Analysis of Stage Matching and Off-Design Performance of Multistage Axial-Flow Compressors Page: 3 of 40
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2 NACA RM E52D0)7
The trend toward increased compressor-pressure ratios aimed at
increasing the fuel economy of turbojet engines having high turbine-
nlet temperatures has introduced serious problems relating to the off-
design-speed performance of the engine. At speeds below design, it has
been found that the operating line of the engine often passes close to
or through the compressor surge region, making rapid acceleration and
sometimes even stable operation impossible. Below 50 percent of design
speed, engine starting problems are introduced by low compressor effi-
ciencies and the surge-line location. Both of these problems are
associated with the matching of the stages in the multistage unit. The
higher the over-all compressor-pressure ratio, the more consideration
must be given to means of matching stages for improved off-design-speed
General discussions of the problems related to stage matching and
to the effects on stage performance of off-design operation are pre-
sented i- references 1 and 2. The discussion in reference 2 indicates
the compromise which must be made to obtain good engine-starting char-
acteristics as well as good design-point operation. A one-dimensional
analysis similar to that used in the present report is applied in ref-
erence 3 to the determination of the effect of stage loading on range of
operation-in a multistage compressor. The analysis of reference 3 is
based on single-stage and two-dimensional cascade data and on the assump-
tion that surge of the compressor occurred when any given blade row
stalled. A similar analysis is presented in reference 4. Both analyses
indicate that the use of highly loaded stages in a multistage compressor
will give better off-design-speed performance than lightly loaded stages
because of the smaller departure from the design axial velocity across
The present report is intended to give a qualitative picture of the
operation of each stage of a multistage compressor over a range of actual
operating conditiona from 50 to 100 percent design speed and to point
out some means of achieving improved off-design performance. The method
used in the report is primaily analytical and is based on single-stage-
performance results. The performance curves of the single-stage unit
have been "stacked" to form a multistage compressor in which the design
or match point of each stage has been arbitrarily selected. This stack-
ing procedure is described in detail. The final results obtained by
this procedure are qualitative in that they point to improved off-design
performance but do not contain numerical values for the improvements
obtained or the design changes required.
Consideration is.given first to the effect of the single-stage
performance on the over-all performance of the unit, then to the various
means of matching the stages of a multistage compressor, Eid finally to
loading and unloading the stages in a given compressor by adjusting the
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Finger, Harold B. & Dugan, James F., Jr. Analysis of Stage Matching and Off-Design Performance of Multistage Axial-Flow Compressors, report, June 27, 1952; (https://digital.library.unt.edu/ark:/67531/metadc59396/m1/3/?rotate=90: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.