Download - Dual-Pump CARS and OH PLIF PI: Andrew Cutler
National Center for Hypersonic Combined Cycle Propulsion
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Dual-Pump CARS and OH PLIF
PI: Andrew CutlerPhD Students: Gaetano Magnotti, Luca Cantu, Emanuela Gallo
The George Washington University
CollaboratorsPaul Danehy (ASOMB), Craig Johansen (NIA)
NASA Langley Research Center
June 16, 2011
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National Center for Hypersonic Combined Cycle Propulsion
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Background
• CFD methods employing semi-empirical models used in analysis of hypersonic airbreathing engine flow paths
– RANS (or Favre averaged) codes have models for turbulent stresses, mass and energy transport, turbulence chemistry interactions
– LES methods have models for subgrid scale turbulence
• Models depend upon experimental data for validation– Information on mean flow and statistics of the turbulent fluctuation in flow
properties
• Data requirements– Simple well defined supersonic combustion flows with well-known boundary
conditions– Time and spatially resolved– Good instrument precision– Converged statistics
• Approach– Dual-pump CARS
• T, mole fraction species
– Planar laser-induced fluorescence imaging of OH radical (PLIF)• Flow-visualization
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Flow Fields
Scramjet combustor
University of Virginia’s Dual-Mode Scramjet
Supersonic free jet flames (hot center jet with H2 co-flow)
Laboratory supersonic flame with 10 mm center jet
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Dual-Pump CARS
• Four-color mixing process at beam focus / intersection– Two narrowband pump (Green+Yellow) + broadband Stokes (Red) laser– Probe molecular Raman transitions– Two simultaneous processes
• Coherent signal beam (Blue)– Analyze with spectrometer– Fit spectra to theory to obtain T, mole fraction species
• Measurement time ~10 ns, volume ~50 µm x 1.5 mm
Rot.-vib. states
Virtual states
P1
S
P2
AS
P2
S
P1
AS
Process 1 (N2, H2) Process 2 (O2, H2)
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“Mobile” CARS System
• Laser cart– Nd:YAG (532 nm,1.2 J, 20 Hz, seeded) =
Green– YAG-pumped broad-band dye laser (~9.6
nm or 263 cm-1 at 605 nm) = Red– YAG-pump narrow-band dye laser (~ 0.07
cm-1 at 553 nm) = Yellow– Remote beam steering (picomotor mounts)– Control of beam size (telescopes) and
energy (polarizers/waveplates)
• Detection system– Focusing lens, polarizer, remote beam
steering– 1 m monochromator– Cooled CCD
Laser cartTransmission Collection Detection
Beam Relay System (BRS)
Narrowband and broad-band dye laser beams on top of each other
Flame
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Beam Relay System at UVa• Moves beam crossing within flow field; 3 components, typically 2
motorized
Detection
Beams from laser cart
Mot
ion
Motion
Mot
ion
Linear bearings/rails*
*Horizontal motions are motor driven
5’x8’ optical table
Dual mode combustor
Measurement point
Transmission
CollectionCARS window
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Transmission and Collection Optics
• Laser beams focused and combined with separate mirrors and lenses• Focal plane imaging system
– Beam focusing– Beam crossing
From BRS and Cart
Focal plane imaging system: microscope objective and CCD
camera
To BRS and Detection
Measurement volume
Multi-pass dichroic splitter
Signal beam
Lenses
ND filter
Uncoated glass splitter
Focal plane images
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Optical Implementation at UVa
Transmission optics platform
Monochrometer
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Method for Fitting Spectra
• Sandia CARSFT code– Computes theoretical spectra
• New algorithm* fits spectra by interpolating from library of pre-computed spectra
• Novel feature is structure of library (sparsely packed) and method for interpolation of spectra from library– Smaller libraries, faster library generation, faster fitting
• Allows fitting of more chemical species and faster turn around– Enables “WIDECARS”
*Cutler, A.D., Magnotti, G., J. Raman Spectroscopy, 2011.
