45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit
2 - 5 August 2009, Denver, Colorado
AIAA 2009-5052
Dual-pump CARS Temperature and Major Species Concentration Measurements in
a Gas Turbine Combustor Facility
Mathew P. Thariyan, Aizaz H. Bhuiyan, Ning Chai,
Sameer V. Naik, Robert P. Lucht, and Jay P. Gore
Maurice J. Zucrow Laboratories, Purdue University, West Lafayette, IN 47906
ABSTRACT
A gas turbine combustor facility (GTCF) has been built and operated for
measuring temperature and major species concentrations using dual-pump CARS in
combusting flows at above atmospheric pressures. The facility includes a stainless steel
window assembly that allows optical access from three sides with a pair of thin and thick
windows on each side. It is water-cooled and provides air film-cooling passages; thin
windows are designed for thermal load while thick windows are designed for pressure
loading. High-speed imaging of combusting flows is performed using the center injector
of a 9-point lean direct injection (LDI) device developed at NASA Glenn Research
Center. The combustor has been operated using Jet A fuel at inlet air temperatures up to
725 K and combustor pressures up to 10 atm. Dual-pump CARS temperature and major
species concentration measurements have been performed in the GTCF at inlet air
temperatures up to 725 K and combustor pressures up to 7 atm. Spatial maps of
temperature and major species concentrations have been obtained by translating the
CARS probe volume in axial and radial directions inside the combustor rig. These
measurements will be used for validation of CFD codes under development at NASA
Glenn Research Center.
INTRODUCTION
A gas turbine combustion facility (GTCF) has been developed at the HighPressure Laboratory (HPL) in Purdue’s Zucrow Laboratory complex. The development
of the facility began under a collaborative project with Rolls Royce Corporation in
Indianapolis, Indiana, funded by the State of Indiana’s 21st Century program. The
Currently, we have performed dual-pump coherent anti-Stokes Raman scattering (CARS)
measurements in the GTCF at pressure up to 7 atm. at 725 K inlet air temperature. These
recent activities are funded under a cooperative agreement with NASA Glenn Research
Center in Cleveland, Ohio.
As part of this effort, a new laser system has been developed to measure
temperature and species concentrations using dual-pump N2-CO2 coherent anti-Stokes
Raman scattering (CARS) [1]. An injection-seeded optical parametric oscillator (OPO) is
used as a narrowband pump laser source to improve the accuracy and precision of dualpump CARS measurements. For the CO2 molecule, Raman transitions near 1300 cm-1
are excited by 532-nm pump beam and 607-nm Stokes beam; while the probe beam at
Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
560 nm is scattered from the induced polarization to generate the CARS signal near 496
nm. For the N2 molecule, the pump and probe beams are reversed with the CARS signal
produced at the same wavelength. The second harmonic output (~532 nm) from an
injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. An OPO
consisting of two nonlinear E-BBO crystals is pumped using the third harmonic output
(~355 nm) of the same Nd:YAG laser and injection-seeded using an external cavity diode
laser at an idler wavelength around 970 nm to generate signal near 560 nm [2]. Output
from a broadband dye laser (BBDL), pumped using the second harmonic output of an
unseeded Nd:YAG laser to generate radiation (FWHM ~ 250 cm-1) centered near 607 nm,
is employed as the Stokes beam. This system was used for an extensive set of
measurements in laminar counterflow flames and the system is currently being installed
for measurements in the GTCF.
EXPERIMENTAL SYSTEMS
Dual-Pump CARS System
A laser system, comprised of two narrowband pump beams and a broadband Stokes
beam, was developed to measure temperature and species concentrations using dualpump coherent anti-Stokes Raman scattering in a counter-flow burner facility. A
diagram showing the generation of the three laser beams required for the dual-pump
CARS process is shown in Fig. 1. An injection-seeded, Q-switched Nd:YAG laser
(SpectraPhysics Model Pro 290-10) with a repetition rate of 10 Hz and a pulse width of
approximately 8 ns (FWHM) was used to produce one of the pump beams in the CARS
process and to pump the optical parametric process. The Nd:YAG laser is equipped with
active-feedback-control systems to obtain both excellent pointing stability (BeamLok)
and minimum divergence (D-Lok) of the output laser beams. The second harmonic
output of the Nd:YAG laser at 532 nm was passed through a combination of a half-wave
plate and a Glan polarizer to provide variable power attenuation of the output beam
during alignment and measurements. The 532-nm pump beam was directed through a
delay line so that it arrived at the probe volume coincident in time with the remaining two
beams. Approximately 175 mJ of the 532-nm output from the same Nd:YAG laser is
used to pump a broadband dye laser.
