The NASA Lewis Research Center has developed and implemented a unique process for forming flawless compound curvature laser windows. These windows represent a major part of specialized, nonintrusive laser data acquisition systems used in a variety of compressor and turbine research test facilities. This report summarizes the main aspects of compound curvature laser window development. It is an overview of the methodology and the peculiarities associated with the formulation of these windows. Included in this discussion is new information regarding procedures for compound curvature laser window development

Verhoff, Vincent G.

English

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C- '? r/ q/
NASA Technical Memorandum 106077
AIAA-93-2177
Compound Curvature Laser
Window Development
Vincent G. Verhoff
Lewis Research Center
Cleveland, Ohio
Prepared for the
29th Joint Propulsion Conference and Exhibit
cosponsored by the AIAA, SAE, ASME, and ASEE
Monterey, California, June 28-30, 1993
PJASA
https://ntrs.nasa.gov/search.jsp?R=19960001702 2020-06-16T06:54:58+00:00Z
COMPOUND CURVATURE LASER WINDOW DEVELOPMENT
Vincent G. Verhoff
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
0
Abstract
The NASA Lewis Research Center has developed
and implemented a unique process for forming flawless
compound curvature laser windows. These windows
represent a major part of specialized, nonintrusive laser
data acquisition systems used in a variety of compressor
and turbine research test facilities. This report sum-
marizes the main aspects of compound curvature laser
window development. It is an overview of the method-
ology and the peculiarities associated with the formula-
tion of these windows. Included in this discussion is new
information regarding procedures for compound curva-
ture laser window development.
Introduction
An increased interest in fundamental research in
turbines and compressors has created a need for non-
intrusive optical flow measurement systems. The state-
of-the-art systems used in obtaining detailed velocity
data are called nonintrusive laser data acquisition sys-
tems. These systems seed the airflow with small particles
that flow through a fringe pattern created by intersect-
ing laser beams. Data are obtained by measuring the
pulsating light reflected as the seed particles pass
through the fringe pattern.
Optically clear laser windows are used for the laser
beams and reflected light to pass through. Normally the
laser window glass used in wind tunnels is approxi-
mately 1.0-in.-thick flat quartz. For turbine and com-
pressor testing facilities, however, the windows are
approximately 0.100 in. thick and have compound curv-
atures. The difference between the two windows is
shown in Fig. 1.
Since laser windows are molded to the flow surface
profile of the turbine and compressor casings, their size
and curvature cannot be altered. Window thickness, on
the other hand, can vary. Thinner windows are desirable
because they minimize error, but the glass must main-
tain high strength with reduced thickness. Reference 1
provides a detailed explanation of compound curvature
laser window development. This paper summarizes the
results from Ref. 1 and provides some new information
regarding the development process.
Background
Laser systems are commonly used at NASA Lewis
Research Center. Most of these systems are located in
the engine component test facilities that test compressor
and turbine rotors and components. The windows are
usually located over rotating hardware where typical
instrumentation cannot be used.
Overall safety is of primary concern. To uphold
safety practices, the laser windows are hydrostatically
pressure tested to 1.5 times the maximum operating
pressures of the facilities. Operating pressures for various
facilities range from 1.3 psia to 72 psia. Laser windows
are also thermally qualified at facility temperatures and
pressures if thermal differentials are significant. The
laser windows are tested on both concave and convex
surfaces to uphold the safety requirements.
Glass Selection
Several types of glass have been used for laser
window development at NASA Lewis. Sodium-lime,
borosilicate, and sodium-alum ino- silicate glass are
among these types. On the basis of the Corning glass
code 0317, 2
 sodium- alumino-silicate glass is the preferred
glass for laser windows where high strength is needed.
This high strength is achieved through chemical
strengthening, which is not possible with other glass
types. This increased strength from chemical strengthen-
ing allows the use of thinner laser windows, which
reduce spatial error and produce higher quality laser
data. At near ambient temperature and pressure opera-
ting conditions, high-strength windows are generally not
required and any of the glass types could be suitable.
