Compressor operation at low air flows for a given speed is limited by unstable flow conditions, commonly called surge. An investigation of surge in centrifugal compressors (reference 1) showed that the pulsation of pressures and velocities occurred when the slope of the compressor characteristic curve was positive and that the magnitude and frequency, as well as the incidence of surge, depended on the capacity and resistance of the total system. Although the theory presented in reference 1 is applicable to axial-floe compressors, little experimental information is available on the surge characteristics of the individual stages of axial-flow compressors, or on the variation of the surge characteristics with operating conditions. During the investigation to determine the performance of the X24C-2 compressor (references 2 and 3), instrumentation was added to study the surge characteristics and to determine the effect of speed and inlet pressure on the frequency, amplitude, and phase relation of the pressure pulsations behind each stage

Buckner, Howard A., Jr.

Downing, Richard M.

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RESEARCH MOCRAIMUM
for the
Bureau of Aeronautics, Department of the Navy
INVESTIGATION OF Z24C®2 10-STAGE AXIAL-FLOW CONFRESSOR
III - SURGE CHARAC=STICS
By Howard A. Buckner, Jr. and Richard M. Downing
INTRODUCTION
Compressor operation at low air flows for a given speed is
limited by unstable flow conditions, commonly called surge. An
investigation of surge in centrifugal compressors (reference 1)
showed that the pulsation of pressures and velocities occurred when
the slope of the compressor characteristic curve was positive and
1, A
NACA
 RM No. SE8H06	 ^ CONFIDKNTIAL	 2
that the magnitude and frequency, as well as the incidence of surge,
depended on the capacity and resistance of the total system. Although
the theory presented in reference 1 is applicable to axial-floe com-
pressors, little experimental information is available on the surge
characteristics of the individual stages of axial-flow compressors,
or on the variation of the surge characteristics with operating
conditions.
During the investigation to determine the performance of the
g24C-2 compressor (references 2 and 3), instrumentation was added to
study the surge characteristics and to determine the effect of speed
and inlet pressure on the frequency, amplitude, and phase relation
of the pressure pulsations behind each stage.
APPAL UM AND PROCEDURE
Compressor. - The 124C-2 compressor, described in reference 2,
was designed for an air weight flow of 54.6 pounds per second and a
pressure ratio of 4 at 12,000 rpm with sea-level inlet conditions.
The blading consisted of a row of inlet guide vanes; ten rows of
rotor blades, nine rows of stator blades, and two rows of outlet
guide vanes. The maximum diameter was 238 inches; the length,
448 inches; and the constant rotor-blade tip diameter, 18.91 inches.
The hub-tip ratio at the inlet and first two stages was approximately
0.5 and gradually increased through succeeding stages to about 0.74
at the exit. The clearance between the rotor-blade tips and the
casing was 0.045 inch.
Setup. The compressor was driven by a 9000-horsepawer
variable-frequency induction motor through a gearbox with a step-up
ratio of 8.974. A floating-spline coupling connected the gearbox
to the drive shaft, which was bolted to the compressor rotor.
Air entered the compressor through a submerged adjustable
orifice in the inlet duct, a butterfly valve, and a depression tank
6 feet in diameter and 10 feet in length. A series of three screens
was fitted into the midsection of the tank to remove any foreign
particles and equalize the air distribution. A wooden nozzle
between the depression tank and the compressor inlet provided smooth
air entry into the compressor. The compressor discharged through a
straight annular passage into the collector. Two 20-inch radial
ducts diametrically opposite connected the collector to a common
20-inch duct, which in turn was connected to the laboratory altitude
exhaust. The outlet pressure was regulated by a butterfly valve in
the discharge duct.
CO
NACA RM No. SE8H06 	 CONFIDENTIAL	 3
Instrumentation. - The instrumentation for determining the air
weight flow and over-all compressor performance was that recommended
in reference 4. The static pressure at the outer casing was measured
immediately downstream of e&ch row of stator blades. Each tap was
circumferentially located at the center of the passage between two
adjacent stator blades. The taps for the different stages were in
approximately the same axial plane. As shown schematically in fig-
ure 1, two differential-pressure recorders of five cells each were
connected to these static-pressure taps to obtain simultaneous
recordings of the pulsations in all stages on two film units. The
