 4(c) represent the magnitude and the phase of each stage, respectively. A gradual increase in magnitude is detected at the first stage from ①, signifying instability. Fur-thermore, in Fig. 4(c), the slope of phase increases linearly at the first stage from ①. It represents the steady rotational speed of the disturbance mode. Based on this observation, the tip injection nozzles were installed at the 1st stage.

3.3 Stability enhancement by injection

To investigate the effect of shaft speed on operation range extension by injection, the injection test was performed at N = 394, 453, 541 and 642. As the throttle valve was closed slowly, the flow rate and the pressure ratio were measured continu-ously. The rate of the injected air was 5.1% of Φ = 0.35, which is the compressor flow rate when the compressor per-formance slope becomes zero. When the compressor runs near the surge line, the injection is started.

Fig. 5(a) represents the performance of the first stage for four different shaft speeds. When the compressor was oper-ated beyond the surge line, the performance was abruptly de-graded due to the occurrence of rotating stall. In SNU com-pressor, the mode was detected as the precursor of rotating stall. As shown in Fig. 5(a), the operation range was extended and the pressure rise was increased by injection at all speeds.

Weigl et al. [2], Suder et al. [3] and R.D’ Andrea et al. [5] installed injection nozzles just before the first stage rotor and analyzed the compressor performance with one-stage com-pressor. As shown in Fig. 5(a), the operation range and the pressure ratio were increased with injection in their research.

In Fig. 5(b), the four-stage performances and the stall onset points are illustrated for N = 394, 453, 541 and 642. The white and black circles represent the stall onset points at no injection and injection condition, respectively, and the group of circle was acquired from several repeat tests. The surge line was moved to a lower flow rate by injection and this change repre-sents the extension of the operation range.

However, there was a difference in performance pattern be-tween the first stage and the four-stage during injection. With injection, the pressure rise was increased at the first stage but that of the four-stage was decreased. In the SNU compressor, there is a downstream stage in which the pressure rise de-creased with injection. The pressure drop of a specific stage causes the overall performance decrease. For this reason, to stabilize the multistage compressor, injection nozzles were

Φ

SFC Phase

SFC Magnitude

Stall onset

emodω

stallω

Ψ

① ②

Fig. 3. The definition of the stall onset point.

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installed at several stages. Strazisar et al. [4] investigated the compressor stability en-

hancement with six-stage axial compressor. High pressure air was injected at 1st, 3rd and 5th stages simultaneously. Hiller et al. [6] performed an injection test with eight-stage compressor, and this compressor used three variable guide vanes at front three stages.

Therefore, in the case of first stage injection with fixed mul-tistage blade angle, the first stage pressure rise is increased (Fig. 5(a)). However, the overall pressure rise was decreased as shown in Fig. 5(b).

3.4 Selection of shaft speed

In Fig. 6, the relation between Re and operation range ex-

tension is analyzed. Extension of the operation range by tip injection can be quantified as:

100)

))×

Φ

Φ−Φ=∆Φ

injNostall

injstallinjNostall . (2)

As shown in Fig. 6, the operation range extension has a rela-tion with Re. Even though, the same percentage of nozzle flow was injected about different shaft speed as %1.5=nozzlem , and the operation range extension was higher at low Re than high Re. For example, ∆Φ≒7 at Re = 51088.0 × , but ∆Φ≒2 at Re = 51043.1 × . This tendency is similar to the results of Weigl et al. [2] and Nie et al. [7].

The research of Cheng et al. [8], Lee et al. [9] and Day et al.

(a) (b) (c) Fig. 4. Analysis of stall onset stage (a) Instant static pressure signal; (b) SFC magnitude for the 1st mode; (c) SFC phase for the 1st mode.

No injectionNo injectionInjectionInjection

Surge lineSurge lineN=642

N=541

N=453N=394

N=642

N=541

N=453N=394

Stall onset pointStall onset point

Surge lineSurge line

No injectionNo injectionInjectionInjection

(a) (b) Fig. 5. The trajectory of compressor performance with/without injection at N = 394, 453, 541, 642 ( 5.1%nozzlem = ) (a) 1st stage performance; (b) 4-stage performance with stall onset points.

H.-S. Lim et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507 1505

[10] mentioned that the stall onset mechanism and its charac-teristic are not affected by Re, if Re is greater than 5100.1 × . On the basis of these results, shaft speed was selected as N = 453 for injection flow rate control.

3.5 Injection flow rate control

In this section, the injection flow rate has been changed dur-ing injection process to examine the possibility of attaining additional stall margin improvement.

In Fig. 7(a), the compressor performance curves with and without injection rate control are illustrated. The injection flow rate control was performed by 1-step injection, multi-step injection and continuous injection.

The one-step injection is shown in Fig. 7(b). The injection flow rate is promptly increased from zero to target flow rate near the surge line. When the nozzle starts the operation, the injection flow rate gets to the target flow rate, and it is main-tained until stall inception. During the injection, the nozzle flow is the steady injection. In this research, the injection flow rate was maintained with 5.1% from Φ = 0.343 to stall onset.

Fig. 7(c) illustrates the multi-step injection. The injection flow rate is increased from zero to the target flow rate in sev-eral steps. The operator can choose the number of steps and the nozzle flow rate for each step. During the injection, when the compressor operation point reaches a certain limit, the injection flow rate is promptly increased as 1-step injection. However, the amount of the increased injection flow rate is just a certain portion of the target flow rate. The increase of injection flow rate with a sequence of steps is repeated until the injection flow rate reaches the target flow rate.

