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TMP-11-1 Copyright 2001 by ASME

Proceedings of ICFDP7:Seventh International Congress of Fluid Dynamics and Propulsion

December 19-21, 2001, Sharm El-sheik, EGYPT

TMP-11

MEASUREMENTS OF UNSTEADY FLOW OF CENTRIFUGAL COMPRESSOR WITHVANED DIFFUSER

Kwang Ho KimThermal/Flow Control Research Center

Korea Institute of Science and Technology

You Hwan ShinThermal/Flow Control Research Center

Korea Institute of Science and Technology

ABSTRACTIn this study, the instability of a centrifugal compressor

with vaned diffuser has been investigated. Unsteady pressure

fluctuation with the variation of flow rate and impeller rotating

speed at diffuser inlet and exit were measured by using high

frequency pressure transducers.

From the spectrum analysis of measured signal, a transient

zone was observed at a certain flow rate where two different

frequency peaks coexisted, whereas there was only onedominant frequency component at other flow rates. And the

result of Wavelet transform analysis also showed such feature

which is distinguished from that of other flow rates.

In this zone, the compressor performance was steeply

deteriorated, propagation speed of stall cell was rapidly

decreased and pressure fluctuation amplitude was quickly

increased.

INTRODUCTIONThe performance of a compressor is typically characterized

with its pressure ratio, flow rate, and efficiency. Stability is also

considered as one of the important performance factor. Rotating

stall and surge in compressor, which are unstable and

undesirable phenomena for the operation, cause the fluctuation

of pressure and velocity and result in vibration and mechanical

damages. Especially, in case of turbomachines that their

operating conditions frequently change, turbochargers and gas

turbine engines for aircraft, the extension of the stable operating

range for a turbo-compressor is more important factor.

Most previous studies on rotating stall in centrifugal

compressor have been dependent upon experiment and

accomplished in vaneless diffusers. Fringe et al. [1], Shin et al.

[2] investigated the characteristics of rotating stall with the

number of stall cell, the propagation speed and the amplitude.

Abdelhamid et al. [3] reported the behavior of pressure

fluctuations. Shin et al. [4] investigated two different

mechanisms that were recognized for the extension of the

reverse flow with flow rate.

However, for the industrial compressors which demand

high pressure ratio and high efficiency, vaned diffusers are often

used. Hunziker et al. [5], Seidel et al. [6] investigated the

characteristics of stall with variation of vane angle. Hunziker et

al. found that the diffuser channel dominated the instability of

compressor. Seidel et al. examined that the number and thepropagation speed of stall cell changed a lot with rotating speed

and flow rate.

In this study flow measurements have been conducted at

rotating stall in a centrifugal compressor with vaned diffuser.

Particularly the transient behavior, which occurs with the

reducing flow rates from the stable to unstable operating range,

is focused.

NOMENCLATURECp : static pressure rise coefficient

p : static pressure

Q : flow rate

: flow coefficientr : radius

D : diameter

b : diffuser axial width

: diffuser peripheral angle: diffuser vane angle

Subscripts

1 : impeller inlet

2 : impeller exit or diffuser inlet

3 : diffuser exit

4 : discharge duct


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Fig. 1 Centrifugal compressor test rig

EXPERIMENTAL FACILITY AND INSTRUMENTATIONA schematic view of the test section of the low-speed

centrifugal compressor is shown in Fig. 1. This compressor has

a single stage with an unshrouded radial impeller and a parallelwall vaned diffuser. The dimensions of the impeller and diffuser

are summarized in Table 1. The test compressor was driven by a

15 kW electric motor with a frequency inverter. The flow rate

was controlled by the throttle valve at the end of the discharge

duct and measured by using the orifice plate in the discharge

duct.

