Paul Ewart

Oxford Institute for Laser Science

Combustion Physics and Nonlinear Optics Group

From quantum mechanics to auto-mechanics

Frontiers in Spectroscopy. Ohio State University, March 2004

Lecture Outline

• Lecture 1: Linear and Nonlinear OpticsNonlinear spectroscopic techniques

Lasers for nonlinear spectroscopy

• Lecture 2: Basic theory of wave mixingCoherent signal generation

Spectral simulation

• Lecture 3: Spectroscopy and diagnosticsHigh resolution spectroscopy

Combustion diagnostics

DFWM spectroscopy of C2 in oxy-acetylene flame

DFWM spectrum of C2 in oxy-acetylene flame

Note: High spectral resolution High signal-to-noise in luminous environment

515.0 515.2 515.4 515.6 515.8 516.0

Simulation (upper graph)Experiment (lower graph)

Inte

nsi

ty (

arb

. units

)

Wavelength / nm

Simulation of C2 DFWM spectra

515.895 515.900 515.905 515.910 515.915 515.920 515.925

P3(28)P2(29)P1(30)

Simulation Experiment Line positions

Inte

nsi

ty (

arb

. units

)

Wavelength / nm

Effects of incorrect line position on simulation

515.0 515.2 515.4 515.6 515.8 516.0

Simulation (upper graph)Experiment (lower graph)

Inte

nsity

(ar

b. u

nits

)

Wavelength / nm

Improved line positions from DFWM measurements

516.0 516.2 516.4 516.6

Simulation (upper graph)Experiment (lower graph)

Inte

nsi

ty (

arb

. units

)

Wavelength / nm

DFWM Spectra of C2 in oxy-acetylene flame

Swan band (0,0) Band head• Corrected line positions• Coherent addition

Multiplex DFWM spectroscopy in flames

1. Broad laser spectrum overlaps molecular resonances

2. Broadband FWM spectrum recorded on CCD camera

3. Theoretical spectrum fitted to find temperature.

C2 spectrum in oxy-acetylene flame

1

2

3

513.0 513.5 514.0 514.5 515.0 515.5 516.0

0.0

0.2

0.4

0.6

0.8

1.0 Experimental spectrum

Theoretical fit

Laser spectrum

Inte

nsity

(ar

b. u

nits

)

Wavelength / nm

(3)(3)

(3)

(45)(45)

(5)(15)(5)

(5)(15)(15) (10)

(10)

(10)

(35)(35)

(35)(40)

(40)

(40)

(45)P3

R1

R2

R3

P1

P2

Broadband/Multiplex DFWM spectroscopy of C2

Multiplex FWM thermometry in flames

• Time resolved measurement of temperature by single laser shot of broadband modeless laser

• Single shot precision of ~4%

500 1000 1500 2000 2500 30000

10

20

Num

ber

Oxy-acetylene flame, Histogram, 100 single shot spectra

Mean: 2936 K

S.D.: 114 K, 3.9%

Thermometry by multiplex DFWM of C2

Temperature / K

1-D line imaging by FWM

Line imaged on Spectrograph slit

Multiplex FWM along a line

• Line formed by intersecting planar laser beams

• FWM signal is induced by broadband laser

• Signal line is mapped onto spectrograph slit

• Spatially resolved spectra recorded on CCD camera

Simultaneous measurement of Temperature and Concentration of C2 along a line

Spectrum at each position

yields temperature T(x)

Spectral intensity along line

yields concentration

Simultaneous measurement of C2 concentration and temperature along 1-D line

DFWM for detection of NOx in

a firing s.i. engine

Detection of combustion generated

NO in s.i.engine using DFWM

• BOXCARS plates: simple, stable, reproducible alignment of input laser beams.

• Collimated beams in interaction region minimizes noise from windows etc.

DFWM spectrum of NO in firing s.i.

engine (methane/air)

DFWM spectrum of NO in firing s.i. engine (methane/air)

Skip fire 1 in 9Speed 1200 rpmIgnition timing: 40o BTDCLaser timing: BDC

Upper trace:NO absorption(line of sight)

Lower trace:DFWM spectrum(space resolved)

LITGS: Laser Induced Thermal Gratings

LITGS in OH in high pressure CH4/air flame

Recorded using cw Ar-ion Laser:1 Watt in ~ 1 s

Flashlamp pumped dye Laser:106 Watt in ~ 1 s

LITGS using long pulse probe

Temperature precision + 0.1% Pressure precision + 2 %

NO2:N2 5 bar 300K

-0.1

0

0.1R

esid

ua

l

0 200 400 600 800 1000 1200

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time [ns]

Inte

nsity [a

.u.]

FLPDL PulseDatapoints Model Fit

LITGS in NO2:N2 at 40 bar, 300 K

-5

0

5A

bs

olu

teR

es

idu

al

290 300 310 320 330 340 350 360 370 380280

300

320

340

360

380

Thermocouple Temperature [K]

LIT

GS

De

riv

ed

Te

mp

era

ture

[K

] Line of EqualityLITGS data

-1

0

1

Ab

so

lute

Re

sid

ua

l

0 5 10 15 20 25 30 35 40 450

5

10

15

20

25

30

35

40

45

Dial Gauge Pressure [bar]

LIT

GS

De

riv

ed

Pre

ss

ure

[b

ar]

Line of EqualityLITGS data

Simultaneous measurement of Temperature and Pressure along a line

using LITGS

Streak image of LITGS signal from line: NO2 in N2 at 2.5 bar

Time nsec 0 120

Pos

ition

x

Oscillation frequency yields Temperature, T(x)Decay function in time yields Pressure, P(x)

5 mm

Time [ns]

Po

sit

on

[m

m]

0 50 100 150 200 250 300 350

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

50

100

150

200

250

300

350

400

Position [mm]

Tem

pera

ture

[K

]

0 1 2 3 4 5292

294

296

298

300

302

LITGS: Laser Induced Thermal Gratings

TGV, Thermal Grating Velocimetry

TGV in NO2 seeded air flow

• Thermal grating written by two beams at 532 nm

• Signal read by delayed pulses at 1064 nm from SLM laser

• Forward and Back scattered signals are Doppler shifted up and down in frequency by

TGV air flow measurements

Conclusions

• Laser Induced Grating techniques:

• Temperature CARS

• Minor Species, Temperature DFWM

• 1-D Concentration and Temp. DFWM

• Velocity LITGS

• Pressure and Temperature LITGS

• 1-D Pressure and Temperature LITGS

Acknowledgements

• Karen Bultitude• Rob Stevens• Geraint Lloyd• Radu Bratfalean• Andrew Grant• Duncan Walker• University of Heidelberg PCI• University of Stuttgart, ITV and DLR• EPSRC, British Gas, Rover plc

Potential of LITGS diagnostics

• Non-invasive temperature and pressure measurement

• Time and space resolved data

• Detection of local temperature and pressure

• Diagnostics of auto-ignition and engine “knock”

Schlieren film of autoignitionCourtesy of Prof CWG SheppardUniversity of Leeds