Discussion of boundary-layer characteristics near the wall of an axial-flow compressor Page: 2 of 20
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REPORT 1085-NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
thle boundary layer. Accordingly, the effects can be ex-
pected to be of the same order of magnitude on the pressure
surface as on the suction surface of the blade. Oil and dirt
patterns on the casing of axial-flow compressors have been
used to indicate the direction of the boundary-layer flow.
In all compressors investigated, these patterns have shown
strong deflections on the suction surface and practically no
deflection on the pressure surface. For the most part,
therefore, the observed boundary-layer deflections must be
due to forces other than the centrifugal forces on the blade.
Appreciable centrifugal force may exist in some cases, but
the process through which the observed boundary-layer
deflections take place must depend principally on effects
other than centrifugation of the boundary layer. Similar
conclusions were reached by Carter (reference 6) on the basis
of experiments and by Fogarty (reference 8) on thc"basis of
actual computations of laminar boundary-layer flow over a
rotating, slender, infinite blade.
Recently, it was shown by Squire and Winter (reference 9)
and Hawthorne (reference 10) that, in the case of thick
boundary layers with turning, a redistribution of vorticity
takes place, such that the ratio of the component of vorticity
in the direction of flow to. the initial vorticity is directly
proportional to the turning. A mechanism for secondary
flows was thereby provided that was independent of the
existence of rolled-up trailing vortices. The indication was
then that the losses may occur because of secondary flows,
which in turn occur because of the redistribution of vorticity
due to turning.
A logical step was next to investigate three-dimensional
boundary-layer characteristics on the walls of the cortpressor.
This study was facilitated by the flow equations presented
in reference 7. In order to apply these equations the
velocity profiles in the boundary layer must be known.
Velocity profiles near the tip of a single-stage axial-flow
compressor were measured at the NACA Lewis laboratory
by use of total-pressure and yaw probes. The results of this
investigation, together with an interpretation of boundary-
layer characteristics on axial compressor walls, carried out
in 1950-51 are reported herein.
Because of the great complexity of the problem and
mechanical difficulties, the information obtained herein is
not detailed enough to permit even an approximate com-
putation of boundary-layer characteristics on the walls of
the whole compressor. Furthermore, since these results are
preliminary and of a piloting nature only, certain simplifica-
tions had to be made, both in the analysisof boundary-layer
low and in the interpretation of the data. The following
assumptions were therefore made: The flow was considered
incompressible, with a zero velocity gradient normal to the
surface at the outer extremities of the boundary layer; the
pressure probes were assumed to read the correct mean
values, not only of turbulent fluctuations but also of the
variation due to blade rotation; and the concept of boundary-
layer thickness was interpreted as the region where frictionaleffects were noticeable. Such regions may be quite thick
and reach well into the passage.
The information obtained is still largely qualitative,
although some quantitative results appear to be of the rightorder of magnitude. The main purpose of this report is,
however, to show that many of the hitherto unexplained
phenomena appear to fall into a logical and well-defined
pattern when the problem is examined with regard to three-
dimensional boundary-layer flow on' the walls. Thus,
although the picture of flow obtained by this analysis is not
accurate in all details (such a picture is still somewhat
difficult to grasp), the model seems to be good enough to
warrant a considerably more extensive study. In addition,
the investigation discloses certain design parameters, the
importance of which has previously been overlooked.
APPARATUS
Experimental data for this report were obtained from
boundary-layer investigations on a 30-inch-tip-diameter
axial-flow compressor stage, which consisted of 40 circular-
are constant-thickness guide vanes and 29 constant-chord
NACA 65-(12)10 rotor blades (reference 11). The guide
vanes imparted a wheel-type rotation, and the rotor imposed
a. vortex-type rotation on the air. A cross-sectional view of
the compressor and inlet bellnouth is shown in figure 1,
which also shows the location of the measuring instruments.
For part of the investigation, a %-inch spoiler consisting of a
circular ring (fig. 1) was installed , inch upstream of tlie
leading edge of the inlet guide vanes. The stationary
clearance between the rotor-blade tips and the compressor
casing was 0.030 inch.
INSTRUMENTATION
Compressor speed, weight flow, and inlet conditions were
measured as described in reference 11. Boundary-layer con-
ditions near the tip were investigated at one circumferential
position and axially at stations I, II, and III (fig. 1).
Station.I was located approximately Y rotor-chord length
upstream of the leading edge of the rotor-blade tips. . The
one circumferential position at this station was chosen to
deviate least from the mean flow values over three passages.Air
flow 4--Instrument
circumferentkrl
position
Plan vie Station I
I~pFIURE 1.--Cross-sectonal view of compressor showing Instrument locations.
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Mager, Artur; Mahoney, John J. & Budinger, Ray E. Discussion of boundary-layer characteristics near the wall of an axial-flow compressor, report, June 7, 1951; (https://digital.library.unt.edu/ark:/67531/metadc60446/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.