Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL MEASUREMENT OF VANE RING THROAT AREA
TECHNICAL FIELD
[0001] The invention relates to a method.. of optically
measuring the throat area of a vane ring for a gas
turbine engine, by irradiating or illuminating the vanes
and shrouds bounding a selected throat, then measuring
the resulting shadow area which corresponds with the
throat area and repeating the process for each throat in
a vane ring to determine the total vane ring throat area.
BACKGROUND OF THE ART
[0002] The invention relates to a method. and a device for
measuring the flow area at the point of maximal
restriction in a gas flow passage, in particular for
measuring the vane or nozzle area of a gas turbine
engine.
[0003]Stators, also known as vane rings, are an array of
stationary airfoils that are used to change the direction
of an annular airflow as it approaches or depaxts from an
array of rotating blades on a turbine or compressor rotor
for example. In order to change the minimum flow area
through a stator vane ring, adjustments to the trailing
edge angle of stator vane blades are made. The minimum
flow are is determined by the distance between a vane
trailing edge and the next cane's pressure side, and so
changing the trailing edge angle changes the minimum flow
area. The minimum flow are controls the pressure ratio
of the turbine and the mass flow of the engine, and
therefore the compressor's running line.
[0004]In a new engine the process of tuning the stator to
~0 the rotor is relatively simple since the rotating blades
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are exactly the same. However, as the engine wears and
is overhauled, the stator airfoils must be individually
adjusted to retune the stator to achieve optimal engine
performance. These adjustments may involve simply
bending the trailing edge of a stator airfoil, cutting
back the trailing edge or in the case of a segmented vane
ring, vane segment replacement. However, generally
hundreds of minute bends or adjustments must be performed
around the stator ring, which accumulate to affect the
flow area of the stator.
[0005]To calibrate the stator ring relative to the gas
turbine engine, the flow area of the stator must be
determined. A change in the stator flow area changes the
compressor running line, which effects gas generator
speed, compressor pressure ratio, temperature, mass flow
at constant power and engine surge margin in a transient
regime. At constant power, increasing the power turbine
stator flow area while maintaining a constant compressor
turbine stator flow area increases gas generator speed
and mass flow but decreases the compressor pressure ratio
slightly. Therefore, vane matching based on effective
flow area is a critical engine overhaul procedure for
predicting optimum engine performance and achieving
optimum efficiency and energy consumption.
[0006]Conventionally, the flow area of a stator ring has
been determined by use of a flow rate as for example
shown in U.S. Patent No. 6,148,677 to Evangelista. The
flow rig comprises a wind tunnel set up that measures the
pressure drop of an airflow as air passes through the
stator ring in a controlled experimental environment.
The flow rig may be precalibrated so that a known flow
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area results in a known pressure drop. Measuring the
pressure drop across a particular stator ring therefore
can be used to calculate the approximate flow area of
that stator ring.
[0007]Flow rigs such as that described in US 6,148,677 to
Evangelista require significant set up and the time
required to run the flow ring is approximately 45-60
minutes. As well, although flow rigs provide reliable
results when stator blades are relatively new and
regular, once a worn stator is used with adjustments made
to the leading or trailing edges, local pressure effects
create significant inaccuracies. In cases where the
stator ring has been refurbished, subjected to wear and
tear, or has been adjusted excessively, conventional gas
flow area measurement with comparator methods are
unreliable. A stator ring is an extremely expensive
component and therefore an accurate reliable means of
measuring the flow area is required.
[0008] Another conventional method of determining the flow
area of the stator ring involves mechanically measuring
the dimensions of the throat area. U.S. Patent No.
4,222,172 to Mason describes a vane area measurement
device using a dial gauge mounted in a specialized
fixture to measure the dimension of the throat area.
Coordinate measuring machines (CCM) can also trace the
area mechanically and calculate the enclosed area which
represents the vane throat area.
[0009]Mechanical measuring devices may be imprecise, slow
and relatively expensive. Coordinate measuring machines
currently use two measurement methods to determine throat
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area. One method measures the throat opening width at
three sections and measures the height to extrapolate to
obtain the throat area. The first method is imprecise
where the trailing edge is irregular between the measured
sections. The second method traces the throat opening
with a probe without breaking contact. The probe traces
the opening at a pre-determined axial distance from the
airfoil stacking line, the reference axis of the airfoil.
The throat area value calculation assumes that the chord
length is constant and that there is no profile deviation...
between the path traced and the actual trailing edge.
However, deviations are common in all but new parts since
vane adjustments, refurbishment and normal wear cause
profile deviation, chord length deviation or both. The
distance between the probe tracing path and the trailing
edge may be between 0.050-0.100 inches whereas profile
deviation may extend to 0.300-0.400 inches from the
trailing edge. Therefore the deviations are not entirely
missed by the tracing probe, although~a degree of error
is introduced. Another technique is demonstrated in
JP62-182604.
[0010] It is an object of the present invention to provide
a fast, inexpensive and reliable method of calculating
the throat area of a vane ring.
[0011]It is a further object of the invention to utilize
optical measurement 'of the vane ring throat area to avoid
innacuracies of prior art mechanical and airflow
measurement methods.