An optical parametric oscillator (OPO) was employed to produce the second
pump beam. The OPO consisted of a pair of counter-rotating beta barium borate (EBBO) crystals, with two flat mirrors on either side of the crystals to form a feedback
cavity. The E-BBO crystals were cut at 30q to the optical axis and were supplied by
Fujian Casix. The face of each crystal has cross sectional dimensions of 8 mm u 10 mm.
The crystals were 12 mm long and were coated with MgF2 protective coating. These
crystals were pumped using the 100 mJ/pulse of the third harmonic output at 355 nm
from the above-mentioned injection-seeded Nd:YAG laser.
One of the flat mirrors had nearly 100% reflectivity at the signal wavelength, but
low reflectivity at the pump and idler (seed) wavelengths. The other flat mirror with a
reflectivity of 70% served as the output coupler. The optical parametric process was
initiated using a continuous wave (cw) external cavity diode laser (ECDL) (Toptica DL100) at the idler wavelength of 970 nm. The ECDL produced 100 mW of single-
longitudinal-mode output and its wavelength was adjusted by manually rotating the
diffraction grating to achieve coarse tuning or by varying both the diode temperature and
current using a diode laser controller (TuiOptics Series DC 100). The angles of the EBBO crystals were adjusted to obtain the signal at the ECDL wavelength. Type 1 phase
matching was for the optical parametric process, with the pump beam horizontally
polarized and both the idler and signal beams vertically polarized. 100 mJ/pulse of 355
nm pump radiation and 20.5 mW of ECDL radiation at 970 nm were incident on the first
E-BBO crystal. The OPO signal intensity can increase by a factor of at least 50 times
when it was injection-seeded [2]. Two Faraday isolators, with a total attenuation of 60dB, were used to prevent any back reflection into the ECDL that could cause instability in
the diode laser. The 560-nm output signal beam was separated from the residual 355 nm
pump beam using a dichroic mirror and a polarizer. The output signal from the OPO was
amplified using a pulsed dye amplifier (PDA) consisting of a dye cell filled with 25 mg
Rhodamine 590 in 750 ml of methanol. The dye cell was pumped by the second
harmonic output (~532 nm) of an unseeded Nd:YAG laser (Quanta-Ray GCR4). A lensaperture combination was used to reduce the undesirable amplified stimulated emission
(ASE) produced in the PDA. The 560-nm signal beam from the OPO was directed
through the dye cell and the overall temporal overlap of the 532-nm pump and signal
beam in the dye cell was maintained using a delay generator (Berkeley Nucleonic
Corporation Model 55) which adjusted the Q-switch delay between the OPO pump and
PDA pump lasers. The dye cell was tilted at Brewster’s angle to allow maximum
transmission of the signal beam. The overall experimental arrangement to produce the
560-nm beam can be seen from Fig. 1; while the optical details regarding the OPO can be
seen from Fig. 2.
A broadband dye laser (BBDL) was pumped using 175 mJ/pulse of the second
harmonic output (~532 nm) of the above mentioned injection-seeded Nd:YAG laser so as
to produce the Stokes beam required in the CARS process. 25 mg of Rhodamine 640 dye
in 750 ml of methanol was used to produce laser radiation with a center wavelength near
607 nm (Ȧs = 16474 cm-1) and linewidth of approximately 250 cm-1 FWHM. Two flat
mirrors, coated at 600 nm, with 100% and 30% reflectivity were used at the front and
back of the oscillator cavity, respectively. The 30% reflectivity mirror with a wedged,
uncoated glass flat acted as the output coupler. The Stokes beam was directed through a
delay line so that it arrived at the probe volume coincident in time with the two pump
beams. The beams were directed to the cell containing the windowed-combustor.
The two pump beams and the Stokes beam, arranged in a folded BOXCARS
configuration, were passed through prisms fixed to high-precision mounts so as to allow
fine tuning of their spatial overlap in the probe volume inside the windowed-combustor.
The beams were passed through 2 in. prisms mounted on horizontal and vertical
translation stages in order to facilitate traversing the probe volume inside the combustion
chamber. The timing of the three beams at the probe volume was checked using a fast
photodiode and an oscilloscope to ensure that all beams arrived within 1 ns of each other.
