lecture 10 cars [kompatibilitetsläge] - princeton … lecture... · 532 nm and a dye laser...
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10. CARS10. CARS Photo: Henrik Bladh
• IntroductionTh• Theory
• Rotational CARS/vibrationalCARS/vibrational CARS
• Temp & conc• Temp . & conc. measurements
• ApplicationsApplications
Background - CARS
Coherent anti-Stokes Raman scattering/spectroscopy (CARS) is a technique that is used for • temperature measurements• concentration measurements
Species with high concentrations (> ~1%)Species with high concentrations (> 1%) are probed N2, O2, CO2, H2O, CO, H2
The first demonstration of CARS in flames was made in 1973 by Taran et al and in anwas made in 1973 by Taran et al. and in an IC engine by Stenhouse et al. (1979)
Characteristics of CARS
• Signal generated as a new laser beam• Pointwise thermometry with high accuracy• Signal blue shifted relative to the primary
laserslasers• Complex theory• Relatively complicated experiments• Operator skill is needed• Operator skill is needed• Two approaches: - Vibrational CARS pp
- Rotational CARS
Nonlinear opticsThus far in the course, the induced polarization of molecules has almost always (except for frequency doubling/mixing) been assumed to depend y ( p q y g g) plinearly on the applied electromagnetic field. This is, however, only valid for incident radiation of low intensity.
Generally, the induced polarization is a nonlinear function of theapplied electromagnetic field:
.....321 PPPP
For gases hich are isotropic (in ersion .....332210 EEEP
For gases, which are isotropic (inversion
symmetry), the even order polarizations vanish
CARS is a four-wave mixing process based on the nonlinear
Joakim Bood
response via the third-order susceptility (
CARS theory
2222 sin
2lklS 22
3212 sin
2lk
2lklIIIS CARSCARS NRRCARS ,
• CARS is the CARS signal frequency ,
• Ii is the irradiance in laser beam i,
• CARS is the CARS susceptibility, CARS p y
• l is the interaction length,
th l t f t i th h t hi diti• the last factor is the phase-matching condition, that takes the value 1 if perfect phase-matching is achieved.
Phase matching implication on the CARS set-upCARS set-up
2 kkk ink 2
Collinear phase-matching
Lens Lens DichroicmirrorFocal point CARS
212 kkkCARS ik
p g
Red +Green
CARS signal
Planar BOXCARS phase-matching
Lens Lens DichroicmirrorFocal point
Red + CARSRed +Green
CARS signal
Green
Lens LensFolded BOXCARS phase-matching
Focal point
Green
CARSsignal
Red
Green
Generation of CARS spectra
Scanning Drawback:It takes time to scan a
CARSWavelength
(nm)
spectrum can be used in stationary flames only
500 600y y
BroadbandAdvantage:Spectra is obtainedBroadband
CARSWavelength
(nm)
Spectra is obtained within 10 ns can be used in turbulent flames
500 600(nm) turbulent flames
© Per-Erik Bengtsson
The selection of wavelengths• The energy difference between 1 and
2 must match a vibrational Raman resonance in the molecule =
Beam arrangement2 asresonance in the molecule. 1- 2 = R
• For nitrogen, with R = 2331 cm-1, it
3as
g , R ,means a Nd:YAG laser wavelength of 532 nm and a dye laser spectral profile centred around 607 nm.
1
Ene
rgy
profile centred around 607 nm.
• The CARS signal is at frequency:
1 3 as2
CARS = 1-2 +3, which for nitrogen is at a wavelength of 473 nm. (1 = 3)
v=0
v=1• By operating the dye laser in
broadband mode all transitions can be monitored simultaneously.monitored simultaneously.
Experimental setup (vibrational CARS)
Nd:YAGLaser source
Nd:YAGlaser 1-m
spectrograph
CCD-cameraM
M
The most common laser system for CARS is a Nd:YAG + dye laser system
Dye laser
BC L LSPNDAL
AMeasurement
volume
with repetition rate 10 Hz and pulse duration of ~10 ns.
O ti l t
M M
MBC L
BS
A
BS
Optical componentsSpecial optics that can stand high pulse energies are neededM M
M = Mirror ND = Neutral density filterBS = Beam splitter CCD = Charge-coupled deviceBC = Beam combiner L = Lens
needed.
Detection systemBC Beam combiner L Lens A = Aperture SP = Short-pass filter High-resolution spectrometer
+ CCD-camera
Temperature measurements• The temperature is normally
measured from the CARS spectrum of nitrogen since
Nitrogen
spectrum of nitrogen, since nitrogen often is present at high concentration before as well as ft b ti si
ty
after combustion.
• The temperature is evaluated 2100 Ksed
inte
ns
The temperature is evaluated from the spectral shape and not the total intensity.
