Energy and Power Engineering, 2010, 2, 131-136
doi:10.4236/epe.2010.22019 Published Online May 2010 (http://www. SciRP.org/journal/epe)
131
Experimental Investigation of Unsteady Pressure on an
Axial Compressor Rotor Blade Surface
Qingwei Wang, Bo Liu, Xiaorong Xiang, Xiangfeng Bo, Weimin Hou
School of Engine and Energy, Northwestern Polytechnical University, Xi’an, China
Received March 16, 2010; revised April 21, 2010; accepted May 15, 2010
Abstract
The inherent unsteady pressure fluctuations on the rotating blade suction surface of an axial compressor were
experimentally measured by directly mounting five high response miniature pressure transducers into the
rotor blade along a streamline at 50% span respectively. The results show that the unsteady pressure fluctua-
tions of rotor blade surface could be measured successfully by this means. The relations about the period,
altitude of unsteady pressure with rotating speed, the discipline of pressure fluctuation along the streamwise
direction were obtained.
Keywords: Kulite Transducer, Rotating Blade, Unsteady Pressure, Experimental Measurement
1. Introduction
It is well known that the flow fields in turbomachinery is
inherently unsteady because of the aerodynamic blade
row interaction, the viscous flows, secondary flows, tip
clearance flows and so on. The efficiency of turbo-
machine blades and the overall performance of the ma-
chine strongly depend on the unsteady flow. Furthermore,
the unsteady flow substantially influences blade forces
and the high-cycle fatigue of blades. Therefore the re-
search about unsteady pressure on blade surface, espe-
cially the rotor blade surface, is very necessary. During
the past decades, an increasing experimental investiga-
tion of the unsteady pressure fluctuation on rotating
blade surface have been carried out by directly embed-
ding the high response miniature pressure transducers in
rotor blades and got great successes [1-6].
In this paper, five high response miniature pressure
transducers were directly mounted in the rotating blade
of an axial compressor along a streamline at 50% span,
to measure the unsteady pressure fluctuations on the ro-
tor blade suction surface. During experiment, the data
logger was fixed and rotated with the compressor axle-
tree, which could directly sample, amplify and store the
pressure signals.
2. Experimental Facilities
2.1. Axial Compressor Test Rig
The experimental study was conducted in a single stage
axial compressor test rig at the Northwestern Polytech-
nical University, which consisted of 20 rotating blades
and 23 stationary vanes, and work at the rotating speed
from 0 to 3000 r/min. Figure 1 shows the scheme of the
axial compressor. Some the geometric data were: the tip
radius was 290 mm, the hub radius was 174 mm, and the
rotor blade tip clearance size was 1.0 mm. Design pa-
rameters of the compressor: design speed was 3000 r/min,
mass flow was 5.97 kg/s, total pressure rise was 1870 Pa,
and isentropic efficiency was 0.875. The compressor was
driven by an AC motor (15 KW). The area of outlet was
controlled by moving a throttle cone sited downstream of
the test rig.
2.2. Pressure Transducer
Kulite model LQ-062 high response miniature pressure
transducers were used in this study, as shown in Figure 2.
The range of the transducer was ± 5PSI, sensitivity was
18.318 mV/PSI, and response frequency was 150 KHz.
The pressure range of blade surface was about ± 0.5PSI
relative to environmental pressure in this study, and the
maximal response frequency of pressure fluctuations was
low to 1000 Hz, so the transducers could content the ex-
periment requirements. In order to prevent breaking the
transducers during installing or taking down, the trans-
ducers were stuck into the guard sheaths firstly, and then
be mounted into the holes drilled through the rotor blade.
The scheme of guard sheath is shown in Figure 3. In the
front of the guard sheath there was a hole with the di-
ameter 2 mm where the transducer was installed. The

Q. W. WANG ET AL. 132
Figure 1. Axial compressor test rig.
Figure 2. Kulite pressure transducer.