The continuous injection is shown in Fig. 7(d). The injec-tion flow rate is continuously increased from zero to the target value. The increasing rate of injection flow about Φ can be chosen by the compressor characteristic.

As shown in Fig. 7(a), even though the injection profiles are different, the location of stall onset is coincident at Φ = 0.326.

The compressor can be stabilized and the injection flow rate can be saved with multi-step injection and continuous injec-

tion in extending the operation range. To compare the amount of injection flow rate among 1-step,

multi-step and continuous injection, the following assump-tions are made: the coincidence of the stall onset point, and the coincidence of the velocity of the operation point trajectory from Φ = 0.343 to 0.326.

As shown in Fig. 7(c) and (d), multi-step injection and con-tinuous injection can save the injection flow rate to 10.6% and 18.6%, respectively, relative to the one-step injection. In the case of continuous injection, this reduced injection flow rate is equivalent to one-step injection with nozzlem = 4.15%. Thus, if the injection flow rate is controlled properly (multi-step injection or continuous), the operation range can be extended with less injection flow than one-step injection.

ΔΦ

Re

N=394

N=453

N=541

N=642

Fig. 6. The variations of the operation range extension by shaft speed( 5.1%nozzlem = ).

Ψ

No injection1-step injectionMulti-step injectionConti. injection

Φ

Operation range Extension (ΔΦ )

Φ : 0.343Φ : 0.326

(a)

Φ

ΔΦ

(b)

Φ

ΔΦ Injection flow 10.6 % reduced than 1-step injection

(c)

Φ

ΔΦInjection flow 18.6 % reduced than 1-step injection

(d) Fig. 7. Extension of operation range with injection flow rate control (a) Trajectory of 4-stage performance; (b) Injection flow at 1-step injec-tion; (c) Injection flow at multi-step injection; (d) Injection flow at continuous injection.

1506 H.-S. Lim et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507

Fig. 8 illustrates the variation of compressor stability for multi-step injection condition. Even in injection condition, the mode (a long length scale disturbance) occurred from ①, and it started to rotate in revolution direction. When the injection flow rate was increased from ② to ③, the mode was weak-ened and the compressor was stabilized (SFC magnitude re-duced). Eventually, an instability recurred at ④, and it initi-ated the rotating stall because no additional injection flow rate was performed.

Thus, increasing the injection rate during injection can bring about additional operation range extension by weakening the compressor unsteadiness like mode.

4. Conclusions

The enhancement of the compressor stability with air injec-tion has been investigated experimentally, and the results can be summarized as follows:

(1) To increase the operation range of the SNU compressor, eight Coanda-shaped nozzles have been designed and installed. The operation range extension was verified at N = 394, 453, 541 and 642 with 5.1% steady injection flow rate.

(2) The reduction of the injection flow rate by injection rate control during steady injection has been investigated. The executed injection profiles were one-step injection, multi-step injection and continuous injection. The compressor operation range can be extended continuously by multi-step injection and continuous injection. The injection air could be saved up to 18.6% with continuous injection.

(3) Under the multi-step injection, the compressor stability variation was investigated by SFT analysis. It was verified that increasing the injection flow rate during the injection weak-ened the growth and propagation of compressor unsteadiness.

Acknowledgment

This study has been financially supported by KATRA08_ A00133_ki 4 Program of the Aerospace Components Tech-nology Development Project of the Ministry of Knowledge Economy. Also, support from the Institute of Advanced Ma-chinery and Design and the BK21 Program of Seoul National University is gratefully acknowledged.

Nomenclature

)(ta : Coefficient of spatial fourier transform xC : Axial flow velocity, m/s

k : Number of modes correctedm : Corrected mass flow (= δθ /m ) nozzlem : Mass flow rate of nozzle, %

M : Number of sensors in the annulus N : Corrected shaft speed (= θ/rpmN )

sP : Static pressure, Pa 0tP : Inlet total pressure, Pa

Re : Reynolds number (= ν/ChordU ⋅ ) t : Time, s U : Rotor midspan speed, m/s

Pδ : Pressure perturbation, Pa π : Total to total pressure ratio Φ : Flow coefficient (= nozzlecompressor Φ+Φ ) Ψ : Pressure coefficient (= )2/1/()(

20 UPP ts ρ− )

ξ : Circumferential sensor location, deg

Subscripts

01 : Inlet to 1st stage 04 : Inlet to 4th stage

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SFC magnitude

SFC phase

nozzlem

Ψ

emodω

emodω

④①

Φ

②③

Fig. 8. The effect of injection flow rate control (multi-step injection)on compressor stabilization.

H.-S. Lim et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1501~1507 1507

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Hyung-Soo Lim received a MS in Me-chanical engineering from Seoul Na-tional University in 2004. He has fo-cused on the research of compressor stability issues including rotating stall inception and stability control. He cur-rently works as a PhD course student in Seoul National University.

Seung-Jin Song is a professor of Mechanical and Aerospace Engineering at Seoul National University. His current research interests include aerodynamics and fluid-structure interactions in turbomachinery, analysis of propul-sion/power generation systems, and related areas of fluid mechanics and

renewable energy.

Shin-Hyoung Kang is a professor of Mechanical and Aerospace Engineering at Seoul National University. He has over 40 years of experience in the field of turbomachinery in both industry and academia, previously serving as the Chairman of the KSME and SAREK. Currently his research activities are

directed toward the optimization of turbomachinery perform-ance by using both CFD and experimental methods.