Total pressure, temperature and wall static pressure were

measured at the inlet plenum, impeller inlet and exit, diffuser

exit, and discharge duct. A pressure scanner (PSI system 8400)

for the pressure measurement and K-type thermocouples for the

temperature measurement were used. For investigating

characteristics of rotating stall, unsteady fluctuations of pressure

at diffuser inlet and exit were measured by using 4 high

frequency pressure transducers (Kulite XCS-062). 2 high

frequency pressure transducers are mounted on the diffuser inlet

(r/r2=1.02) wall with the interval of 21 in the circumferentialdirection (=180 and 201). The others are mounted on thediffuser exit (r/r2=1.56) wall with the circumferential interval of

90 (=90 and 180). A low-pass filter (Krohn-Hite 3384)filtered the signal from 4 high frequency pressure transducers

and the signal was processed with a waveform analyzer

(Analogic D6500E). The low-pass filter has removed the signal

higher than 200Hz and sampling period is 0.9 milliseconds. For

analyzing the signal, correlation function, FFT, and wavelet

transform were used.

RESULTS AND DISCUSSION

Compressor PerformanceThe static pressure rise was measured to obtain the

performance of impeller, diffuser, and total compressor. The

flow coefficient and the static pressure rise coefficient are

defined by

222 UbD

Q

= (1)

Table 1 Geometry of impeller and diffuser (mm)Impeller exit diameter 418

Impeller hub diameter 110

Impeller tip diameter 240

Number of impeller blade 17 (no splitter)Impeller exit blade angle 90 (radial type)Diffuser inlet diameter 420

Diffuser outlet diameter 720

Diffuser inlet width 19.4

Diffuser outlet width 19.4

Number of vanes 16

Vane type Plate (straight)

Vane stagger angle 23Vane inlet radius ratio (r3/r2) 1.09

Vane length 120

Vane solidity 1.33

Vane thickness 2

flow coefficient ()

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

stat

icpressurerisecoefficient(Cp

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1800 rpm2400

3000

3560

N

total

impeller

diffuser

rotating stallonset

Fig. 2 Centrifugal compressor test rig

222/1 U

pCp

=

(2)

Fig. 2 shows the performance characteristics of the tested

compressor with variation of impeller rotating speed. In this

case, the diffuser vane angle was maintained at 23. This figurealso shows the similarity of a turbo-machinery, because the

change of the pressure coefficient in the impeller of this tested

compressor is independent of the rotating speed. It can be found

that the drop of the total compressor performance in the stable

operating range is dominated by the static pressure rise in the

diffuser. It means that the flow from the impeller is blocked to

the suction side of the diffuser vane.

As the flow rate decreases, the static pressure coefficient

increases and then drops at about =0.33. The rotating stalloccurs at this flow rate and the condition of the compressor

turns into unstable condition. The stall phenomenon was


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0 20 40 60 80 100

amplitude(Pa)

0

300

600

900

1200

0.32

0.29

0.27

0.15

0.09

0.04

()N = 3000 rpm

Pamp, max

= 704 Pa ( = 0.15, 16.6 Hz )

Fig. 3 Pressure fluctuation amplitude spectra with

flow rate at impeller exit (r/r2=1.02, =180)

flow coefficient ()

0.0 0.1 0.2 0.3 0.4

propagationspeed(frs/fi)

0.3

0.4

0.5

1800 rpm

2400

3000

3560

N

Fig. 4 Stall cell propagation speed with flow rate andimpeller rotating speed

observed intermittently at the flow ranges =0.330.31. As theflow rate decreases, the performance characteristics of the

impeller in the unstable operating range increases continuously

but that of the diffuser decreases. Therefore, it is believed that

the total compressor performance in the unstable operatingrange is also dominated by the characteristics of the diffuser.

Characteristics of Rotating StallFig. 3 shows the spectrum of the measured pressure

fluctuation amplitude with impeller rotating speed and flow rate

at impeller exit (r/r2=1.02). The pressure fluctuation spectrum

before and after the onset of rotating stall was compared. The

stall frequency was not observed at the flow rate coefficient of

=0.35. From the experiment, the number of rotating stall cellwas one at all speed and flow rate. The cell rotates with the

direction of the impeller rotation. The specific frequency shown

in Fig. 3 is the propagation speed of the rotating stall cell. Onedominant frequency peak and harmonics were observed at the

entire flow ranges except =0.29 where two different frequencypeaks existed simultaneously.