The three beams were focused inside the combustion chamber using a plano-convex lens
having a focal length of 180 mm. For the N2-CO2 dual-pump CARS system, spatial
overlap of the 532 nm pump beam with the 607 nm Stokes beam produces a rovibrational N2 Raman polarization that coherently scatters the 560 nm pump beam,
yielding N2 CARS spectrum near 496.5 nm. Simulataneously, the 560 nm pump beam
and the 607 nm Stokes beam produce a ro-vibrational CO2 Raman polarization that
scatters the 532 nm pump beam, yielding a CO2 CARS spectrum that also appears near
496.5 nm. The energies of the 532-nm, 560-nm, and the Stokes beam were nominally 20
mJ/pulse. The three beams and the CARS signal were re-collimated using another 180
mm plano-convex lens. The input beams were separated using prisms and beam dumps.
The CARS signal was focused onto an adjustable entrance slit of a 0.5-meter
spectrometer using a spherical convex lens having 200 mm focal length. The entrance
slit was typically adjusted to a width of 400ȝm, eliminating most of the residual scatter
due to the pump and Stokes lasers. The re-collimating lens and the mirrors for guiding
the signal beam into the spectrometer were mounted on another pair of translation stages
to keep the signal beam focused on the spectrometer slit when translating the probe
volume inside the combustion chamber. A Macor aperture was placed after the recollimating lens to reduce the flame emissions. An interference filter was placed in the
CARS beam path to block the remaining scattering from the pump and Stokes beams. A
mechanical shutter was also incorporated in front of the spectrometer to minimize the
interference from flame emission and to block the scattered light. A picture of the optical
arrangement to generate the CARS signal using the three laser beams mentioned above is
shown in Fig. 3.
In order to correct for the spectral intensity variation of the BBDL, the three
CARS input beams were taken through another leg that also focuses the signal beam into
the spectrometer. This “reference leg” was established using 2 in. prisms and mirrors
mounted on translation stages. The non-resonant signal was collected by flowing an inert
gas through the probe volume. This leg was used to align the three incoming laser beams
to optimize the dual-pump CARS signal before passing them through the windowedcombustor. A picture of the optical arrangement in the reference leg to generate the
CARS signal using the incoming laser beams is shown in Fig. 4.
The optical grating of the spectrometer had 1200 grooves/mm. Two camera
lenses were used at the exit slit of the spectrometer so as to image the CARS signal onto
the detector. A 28-mm focal length, f/2.8 Nikon camera lens and a 70-210 mm adjustable
focal length, f/4.5-f/5.6 Nikon telephoto lens were used to increase the system dispersion.
An Andor back-illuminated, unintensified CCD camera (Andor MCD DU-440) with a
2048 × 512 pixel array was used to detect the CARS signals. Each pixel had an active
area of 13.5 ȝm2. The spectral dispersion of the CARS signal was 0.25 cmí1/pixel and
the resolution of the CARS detection system was approximately 0.8 cmí1. The N2 CARS
signal was shifted relative to the CO2 signal by scanning the wavelength of the
narrowband OPO pump source from 560 nm to 561.4 nm by tuning the ECDL. The
frequency separation of the two CARS signals was adjusted and optimized so that the
main features for the two molecules did not overlap, allowing the CARS spectra to be
imaged onto the CCD camera with the highest possible resolution.
Gas Turbine Combustion Facility (GTCF)
The gas turbine combustor facility (GTCF) at Purdue University, shown in Fig. 3,
has been built for conducting combustion tests at higher than atmospheric pressure using
commercial and research injectors at various flight operating conditions. A compressor
system produces 137 atm air at a flow rate of 0.45 kg/sec. There are 56.6 actual cubic
meters of air storage for 137 atm pressure. Heated high-pressure air is introduced into the
facility using the remotely-actuated valves and regulators. A natural gas-fired air heater
with a maximum discharge temperature up to 750 K at 4 kg/sec and 48 atm is utilized for
heating the inlet air. The inlet air mass flow rates are precisely controlled and measured
using dome-loaded pressure regulators and sonic orifices. After flow straightening, inlet
air enters the NASA top-hat LDI assembly where the flow passage is transitioned from
circular to square cross-section measuring 76.2 mm on each side. The combustor
window assembly (CWA) is installed downstream of the NASA top-hat LDI assembly
and has internal square flow passage of side 76.2 mm each. The LDI assembly protrudes
into the CWA such that the injector exit aligns with the upstream end of the windows
used in the window assembly, thereby allowing optical access to the entire combustion
zone. The optical access is 63.5 mm (2.5 in) in the axial location and 38.1 mm (1.5 in)
high with location of the middle line fuel injectors coinciding with the center of the
window. This can allow visualization of the combustion process in the middle line of
injectors and studying the interaction between adjacent combusting swirling streams from
the two sides. An annular effusion-cooled liner, utilizes a small percent of the relatively
lower temperature upstream inlet air, is installed downstream of the CWA. A waterquench section is installed downstream of the liner, where the sampling of gases is
performed for further analysis if necessary. A high-pressure pump with a maximum
capacity of 75 liters per minute of water at 31 atm is used to pump high-pressure cooling
water into the combustor. The water circulates through the quench section and mixes with
combustion exhaust products. A butterfly valve is used to control the pressure inside the
combustion chamber. The cooling water helps maintain the temperature of the butterfly
valve in its operating range.