Figure 3. Guard sheath.
transducer sensed the pressure through a 0.8 mm diame-
ter hole at another end of the guard sheath. The sketch of
the transducer distributions is shown in Figure 4. Five
transducers and guard sheaths were mounted at 5%, 20%,
40%, 60%, and 90% of axial chord position along the
streamline at 50% span respectively. Just only five
transducers were used in the investigation, to test and
verify the experiment method and obtain some experi-
ences. Next step, we will dispose more transducers on
both the rotor suction and pressure surface, to obtain
Figure 4. Distribution of pressure transducers.
more detailed unsteady pressure fluctuations on the rotor
blade. The guard sheaths had some influence on flow
fields of pressure surface side passage. However, the
main purpose in this study was to measure the unsteady
pressure on suction surface, and the suction surface con-
figured a flow passage with the pressure surface of adja-
cent blade, so the effects of guard sheaths on the meas-
urement flow fields were ignored.
2.3. Data Acquisition System
Figure 5 shows the data logger used in current study,
which consisted of signal modulator, AD transform
module, timer module, USB module and memorizer. The
data logger had 8 channels and each channel sampled at
the same time. The system sampling frequency was 100
KHz, the amplifying multiple was 186, and the memory
capability was 1 G. The data would be continuously
sampled for 10 minutes after the trigger infrared light
aiming at the data logger. During experiment, the data
logger was fixed and rotated with the drive shaft of the
compressor as shown in Figure 6. The electrical wires of
transducers were routed down along the blade pressure
surface and through the hub to connect with the data
logger, thus the data logger could supply powers for
transducers. In additional, while the rotor was working,
the data logger could directly amplified the electrical
signals of transducers, and transform them to digital data,
and then store to the memorizer. After the compressor
stopping, the digital data stored in the memorizer was
transferred to computer through a USB data line. Using
the data logger could effectively avoid the distortion
during data transmission and increased the data reliabil-
ity.
3. Experimental Results and Discussion
In order to eliminate the electromagnetic interference of
the AC motor, the shell of AC motor was connected with
earth, and the electric wires of pressure transducers were
shielded. In addition, the waves up 3000 Hz were sieved
by software to reduce the effect of transducer noise sig-
nals, and that had little influence on the study. The inter-
ferences were weakened greatly by adopting the means
above, which enhanced the reliability of pressure signals.
The acquired dynamic pressure signals data consisted of
time-average values and fluctuation values. For easy to
analyze the unsteady pressure pulses, the time-average
values was discarded from the gained signals data and

Q. W. WANG ET AL. 133
Figure 5. The data logger.
Figure 6. The fixing of data logger.
the fluctuation values was discussed only. The fluctua-
tion values was defined using the following formula:
ppp 
~
. The p
~
was fluctuation value, the was
dynamic pressure sampled and the
p
p was the
time-average value of the sampled pressure using the
formula 1
0
()/
i
Pn
n
i
P


. Before experiment, the whole
acquisition system had been static calibrated precisely,
that ensure the veracity of the measurement results.
The unsteady pressure measurement covered different
speeds and an extensive range of mass flow. The inves-
tigation focus on the inherent unsteady pressure on rotor
blade suction surface, so there was no any disturb source
in front of the rotor. Figures 7-16 show the time-domain
waves of 5 measurement points under 2000 r/min and
1000 r/min rotating speed while keeping the same outlet
area. At the outlet area, the compressor was both near the
peak value point of total pressure rise under 2000 r/min
and 1000 r/min speed. From the figures it can be ob-
served that the pressure fluctuations of rotor blade suc-
tion surface were periodical obviously. The period of
waveforms at 2000 r/min speed was approximately 0.03
s, and was about 0.06 s at 1000 r/min speed. All these
periods were just the times that the compressor revolving
one circle at each rotating speed, which shows that the
frequency response of inherent unsteady pressure were
related to rotating speed. It is obviously that the altitudes
of unsteady fluctuations declined as the rotating speed
down. The maximal altitude of fluctuation was about 300
Pa at 2500 r/min speed, which was about 85 Pa at 1000
r/min speed. These show that the altitudes of the inherent
pressure fluctuations were also related to the rotating
speed. The periodical waveforms also contain some other
complex pulses, which show the unsteady pressure fluc-
tuations were very complicated. Because of the compli-
cation of unsteady flow fields, as well as the errors lead-
ing by the zero drift of transducers, each of the periodical
waveforms is not absolutely the same with the other
ones.