Fig. 4 shows the propagation speed of the stall cell which is

normalized by the impeller rotating speed. It was found that the

propagation speed of the stall cell is independent of the impeller

rotating speed at the same flow rate. As the flow rate decreased

to =0.15, the propagation speed of the stall cell also decreased.With further throttling from the point, the stall cell speed

increased gradually. Specially, the propagation speed suddenly

drops at the ranges =0.30.25, where the static pressure rise

frequency (Hz)

0 10 20 30 40 50 60 70 80 90 100

amplitude

(Pa)

0

100

200

300

frs = 21.9721 Hz

2500Pa

frequency (Hz)

0 10 20 30 40 50 60 70 80 90 100

amplitude(Pa)

0

100

200

300

frs = 21.9721 Hz

18.1744 Hz

2500Pa

frequency (Hz)

0 10 20 30 40 50 60 70 80 90 100

amplitude(Pa)

0

100

200

300

400

500

600

700

800

frs = 18.1744 Hz

2500Pa

Fig. 5 Pressure fluctuations and amplitude spectrawith flow rate at impeller exit (N=3000 rpm)

coefficient suddenly drops and two specific frequency peaks

appeared.

Spectrum AnalysisFig. 5 shows the pressure fluctuation and amplitude

spectrum which measured from the impeller exit with flow

rates. One dominant frequency of 21.9721Hz was observed at

=0.32 but there was two different frequency peaks when theflow rate decreased to =0.29. The frequency of 21.9721Hz isidentical with specific frequency element and pressure wave of

=0.32. When the flow rate reduces further to =0.27, only onesignal of specific frequency, 18.1744Hz is observed. This

frequency is identical with the frequency which is observed

from =0.29.

Wavelet AnalysisThe Wavelet transform is useful to get local information on

disturbances.[11]

In this study, the size, interval and transient behavior of

stall cells were examined by the Wavelet transform.[10]

(a) =0.32

b) =0.29

(c) =0.27


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The Wavelet transform of pressure signalp(t) is given by

= dttpTa

baW )()(1

),( (4)

where )exp()2sin()(2

TTT = is Morlet wavelet function,

T = ( t - b )/ a , a is scaling parameter and b is translation.

The result of Wavelet transform of the diffuser inlet static

pressure signal is shown in Fig. 6.

In Fig. 6(a), the maximum values ofW(a,b) are located at

a/t 810 at equal intervals ofb/t 50 (where t, 0.0009secis 1/22 rotor rotation, 50Hz). This means that the size of stall

cell is on the order of 67 blade spacings in the circumferentialdirection since the rotor has 17 blades and they appear at

intervals of about 2.3 of a rotor rotation. This value (50 Hz / 2.3

= 21.74 Hz) is similar to that of spectrum Analysis (frs = 21.97

Hz).

At the flow rate =0.29 where two different frequencypeaks coexisted from the spectrum analysis, the Waveletanalysis shows totally different feature in contrast with that of

other flow rates. That is, the size and the intervals are irregular

and this irregular pattern explains the behavior of stall cell in

transient region.

Fig. 6(c) shows slight increase ofa/t(8.89.2) and b/t(5060) as compared against the flow rate (a) =0.32. Thismeans that the size of stall cell increases and propagation speed

decreases after transient region.

In the generally accepted idea, the increase of cell size is

linked with decrease of propagating speed. In this study, The

idea was confirmed by Wavelet analysis.

CONCLUSIONIn order to investigate the unstable characteristics in a

centrifugal compressor with vaned diffuser (=23), the rotatingstall signal was measured by using high frequency pressure

transducers. From the analysis of the measurements, the several

results are obtained as follows.A transient zone was observed at a flow rate =0.29 where

two different frequency peaks existed. In this zone, the pressure

rise of the compressor suddenly drops, which is dominated by a

frequency shift, from 21.9721Hz to 18.1744Hz in this case and

the occurrence of double peaks. Also, the propagation speed of

rotating stall cell in this zone rapidly decreased and pressure

fluctuation amplitude was quickly increased.

