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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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TMP-11-2 Copyright 2001 by ASME
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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TMP-11-3 Copyright 2001 by ASME
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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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
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[3] Abdelhamid, A.N., Colwill, W.H., and Barrows, J.F.,1979, Experimental Investigation of Unsteady
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[5] Hunziker, R., and Gyarmthy, G., 1994, The OperationStability of a Centrifugal Compressor and Its Dependence
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[6] Seidel, U., Chen, J., Jin, D., and Rautenberg, M., 1991,Experimental Investigation of Rotating Stall Behavior
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[7] Shin, Y.H., Kim, K.H., Bae, M.H., and Kim, J.H., 2000,Compressor Performance with Variation of Diffuser
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