5. Laser Induced Fluorescence

GeneralGeneral• Introduction to LIF; Theory• Species concentrations• Temperature measurementsTemperature measurements• Multi-dimensional imaging

Applications– Engines– GasturbinesGasturbines– Furnaces/boilers-Biomass applications

Laser-induced fluorescence

LIF: Measures e.g, NO, OH,LIF: Measures e.g, NO, OH, CH, CN, C2, O2, fuel-tracer

General features:- High sensitivityHigh sensitivity- 2D imaging capabilities- Spontaneous techniquep q- Measures temp. and konc.- Quantification problem

Different LIF approachesExcitation scanFluorescence spectrum

pp

v’

B

J’ = 0J’ = 1J’ = 2

B

J’ = 0J’ = 1J’ = 2

v’ J 0J 0

Xv’’ = 1

Xv’’ = 1

v’’ = 0J’’ = 0J’’ = 1J’’ = 2

v’’ = 0J’’ = 0J’’ = 1J’’ = 2

Joakim Bood

Laser tuned to a specific absorptionline and the spectrometer is scanned

Laser is tuned across the various absorptionlines and the total fluorescence is monitored

Excitation and fluorescence spectra

Excitation spectrum of OH Fluorescence spectrum of CN

I t t l l d t t d b LIFImportant molecules detected by LIF

Table I

Molecule Abs. wavelength (nm)OH ~306C 516C2 ~516CH ~431CN ~388NH ~336NO ~226CH2O ~320-360NO2 450-470

LIF Theory

S ~ N2 , How is N2 related to Ntot ?21A

N2 2

B Bc

B L1221A

cB L21 21Q

N11

Rate equation analysisRate equation analysis

BBtN

cBQAtN

cBtN

ttN LL 21

2121212

11

BQAtNBtNtN LL 21122

c

QAtNc

tNt 212121

21 NNNtot and assume steady state; dN/dt=0

bij=Bij/c

A /B = h/3A21/B21=h/

Results

Ltot

bbQAbNN

)(12

2

LbbQA )( 12212121

Ltot

bbQAbNAS

)(12

21

LbbQA )( 21122121

LLptot bbQA

AbVFNS

)( 21122121

2112

LbbQA )( 21122121

Stern-Vollmer factor = Y

Evaluate Lptot bbQA

AbVFNS

)(

2112

L l li it (b b ) (A Q )

LbbQA )( 21122121

Low laser limit: (b12+b21)L<<(A21+Q21)

21AbVFNS 2121

2112 QA

bVFNS Lptot

LIF in the linear regime

21AbVFNS Fluorescence

efficiency

Th fl i li l ti l t th i t l i di

212112 QA

bVFNS Lptot y

• The fluorescence is linearly proportional to the input laser irradiance

• Since most often A21 << Q21, the fluorescence efficiency is generally <<1,which diminishes the sensitivitywhich diminishes the sensitivity

• For quantitative measurements usually Q21 has to be evaluated

• Q21(p,T,composition) difficult to obtain quantitative data

For quantitative measurements it is critical to measure/estimate Q !!

Joakim Bood

critical to measure/estimate Q !!

Make Q small: High laser intensityMake Q small: High laser intensity

ijLjiijij

ij

QbbAA

Y

)(

(b +b ) >>(A +Q )

ijLjiijij Q)(

(b12+b21)L>>(A21+Q21)

1221 bb

bVNAS tot2112

21 bbtot

Saturation

Fluorescence power versus laser irradianceSaturated regime

1.0

Saturated regime

0.8

pow

er At saturation

0.6

esce

nce

p

0.20

Linear regime •S is independent of both and Q21

• S is maximized sensitivity maximized0.4

Fluo

re

0.05

0.10

0.15

S is maximized sensitivity maximized

0 0

0.20.0 0.5 1.0 1.5 2.0

0.00

0 100 200 300 4000.0

Laser irradiance (arb. unit)

Joakim Bood

Power dependence

SaturationSaturation

• Saturation depends on Q

• Saturation depends on pspatial, temporal and frequency laser wing High Qq y geffects

g

KvvKzF

zRrExp

TtExpvrzt

LL

2

22,,,

Two-photon LIF

Differences between one- and two-photon LIF:

Diff t l ti l- Different selection rules- Doppler-free measurements

possible- Temporal behaviour of laser pulse

important- LIF signal proportional to the laser-g p p

intensity squared

Species Exc (nm) Em (nm)Species Exc. (nm) Em. (nm)H 2x205 656O 2x226 845 (777)N 2x211 870 (822,744)( , )C 2x280 910CO 2x230 400-725H2 2x193 830H2O 2x248 400-500NH3 2x305 550-575 (720)

Difference between one and two-photon LIF

• Ione-photon ~ I/A x A = I , independent on focusing

• Itwo-photon ~ (I/A)2 x A = I2 /A

LIF thermometry

A th d th t fl t th di t ib ti f l ti t• Any method that reflects the distribution of population over two or moreindividual vibrational rotational states can in principle be used fortemperature measurement. LIF is such a method.

• LIF thermometry restricted to high temperatures if molecular radicalsare employed. For OH temperatures above ~1500 K are needed.p y p

• If atomic species, such as metal atoms, are used, these have to be seededinto the flame or flowinto the flame or flow.

• If LIF was used for concentration measurements it is definitelyconvenient to apply it for thermometry tooconvenient to apply it for thermometry too.

Joakim Bood

Temperature masurementsTemperature masurements

jtot kT

EExpNJ

n)(12

rotvib

j QQn

0.010

]

0.005

300 K 1500 K 3000 K

popu

latio

n [a

u]Fr

actio

nal p

0 5 10 15 200.000

Rotational level [J]

Various techniquesq

Excitation Scans Thermally-Assisted

Collisions

Two-Line

2

Electronicallyexcited

CollisionsVibrational

Levels 02

12

2stateLaser variable

with scans from i to j

Laser fixed

RotationalGroundEl i

F21

1F

i j

RotationalLevelsElectronic

State 0F20

Excitation spectraExcitation spectra

NO roomNO room

T~ 300 K

NO flame

T~ 2000 K

Excitation scans of OH A-X (0,0)

T = 500 K

T = 500 K

T = 1500 K

arb.

uni

t)

T 2000 Ktens

ity (a

T = 2000 K Int

T = 2500 K

Joakim Bood

3040 3060 3080 3100 3120 3140 3160

Wavelength (Å)

Plotting approach for yielding TPlotting approach for yielding T

ln (ni ) ~ - Ei /kT

R2(3)

R2(2)

P1(2)

5Temperature 1794 K; =185 K

R2(10)R2(11)

P1(1)

Q1(5)

3

4

Boltz

man

n - f

acto

rR1(12)

R2(13)

R1(14)

0 1000 2000 3000 40001

2

B

Energy (cm-1)

Single shot 2D LIF flame thermometry strategies: Two-line excitationg

T li l l fl OH NOTwo-line molecular fluorescence: e.g OH, NO, ketones

Problems: • Applicable only in areas where the species is located; high T (OH) ,

low T (ketones) E it ti i UV b k d i i t i t• Excitation in UV, may cause background emission, trapping etc,

• Require relatively high laser intensities P bl i f l i h i t• Problems in fuel rich environments

• Complex set up (NO)

Two-Line Atomic Fluorescence (TLAF)

• Atoms usually possesses much higher transitionprobabilities than molecules Very low laser power can be used for excitation

• Excitation can be made in the visible region

• Measurements in sooty flames can be made

• Usually metal atoms are seeded into the flame

The tracer species should be selected based on:• The tracer species should be selected based on:• temperature sensitivity• complexity of the emission spectrum

Joakim Bood

complexity of the emission spectrum• minimum intrusion on the combustion process

Two-line LIF thermometryy

2Basic idea:

12F21

Basic idea:

To measure the relative populationf t t t T f B lt

02

12F21

F20

of two states T from Boltzmannexpression

1Excitation to the same upper state F21 and F20 are equally affected byquenching and energy transfer processes

0

quenching and energy transfer processes

kEE C

II

FF

kEETlnln4lnln 211221

01

C non-dimensional system

dependent calibration constant

Joakim Bood

IF 200220

Two-line fluorescenceTwo-line fluorescence

Species 1(nm) 2(nm) E(eV)

Gallium 403.4 417.3 0.109Indium 410.3 451.3 0.274Thallium 377 6 535 0 0 966Thallium 377.6 535.0 0.966Lead 339.4 461.9 1.320

Temperature sensitivitiesTemperature sensitivities

T sensitivites of different atomic seeder species

0.8

arb.

u.)