Air is pre-heated to the burner inlet temperature (BIT) prescribed for the operating
conditions, with the maximum BIT of 725 K. Two pressurizable fuel tanks filled with Jet
A fuel are used to independently control the fuel flow rate into the injectors. Control valves
and flow-meters are used respectively to control and record the fuel flow in each fuel line to
the combustor. An igniter system is installed on the unused face of the CWA using metal
windows. The CWA is cooled using de-ionized water. During the operation of GTCF,
important parameters involved in the combustion process are monitored real time. These
include the air flow rate, fuel-air ratio, and normalized pressure drop across the injector, fuel
injector face temperature, cooling water flow rates and temperature, combustion exhaust
temperature at the quench section, temperature at back-pressure control valve.
NASA 9-point Top-hat LDI assembly
Lean-direct injection (LDI) concept has been among the several combustion
concepts studied by NASA Glenn Research Center (GRC) to reduce NOx emissions
while maintaining high combustion efficiency. LDI assemblies have been utilized to
develop a broad measurement database for characterizing fuel vaporization, turbulent
mixing and combustion process. The NASA 9-point top-hat LDI assembly is a multiplex
fuel injector containing nine fuel injection tips and multi-burning zones to replace one
conventional fuel-injector. Nine injectors are placed in a 3 × 3 square matrix
arrangement as shown in Fig. 5. The square flow passage, 76.2 mm long on each side,
has three injectors on each side. Each LDI element contains axial swirlers with helical
vanes generating swirling air that passes through a converging diverging venture section.
The simplex fuel injector is inserted through the center of the swirler so that the fuel tip is
at the throat of the venturi. Fuel mixes rapidly with the incoming air, thereby, shortening
the distance for complete combustion. The flame zone is less than 10 mm long in the
axial direction for most operating conditions. This observation was made visually during
combustion tests at Purdue University as well as in measurements and computations
conducted at NASA GRC [3]. The “top-hat” refers to the injector exit face protruding
out as a 76.2 mm × 76.2 mm square section as compared to older versions of the
assembly where the injector exit plane was coincident with the rest of the injector
housing. The LDI assembly allows for various fuel-staging possibilities. In this study,
only the center fuel injector is operated for swirler vane angles of 60 degrees.
Combustor Window Assembly (CWA)
The CWA is designed to have two windows, one thin and one thick window, in
each viewing direction. The CWA consists of a retainer sub-assembly, shown in Fig 6,
with square inner and outer cross-sections that retain the thin windows from one side.
The retainer sub-assembly consists of two flanges that interface with the facility brazed
on end faces of a retainer section. The retainer section contains grooves for the
placement of the thin windows to maintain a consistent square passage section for the
combustion products. The retainer sub-assembly is surrounded by four water-cooled
clamping sections. These clamping sections have grooves for the thicker windows. The
thick windows are held in place using four smaller clamps.
Water cooling passages are machined internally to cool the retainer section during
combustion tests using de-ionized water. Water cooling manifolds are machined on the
faces of the upstream and downstream flanges. The upstream flange has another
manifold for cooling air. The retainer section has corresponding passages for cooling air.