The Wavelet analysis showed the unique feature which is

distinguished from that of other flow rates. The size and interval

were irregular, explaining the transient behavior of stall cell.

From the analyzed results of FFT and Wavelet Transform,

it was confirmed that two different transform are in substantialagreement for understanding the characteristics of stall cell.

ACKNOWLEDGMENTSThis study was accomplished with the support of the

research program, the Machinery Design Technology

Enhancement, from the Ministry of Science and Technology,

Korea. The authors would like to thank for the support.

0 250 500 750 10004

6

8

10

12

14

a/ t

b/ t 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000

4

6

8

10

12

14

a/t

b/t

0 250 500 750 10004

6

8

10

12

14

a/t

b/ t

0 250 500 750 10004

6

8

10

12

14

a/t

b/t

Fig. 6 Wavelet transform with flow rate at diffuser inlet (N=3000 rpm)

(d) =0.29 fourfold enlarged(c) =0.27

b) =0.29a) =0.32


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REFERENCES[1] Frigne, P., and Van Den Braembussche, R., 1984,

Distinction Between Different Types of Impeller and

Diffuser Rotating Stall in a Centrifugal Compressor with

Vaneless Diffuser, J. of Eng. for Gas Turbines andPower, Vol. 106, pp. 469-474.

[2] Shin, Y.H., Kim, K.H., and Son, B.J., 1998, AnExperimental Study on Rotating Stall in Vaneless

Diffuser of a Centrifugal Compressor, KSME, Series B,

Vol. 22, No. 2, pp. 153-161.

[3] Abdelhamid, A.N., Colwill, W.H., and Barrows, J.F.,1979, Experimental Investigation of Unsteady

Phenomena in Vaneless Radial Diffusers, J. of Eng. for

Power, Vol. 101, pp. 53-60.

[4] Shin, Y.H., Kim, K.H., and Son, B.J. 1998, AnExperimental Study on the Development of a Reverse

Flow Zone in a Vaneless Diffuser, JSME Int. J., Series

B, Vol. 41, No. 3, pp. 546-555.

[5] Hunziker, R., and Gyarmthy, G., 1994, The OperationStability of a Centrifugal Compressor and Its Dependence

on the Characteristics of the Subcomponents, J. of

Turbomachinery, Vol. 116, pp. 250-259.

[6] Seidel, U., Chen, J., Jin, D., and Rautenberg, M., 1991,Experimental Investigation of Rotating Stall Behavior

Influenced by Varying Design and Operation Parameters

of Centrifugal Compressors, 91-YOKOHAMA-IGTC-

93, pp. I-89 I-98.

[7] Shin, Y.H., Kim, K.H., Bae, M.H., and Kim, J.H., 2000,Compressor Performance with Variation of Diffuser

Vane Angle, J. of KFMA, Vol.3, No. 2, pp. 36-43.

[8] Shin, Y.H., Kim, K.H., Choi, H.C., and Jeon, J.H., 2000,Unsteady Pressure Fluctuations in Vaned Diffuser of a

Centrifugal Compressor, J. of KSAS, Vol. 28, No. 8, pp.

33-38.

[9] Kang, J.S., and Kang, S.H., 2000, Scale Analysis ofCentrifugal Compressor Surge Using Wavelet

Transform, The First National Congress on Fluids

Engineering, Korea, pp. 575-578.

[10] Inoue, M., Kuroumaru, M., Tanino, T., Yoshida, S., andFurukawa, M., 2001, Comparative Studies on Short and

Long Length-Scale Stall Cell Propagating in an Axial

Compressor Rotor, J. of Turbomachinery, Vol. 123, pp.

24-32.

[11] Farge, M., 1992, Wavelet Transforms and TheirApplications to Turbulence, Annu. Rev. Fluid Mech.,

Vol. 24, pp. 395-457.

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