static-pressure tap was connected directly to one side of the
recorder diaphragm and connected through a relatively high-volume,
high-resistance damping chamber to the other side of the recorder
diaphragm. The diaphragm thus had a fluctuating pressure at one
side and a relatively steady pressure at the other during surge,
but the average pressure on the two sides was nearly the same.
This method of utilizing the pressure recorder eliminated the neces-
sity of providing a mechanical source of pressure for balancing the
mean static pressures of the various compressor stages. Because
the installations for the ten stages were identical, the amplitudes
of the fluctuations for the different stages can be compared when
the small variations in calibration among the recorders are taken
into account (fig. 2). The absolute values of amplitude, however,
are dependent on the dynamic characteristics of the installation
rather than being a true indication of the static-pressure
fluctuation.
Procedure. - With the compressor operating at constant speed
and inlet conditions just outside the surge region, the pressure
recorders were started and the outlet throttle was gradually closed
until surge was obtained. After the unit had surged for a few
seconds, the outlet throttle was opened until surge was stopped;
the recorders were then turned off.
Surge traces were obtained at equivalent compressor speeds of
80, 89, and 100 percent of design speed at an inlet pressure of
12 inches of mercury absolute and at 89 and 100 percent of design
speed for an inlet pressure of 6 inches of mercury absolute. All
runs were made with an inlet temperature of approximately 538 0 R.
Because of accidental plugging of the small orifice holes in the
surge chambers and the delicacy of the pressure recorders, consid-
erable difficulty was encountered in getting the recorders to
function properly. As a result, complete data for all stages were
not obtained for any one run. By repearing runs, however, suffi-
cient data were obtained on all stages for determining the trends
desired.
CONFIDENTIAL
NACA RM No. MR06	 CONFIDMUIAL	 4
Because film exposure could not simultaneously be within the
proper range for all of the pressure recorders, it was necessary to
retouch the surge traces to make duplication possible. This
retouching consisted in increasing the density of the lines on the
film without changing the position or width of the lines.
FMSMS AND DISCUSSION
Surge characteristics. - The surge traces for 89 percent of
design equivalent speed and an inlet pressure of 12 inches of mercury
absolute are presented in figure 3 for all the stages except stage 9.
In general, all stages began to surge at approximately the same time
and, although the end of surge is not clearly defined, all stages
apparently stopped surging at the same time. Data from a preliminary
run at design equivalent speed are presented in figure 4 for stages 8
and 9 to show that stage 9 exhibited the same general trends as the
other stages. Because the sensitivity of the recorders was differ-
ent for this preliminary run, the calibration curves of figure 2 are
inapplicable to these data.
Because of the superimposition of higher harmonics and mechan-
ical vibrations on the pressure fluctuations, it is difficult to
determine whether the fluctuations for the different stages are in
phase. Stages 1 to 5, where the maxima and minima are well defined
and uniformly separated, are in phase (fig. 3(a)), but the time
spacing between the maxima and minima for stages 6 to 10 is so small
that the phase relations are poorly defined (fig. 3(b)). The funda-
mental frequencies of all the different stages, however, are approx-
imately equal. The amplitude tended to decrease from either end of
the compressor toward the center with the minimum amplitude occurring
at approximately the sixth stage. This trend indicates the possibil-
ity of a node near the center of the compressor, which may result
from the large volumes of the depression tank and the collector at
the inlet and outlet of the compressor, respectively. Because of
the superimposition of higher harmonics and mechanical Vibrations of
unknown magnitude on the pressure fluctuations, this phenomenon
could not be further investigated by a harmonic analysis.
In reference 1, it was found that a compressor system with a
large external volume has a low-frequency, high-amplitude fluctua-
tion during surge, whereas a system with a small volume has a high-
frequency, low-amplitude fluctuation. In reference 5 it was found
that the frequency of pulsation could be calculated with reasonable
accuracy for centrifugal blowers by an adaptation of the Helmholtz
resonator equation. In the present case, a slightly different
adaptation of the equation was made for computing frequency as
follows
COMIDEAT!'IAL
NACA RM No. MSE06	 CONFIDENTIAL
	
