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HIGH-TEMPERATURE SURFACE MEASUREMENTS OF TURBINE ENGINE
COMPONENTS USING THERMOGRAPHIC PHOSPHORS
Sami Alaruri and Andy Brewington
Allison Engine Company, P.O. Box 420, M/S W3A,
Indianapolis, IN 46206, USA
ABSTRACT
A laser-based system for single point high-temperature measurements of turbine engine component surfaces coated with thermographic phosphors is described. Decay lifetime calibration measurements obtained for Y203:Eu over the temperature range -530-1000°C
are presented. Further, the results obtained from a coupon placed in
the outlet gas flow of an atmospheric-combustor are described.
INTRODUCTION
New designs of advanced turbine engines strive to increase the
thermal efficiency of engine components, and thus performance, by
introducing sophisticated airfoil cooling schemes, new composite materials and new design methods. As a result, a new generation of advanced engines with shorter combustion chambers, higher inlet temperatures and pressures, lower specific fuel consumption and reduced
weight is emerging. The high-temperature environments of these advanced engines are creating new challenges for the two widely used
temperature measuring sensors, namely thermocouples and pyrometers
(Suarez et al, 1994, Suarez et al. 1990 and Atkinson et al, 1987).
Currently, infrared pyrometers which are used for engine diagnostics, control and general health monitoring purposes, are incorporated into several commercial and military engine tests for gathering
surface temperature measurements (Suarez et al, 1994, Suarez et al,
1990 and Atkinson et al, 1987). Despite the several advances in the
recent years in the areas of radiation detector technology and data
acquisition systems, single-wavelength intensity-based radiation thermometry instruments suffer from three major problems: 1) inferring
the object surface temperature from the spectral radiance measurements without knowing the emissivity of the object at different temperatures, 2) correcting temperature measurements for the extraneous
reflection and emission components (i.e. background radiance) which
can be produced by flames, walls and particles in the field of view (in
advanced engines the magnitude of the reflection component can
constitute up to 70% of the measured signal (Suarez et al, 1990), and
3) correcting for the attenuation of the optical signal due to the variable transtnissivity of the optical path.
The emissivity-temperature dependence can be a significant
source of error in the case of single-wavelength pyrometers (± 60 °C at
1000 °C [DeWitt, 1994 and Alaruri et al, 1996]). Algorithms for correcting pyrometer temperature readings for different uncertainty components have been treated in several publications (Suarez et al, 1994,
Suarez et al, 1990, Atkinson et al, 1987, DeWitt, 1994 and Alaruri et
al, 1996]. Due to the complexity of these multicomponent radiation
problems data reduction algorithms were developed around several
assumptions and empirical approximations. Consequently, the confidence level in the precision and the accuracy of the temperature measurements gathered using these instruments is low, especially when
used for gathering surface temperature measurements from blades
coated with thermal barrier coatings under extreme extraneous radiation conditions.
In the work herein, we describe a prototype laser-based system
for remotely measuring the surface temperature of parts coated with
thermographic phosphor (Y203:Eu-europium-activated yttrium oxide)
over the temperature range -530-1000°C. Unlike infrared pyrometry,
temperature measurements collected utilizing the decay lifetimes of
thermographic phosphors are independent of the target emissivity, the
spectral characteristics of the coated object and reflected radiation
component. Furthermore, the absence of lead wires, as the case in
thermocouples, makes this technique immune to electromagnetic radiation interference and very attractive for measuring the surface temperature of rotating engine parts (i.e. blades). Referenced to the output
of K-type thermocouples, the accuracy of this technique was estimated
to be approximately +2%. Similar work reported thus far by groups at
Oak Ridge National Laboratory, Los Alamos National Laboratory, and
EG&G (Goss et al, 1989, Noel et al, 1990, Tobin et al, 1988 and
Presented at the International Gas Turbine & Aemengine Congress & Exhibition
Orlando,on
Florida
— June
2-June
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Bugos, 1989) has demonstrated the feasibility of this noncontact temperature-sensing technique for turbine engine, furnace, ambient-spin
rig, and centrifuge applications where high temperature and harsh
environments are encountered.
In addition, this paper presents Y203:Eu fluorescence decay lifetime versus temperature data and the measurements obtained from a
proof-of-principle test conducted on a stationary target coated with
Y203:Eu which was placed in the outlet gas flow of an atmospheric
pressure combustor. This new high-temperature measurement technique can provide engine designers with accurate and reliable temperature measurements and can aid them in evaluating the performance of
new engine components and designs.
launch fiber). Generally, the input surface damage of the used high
Olt step-index fiber was measured around 3.5 J/cm 2 (Alaruri et al,
1995). Nine 1000 gm core diameter optical fibers were used to collect the fluorescence signal emanating from the phosphor coated aircooled target. The maximum area inside the housing of the optical
probe was utilized by arranging the collection fibers circumferentially
around the launch fiber. An air-cooled probe was developed and fabricated (Alaruri et al, 1995) to protect the collection and delivery fibers against the engine harsh. thermal and dynamic environment.