no. chem
species
sparse library size
regular library size
size ratio
1 10 10 1.002 60 100 1.673 270 1000 3.704 1080 10000 9.265 4050 100000 24.696 14580 1000000 68.59
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Measurements in Hencken Burner
• Hencken flat flame burner– H2-air flame– Flame temperature and composition known from theory
• Computed from gas flow rates assuming adiabatic reaction to equilibrium
Average DP CARS spectra at 3 equivalence ratios
Hencken burner10
100
1,000
10,000
100,000
0 200 400 600 800 1,000 1,200
Pixel
Inte
nsity
(cou
nts)
H2 S(9)
H2 S(5)O2
N2
H2 S(6)
=2.34 =1.17 =0.23
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10
100
1000
10000
100000
2050 2100 2150 2200 2250 2300 2350
Wavenumber/cm-1
CA
RS
In
te
ns
ity
/Arb
itr. U
nit
s
Theory
Experiment
10
100
1000
10000
2050 2100 2150 2200 2250 2300 2350
Wavenumber/cm-1
CA
RS
In
te
ns
ity
/Arb
itr. U
nit
s
Theory
Experiment
10
100
1000
10000
100000
2050 2100 2150 2200 2250 2300 2350
Wavenumber/cm-1
CA
RS
In
te
ns
ity
/Arb
itr. U
nit
s
Theory
Experiment
10
100
1000
10000
2050 2100 2150 2200 2250 2300 2350
Wavenumber/cm-1
CA
RS
In
ten
sit
y/A
rb
itr. U
nit
s
Theory
Experiment
(a)
(b)
(c)
(d)
Typical spectra normalized and fitted
(a)=0.23 (single shot)
(b) =1.17 (single shot)
(c) =0.23 (mean)
(d) =1.17 (mean)
O2
N2
H2
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Fitted Temperature in Hencken Flame
• Means agree well with theory• Actual standard deviation (SD)
in flame due to unsteadiness is unknown but believed small
• Fitted parameters have noise– Mode noise in broad band dye
laser, camera noise, photon shot noise
• SD of fitted parameters depends on selection of residual minimized by fitter
– R2 = noise-weighted least squares fit to signal intensity (Snelling et al, 1987)
– (R1, R3 commonly used in the literature)
• We found R2 < R3 < R1 • Similar results for fitted N2, O2,
H2 mole fractions
0
500
1000
1500
2000
2500
0 0.5 1 1.5 2 2.5 3 3.5
Equivalence ratio
Te
mp
era
ture
(K
)
Standard deviation of fit x10
Fitted mean
Theory
R1
R3
R2
SD in T reduced x2 by proper selection of residual! Important
for turbulence studies
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Saturation Effects
• Signal Ip1 Ip2 IS d2 L2
– Want to minimize L– Must maximize Ip1 Ip2 IS to maintain
signal – Saturation effects limit Ip1 Ip2 IS
• Saturation effects studied in Hencken flame– Mixture of Stark shift and
stimulated Raman pumping
• Fitted mole fraction more sensitive than temperature– Error in O2 up to 30%– H2 most sensitive to saturation– Saturation thresholds determined– We now know how to avoid
saturation
O2
N2
0.1
0.11
0.12
0.13
0.14
0.15
0.16
1 10 100 1000 10000 100000 1000000
O2
mo
le f
ract
ion
Ip2*Is
Ip1=40 GW/cm2
Ip1=140 Gw/cm2
850 mm f.l 500 mm f.l 300 mm f.l
(GW2/cm4)
0.1
0.11
0.12
0.13
0.14
0.15
0.16
1 10 100 1000 10000 100000 1000000
O2
mo
le f
ract
ion
Ip2*Is
Ip1=40 GW/cm2
Ip1=140 Gw/cm2
850 mm f.l 500 mm f.l 300 mm f.l
(GW2/cm4)
0
2
4
6
8
10
2150 2200 2250 2300
Sq
rt o
f C
AR
S i
nte
ns
ity
(arb
itra
ry u
nit
s)
Raman Shift (cm-1)
Ip2=100 GW/cm2
Ip2=400 GW/cm2
Saturation effects in Hencken burner flame = 0.3
Mean spectra
Fitted O2 mole fraction vs. irradiance
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Status of CARS
• CARS data base in laboratory supersonic flame– Data acquisition completed– Data analysis in progress
• CARS system currently installed at UVa
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Supersonic Flame*• Center jet 10 mm, hydrogen vitiated air,