1200 KNor
mal
i
• The temperature is evaluated by fitting the experimental spectra 300 Kg p pusing a library of theoretical spectra. Raman shift / cm-1
475 474 473 Wavelength / nm
Concentration measurements• Concentration measurements are
normally made from the spectral h
H2
yshape.
• The figure shows an experimental
CON2In
tens
ity
spectrum with contributions from H2, CO, and N2.
H2
Raman shift (cm-1)• The lower figure shows the same experimental data together with a fitted curve The evaluation gave all º Experiment
Thfitted curve. The evaluation gave all three concentrations together with the temperature.
ensi
ty
TheoryT = 2600 KC = 25.3 % N2C = 36.6 % CO
Figures are from Stufflebeam
Inte C 36.6 % CO
C = 23.0 % H2
gand Eckbreth, Combust. Sci. and Tech. 66, 163-169 (1989)
Raman shift (cm-1)
Rotational CARSConventional rotational CARS (C-RCARS)
Nd:YAG
C 5002
Nd:YAG 1, 3
Rot. CARSCARS
1 2 3 CARS
500 600E
1 2 3
J
CARS
500 600Wavelength / nmRamanE J
Dual-broadband rotational CARS (DB-RCARS)
Nd:YAG, 3
1
Rot. CARSCARS
2
,
1 2 3 CARS
500 600Wavelength / nmRaman
RamanE E J
Experimental set-up for rotational CARS
Nd:YAGlaser CCD-
• An experimental setup for dual-
Spectro-graph
CCDcameraM
ACLM
broadband rotational CARS is using basically
Dye laser
EngineL LSPNDACL
A
g ythe same experimental parts as for
M M
BS
M
BCL
f=300 mm f=300 mmBS
A parts as for vibrational CARS.
• Some optical t hM M components such
as mirrors and filters are M = Mirror ND = Neutral density filter
BS = Beam splitter CCD = Charge coupled device camera replaced.BS = Beam splitter CCD = Charge-coupled device cameraBC = Beam combiner L = Lens A = Aperture SP = Short-pass filterCL = Cylindrical lens
Temperature measurements
0.8
1.0
(a.u
.) T=300 KNitrogen
• The spectral
0.4
0.6
Inte
nsity
(shape of a rotational CARS
0.0
0.2spectrum varies strongly as a
0.8
1.0
u.)
0 100 200 300 400 500
T=2100 KNitrogen
function of temperature.
0.4
0.6te
nsity
(a.u T=2100 K
0.0
0.2Int
0 100 200 300 400 500
Raman shift (cm-1)
Concentration measurements
0.8
1
T=300 KNitrogen
• Rotational CARS spectra from many species (N2
0.4
0.6
ten
sity
(a.
u.)from many species (N2,
O2, CO, CO2) appear in the same spectral region.
0
0.2
Int
• Concentration evaluation is made from the relati e 0 50 100 150 200
Raman Shift (cm-1)
T=300 KAir
O2
is made from the relative intensities of the lines. T=300 K
AirAirAir
Raman shift / cm-1
Rotational CARS spectra for different molecules
Li l l i• Linear molecules give clear distinct spectra.
• Smaller molecules have higher B-gconstants and result in wider spectra.
© Per-Erik Bengtsson
Advantages with CARS for practical applicationspractical applications
• Signal as a new laser beamSt i l• Strong signal
• CARS signal on the anti-Stokes sideside
• Measurements on nitrogen molecules (max signal)molecules (max signal)
Special issue: High pressure effectsSpecial issue: High pressure effects
Ref: Eckbreth et al.
Applications to engines
Single-shot temperature measurements in a spark-ignition engine at heat andin a spark ignition engine at heat and power engineering using rotational CARS
© Per-Erik Bengtsson
Vibrational CARS applications
CARS spectra:CARS spectra:
In a burning spray of JET-A Burning of solid propellants
A.C Eckbret et al.