Sodium-lime windows are the least costly alternative.
An additional benefit of the sodium-alumino-
silicate laser window is that a failure will cause the
window to shatter in tiny particles, only millimeters in
cross-sectional area. This characteristic is advantageous
since the smaller particles contain less kinetic energy and
are less likely to damage blades or rotors. The particles
from other failed glass windows are relatively large in
cross-sectional area. Figure 2 shows a comparison of
failed strengthened and unstrengthened laser window
glass.
1
To help select the correct type of laser window
glass for the application intended, an analytical equation
is being developed. This equation includes the param-
eters of glass load area, thickness, working pressure, and
bond/contact area.
Mold Design
The mold material that offers the best results for
forming laser windows is machinable ceramic because it
produces the best accuracies. Machinable ceramic can be
machined to closer tolerances and surface finishes than
graphite. Reference 1 stated that graphite was the best
suitable material; however, further development indi-
cates that machinable ceramic offers the best results.
The molds for laser window development consist of
a male and female mold, as shown in Fig. 3. The male
mold is machined to match the internal flow path sur-
faces, whereas the female mold is machined to these
coordinates plus the glass thickness. The overall dimen-
sions of the molds are 1 in. greater than the actual size
of the laser window glass. Both molds have threaded
holes on the perimeter to fit alignment bars.
whether the temperature should be increased or de-
creased. The ideal slumping temperature for compound-
curvature laser window development is usually within
2 percent of the annealing temperature. The procedure
for laser window development is detailed in Ref. 1.
Visual inspection of the slumped laser window will
provide adequate information to alter the slumping tem-
perature. If the slumping temperature is excessive, sur-
face imperfections will be apparent. These surface
imperfections will appear along the plane of severe
compound curvature or inflection point. This problem
can be solved by decreasing the slumping temperature
by 1 to 2 percent of the annealing temperature.
Laser window development can be a lengthy proc-
ess, but once the parameters are found for a particular
laser window, reproduction is routine. The last process
in the formulation of laser windows is to anneal the
glass. This relieves residual stresses developed in the
glass during the development process. Glass annealing
also helps achieve glass strength consistencies from one
laser window to another.
Glass Edging
To maintain surface quality, accurate machining
and polishing of the molds used for laser window devel-
opment are essential. The molds are machined and pol-
ished to a tolerance of 0.003 in. of the desired contour.
After finish machining, these molds are polished to
within an 8 Ra surface finish.
All slumping components are machined out of same
material to ensure similar coefficients of thermal
expansion. Orientation of the slumping component con-
figuration is shown in Fig. 4.
Glass Molding
Inert gas furnaces are preferred for slumping
because of the heating characteristics of the molds.
Other types of furnaces usually introduce contaminants
into the slumping environment that degrade glass sur-
face quality.
Cleanliness of the molds, glass, and furnace is
critical to glass quality. These components should be
thoroughly cleaned at the beginning of every slumping
operation.
The forming temperature of laser windows is found
by an iterative process. The mean annealing temperature
of about 1100 O F is the theoretical starting temperature
for new laser window development. Adequate visual
inspection of the window after slumping will indicate
Previous methods of edging the laser windows to
size involved scoring the glass to the desired size, then
breaking the glass along this scored edge. During this
process, small fragments of glass are broken away along
the line of scoring, creating voids in the glass. Unfor-
tunately, minute cracks along the edge of the glass
remain after the glass is broken (Fig. 5). These cracks
weaken the glass and cause failure of the glass under
load.
Minimizing cracks is important during the edging
process. A perfected process for edging glass using a
numerically controlled water-jet cutting machine has
been developed and is recommended for cutting laser
windows. The machine abrades glass away along the
desired cut line, reducing glass fragmentation. The
water-jet cut edge of the glass is less densely populated
with cracks, and crack penetration is less severe when
compared with scored glass.
The desired laser window dimensions are program-
med into the numerically controlled water-jet cutting
machine. The laser window glass is then secured into
position by pressing it into clay and the entire assembly
is then submersed under water to further assist in
absorbing the high cutting frequencies of the water jet.