These three sections, namely the upstream flange; the retainer section; and the
downstream flange are brazed together ensuring that air and water cooling lines are
independent of each other. Cooling air lines are designed to use a small quantity of the
inlet air to film-cool the inside faces of all the thin windows that are subjected to
combustion products. The air passage slots are at angle of 15 degree with respect to the
bulk air flow direction. They can be observed in Fig. 7, which shows axial and radial
cutouts of the CWA. Thin windows are 6.25 mm (0.25 in) thick and made out of fused
silica. The edges of the thin windows are chamfered at 45 degree with a total viewing
area 63.5 mm (2.5 in) long and 38.1 mm (1.5 in) high. The retainer section has
complementary grooves for the windows. Three thin windows are installed on three
faces of the retainer section while the fourth face consists of a metal window machined to
incorporate an igniter. Four clamping sections are assembled outside the retainer subassembly. The four outer faces of the retainer section have grooves machined on the
surface to incorporate metal O-rings to ensure sealing when in contact with the clamping
sections. The clamping sections also have internal passages machined for water cooling.
Each clamping section has one groove for a thicker window. The thick windows are 12.7
mm (0.5 in) thick and also made out of fused silica. Graphite gaskets are used on both
sides of the thick windows to ensure sealing. The clamping sections also have a
protrusion on the inner side which constrains the thin windows when assembled. The
thin windows are shielded from the stainless steel by using porous Fiberfrax ceramic on
the contact surfaces. The thin windows endure most of the thermal load due to
combustion. To reduce pressure loading on the thin windows, the pressure on the cooler
face of these windows is maintained close to combustor pressure at the hotter face. This
is ensured by using some fraction of the inlet air to pressurize the chamber between the
thin and the thick windows. The thicker windows, thereby, experience most of the
pressure loading of the combustor. The thermal load on thick windows is very less due to
relatively cooler inlet air in the chamber between the thin and thick windows. The
thicker windows are constrained using four small outer clamps. De-ionized water at 5
atm (75 psia) inlet pressure is used to cool both the retainer sub-assembly and the four
clamping sections. Inlet and outlet water temperatures are monitored during an
experiment to ensure smooth operation.
EXPERIMENTAL RESULTS
Combustion tests were conducted on the GTCF in a single-point injection mode,
with fuel supplied only to the central injector. For ignition, inlet air temperature was
maintained between 645-670 K (700-750 ºF) at a combustor pressure of 4 atm (60 psia)
and a normalized pressure drop of 3 % across the LDI injector. The air flow rate was
approximately 0.21 kg/s (0.46 lbm/s) and the equivalence ratio was approximately 3.5 for
the central injector (0.39 overall). Flames were stabilized at combustor pressures of 5.5,
6.8, 8.2 and 9.9 atm (80, 100, 120 and 145 psia) at equivalence ratios ranging from 0.4 to
1.2, while maintaining the normalized pressure drop across the injector at a value of 4 %.
Table 1 displays the initial operating conditions of the GTCF the flame was stabilized at
in order to choose conditions at which to conduct dual-pump CARS measurements. A
Phantom v7.1 digital high-speed camera and a Canon XL2 digital camcorder were used
to record videos of the combustion tests. Figure 8 shows frames from videos recorded of
the GTCF operating at inlet air temperatures of 690-725 K (780-850ºF), combustor
pressures of 7-8.2 atm (100-120 psia). The windows stayed intact and clean during the
combustion tests. The flame was non-sooting even at center injector equivalence ratios
close to 1.2. It was found that in the single-point injection mode the flame was
susceptible to blow-off and stable operation could not be achieved for air inlet
temperatures below 645 K (700ºF). The inlet air temperature was raised to 725 K (850ºF)
for stable operation of the combustor for the dual-pump CARS measurements.
Dual-pump CARS measurements were obtained inside the flame zone of the
combustor at various pressure and equivalence ratio conditions. Table 2 indicates the
conditions for these measurements. The inlet air temperature was maintained at 725 K
(850ºF) for all the conditions. The location of the measurement point inside the
flamezone was changed using equal movements of the translation stages on both sides of
the combustor. At a combustor pressure of 7 atm (104 psia), measurements have been
conducted at equivalence ratios of 0.4, 0.59, 0.8 and 1.0. At a combustor pressure of 5.4
atm (80 psia), measurements were obtained at an equivalence ratio of 1.0. The closest
axial measurement location to the injector was 10 mm downstream, while the furthest
was 50 mm downstream. Measurements were obtained in steps of 5 mm axially and 3
mm radially. The span in the radial direction was 11 mm. on both sides of the injector
center-line. At least 1000 single-shot spectra were acquired at each location. At
locations closer to the fuel injector exit, many spectra were saturated by scattering from
unevaporated liquid fuel droplets. Therefore, 2000 single-shot spectra were acquired for
these locations.
processing.