5
f 2'X LVo
where
f frequency of pulsation, cycles per second
s minimum flow area in compressor discharge, 0.900 square foot
L distance between inlet depression tank and exhaust collector,
7.07 feet
Vo total volume of system between inlet and outlet throttles,
556 cubic feet
c velocity of sound in air, feet per second
The solution of this expression gave a frequency of 2.74 cycles
per second, whereas the experimental values varied from 1.97 to 2.58.
The volume used in the calculation is apparently smaller than the
effective volume,-which is logical since the inlet and outlet
throttles were partly open. This belief is supported by the surge
traces where the amplitude generally increased and the frequency
decreased as the outlet throttle was opened to bring the unit out
of surge.
Effect of steed. - The effect of speed on the surge character-
istics can be seen by a comparison of figures 3, 5, and 6. For
example, the amplitude for stage 2 increased from 0.80 to
2.20 inches of water with an increase in speed from 80 to 100 per-
cent of design equivalent speed at an inlet pressure of 12 inches
of mercury absolute and the trend was similar for the other stages.
For centrifugal compressors, compressor speed apparently had little
influence on the frequency of pulsation (references 1 and 5) but
for axial-flow compressors, speed apparently has an appreciable
affect. The frequency decreased as the speed was increased. For
example, an increase from 80 to 100 percent of design equivalent
speed at an inlet pressure of 12 inches of mercury absolute
resulted in a decrease of frequency from 2.58 to 1.97 cycles per
second, which may result from the increased pressure and tempera-
ture at the compressor outlet caused by the increase in speed.
Effect of inlet pressure. - Comparison of figure 5 with 7 and
figure 3 with 8 shows that inlet pressure had little effect on the
frequency of pulsation. For 89 percent of design equivalent speed,
the frequency varied only from 2.46 to 2.30 for an increase in
CONF	 IAL
NA.CA RM No. SEBH06	 ' COIWInWIAL	 6
pressure from 6 to 12 inches of mercury absolute; at 100 percent of
design equivalent speed the frequency varied only from 2 . 15 to 1.97.
The amplitude, however, increased appreciably with inlet pressure.
For 89 percent of design equivalent speed, the amplitude for
stage 2 increased from 0.60 to 0.98 inch of water for an increase
in pressure from 6 to 12 inches of mercury; at 100 percent of design
equivalent speed, the variation was from 0 . 98 to 2 . 20 inches of
water. The other stages in general exhibited the same trend.
SUMMARY OF RESULTS
An investigation of the surge characteristics of the Z24C-2
axial-flow compressor produced the following results:
1. All stages started surge at approximately the same time.
Although the end of surge was not clearly defined, it appeared that
surging also ended at approximately the same time for all stages.
2. The amplitude of the pressure pulsations decreased from
either end of the compressor toward the center, which indicated the
possibility of a node near the center of the compressor.
3. When adapted to this compressor system, the equation for
computing the frequency of pulsation for a Helmholtz resonator
gave a frequency of 2.74 as ccmpared with observed frequencies of
1.97 to 2.58 cycles per second.
4. Compressor speed had an appreciable effect on both the
amplitude and the frequency of pulsation.
1^ COINFIMMIAL
NACA RM No. SE8HO6 	 CONFI	 JAL
5. Inlet pressure had an appreciable effect on the amplitude
but very little effect on the frequency of pulsation.
Flight Propulsion Research Laboratory,
National Advisory Committee for Aeronautics,
Cleveland, Ohio, August 6, 1948.
r
7
Richard M. Downing,
Mechanical Engineer.
Approved: GG^G^ l^
Robert 0. Bullock,
Aeronautical Research
Scientist.
scar W. Schey,
Aeronautical Research
Scientist.
rl
RErERENCES
1. Bullock, Robert 0., Wilcox, Ward W., and Moses, Jason J.. Exper-
imental and Theoretical Studies of Surging in Continuous-Flory
Compressors. NACA TN No. 1213, 1947.
2. Schum, Harold J., and Buckner, Howard A., Jr.: Investigation of
10-Stage Axial-Flow %24C-2 Compressor. I - Performance at
Inlet Pressure of 21 Inches Mercury Absolute and Inlet Temper-
ature of 5,380
 R. NACA RM No. E7Gll, Bur. Aero., 1947.
CONFIDENTIAL
NACA RM No. SE8H06	 CONFIDENTIAL	 8
3. Finger, Harold B., Schwa, Harold J., and Buckner, Howard A., Jr.:
Investigation of X24C-2 10-Stage Axial-Flow Compressor. II -
Effect of Inlet-Air Pressure and Temperature on Perfoxmance.
NACA RM No. E7H22, Bur. Aero., 1947.
4. NACA Subcommittee on Compressors: Standard Procedures for Rating
and Testing Multistage Axial-Flow Compressors. NACA TN
No. 1138, 1946.
5. Heath, W. R., and Elliot, W. R.: Control and Prediction of
Pulsation Frequency in a Duct System. Journal of Applied
Mechanics, vol. 13, no. 4, Dec. 1946, pp. A-291 - A-293.
.
CONFIDENTIAL
P
d i aph ragm of pressure
:cording unit
^^	
d
NACA RM No. SE8H06
	 CONFIDENTIAL 9
To manometer
board
Damping chamber
Static—pressure tap
	 /—Compressor casing
NACA
Figure 1. — Schematic diagram of differential —pressure
fecording unit.
CONFIDENTIAL
NACA RM No. SE81-106	 CONFIDENTIAL IO
2
14
12
y 10
ms
c
ai 8
a
CL
6
V
a
e 4
a
2
0	 .2	 .4	 .6	 .8	 1 .0
Amplitude, in.
Figure 2. — Calibration of differential—pressure recorders.
CON F1 DENTIAL
	Stage	 z
D
C-)
D
1
v^	 v \	 z0
	