Other optical probes designed for turbine engine thermographic phosphor measurements have been reported in Refs (Allison et al, 1987
and Tobin et al, 1990). In comparison with these optical probes, Allison's fiber optic probe reduces the interference between the collected
fluorescence pulses and the background pulses generated by the energetic 266 nm laser beam by isolating the launch optical path from the
collection optical path. Since generating microbends along the rigid
2000 gm core-diameter launch fiber or wrapping the fiber around a
mandrel to achieve equilibrium mode distribution (EMD) was not
feasible, the spot size and the spatial profile of the excitation beam
were governed by the initial launch conditions into the numerical aperture of the optical fiber (Hentschel, 1989 and Alaruri et al, 1994).
Usually a surface area approximately 1 mm in diameter was covered
by the excitation laser beam.
The Y203:Eu-611 nm emission wavelength was selected by
coupling the collection fibers into the entrance slit of a Czemy-Tumer
configured-programmable monochromator (f/3.9, 0.275 meter-Acton
Research Corp-Spectra Pro 0.275). The gathered 611 nm optical
radiation was converted into an electrical signal by the means of a
EXPERIMENTAL
The laser induced fluorescence (L1F) stationary target measurement system consists of four major components: laser system, fiberoptic probe, signal collection and data acquisition system. The LIF
system is schematically depicted in Fig. I.
As shown in Fig. 1 the laser beam (266 nm/4-6 ns pulse width at
FWHM) generated from a Nd: YAG laser(Quanta Ray GCR-4-30,
Spectra Physics) was coupled into a 2000 gm core diameter fused
silica-on-fused silica optical fiber (Fiberguide Industries, Superguide
G) using a short focal length fused silica lens. During each data collection cycle the fluctuations in the laser beam energy were monitored
using a pyroelectic joulemeter (Molectron-Model 150). The spot size
of the launched laser beam was adjusted to overfill the numerical aperture of the delivery optical fiber (i.e. overfill the core diameter of the
Collation Furnace
EllocSbode
Fiber Optic
Probe
Dicer*
Mirror
Fused %co Lem
PMT
Monoctromoter
Phosphor Semple
Colorkneler
HV Power Saxer
Fiber Optic
Coble and
Thermocouple
Optics Fber Temperotwe
Control System
L Thermocouple Readout
DAS
Arrpf For
Osaloscope
Wes
Delay Lbe
• Ext. Trigger
NckYAG Loser
Svc Out
•
Mirror
486 P.C. Compote:le
Figure 1. Schematic diagram of the LIE exper'mental system used for measuring fluorescence decay lifetimes as a function
of temperature. DAS is a data acquisition system; PMT is a photomultiplier tube.
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Table 1.
Regression fits calculated for 611 nm Y,0 3:Eu over the temperature range 530-1000°C.
12-stage fast risetime (-2 ns) photomultiplier tube (PhilipsXP2233B) the output of which was amplified and fed into a 350
MHz oscilloscope (LeCroy-Model 9450). During each measurement
cycle, 300 fluorescence pulses were accumulated and averaged by the
oscilloscope. Next, the resultant averaged pulse was transmitted to a
P.C. for additional processing via IEEE-488 communication interface.
Because of the negligible contribution of the fiber's dispersivepulse broadening mechanisms on the constant of the decay lifetimes
(1000 to 1 useconds), employing a deconvolution technique to correct for the impulse response of the collection optical fibers was
deemed unnecessary at 611 nm (Alaruri et al, 1994).
Decay
lifetimes
(Ps)
1392 a.1 > 396.9
396.9 > > 4.03
RESULTS AND DISCUSSION
Utilizing the previously reported L1F calibration system (Alaruri
et al, 1993), fluorescence decay lifetimes over the temperature range
—530-1000°C were collected for Y203:Eu (4.52% Eu). The fluorescence decay lifetimes were calculated by taking the natural logarithm
of the resulting averaged fluorescence pulse, fit), which can be expressed assuming a first-order sample approximation as
F(t)=Fo e.sp(-th)
4.03 > e > 0.48
Values of
constants
Fit
and r2a
T = [el + cr3 ]-1
c = 0.002
410 >T> 610°C
c2 = 2.776 x 10-3
T=ci +c2 In (2)
r2 = 0.949
et = 932.293
610 > T > 850°C
c2 = -53.136
T = exp (ci + c2t)
r2 = 0.999
ci = 7.033
850 > T > 1000°C
Q = -0.177
r2 = 0.998
ar2 is the coefficient of determination
(1)
Field Test of 1-IF System
where t is the average fluorescence decay lifetime and F0 is the signal
amplitude. The best fit for the linear portion of the data (i.e. slope = IA) was computed using linear least-squares regression analysis.
