The LeRC program in research sensors is directed at development of sensors and sensing techniques for research applications on turbine engines and propulsion systems. In general, the sensors are used either to measure to response of an engine component to the imposed environment. Locations of concern are generally within the gas path and, for the most part, are within the hot section of the engine. Since these sensors are used for research testing as opposed to operational use, a sensor lifetime of the order of 50 hr is considered sufficient. The following discussion presents a sample of this work, describing programs to develop a dynamic gas temperature measuring system, total heat flux sensors, a variety of thin-film sensors, and high-temperature strain measuring systems

Englund, David R.

English

NASA Technical Reports Server

N92-22526
II - RESEARCH SENSORS
David R. Englund
The Lewis program in research sensors is directed at development of sen-
sors and sensing techniques for research applications on turbine engines and
propulsion systems. In general, the sensors are used either to measure the
environment at a given location within a turbine engine, or to measure the
response of an engine component to the imposed environment. Locations of con-
cern are generally within the gas path and, for the most part, are within the
hot section of the engine. Since these sensors are used for research testing
as opposed to operational use, a sensor lifetime of the order of 50 hr is con-
sidered sufficient. The following discussion will present a sample of this
work, describing programs to develop a dynamic gas temperature measuring system,
total heat flux sensors, a variety of thin-film sensors, and high-temperature
strain measuring systems.
DYNAMIC GAS TEMPERATURE MEASURING SYSTEM
One of the most important parameters in a turbine engine hot section is
gas temperature. Normally only time-averaged temperature is measured. Fluctu-
ations in gas temperature are, however, of great concern for hot section dura-
bility and combustor modeling activities. The dynamic gas temperature measuring
system uses a probe (fig. If-l) with two wire thermocouples of different diame-
ters, typically 75 and 25 pm (0.01 and 0.003 in.). The thermocouple junctions
are butt welded and are located midway between the supporting posts. The ther-
mocouples are within i mm of each other so that they are measuring essentially
the same gas sample. This probe provides dynamic signals with limited frequency
response. By comparing these signals over a range of frequencies, a compensa-
tion spectrum can be generated sufficient to provide compensated temperature
data over an extended frequency range. The target frequency response for this
system was i000 Hz. This is probably higher than necessary for questions of
durability loss due to thermal cycling, but if one is interested in modeling
combustor processes or the flow in a combustor, i000 Hz is a reasonable target.
The upper limit in achievable frequency response for such a system is deter-
mined by the signal-to-noise ratio.
This system has been developed and used to measure fluctuating tempera-
tures in both combustors and engines (refs. II-I to 11-4). Figure II-2 shows
dynamic temperature data obtained from a probe at the turbine inlet of a PWA
F-100 engine operating at an intermediate power setting and with an average
turbine inlet temperature of 925 °C (1700 °F). The plot on the left is the
dynamic signal from the 25-pm-diameter (0.O03-in.-diam) wire thermocouple with
no frequency compensation. The rms value of the temperature fluctuation is
41 °C (74 °F). The plot on the right is the compensated signal from the same
217
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thermocouple. The rms value of the temperature fluctuation is 218 °C (390 °F)
and the peak-to-peak fluctuation is over i000 °C (1800 °F). Such a large tem-
perature fluctuation implies that there are filaments of primary combustion
gas and dilution gas within the combustor exhaust stream.
TOTAL HEAT FLUX SENSORS
Another environmental parameter of interest for hot section durability
is total heat flux. We have developed miniature total heat flux sensors
(refs. 11-5 to II-I0) which can be welded into combustor liners and built into
cooled turbine airfoils. Figure II-3 shows one sensor configuration based on
the Gardon gage design as it would be built into an airfoil. In this case, the
airfoil must be opened so that the sensor can be installed from the cooled wall
side. A cylindrical cavity 1.5 nun in diameter (0.06 in.) is cut in the wall
leaving a thin membrane at the hot side surface. ISA type K thermocouple wires
are located in the cavity as shown, and then the cavity is filled with ceramic
cement. With this sensor, the temperature difference between the center of the