After the laser window glass is cut to size, the
edges are rounded. The minimum radius of the rounded
edges is equivalent to half the glass thickness, as shown frame. The frame is aligned on the gluing fixture using
in Fig. 6.	 dowel pins.
Glass Strengthening
Sodium-alumino silicate laser windows are chemi-
cally strengthened through ion exchange to endure facil-
ity operating pressures and temperatures, increase their
relative impact strength, and minimize glass fracture
particle size. These windows are chemically strengthened
to contain a 0.010-in. or greater compression layer.
Other types of glass do not benefit from the chemical
strengthening process. Detailed information on the
chemical strengthening process is given in Refs. 3 to 8.
Window Proof-Testing Procedure
At NASA Lewis, the turbine and compressor cas-
ings into which the laser windows are located are evalu-
ated in accordance with boiler vessel codes to ensure
their structural integrity and safety. The normal process
for evaluating facility hardware is hydrostatic testing.
To uphold the structural integrity and safety of these
facilities, the laser windows are also hydrostatically
tested after assembly. A detailed procedure for proof-
testing is found in Ref. 1.
i
j
Glass Bonding to Frame
The laser window frames are specially designed to
pocket and hold the glass. Figure 7 shows this unique
design. The laser window glass is bonded to the window
frame using a high-strength semiflexible adhesive. The
laser window glass is actually floated in the laser
window frame with 0.030- to 0.050-in. bonding medium
between the laser window glass and all laser window
frame surfaces. Figure 8 shows a cross section of the
laser window frame, glass, and bonding medium. The
bonding strength is increased drastically by a semi-
circular groove located in the lower and inner edge of
the laser window frame.
Accurate positioning of laser window glass into the
frame is a crucial step in obtaining quality laser data.
Laser window glass can be positioned precisely by using
a specifically designed gluing fixture to hold the align-
ment of the laser window glass with respect to the laser
window frame. Gluing fixtures are machined to the iden-
tical contour and dimensions of the male molds.
Figure 9 shows the assembly stackup for bonding
the laser window glass to the frame. The gluing fixture
has an inscribed inner channel 0.125 in. in depth which
matches the circumference of the laser window glass.
Without this escape channel, bonding medium would get
trapped over the frame or glass, resulting in a non-
uniform transition between the laser window glass and
frame. Duct seal is used to hold the alignment of the
laser window glass to the gluing fixture.
Figure 10 represents a cross section of the bonding
process. The laser window glass is aligned onto the glu-
ing fixture by positioning the outer perimeter of the
laser window glass with the inner edge of the bonding
medium escape channel. The bonding medium is then
applied to the supporting edge of the laser window
Concluding Remarks
High-quality contoured windows are a key part of
laser anemometry systems for compressor and turbine
facilities. The present development process ensures
accurate tolerances and flawless quality of these laser
windows. Although they are tested for flaws, laser win-
dows should be considered and treated as a fragile
material. Routine visual inspection and hydrostatic
evaluations are considered important to their integrity.
Future experiments indicate the need for larger
laser windows to increase the fields of unobstructed
view. Future windows must also endure higher pressures
and temperature gradients. With the present develop-
ment processes, these laser windows should successfully
withstand these future requirements.
R PfPTPn rPR
1. Verhoff, V.G., "Three-Dimensional Laser Window
Formation." NASA RP-1280, 1992.
2, Corning Glass Code 0317 Specification Sheet,
Corning, Inc., Corning, NY, 1990.
3, McLellan, I., and Shgnd, G.W. II, Glass
Engineering Handbook, Third ed., McGraw Hill,
1984.
4. Nordberg, M.E., Moehel, M.E., Gartinke, H.M.,
and Olcott, J.S., "Strengthening by Ion
Exchange," American Ceramic Society Journal,
Vol. 47, No. 5, May 1964, pp. 215-219.
