Extension of Useful Operating Range of Axial-Flow Compressors by Use of Adjustable Stator Blades Page: 1 of 28
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REPORT No. 915
EXTENSION OF USEFUL OPERATING RANGE OF AXIAL-FLOW COMPRESSORS BY USE
OF ADJUSTABLE STATOR BLADES
By JosN T. SINETm , Ja., and WILLIAM J. VossSUMMARY
A theory has been developed for resetting the blade angles of
an aial-flow compressor in order to improve the performance
at speeds and flows other than the design and thus extend the
useful operating range of the compressor. The theory is
readily applicable to the resetting of both rotor and stator blades
or to the resetting of only the stator blades and is based on
adjustment of the blade angles to obtain lift coefficients at which
the blades will operate efficiently.
Calculations were made for resetting the stator blades of the
NACA eight-stage axial-flow compressor for 75 percent of
design speed and a series of load coefficients ranging from 0.28
to 0.70 with rotor blades left at the design setting. The NACA
compressor was investigated with three different blade settings:
(1) the design blade setting, (2) the stator blades reset for 75
percent of design speed and a load coefficient of 0.48, and (8)
the stator blades reset for 75 percent of design speed and a
load coefficient oJ 0.65. Most of the tests were conducted at
an inlet-air pressure of 10.16 inches of mercury absolute and
an inlet-air temperature of 0 F and extend over a range of
compressor AMach numbers from 0.2 to 0.8 for the three blade
settings. In order to investigate the effect of inlet-air condi-
tions at low speeds, tests at a compressor Mach number of 0.2
were repeated for the first blade resetting with standard sea-
level inlet-air conditions.
The experimental results show that a substantial increase in
the useful operating range of axial-flow compressors can be
obtained by adjustment of only the stator-blade angles. Con-
siderable improvement in efficiency at compressor Mach num-
bers appreciably below the design value was obtained for both
stator-blade resettings. A difference of 0 to 80 percent in the
peak-efficiency flows for the two stator-blade resettings also was
obtained, which at high compressor speeds was about seven
times the maximum flow range for a particular blade setting.
Peak pressure ratios were increased with the stator blades reset
for a load coefficient of 0.65 and were substantially the same as
for the design blade settings with the stator blades reset for a
load coefficient of 0.48. Inlet-air conditions were found to
have a very large effect on the adiabatic temperature-rise effi-
ciency at low compressor M4fach numbers, which is believed to
be caused by heat-transfer and Reynolds number effects. No
definite surging of the compressor was observed at Mach num-
bers lower than 0.4; instead a sudden transition to a lower
pressure ratio occurred when the flow was decreased appreci-
ably below the peak-efciency point.INTRODUCTION
The axial-flow compressor, in spite of the high peak effi-
ciencies obtainable in modern designs (references 1 to 3),
has had somewhat limited application because of its in-
herently narrow operating range. The useful flow range of a
compressor at a given speed may be limited because of one or
more of the following characteristics: (1) a rapid change in
pressure ratio with a small change in flow; (2) a marked drop
in the efficiency with changes of flow; and (3) unstable
operation, referred to as "surging," which occurs when the
flow is decreased beyond a certain point. The useful speed
range of a compressor may be limited by a marked drop in
peak efficiency with change in speed or by large differences
between the peak-efficiency flow and the required flow at
speeds appreciably different from the design speed. The
characteristics that limit the useful range of axial-flow
compressors are explained by the fact that the angles of
attack, and hence the lift coefficients, for some of the blade
rows are greatly affected by changes in flow and speed; some
of the blade rows are therefore operating far from their
optimum lift coefficients for speeds and flows appreciably
different from the design.
An obvious remedy for these limitations on the useful
range is to change the blade-angle settings with changes in
operating conditions to maintain optimum lift coefficients.
This method has been very successfully employed in extend-
ing the high-efficiency range of propeller turbines (references
4 and 5). Tests on axial-flow fans show that a very pro-
nounced extension of the flow range can be obtained by the
same method (references 6, pp. 100-104, 7, and 8). For
high-speed multistage compressors, however, the adjustment
of the many rows of rotor and stator blades during operation
would involve mechanical problems of considerable difficulty.
The alternative method of adjusting only the stator blades,
which is mechanically more feasible, has been given particular
consideration in the present investigation.
A general method of calculating blade-angle settings for a
wide range of operating conditions has been developed. The
method is applicable to the resetting of both rotor and stator
blades or to the resetting of the stator blades alone and is
based on the adjustment of blade angles to obtain lift coeffi-
cients at which the blades operate efficiently with special
attention to the avoidance of blade stalling. Because of the
extreme complexity of exact flow calculations, several
simplifying approximations are made in the theoretical
425
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Sinnette, John T., Jr. & Voss, William J. Extension of Useful Operating Range of Axial-Flow Compressors by Use of Adjustable Stator Blades, report, December 29, 1944; (https://digital.library.unt.edu/ark:/67531/metadc60225/m1/1/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.