variable Mach (0.75-2.0) and temperature• Unheated, low-speed coflow • 10 test cases; variables
– Nozzle exit Mach number (Me)– Center jet temperature (equivalent flight Mach
Mh)– Coflow gas
• C = H2 coflow, combustion• M = N2 coflow, mixing but no combustion
• Nominally 100,000 spectra for each test case, fitted for T, and mole fractions N2, O2, H2
– Sampled at 5 axial locations, 23-30 radial locations at each
• Will be used by NCHCCP modelers
Me/Mh 5 6 70.75 C+M
1 C+M1.6 C C+M C+M2 M
Exit Mach number (Me)
Equivalent flight Mach number (Mh)
*This work mostly supported by NASA Hypersonics NRA grant (NNX08AB31A)
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Typical Results in Supersonic Flame
0
400
800
1200
1600
2000
2400
-20 -10 0 10 20
1
15
35
65
100
0
400
800
1200
1600
2000
2400
-40 -30 -20 -10 0 10 20 30 40
2
16.2
36.6
63.4
98.9
Mixing (coflow = N2)Me=1.6Mh=7
Combustion(coflow = H2)Me=1.6Mh=6
Plots of mean T vs. radial distance at several axial locations
Center jet diameter = 10 mm, surveys are at ~1, 15, 35, 65, and 100 mm
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“WIDECARS”*
• Enable DP CARS in C2H4 + H2 – air flames
• New broadband Stokes laser– 2x increase in spectral width allows
additional chemical species to be simultaneously probed
• Temperature, mole fraction N2, O2, CO2, C2H4, H2 and CO measured at 300 K.– Most species ever simultaneously
measured with CARS!– Fit with new software
• Future development required– Validate at flame temperature– Modeling/calibration for C2H4
Stokes laser Spectrum
N2
CO2
CO2
CO
C2H4
H2 S(3)
H2 S(4)
3% CO2, 5% CO, 5% C2H4, 25% H2, 62% N2
Demonstration in a room T gas cell
*Collaboration with Tedder and Danehy (LaRC)Tedder et al., Appl. Optics, 2010
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OH PLIF Preliminary Setup
• Modified CARS laser cart to produce UV light needed for OH PLIF– Installed doubling crystal– Optimized laser for power
• Performed OH PLIF measurements in laboratory– Set up sheet forming optics– Set up laminar OH combustion with water welder– Investigated camera settings– Determined optimal PLIF transitions
• Flow visualization only– Signal roughly ~ OH density (not calibrated)
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Tunable Laser
Planar Laser-Induced Fluorescence (PLIF)
CCD camera detects
Laser sheet excites molecules
Excited molecules fluoresce
Gas flowGround state
Excited state
LIF ~ nOH
H2 O2
Water welder provides reactants for combustion
λ =
281
.135
nm
A2∑+←X2ΠΩ(1,0)
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Ongoing Work and Future• Summer 2011
– Acquire CARS data bases in UVa Dual Mode Scramjet Configuration B (no isolator) and C (with isolator)
– OH PLIF flow visualization (same cases)
• 2011-2013– Develop and validate “WIDECARS” for C2H4 (+ H2) in Hencken
flame – Acquire CARS data base supersonic flame with C2H4 + H2 coflow– Acquire CARS/PLIF data base in UVa scramjet with ethylene
flame/cavity flame holder
Combustor ExtenderTDLATCombustor ExtenderTDLAT
ExtenderCombustorIsolator TDLAT ExtenderCombustorIsolator TDLAT
Config. C
Config. B
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Summary• Motivation• Dual pump CARS• “Mobile” system and implementation at UVa• Method of fitting spectra• Measurements in a Hencken burner (H2-air) and
supersonic jet• Saturation effects• Measurements in a supersonic flame• WIDECARS• OH PLIF setup and preliminary data• Ongoing and future work