5, Hagy, H.E., "Design Strength of a Chemically
Strengthened Glass," Central Glass Ceramic
Research Institute Bulletin, Vol 13, No. 1, 1966,
pp, 29-31.
3
(a) Wind tunnel window.
(b) Turbine/compressor window.
Blizard, J.R., "Chemically Strengthened Glass,"
SAE Paper 680485, 1968.
7. Miska, H.A., "Understanding the Basics of Chemi-
cally Strengthened Glass," Materials Engineering,
Vol. 83, No. 6, 1976, pp. 38-40.
Olcott, J.S., "Strengthening of Glass," Science,
Vol. 140, No. 3572, June 14, 1963, pp. 1189-1193.
Figure 1.—Typical laser windows.
IC-92-06406
(a) Laser window before failure.
AN
illl^	 C-91-02492
Figure 3.—Laser window male and female mold,
A
s	 ^
'	 C-92-09597
(b)Unstrengthened laser window after failure.
(c)Strengthened laser window after failure.
Figure 2.---Comparison of failed strengthened and
unstrengthened laser window glass.
Stainless steel bolt with drilled
through center hole
Graphite alignment bar
/^_l-Graphite male mold
Casing
Glass
 \ cu rvature
Glass	 i / ,/	 ///
^^/ / ithickness-L l
Graphite female
'^?Ilimold-,
/
Casing curvature plus
glass thickness
Figure 4.—Slumping components configuration.
Remaining cracks after glass is broken
i Scored surface
Glass thickne s 
^—^i
Figure 5.—Edged window glass by scoring.
5
Typical laser
window glass-
0.030 to 0.050-in. bonding medium
)E^E
qthickness
hickness
Laser window glass
Figure 6.—Rounded edges of typical laser window.
- 30 to 50 mils
bonding medium
nal
I—Laser window frame
Figure 7.—Typical laser window frame.
Figure 8.—Cross section of laser window frame, glass, and
bonding medium.
6
w frame
ng medium
Laser window frame— , 	 0.001-in.
clearance
/Bonding medium ^\	 / i r--Dowel pin
Laser window glasses
Laser window glass	 Duct seal	 Bonding medium
escape channel
Gluing fixture
	 (0.125-in diam)
Figure 10.—Cross section of laser window bonding process.
Duct seal
Figure 9.—Assembly stackup of laser window bonding process.
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
June 1993 Technical Memorandum
4- TITLE AND SUBTITLE 5. FUNDING NUMBERS
Compound Curvature Laser Window Development
WU-505-62-846. AUTHOR(S)
Vincent G. Verhoff
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
National Aeronautics and Space Administration
Lewis Research Center E-7691
Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
National Aeronautics and Space Administration
Washington, D.C. 20546-0001 NASA TM-106077
AIAA-93-2177
11. SUPPLEMENTARY NOTES
Prepared for the 29th Joint Propulsion Conference and Exhibit cosponsored by the AIAA, SAE, ASME, and ASEE,
Monterey, California, June 28-30, 1993. Responsible person, Vincent G. Verhoff, NASA Lewis Research Center,
(216) 433-5708.
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b- DISTRIBUTION CODE
Unclassified - Unlimited
Subject Category 14
13. ABSTRACT (Maximum 200 words)
The NASA Lewis Research Center has developed and implemented a unique process for forming flawless com-
pound curvature laser windows. These windows represent a major part of specialized, nonintrusive laser data
acquisition systems used in a variety of compressor and turbine research test facilities. This report summarizes the
main aspects of compound curvature laser window development. It is an overview of the methodology and the
peculiarities associated with the formulation of these windows. Included in this discussion is new information
regarding procedures for compound curvature laser window development.
14. SUBJECT TERMS 15. NUMBER OF PAGES
Windows; Laser windows; Three-dimensional glass; Curved glass; Slumped glass; 8
16. PRICE CODEMolded glass; Chemical strengthened glass
AOa
17. SECURITY CLASSIFICATION 18- SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACTOF REPORT OF THIS PAGE OF ABSTRACT
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