membrane and the side wall is a measure of heat flux. This temperature differ-
ence is measured with the Alumel-bladewall-Alumel differential thermocouple.
The use of the burner liner or airfoil material as part of a differential ther-
mocouple circuit is an innovation that considerably simplifies construction,
but it requires calibration of the materials involved. Calibration tests
showed that this technique could provide acceptable signals. These miniature
heat flux sensors must be calibrated over the temperature range in which they
will be used because of the nonstandard differential thermocouple and because
of the uncertainty in positioning the thermocouple junctions.
Total heat flux sensors have been used in tests on combustor liners and
on turbine airfoils. Figure 11-4 shows a segment of a combustor liner which
has been instrumented with five total heat flux sensors. The sensors are
7.5-mm-diameter (0.3-in.-diam) disks with thermocouple leads radiating from
the edge of the disk. The actual sensor part of the unit is at the center of
the disk and is only 1.5 mm (0.06 in.) in diameter. The sensors are individu-
ally calibrated and then welded into holes cut in the liner. Tests on combus-
tors such as this one have produced useful heat flux data over a range of
combustor operating conditions. Similar sensors built into turbine airfoils
have been less successful because of the sensitivity of these sensors to tem-
perature and/or heat flux gradients, which are more prevalent in turbine
airfoils.
As noted previously in this section, calibration of total heat flux sen-
sors over the range of temperatures and heat fluxes that will be encountered
is a requirement. Figure 11-5 shows a photograph of a heat flux sensor cali-
bration system developed at Lewis. The heat source is a 100-kW arc lamp. A
reflector is used to focus the energy from the arc onto a ceramic sensor holder.
This system can supply a maximum flux of 6 MW/m 2 (500 Btu/ft2-sec), which is
higher than the heat fluxes in present-day turbine engines. The system can
operate in both steady-state and transient modes. Two other roughly comparable
calibration facilities exist in this country; efforts to cross-compare calibra-
tion of test sensors have been started. This is especially important since a
national standard for heat flux sensor calibration does not exist for these
high levels of heat flux.
218
THIN-FILM SENSORS
Lewis has been the major advocate and sponsor for development of thin-
film sensors for turbine engine applications (refs. II-II to II-19). Thin-film
sensors applicable to turbine engines include temperature sensors, strain gages,
and heat flux sensors. Thin-film sensors are formed directly on the component
to be instrumented (fig. 11-6) by first depositing a suitable insulating film
and then depositing sensor and protective films as required. A stable, adher-
ent, pinhole-free insulating film is the base for the whole structure and is
the most critical element of the sensor.
An excellent application for thin-film thermocouples is the measurement
of the surface temperature of a cooled turbine vane such as shown in figure II-7.
The surface of the vane is covered with AI203 thermally grown from an anticor-
rosion coating and augmented with sputtered AI203. Pt and Pt-Rh films are
sputter-deposited with thermocouple junctions formed by overlapping the two
films at the desired spot. The films extend to the base of the vane where
leadwires are connected. Typical thicknesses are 3 _m for the AI203 and 5 _m
for each of the thermocouple alloy films; in this case, no cover film was used.
The advantage of this technique over the previous technology, which required
swaged thermocouple wires to be buried into grooves cut into the surface,
should be obvious.
The present state of the thin-film sensor technology is sufficiently
advanced that dynamic strain gages have been used on compressor blades, and
thermocouples have been used to measure turbine airfoil surface temperatures
in some turbine engine test facilities in the United States. Thin-film, high-
temperature static strain gages and thin-film heat flux sensors are still under
development. In addition, work is continuing on the basic thin-film sensor
technology with the goals of simplifying and improving sensor processing and
adapting the presently used techniques to other substrate and sensor materials.
One of our goals is to make the thin-film sensor technology available to
the whole U.S. turbine engine community. Impediments to wider usage of this
technology are many. One problem is that sensor fabrication is material spe-
cific; technology has not been established for a wide variety of materials.
Another problem is that the investment required to establish a thln-film sensor