The saturated spectra were manually discarded later during data
Due to potential of steering the CARS signal beam as a result of the turbulence
and refractive index gradients inside the combustor, the spectrometer slit was kept
relatively widely open at 400 ȝm. In order to minimize misalignment of the optics
surrounding the combustor while operating for longer durations, metal plates were placed
on both sides of the combustor. This helped in shielding the optics from the heat load of
the combustor, especially for higher equivalence ratio cases. High quality optical mounts
were used for stable operation of the optical arrangement even at such harsh conditions.
To reduce the background flame emission interference with the CARS signal beam, the
CCD camera was used in conjunction with a fast mechanical shutter from Uniblitz
Shutters to acquire data only for a time interval of approximately 3 ms. This reduces the
flame emissions and also helps reduce the noise in the dual-pump CARS spectra.
Background-corrected CARS spectra were normalized using a non-resonant
spectrum obtained by flowing pure argon gas through the reference leg to account for the
spectral variations in the BBDL. The square-root of background corrected dual-pump
CARS signal was compared with theoretical CARS spectra calculated using the Sandia
CARSFT code [5]. Experimental CARS spectra were analyzed in two steps. First, the
N2 part of the spectrum was analyzed using the CARSFT code to obtain temperature at
the measurement location. The CO2 concentration was then determined by fixing the
temperature at the value obtained by analyzing the N2 part of the spectrum. The
CARSFT process requires a multi-variable least squares fit analysis to optimize values of
variables such as temperature, major species concentrations, spectrometer instrument
linewidth.
Figure 9 compares typical experimental single-shot dual-pump CARS spectra to
the theoretical CARS spectra calculated using the Sandia CARSFT code at 30 mm and 40
mm downstream of the injector exit along the injector centerline at a nozzle equivalence
ratio of 0.8, 6.8 atm (100 psia) combustor pressure, and an inlet air temperature of 725 K
(850ºF). The measured temperature for the 30 mm location is 1138 K and the CO2/N2
concentration ratio is 0.0477. At each location, single-shot spectra are analyzed by
running the Sandia CARSFT code in a batch mode. Figure 10 compares typical
experimental single-shot dual-pump CARS spectra to the theoretical CARS spectra
calculated using the Sandia CARSFT code at 30 mm and 40 mm downstream of the
injector exit along the injector centerline at a nozzle equivalence ratio of 0.59, 7.0 atm
(104 psia) combustor pressure, and an inlet air temperature of 725 K (850ºF). The singleshot spectra are used to obtain flowfield temperature and CO2/N2 concentration statistics
for a given measurement location. Figure 11 probability density functions (PDF) of the
temperature for single-shot dual-pump CARS spectra obtained for various operating
conditions. From such data, we can find the mean temperatures and standard deviations.
We are currently working on the remainder of the data analysis to generate such
histograms throughout the combustor. Similar PDFs for major species concentrations (N2
and CO2) will also be generated. From the spectral fits to individual single-shot spectra
as well as from the resulting probability distribution function of temperature, we can use
the current optical setup to determine flowfield statistics inside the combustion zone at
high pressures in such a harsh environment using the dual-pump CARS technique. More
detailed analysis of the dual-pump CARS spectra is being conducted for all operating
conditions and measurement locations. In the near future, we plan to conduct dual-pump
CARS temperature and CO2/N2 concentration ratio measurements at pressures up to 10
atm and fuel-lean conditions using the same injector.
CONCLUSIONS
A laser system has been developed to measure temperature and species
concentrations using dual-pump N2-CO2 coherent anti-Stokes Raman scattering. For the
CO2 molecule, Raman transitions near 1300 cm-1 are excited by 532-nm pump beam and
607-nm Stokes beam; while the probe beam at 560 nm is scattered from the induced
polarization to generate the CARS signal near 496 nm. For the N2 molecule, the pump
and probe beams are reversed with the CARS signal produced nearly at the same
wavelength. A windowed-gas turbine combustor facility (GTCF) has been built and
successfully operated at inlet air temperatures up to 725 K (850ºF) and combustor
pressure up to 10 atm. (145 psia) for studying above atmospheric combusting flows. The
facility includes a stainless steel window assembly that allows optical access from three
sides with a pair of thin and thick windows on each side. High-speed imaging of
combusting flows is performed using the center injector of a 9-point lean direct injection
(LDI) device developed at NASA Glenn. Dual-pump CARS temperature and major
species concentration measurements have been performed at various equivalence ratios
for combustor pressures up to 7.0 atm and inlet air temperatures up to 725 K. The CARS
probe volume has been successfully traversed in axial and radial directions inside the
combustor rig while maintaining the optical alignment at such harsh conditions.
