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Home > Documents > Lecture 10 CARS [Kompatibilitetsläge] - Princeton … Lecture... · 532 nm and a dye laser...

Lecture 10 CARS [Kompatibilitetsläge] - Princeton … Lecture... · 532 nm and a dye laser...

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10. CARS 10. CARS Photo: Henrik Bladh Introduction Th Theory Rotational CARS/vibrational CARS/vibrational CARS Temp & conc Temp . & conc. measurements Applications Applications
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10. CARS10. CARS Photo: Henrik Bladh

• IntroductionTh• Theory

• Rotational CARS/vibrationalCARS/vibrational CARS

• Temp & conc• Temp . & conc. measurements

• ApplicationsApplications

Classification of techniques

PS/DFWM

CARS

CARS conceptual behaviour

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.

Calculated CARS spectra of CO

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

Experimental CARS spectra (scanned)

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

Broadband CARS (N )Broadband CARS (N2)

T = 1900 K

CARS on SOCARS on SO2

What does thermal equilibrium mean?

Flame: Tvib=Trot Discharge: T ibT tvib rot Discharge: TvibTrot

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

Comparison between collinear CARS and BOXCARS

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

One measuring situation where 2- CARS has to be used

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

Experimental approach to produce a two colour dye laser

Wavelength selection in 2- CARSWavelength selection in 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 g- CARS

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

Sooty flame diagnostics: Rot. CARS

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


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