fabrication capability is considerable, and commercial services for custom fab-
rication of thin-film sensors are not yet available.
HIGH-TEMPERATURE STRAIN MEASURING SYSTEMS
The most ambitious goal of the research sensor program is development of
980 °C (1800 °F) strain measuring systems. Approaches being followed in this
work include both wire and thin-film resistance strain gages and remote strain
measuring systems. The resistance strain gage work involves development of new
strain gage materials and extensive testing of available strain gages. Work on
remote strain measuring systems has involved three different system concepts
based on laser speckle patterns.
A major part of our work in resistance strain gages has been the develop-
ment of a new palladium-based strain gage alloy (refs. 11-20 and 11-21). The
219
outstanding property of this alloy is its repeatability of resistance over the
temperature range up to 980 °C (1800 °F). Repeatability of resistance over the
temperature range within a few hundred parts per million is a fundamental
requirement for a high-temperature strain gage alloy. Work is now underway to
develop thin-film and wire strain gage systems using this alloy. Work with
other strain gages has involved high-temperature evaluation testing (refs. 11-22
to 11-24) including a 700 °C (1300 °F) strain gage available from the People's
Republic of China.
Resistance strain gages are not the only approach to high-temperature
strain measurements. Attractive alternatives are found in a variety of remote
optical strain measuring systems, manyof which use the laser speckle pattern
as a basis for measurementsof in-plane surface deformation. Wehave worked
with two laser speckle systems: a photographic system in which speckle pattern
photographs are analyzed in an interferometric photocomparator, and an elec-
tronic system in which strain is determined from the shift in the speckle pat-
tern falling on a linear photodiode array. This work has included both
laboratory development and evaluation in test cell environments (refs. 11-23
and 11-25 to 11-27). A fundamental problem with remote optical systems in
test cell environments is interference generated within the viewing path;
methods for minimizing or eliminating this problem are being studied.
FUTURETHRUSTSIN RESEARCHSENSORS
Future work in research sensors will be strongly influenced by programs
to develop new materials for turbine engines which will permit significantly
higher hot section temperatures. These materials are expected to be in the
forms of metallic or intermetallic and ceramic matrix composites. The impact
of such developments will be twofold. First, these new materials will have
markedly different properties compared to the metals involved in the sensors
developments already described. The emphasis for surface-mounted sensors will
be on thin films, and extensive development of fabrication procedures for
these new substrate materials will be required. For remote sensors, there
will be requirements to adapt or develop new sensing techniques suitable for
these new materials.
The second area of impact is the higher temperatures that will be encoun-
tered. Higher temperatures may require new sensor materials and may ultimately
require abandonment of surface-mounted sensor techniques. Work is already in
progress on development of sensors and sensing techniques for ceramic compo-
nents for turbine engines. Results of studies on applicable sensor techniques
for measurements of surface temperature, strain, and heat flux are available
(refs. 11-28 and 11-29).
220
"':" 7_'HITE I.I-_OI,'I,_IiIAPH
CD-87-29397
Figure II-i. - Dynamic gas temperature probe.
TEMPERATURE,
oF
+ 4OO
+ 200
- 200
-400
74°F RMS
I
0 250 500
TEMPERATURE,
oF
+ 1500
+,°°I
-1000
- 1500
0 250 500
TIME, msec
CD-87-29398
Figure 11-2. - Fluctuating gas temperature measured at turbine inlet of PWA
F-100 engine. Left plot shows as-recorded data from 25-_m-diameter
(0.003-in.-diam) wire thermocouple with no compensation; right plot shows
same data compensated for flat frequency response to i000 Hz.
221
_TIIID SIDE
41 _, ..._liiiit!',iiii_;iiii:
__'" . CHROMEL {iiiiii!iiii!
"-BLADE WALL "--HOT SIDE
CD-87-29399
Figure II-3. - High-temperature heat flux sensor based on Gardon gage design.
ORIGINAL PAGE
BLACK AND WHITE PHOTOG_IAPH
..... CD-87-29400
Figure 11-4. - Segment of combustor liner instrumented with heat flux sensors.
222
CD-87-29401
Figure II-5. - Heat flux sensor calibration system at Lewis Research Center.
F- - INSULATING FILM
\ - COVER FILM
\
"_ k_'_ • • " x ". x \\
• ,\SENSOR FILM(S)k\,\
J
SENSOR STRUCTURE
CD-87-29402
Figure TT-6. - Cross section of thin-film sensor on blade.
223
CD-87-29402
Figure 11-7. - Cooled turbine vane instrumented with four thin-film
thermocouples.
224


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