Temperature probability density functions have been obtained for some measurement
locations inside the flamezone by analyzing the single-shot CARS spectra. Spatial maps
of both temperature and major species concentrations are being generated using the
single-shot spectra acquired at various locations inside the combustor rig by translating
the CARS probe volume. Such measurements will be helpful for validation of CFD code
development activities at NASA Glenn.
ACKNOWLEDGEMENTS
Funding for this work was provided by NASA Glenn Research Center under
Cooperative Agreement Number NNX07AC90A. The technical monitor of the project is
Dr. Yolanda Hicks. We wish to thank Dr. Hicks and Drs. Clarence Chang and Randy
Locke at NASA Glenn for numerous helpful suggestions concerning the design and
fabrication of the GTCF, and for the loan of the 9-point LDI device and other equipment.
We thank Drs. Nader Rizk, Mohan Razdan, Vic Oechsle, Dan Nickolaus, and Duane
Schmith at Rolls Royce Corporation in Indianapolis, Indiana for helpful technical
discussions during the course of the project. We would especially like to thank Dr. Vic
Oechsle for sending us solid diagrams of the window assembly.
REFERENCES
1) Lucht, R. P., Velur, V. N., Carter, C. D., Grinstead, Jr., K. D., Gord, J. R.,
Danehy, P. M., Fiechtner, G. J., and Farrow, R. L., “Measurements of
Temperature and CO2 Concentration by Dual-Pump Coherent Anti-Stokes Raman
Scattering,” AIAA Journal, Vol.41, No.4, 2003, pp. 679-686.
2) Kulatilaka, W.D., Anderson, T.N., Bougher, T.L., Lucht, R.P., “Development of
Injection-Seeded, Pulsed Optical Parametric Generator/Oscillator Systems for
High-Resolution Spectroscopy,” Applied Physics B, Vol. 80, No. 6, 2005, pp.
669-680.
3) Tacina, K. M., Lee C. M., and Wey C., “NASA Glenn High Pressure Low NOx
Emissions Research,” NASA Technical Memorandum, 2008-214974, 2008.
4) N. Vora, J. E. Siow, N. M. Laurendeau, “Chemical Scavenging Activity of
Gaseous Suppressants by using Laser-induced Fluorescence Measurements of
Hydroxyl,” Combustion and Flame, Vol. 126, No. 1-2, 2001, pp. 1393-1401.
5) R. E. Palmer, “The CARSFT Computer Code for Calculating Coherent AntiStokes Raman Spectra: User and Programmer Information,” Sandia National
Laboratories, Report No. SAND89-8206, Livermore, CA, 1989.
Tables and Figures:
Table 1: Operating conditions for the GTCF at a normalized pressure drop of 4% across
the injector.
Inlet
Temp
(K)
Combustor
Pressure (atm)
Air Mass Flow
Rate (kg/s)
Equivalence Ratio
Center Injector
Fuel Flow
Rate (kg/hr)
625
650
665
665
665
660
690
690
5.5
5.5
6.8
6.8
6.8
8.2
8.2
9.9
0.38
0.36
0.40
0.40
0.42
0.54
0.52
0.76
0.79
1.2
0.98
0.75
0.36
0.44
0.52
0.50
8.2
11.5
10.6
8.1
4.1
6.5
7.4
10.4
Table 2: Operating conditions for the GTCF at burner inlet temperature of 725 K (850ºF).
Normalized
Injector
Pressure
Drop (%)
Combustor
Pressure
(atm)
Air Mass Flow
Rate
(kg/s)
Equivalence Ratio
Center Injector
Fuel Flow
Rate
(kg/hr)
3
3
4
4
4
4
5.5
6.8
6.8
6.8
7.0
7.0
0.34
0.36
0.40
0.40
0.42
0.42
1.00
1.00
1.20
0.80
0.59
0.40
9.4
10.0
13.3
8.8
6.7
4.5
To Purdue GTCF
607 nm
BBDL
PDA 2
0q Mirror
355 nm
970 nm
BS
PDA 1
0q Mirror
ECDL
BS
BS
532 nm
Seeded Nd:YAG
560 nm
OPO
Figure 1: Schematic diagram of generation of laser beams for dual-pump CARS system
using an injection-seeded YAG laser and an injection-seeded OPO for the two pump
beams and a BBDL for the Stokes beam.