[0012]Further objects of the invention will be apparent
from review of the disclosure, drawings and description
of the invention below.
AMENDED SHEET
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DISCLOSURE OF THE INVENTION
[0013]The invention provides a method and device for
optically measuring throat area in a vane ring for a gas
turbine engine following the steps of: placing the vane
5 ring on a fixture with the periphery of each throat
within an optical measuring field of view; positioning a
primary radiation source to cast an area of shadow on the
vane ring delineating the throat as a dark area
surrounded by an area of reflectance; capturing an image
of the dark area with a radiation detector; analyzing the
image to acquire a dimensional data of the dark area,
proportional to the true throat dimensions; progressively
capturing and analyzing images from each of the
individual throats; then processing and calibrating the
dimensional data of each image to account for scaling and
viewing direction (perspective distortion) to acquire a
true value for the composite throat area of the vane
ring.
[0014]In the preferred embodiment, the source of radiation
is light in the visible, ultrasonic, or infrared spectrum
since these are easily and safely utilized in most
environments. Of significant benefit the optical
measuring method is very fast requiring only 25 to 30
seconds for a very accurate measurement that accounts for
modifications to the stator vane trailing edges, is
easily repeatable, inexpensive and avoids unnecessary
scraping of older vane arrays that usually provide
inaccurate results if prior art methods are utilized.
Leading edge profile deviations significantly effect
conventional flow comparator methods but do not affect
the engine. It is an advantage of the present method,
which views the trailing edge only, that leading edge
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profile deviations do not lead to inaccuracies in the
measurement of vane ring throat area.
DESCRIPTION OF THE DRATnIINGS
[0015]In order that the invention may be readily
understood, one embodiment of the invention is
illustrated by way of example in the accompanying
drawings.
[0016]Figure 1 is an axial cross-sectional view through a
turbo fan engine to indicate the usual location of stator
vane rings adjacent turbine rotors and compressor rotors.
[0017]Figure 2 is a perspective view of a single vane ring
showing the trailing edges of an array of stator vanes
confined between an inner and an outer shroud.
[0018]Figure 3 is an elevation sectional view of a device
according to the invention showing a partial Cross-
sectional view through the stator vane mounted on a
rotary fixture with primary and auxiliary light sources
creating an area of shadow surrounded by an area of
illumination to visually depict the throat area which is
viewed by an optical camera.
j0019]Figure 4 shows an elevation view along a plane
perpendicular to that of Figure 3 showing the primary
illumination sources and camera.
[0020]Figure 5 is a perspective view as seen by the camera
showing the area of shadow corresponding to the throat
area surrounded by an area of illumination.
[0021]Figure 6 is a sectional view along lines 6-6 of
Figure 5 showing the primary and auxiliary light sources
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similar to that of Figure 3 with the field of view of the
camera indicated as a rectangle capturing an image of the
area of shadow representing the throat area between the
stator vanes.
[0022]Further details of the invention and its advantages
will be apparent from the detailed description included
below.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0023]Figure 1 shows a typical axial cross-sectional view
through a turbofan engine, although the invention is
equally applicable to turbo shaft and turboprop engines.
Intake air passes over rotating fan blades 1 within the
fan casing 2 and is split into a bypass flow that
progresses through bypass duct 3 and the internal engine
core. The internal portion of the airflow passes through
low-pressure axial compressor 4 and centrifugal
compressor 5 into the combustor 6. Fuel is injected and
ignited within the combustor 6 and hot gases pass over
turbines 7 to be ejected through the rear exhaust portion
of the engine
[0024]Figure 2 illustrates a perspective view of a single
vane ring 8 that is conventionally disposed upstream of
the turbines 7 or upstream of compressor turbines in an
axial flow compressor 4.
[0025]The vane ring 8 has an annular array of stator vanes
9 that define a plurality of individual throats 10
between each set of adjacent vanes 9. The detailed views
in Figure 5 and Figure 6 illustrate the individual vane
throats 10 as a dark area of shadow surrounded by an area
of illumination created by means described below. Each
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individual throat 10 (as best seen in Figure 6) is a
planar opening with a periphery bounded in a radial
direction by an inner vane shroud 11 and an outer vane
shroud 12. The individual vane throat 10 has a periphery
bounded in the Circumferential direction by the trailing
edge 13 of the leading vane 9a and a projected Co-planar
line 14 on the convex surface of the adjacent following
vane 9b .
[0026]The method of the invention commences with the
following steps as illustrated in Figures 3 and 4. The
vane ring 8 is placed on a fixture 15 preferably with a
rotary indexing table 16 so that the ring 8 can be
progressively rotated about axis 17 to progressively
capture images for each individual throat and then
collect and process the data to obtain a composite throat
area for the entire ring 8.
[0027]As shown in Figures 3 and 4, the vane ring 8 is
placed in the fixture 15 in an imaging position such that
the periphery of a selected individual throat 10 is
within the optical measuring field of view of camera 18.