In Ga TlPb

0 4

0.6

T-se

nsiti

vity

(a

0.2

0.4

rela

tive

T

0 1000 2000 3000 4000

0.0

0 1000 2000 3000 4000Temperature (K)

LIF set up: 0DLIF set-up: 0D

Multi-point visualization: 1DMulti-point visualization: 1D

Multi-point visualization: 1DMulti-point visualization: 1D

Two-dimensional visualization: 2D

Two-dimensional measurements

FlameLaser beam

Cylindrical lensLensLens

2D-detector

CH visualization

Motivation: Intermediate in NOx formationFlame front markerFlame front marker

Approach: Excitation X-B at ~ 387 nmEmission B-X, A-X; ~430 nmBroadband excitation

~ two orders of magnitudeincreased detectability

Kiefer et al, 31st Comb. Symp

CH visualization in a turbulent flameCH visualization in a turbulent flame

Kiefer et al, 31st Comb. Symp

Multi species pvisualization CH, OH

Kiefer et al, Comb.Flame 2008

Simultaneously multi-species visualization CH, CH2O

Courtesy: Z-S. Li et al.Li et al. ECM 2009, poster 259

Exp set-up for 2D temperature measurements i TLAFusing TLAF

YAG 1

YAG 2

355

532 667

451mixing

410

451dye 1

dye 2

1064

451crystal 410

/2 platedelay line

polarizerCCD 1( )F20

CCD 2( )F21

cylindricaltelescope

( )21

signal beamsplitter

photodiode

filters

engine

CCDcontrol

CCDcontrol

boxcarint.

PC1 PC2boxcar

int.

2D TLAF

Temperature field in a Bunsen burnerTemperature field in a Bunsen burner

High speed visualization I: Turbulent non premixed CH /air flame Re=5500

AirCH4CH4

Turbulent non-premixed CH4/air flame, Re=5500

High speed visualization II.Flame extinction – simultaneous scalar/flow measurements

Simultaneous PIV and OH PLIF

• Turbulent opposed jet flames as model-systems for aerodynamically Simultaneous PIV and OH PLIF

at 5 kHz repetition ratemodel systems for aerodynamically induced extinction

Oxidant

• White area: Instantaneous flame position from OH PLIF

• Arrows: Velocity vectors from PIV• Color-code: Out-of-plane vorticity

(blue: clockwise rotation, red: counter-clockwise rotation)Fuel

Courtesy: Andreas Dreizler (Böhm, Heeger, Boxx, Meier and Dreizler. Proc. of the Comb Inst 32 (2009) 1647)

Fuel

3D measurements

Formaldehyde visualization

Multi-Species Measurements

Multi-Species Measurements

Jet speed 120m/sJet speed 60m/s

OH CH O CH OOH CH O CH O OH CH2O CH2OCH CH Toluen

OH CH2O CH2OCH CH Toluen

Summary: LIF• Can measure 2D temperatures and species distributions• Measure preferably atoms (O, H, N, C) and diatomic molecules (NO, OH,

CH CN C O CO H ) S l t i i (NH H O HCO CH O)CH, CN, C2, O2, CO, H2). Some polyatomic species (NH3, H2O, HCO, CH2O)• High sensitivity (ppm)• Mainly used for one species detection• Accuracy; concentration: ~10% (stationary flame, all quenching

parameters known)~30% (turbulent flame, no quenching

corrections)corrections)temperature: ~2% (point measurement)

~8 % (2D measurement)

• Resonant excitation Tunable laser needed• Incoherent non-elastic scattering Signal emitted in all directions

S ff f hi i lli i l d it ti tit ti i• Suffers from quenching, i.e. collisional deexitation quantitative speciesconcentration measurements not trivial

• Problem with separation of multi-atomic species, e.g. PAH’s• Outlook: Further 3D visualization high speed imaging (>kHz) velocity• Outlook: Further 3D visualization, high speed imaging (>kHz), velocity

visualization without molecule seeding