The reasons of blade surface unsteady pressure were
various. The downstream stator was very far from the
rotor. The axial distance between the rotor and stator was
268% of the rotor blade chord length at 50% span, so the
stator gave little interaction to the flow fields of rotor
blade passage. There were three main reasons of rotor
blade surface unsteady pressure fluctuations. Firstly,
although there were no any disturb source in front of the
test rig, the flow fields of the blade passage was essen-
tially unsteady because of the viscous flows, secondary
flows, tip clearance flows and so on. Secondly, the inlet
fairing of the compressor had a little warp and a bad
concentricity, which resulted that the airflows passing by
the inlet fairing and then flowing into the compressor
were not absolutely uniform. Finally, there was a little
vibration of test rig during experiment. As a result of
fluid inherent characteristics, the inlet fairing and vibra-
tion of compressor, the pressure signals of the rotor blade
surface were unsteady, and all the reasons were relate to
the rotating speed, which were the reasons that the
waveforms periods were approximately the same with
the time compressor revolving one circle.
Figures 17-21 show the fairly typical time-domain
waves and frequency spectral curves of 5 measurement
transducers near peak value point of total pressure rise
under 1500 r/min rotating speed. From the time-domain
waves figures it can be observed that the waveforms pe-
riods of every measurement point were all approximately
0.04 s, which was the time the compressor revolving one
circle. From the frequency spectral curves, it also can be
seen that the dominant frequency was the fundamental
frequencies of 25 Hz and its hormonics, which was the
compressor rotating frequency. All these further prove
that the periods of waveforms were related to rotating
speed. From the frequency spectral curves, it can be

Q. W. WANG ET AL. 134
Figure 7. The time-domain waves of A under 2000 r/min. Figure 8. The time-domain waves of A under 1000 r/min.
Figure 9. The time-domain waves of B under 2000 r/min. Figure 10. The time-domain waves of B under 1000 r/min.
Figure 11. The time-domain waves of C under 2000 r/min.Figure 12. The time-domain waves of C under 1000 r/min.
Figure 13. The time-domain waves of D under 2000 r/min.Figure 14. The time-domain waves of D under 1000 r/min.
Figure 15. The time-domain waves of E under 2000 r/min. Figure 16. The time-domain waves of E under 1000 r/min.
seen that the peak values of the second, third, fourth and
fifth order spectral were all remarkable at A point. From
the relative Mach number contours at 50% span of the
simulation as shown in Figure 22, it can be seen that the
grads of the airflow velocity varied greatly near the blade
leading edge, where the airflow began accelerating. Be-
sides, the transducer at A point was located in the front
of the blade, where the inlet no-uniform airflows had
great influence. As a result of the two main reasons, the
unsteady pressure fluctuation was complex at A point.
Figure 23 represents the peak values of the first order
spectral of every measurement point. It was obvious that
the peak values of the first order spectral gradually in-
creased from A to D point, and then declined at E point.
Because the blade surface pressure gradually increased
from blade leading edge to trailing edge, accordingly the
altitude of pressure fluctuation increased from A to D
point. From Figure 22 it can be seen that the boundary
layers were thick near the blade trailing edge region,
where the velocity and pulse of airflow were weak, so the
boundary layers maybe the major contribution to the alti-
tude of pressure fluctuation declining at E point.
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Q. W. WANG ET AL. 135
Figure 17. The time-domain waves and frequency spectral curves of A.
Figure 18. The time-domain waves and frequency spectral curves of B.
Figure 19. The time-domain waves and frequency spectral curves of C.
Figure 20. The time-domain waves and frequency spectral curves of D.
Figure 21. The time-domain waves and frequency spectral curves of E.
4. Conclusions
The unsteady pressure on rotor blade suction surface
were measured using five miniature pressure transducers,
directly mounted in the rotating blade along a streamline
at 50% span. The results show:
1) The unsteady pressure fluctuations of blade surface
could be measured successfully by this means. The
pressure fluctuations were periodical obviously, and the
periods and the altitudes of unsteady fluctuations were
all related to the compressor rotating speed.
2) The periods of waveforms were the same with the
time of compressor revolving one circle; the fluctuations
were weakened as the rotating speed down.
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Q. W. WANG ET AL. 136
Figure 22. The relative mach number contours at 50%
span. Figure 23. The peak values of the first order spectral.
3) The pressure fluctuations gradually increased from
leading edge to trailing edge. But owing to the boundary
layers near the blade trailing edge region, the fluctuations
declined.
5
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