Limitations with conventional CARS
• Low spatial resolution - In order to keep a sufficient signal strength it has been important to keep the crossing angle between the laser beams small rather low spatial resolution (~ >1-3 mm)
• Limited signal strength in an environment with low transmission - The CARS signal scales as (laser intensity)3 at strong laser attenuation, the signal strength is too low
Probe volume considerations
Reaction zone Thickness: ~100 m
Probe volume diam.: ~100 mProbe volume length: ~1-3 mm
CARS probe volume
Burned gasUnburned gas~300 K
Burned gas~2000 K
The result will be a “mixed spectrum” pwith a strong low-temperature part
© Per-Erik Bengtsson
Limitations with conventional CARS
• Low spatial resolution - In order to keep a sufficient signal strength it has been important to keep the crossing angle between the laser beams small rather low spatial resolution (~ >1-3 mm)
• Limited signal strength in an environment with low transmission - The CARS signal scales as (laser intensity)3 at strong laser attenuation, the signal strength is too low
Potential solution: 2- CARSPotential solution: 2 CARS
Concept of 2- CARS:Use a two colour dye laser instead of a broad-band
dye laserhigher spectral intensity
Broadband CARS
Broadband CARS
2 CARS
Features of 2- CARS
Advantages:• ~ 30 times higher signal intensity
Higher spatial resolution, experiments in very applied areas possible
Disadvantages:• More complex experimental set up• More complex experimental set-up• Somewhat lower temperature precision
(~5 % comp to ~3% with broadband CARS)
Application: High spatially resolved temperature measurement in model of an after-burnermeasurement in model of an after-burner
(VOLVO Aero Corporation, Trollhättan)
Application: High spatially resolved temperature measurement in model of an after-burner
(VOLVO Aero Corporation, Trollhättan)
Application: High spatially resolved temperature measurement in model of an after-burner
(VOLVO Aero Corporation, Trollhättan)
Temperature pdf:s at apdf:s at a
distance 150 mm from themm from the bluff body at
diff t di ldifferent radial distancesdistances
Practical diagnostics - CARSCARS, Single shot (10ns)
constant thermal load 72 MW 4.5 m from focusing lens
1
23
4
5
mean value 1352 Kstd. deviation 187K
0
1
700 760 820 880 940 1000 1060 1120 1180 1240 1300 1360 1420 1480 1540 1600 1660
t emperat ure (K)
Suction Pyrometerconstant thermal load 72 MW
1380
13301340135013601370
ratu
re (K
)
12801290130013101320
tem
per
4 m from focusing lens
1280
10:1
0
10:1
3
10:1
5
10:1
7
10:1
9
10:2
1
10:2
4
10:2
6
10:2
8
10:3
0
10:3
3
10:3
5
10:3
7
10:3
9
10:4
2
10:4
4
10:4
6
10:4
8
10:5
1
10:5
3
10:5
5
10:5
7
10:5
9
time (h,m)
Pressure dependence f t ti l CARSfor rotational CARS
If pressure increases
the signal increases
If pressure increases
the signal increases• the signal increases• the linewidths will be broader• the signal increases• the linewidths will be broader
© Per-Erik Bengtsson
Rotational CARS spectra from an engineP = 4 BarT = 490 K
• Single-shot spectra in the compression phase of engine
N2
O2
burning natural gas.
• Both nitrogen and oxygen lines s
g ygare observed.
• Below each experimental arb.
uni
ts
P = 16 BarT = 706 K
Below each experimental spectrum the difference between the experimental spectrum and the best fit theoretical spectrum In
tens
ity /
the best-fit theoretical spectrum is shown, illustrating good spectral fits.
I
Raman shift / cm-1Figures are from Bengtsson et al., Proc. Combust. Inst. 25,: 1735-1742 (1994)
Rotational CARS spectra from an engine
• Single-shot spectra in the compression phase of engine
c
burning natural gas.
• Both nitrogen and oxygen lines g ygare observed.
• Below each experimental dP = 16 BarT = 706 K
Below each experimental spectrum the difference between the experimental spectrum and the best fit theoretical spectrumthe best-fit theoretical spectrum is shown, illustrating good spectral fits.
c: 32 CAD ATDC, 32 bar, 2550 K
Figures are from Bengtsson et al., Proc. Combust. Inst. 25,: 1735-1742 (1994)
d: 56 CAD ATDC, 15.5 bar, 2435 K
Sooty flame diagnostics: Vib CARS
a) Sooty flame
b) ”Clean” flame
a) fitted and T
a) N2 flame
b) Ar flame a) fitted NR and T
b) Only fit T with
estimated
b) Ar flame
c) C2 emission
Summary: CARS
• strong coherent signal
i t t t h i• point measurement technique
• complex theory
• double-ended technique
• can be applied to harsh environments, sincecan be applied to harsh environments, since background radiation is easily discriminated, anti-Stokes signal (low fluorescence interferences)
• mainly for temperature measurements, where temperature is measured from spectral profile
• high accuracy for temperature measurements• can sometimes be used for concentrationcan sometimes be used for concentration
measurements, but for major species only.Per-Erik Bengtsson
Summary: Rotational CARS vs Vibrational CARS
S i t l l it
Rotational CARS vs Vibrational CARS
• Same experimental complexity
• Both methods are very accurate thermometers: Uncertainty 1-2% of T
• Rotational CARS: better at high P low T• Rotational CARS: better at high P, low TVibrational CARS: better at low P, high T
• Rotational CARS: many species simultaneouslyVibrational CARS: often one species only with a setup
• Rotational CARS: Problems may arise with rejection of light at 532 nm because of spectral closeness.g p
Per-Erik Bengtsson