BD: Beam Dump
CM: Curved Mirror
ECDL: External Cavity Diode Laser
O/2: Half-wave-plate
M: Mirror
MA: Makor Aperture
OC: Output Coupler
OPO: Optical Parametric Oscillator
PDA: Pulsed Dye Amplifier
Pol: Polarizer
T: Telescope
CC: Concave lens
Pol. O/2
355nm
M
To PDA
ECDL
MA
OC
970 nm
T
560 nm
BD
Seeded Nd:YAG
Faraday
Isolator
M
CC
E-BBO Crystals
CM
BD
OPO Cavity
Figure 2: Experimental system to produce single-longitudinal-model laser radiation near
560 nm using an optical parametric oscillator.
Figure 3: Purdue gas turbine combustor facility (GTCF).
Figure 4: The “reference leg” of optics for acquiring the non-resonant CARS signal.
Figure 5: NASA 9-point top-hat LDI assembly.
Figure 6: Retainer sub-assembly of the combustor window assembly (CWA).
Figure 7: Schematic of axial and radial cut-outs of the window assembly.
(a)
(b)
Figure 8: Purdue GTCF operating at (a) 7 atm (104 psia) combustor pressure, 725 K
(850ºF) inlet air temperature, and nozzle equivalence ratio of 0.40, and (b) 8.2 atm (120
psia) combustor pressure, 690 K (780ºF) inlet air temperature, and nozzle equivalence
ratio of 0.52.
(arb. units)
1/2
(CARS Intensity)
140
30 mm downstream
Data
Theory
120
100
T = 1138 K
XCO2 / XN2 = 0.0477
80
60
N2
40
CO2
20
0
1315
1330
1345
1360
1375
1390
(CARS Intensity)
1/2
(arb. units)
Raman Shift (cm-1)
140
40 mm downstream
Data
Theory
120
100
T = 1222 K
XCO2 / XN2 = 0.0555
80
60
40
20
0
1315
1330
1345
1360
1375
1390
Raman Shift (cm-1)
Figure 9: Comparison between experimental and theoretical dual-pump CARS spectra at
two downstream locations at combustor pressure of 6.8 atm (100 psia), inlet air
temperature of 725 K (850ºF) at a nozzle equivalence ratio of 0.8.
(arb. units)
1/2
(CARS Intensity)
140
30 mm downstream
Data
Theory
120
100
T = 1138 K
XCO2 / XN2 = 0.0572
80
60
40
20
0
1315
1330
1345
1360
1375
1390
-1
(CARS Intensity)
1/2
(arb. units)
Raman Shift (cm )
100
40 mm downstream
80
Data
Theory
T = 1227 K
XCO2 / XN2= 0.0515
60
40
20
0
1315
1330
1345
1360
1375
1390
-1
Raman Shift (cm )
Figure 10: Comparison between experimental and theoretical dual-pump CARS spectra at
two downstream locations at a combustor pressure of 7.0 atm (104 psia), inlet air
temperature of 725 K (850ºF) at a nozzle equivalence ratio of 0.59.
Phi 0.45_100 psi_30 mm_11 mm Up
Phi 0.6_100 psi_30 mm_11 mm Up
500.0
400.0
Mean = 1023.1
Std. Deviation = 90.6
Std. Deviation/Mean = 0.088
Mean = 1073.1
Std. Deviation = 61.8
Std. Deviation/Mean = 0.058
350.0
400.0
300.0
Frequency
Frequency
300.0
200.0
250.0
200.0
150.0
100.0
100.0
50.0
0.0
600.0
800.0
1000.0
1200.0
1400.0
1600.0
0.0
800.0
1800.0
900.0
1000.0
Temperature (K)
Phi 0.8_100 psi_50 mm
1300.0
1400.0
1500.0
Phi 1.0_100 psi_40 mm
250.0
Mean = 1298.6
Std. Deviation = 162.4
Std. Deviation/Mean = 0.125
Mean = 1763.9
Std. Deviation = 357.7
Std. Deviation/Mean = 0.203
200.0
Frequency
200.0
Frequency
1200.0
Temperature (K)
250.0
150.0
100.0
50.0
0.0
800.0
1100.0
150.0
100.0
50.0
1000.0
1200.0
1400.0
1600.0
Temperature (K)
1800.0
2000.0
0.0
1000.0
1500.0
2000.0
2500.0
3000.0
Temperature (K)
Figure 11: Probability density functions for temperature determined from spectral fits to
single-shot dual-pump CARS spectra measured at few measurement locations for
different operating conditions of the GTCF.