3 
1 ^^~ ^._	 mOD
5
4
	
0	 2	 4	 6-	 8	 10	 12 NES.iyio
(a) Stages 1 to S.
Stage
0	 6 ^^	 Oz
M
	
m
8
r	 1	
r
0
10
a
0	 2	 4	 6	 8	 10	 12	 P 14
new.^lt
Time, sec
	
(b) Stages 6 to 10.	 NACA
Figure 3. - Surge traces for 89 percent of design equivalent speed, inlet pressure of 12 inches mercury absolute, and inlet
temperature of 5380
 R.
^ v ^
1
0
1	 t 0 1	 1	 1 f t	 1 1	 1 i r ^	 a
0 2 4 6 8
e
10 12 14
0
_z
	
8
vMz
nr
9
0
n
z0
M
OD
z0ON
0z
_-n
v
Mz
nr
z
n
HEy . W i
Time, sec	 NACA
Figure 4. - Sorge traces for stages 8 and 9 for 100 percent of design speed, inlet pressure of 12 inches mercury absolute and
inlet temperature of 5380 R. (Calibration curves of fig. 2 inapplicable.)
stage
2
3 J
5
4
K
a
D
z
0
U)
m
OD
O
CT
Stage
6
7
8
nOz
D
mz
ar
z
0.1
0
	 2	 4	 6	 8	 10
(a) Stages 1 to 5.
C-)O
z
0
m
z
D
r
a
^k^
10 iv v	 - ^ v VV V UVUU UV `-,—
0	 2	 4	 6	 8	 10	 12aas.rs
Time, sec
(b) Stages 6 to 10.	
NACA
Figure 5. - Surge traces for 100 percent of design equivalent speed, inlet pressure of 12 inches mercury absolute, and inlet
temperature of 5380
 R.
kA
0Oz
Mz
Dr
z
n
b
z
0
M
CO
O
0`
Stage
1 -
2 -
310
5 --
4 —
0	 2	 4	 6	
"M. O:
(a) Stages 1 to 5.
Stage
n
O
z
-n 6
m 7
z
n
r 8
l
0•10 --
0	 2	 4	 6	 8	 10	 12	 14 n^ wa
Time, sec
NACA(b) Stages 6 to 10.•
Figure 6. - Surge traces for 80 percent of design equivalent speed, inlet pressure of 12 inches mercury absolute, and inlet
temperature of 5380 R.
z
D
Stage	 n
1	 ^
2	 z
3	 ^'	 m
	
1	 O
O
0
p,
5
4
	
0	 2	 4	 6	 8	 10 r+ss. #►d
(a) Stages 1 to 5.
Stage
C)O	 6	 nz	 O
-n	 7	 n
°
m m
-^	 z
8.1	
—i
r	 Dr
0
10
0	 2	 4	 6	 8	 10
Time, sec
(b) Stages 6 to 10.
z
D_
Stage D
ys
3 --^'—'—_
M
^^	 os
l 00
0
4 _—^ 3 	 ^^
0	 2 4	 6	 8	 10	 12	 14	 ncs`
(a) Stages 1 to 5.
Stage
C q	 /)
',	 o
7
-
e 8
10 _,
i	 I	 i	 o	 i	 i	 I	 I	 i	 i	 i	 i	 I
0	 2	 4	 6	 8	 10	 12	 14	 nv
Time, sec
(b) Stages 6 to 10.	
AA
Figure 8. - Surge traces for 89 percent of design equivalent speed, inlet pressure of 6 inches mercury absolute, and inlet
temperature of 5380 R.	 _
-	
^
Restriction/
Classification 
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R
estriction/Classification Cancelled


Investigation of X24C-2 10-Stage Axial-Flow Compressor. III - Surge Characteristics

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