Figures 5 and 6 illustrate the optical measuring field of
view as a rectangular plane 19. It will be understood
that any shape of the field of view may be used and
multiple cameras 18 may be employed. Cameras 18 may each
measure a defined portion of the throat area which is
then summed to obtain a total, or each camera may measure
the entire throat and results are averaged, to improve
accuracy.
[0028]As shown in Figures 3 and 4, two primary radiation
sources 20 are disposed in a throat defining position to
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cast an area of shadow (as best seen in Figures 5 and
6)on the vane ring 8 initiating in the plane of the
selected throat 20. As indicated in Figures 5 and 6, the
area of shadow is surrounded by an area of reflectance
thereby visually delineating the throat area.
[0029]In order to provide as superior delineation
especially of the trailing edge 13, an auxiliary
radiation source 21 is.positioned to illuminate the
trailing edge of the leading vane 9a. This reduces
undesirable shadow on the following vanes. In order to
optimize the contrast between the areas of reflectance
and areas of shadow, the primary radiation sources 20 are
adjusted by the operator. Preferably, the primary
radiation source 20 and auxiliary radiation source 21 are
collimated for improved accuracy. Collimated light
sources produce little or no diffusion of light rays
around the edges of an illuminated area.
[0030]Although the radiation sources 20 and.21 can be of
any known radiation type, for safety and ease of use
radiation within the light spectrum is preferred and may
be chosen from visible light, infrared light or
ultraviolet light to equal advantage. Further, the
radiation sources 20 and 21 may provide a pattern or
radiation having a low intensity portions contrasted with
high intensity portions to improve detection by the
camera 18. For example " a checkered pattern of radiation
or straight lined pattern of radiation may improve
detection of the contrasting areas of shadow and
illumination in certain circumstances.
[0031]Further, in the embodiment illustrated in Figures 3,
5 and 6, the viewing direction 22 of the camera 18 is
AMENDED SHEET
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shown as being perpendicular to the plane of the selected
throat 10 (depicted in the illustrations as the plane of
field of view 19). The preferred viewing direction 22 is
from slightly above a perpendicular orientation to more
5 precisely define the lower boundary of the shadow (along
line 14 of Figure 6).
(0032jWith the vane ring 8 positioned in the fixture 15 in
the imaging position shown with the periphery of the
individual throat 10 within the optical measuring field
10 of view 19, the operator can proceed to capture an image
of a portion of the vane ring as shown in Figures 5 and 6
within the field of view 19 with the camera 18 or other
radiation detector to suit the radiation sources 20 and
21. The light or radiation sources 20, 21 should have a
broad enough area to illuminate a sufficient area around
the throat 10 opening to ensure a good contrast all
around the throat 10 opening and preferably any light
source is collimated or near collimated.
[0033] Where radiation in the form of light sources 20 and
21 are utilized, the image can be analized lay pixel
counting to acquire dimensional data of the dark portion
of the image which is proportional to the individual
throat area of the selected throat 10.
[0034jWhere an accurate composite throat area for the vane
ring 8 is required, the method can proceed to
progressively capture and analyze images from each of the
individual throats 10 and then process the dimensional
data for each image to acquire a composite throat area
for the entire vane ring 8.
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[0035]However, in some circumstances an estimated value
for the approximate throat area is adequate. In such
case, the method may include progressively capturing and
analysing selected images from a selected plurality of
individual throats 10. An estimate for the approximate
throat area for the entire vane ring 8 can be obtained by
prorating the dimensional data for the selected images
over the entire vane ring 8.
[0036]An absolute value for the actual throat area in the
plane of the selected throat 10 (plane shown as
rectangular field of view 19 in Figures 5 and 6) may be
obtained by calibrating the dimensional data applying at
least one scaling factor to the dimensional data
obtained. Since the camera 8 is positioned at a distance
from the vane ring 8, it is normally necessary even when
the field of view 19 is perpendicular to the viewing
direction 22 to provide a scaling factor to obtain and
actual measure of throat area for the vane ring 8. In
the event however that the viewing direction 22 is not
perpendicular to the plane, of the vane throat 10
(illustrated as rectangular plane 19)a first scaling
factor may be applied to the radial dimension of the data
and a second scaling factor applied to the
circumferential dimension of the data obtained. The
image data may be transmitted to a computer 23 for
analysing the image data and obtaining throat area for
each individual throat 10 or acquiring the composite
throat area for the entire vane ring as the fixture 10 is
progressed in a rotary fashion.
[0037]More complex calibration or scaling of the image
analysis may be applied to compensate for lens distortion
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and image perspective. The system is initially
calibrated by imaging a known grid or pattern of holes
located at the same distance and orientation as the
throat area 10 to be measured.
[0038]A secondary calibration involves using master vane
rings having a known vane throat area for each
manufactured model. The optical measurement method
described above is performed on each master vane to
calibrate the optical measuring system and to establish a
calibration curve against which measured values are
compared to determine the actual vane throat area.
[0039]Although the above description relates to a specific
preferred embodiment as presently contemplated by the
inventor, it will be understood that the invention in its
broad aspect includes mechanical and functional
equivalents of the elements described herein.