Figure 2 depicts three sets of fluorescence decay lifetime measurements obtained for the 611.0+0.2 nm-Y203:Eu emission line.
Clearly, the onset quenching temperature for Y203:Eu is approximately 510.01-0.6°C. No variation in the fluorescence decay lifetime
was recorded beyond 1000°C. At 1000°C the measured average
decay lifetime for the 611 nm emission line was 0.480+0.005 Ms.
The thermal quenching of rare-earth doped thermographic phosphors
at the onset quenching temperature can be ascribed to a chargetransfer phenomenon which is best described by Struck and Fonger
(Struck et al, 1971 and 1970). Utilizing Mathematica® (Wolfram,
1994), empirical regression fits were calculated for the temperature
response of Y 203:Eu as a function of decay lifetimes. The obtained
functional forms are listed in Table I.
The solid line which appears in Fig. 2 represents the empirical
fit calculated using the regression relationships given in Table I. An
average percentage error of ±2% was calculated between the experimental temperature measurements and the computed temperatures
employing these empirical fits.
Other phosphors such as YAG:Tb (terbium activated yttrium
aluminum gallium oxide) may be used for obtaining high surface
temperature measurements from engine parts. Because YAG:Tb
fluorescence lies in the green region of the electromagnetic spectrum
(544 am), it is particularly useful phosphor when used in engine locations where a large background signal component would be detected. However, the quantum yield of YAG:lb-544 nm emission
line is smaller than the Y203:Eu-611 am by roughly a factor of two
which makes Y203:Eu the favored phosphor for turbine engine applications (Alaruri et al, 1993).
Two experiments were conducted using the LIF system, to read
the surface temperature of a superalloy single-crystal coupon coated
with Y203:Eu. During these experiments, the thermographic phosphor coated-coupon was mounted in an open burner rig, as illustrated
in Fig. 3. The objectives of the field test were to examine the magnitude and the decay lifetimes of the fluorescence and background signals in a combustion environment and to estimate the survivability of
a phosphor-chemical binder in an engine-like environment. In this
test the coupon was subjected to gas flow velocities ranging between
407 and 245 mis and pressures of — 14-7 psi. Typical pressures and
gas velocities in a turbine engine range between Mach 0.8-0.9 and
—300 psi, respectively.
The Y203:Eu was applied to the surface of the coupon using a
proprietary chemical binder. An air brush was utilized in applying
the mixture of phosphor and chemical binder to the coupon front
surface. The average thickness of the deposited films was 25+5 gm.
The temperature of the phosphor-coated coupon was monitored using
a K-type thermocouples embedded into the back of the coupon.
Using an identical coupon made of the same material thickness
and instrumented with small diameter K-type thermocouples at both
surfaces (six thermocouples embedded into the backside and six
thermocouples embedded into the front side), the temperature gradient between the two surfaces at different temperature settings was
established for the open burner rig test. The collected temperature
readings for the front and back surfaces were used to compute an
empirical relationship correlating the back surface to the front surface
temperature measurements at different temperature settings. A data
acquisition system (HP 75000B) equipped with a thermocouples
relay multiplexer card was used to read the thermocouples outputs.
Figure 4 depicts the decay lifetime versus the corrected front
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10000 7
TEST 1
Decay Lifetimes 41S)
1000 -
•
9
•
TEST 2
*
TEST 3
FIT
100
10
-
0.1
0
200
400
600
Temperature ( °C)
800
1000
1200
Figure 2. Decay lifetimes versus temperature collected for the 1 1 3 02:Eu-611.0±0.2 nm emission line using 266 nm excitation.
surface temperature measurements obtained for a single crystal coupon
coated with Y 203:Eu, during the open burner rig test. The figure represents the measurements obtained for the same sample after two consecutive periods, 7.5 and 14.7 hours, of thermal exposure in the
burner-rig. An excellent reproducibility between the calibration data
and the two sets of measurements (i.e. 7.5 and 14.7 h) was obtained.
In this test, the chemical binder coating started to degrade after approximately 15 hours of continuous thermal exposure in the burnerrig. Approximately 50-60% of the coating was lost after 15 hours.
CONCLUSION
Rare-earth doped thermographic phosphors can be used for
monitoring the surface temperature of turbine engine parts. The
overall accuracy of the temperature measurements using the described
above technique was estimated to be + 2%.
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Figure 3. Coupon coated with YA:Eu mounted in the open
burner rig. As this photograph illustrates, the coupon was
rotated with respect to the flow direction to allow for optical access to the front surface.
4
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Figure 4. Plot of the decay Iletime-temperature measurements obtained from a coupon coated with Y,0,:Eu after two consecutive periods, 7.5 and 14.7 h, of thermal exposure in the open burner rig.
5
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