4 ”S i i t i ” t h i4. ”Semi-intrusive” techniques

I M t f t LI. Measurements of soot - Laser induced incandescence, LII ,

II L i d d b kdII. Laser-induced breakdown spectroscopy, LIBS

I. Optical diagnostics of soot in flames

• Soot formation

• Soot measurements using

scattering/extinction

• Soot measurements using Laser-Induced gIncandescence, LII

Soot in combustion processes

• For health and environment hazardous emissions

I d di ti ff ti h t t f• Increased radiation, more effective heat transfer. Important in boilers, furnaces, camp fires, candles etc. crucial component in fire spread

• Incomplete combustion, reduced engine efficiencyIncomplete combustion, reduced engine efficiency

• Increased wear leading to reduced lifetimes of components

• Deposits in engines, turbines, furnaces

Engine EmissionsEngine Emissions

From Kittleson (1998)

S t f tiSoot formation

2Fuel + O

C3H3

GAS PHASE POLYCYCLIC AROMATIC HYDROCARBONS

...

P ti l I tiC d i

C3H3

C2H3

C HO...

..... Particle InceptionCondensation

O2,O H+

+ C2H2

CO2+ OH2

PRIMARY SOOT PARTICLES

..

CoagulationAgglomerationAgglomeration

AGGREGATES

F. Mauss

Soot characteristicsSoot characteristics

Primary soot particles 10 40nm10-40nm

Black body radiationBlack body radiation

Black body radiation has a continuous spectrum described byPlanck’s radiation law

112)( /5

2

kThchcI () – emissivity !

The radiation has a maximum that is shifted to shorter wavelengthswith increased temperature. This is described by Wien’s

1)( /5 kThce

( ) y

with increased temperature. This is described by Wien sdisplacement law:

mK10898,2 3max T

The total intensity of the radiation is increasing with temperatureaccording to Stefan Boltzmann’s law:according to Stefan-Boltzmann’s law:

4TI

Henrik Bladh, Division of Combustion Physics

Planck emission from soot particles

Soot extinctionSoot extinctionAssuming d <<

I0/IT = exp(LKext)

N; number of soot particles

d; particle diameterd; particle diameterm; complex refractive index of soot particle

Conclusion:

Soot volume fraction measurable

Li f i ht t h iLine of sight technique

No size information

Soot scatteringSoot scattering Pol. of laser light

Soot scattering;

o o ase g t

g;

Pol. of the scattered lightscattered light

Soot scattering/extinctiongCombined scattering /extinction can be used for measurements ofcan be used for measurements of N and d

Laser-Induced Incandescence, LII,Soot particles in a well-defined region are heated by means of laser radiation

Heating of the soot particle leads to increased thermal radiationradiation

The increased radiation is detected

2000 K4000 K

Applications of laser‐induced incandescence

In-cylinder engine meas.

Aero-engine exhaust meas.

Joh

Flame studies

Bla

Desgro

hnsson et a adh et al. 20

oux et al. 20

l. 2008 (pos

Environmental 006

008

ster)

Nanoparticle charact. Welding fumes

Sm

al

monitoring

Gure Lllw

ood et al

entsov et al

Lucas et al.l. 2006

l. 2005

. 2006

Courtesy: Henrik Bladh

Increased black body radiation by LIIIncreased black body radiation by LIIThe red curve shows the black body radiation as a function of the wavelengthfor T=1800 K, which is a typical flame temperature. The blue curve shows thesame radiation for T=4500 K, which is within the same temperature range as alaser heated soot particle.

The left figure shows the real intensity difference for a large wavelengthinterval, while the right figure shows the normalised signal strength for visiblelight (normalization factor ~950).

2

2.5x 1013

2

2.5x 1010

3 ) 3 )4500K4500K

1

1.5

1

1.5

tens

ity (W

/m3

tens

ity (W

/m3

1800K

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.5

400 450 500 550 600 650 7000

0.5

Int

Int

1800K

Henrik Bladh, Division of Combustion Physics

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 400 450 500 550 600 650 700

Wavelength (nm) Wavelength (nm)

Background and LIIBackground and LIISpectral behavior:p

100%A spectral short-pass filter can

be used to suppress longerwavelengths. The laser-heated (W

/m)

%

soot radiates more at shorterwavelengths than the flamedoes. In

tens

ityWien’s displacement law!

Wavelength (nm)0%

Wavelength (nm)

Henrik Bladh, Division of Combustion Physics

Soot temperature and LII signalSoot temperature and LII signal

LII - The laser wavelengthLII The laser wavelength

Which wavelength is suitablefor LII?

S t b b ll i lSoot absorbs well in a largewavelength interval (UV -visible – IR). There are,however other species thathowever, other species thatcan be excited with UV lightand cause disturbingfluorescence Thereforefluorescence. Thereforewavelengths in the UV shouldnot be used. Most common isthe Nd:YAG laser at 1064 nmthe Nd:YAG laser at 1064 nm(IR) and its second harmonicat 532 nm.

Henrik Bladh, Division of Combustion Physics

The radiation-particle interactionThe radiation particle interaction

A soot particle is exposed toA soot particle is exposed tolaser radiation.

The following processes arePlanck radiationInternal energy

g pintitiated

Heat transfer Heat TransferAbsorption

Vaporization Emission of Planck

radiationV i ti

Heat Transfer

The energy balance equation:Vaporisation

2

061),(),(

)(2)()(

ChangeEnergy Internal

3

RadiationPlanck

2

onVaporizatiTransferHeat

2

Absorption

32

dt

dTcDdTMDDdt

dMMH

GDDTTk

tqmEDss

b

v

v

MFP

ga

Henrik Bladh, Division of Combustion Physics

LII f ti f l flLII as a function of laser fluence

10

1.u.

0 1sity

/ a

0.1

Inte

ns

0.0 0.2 0.4 0.6 0.8 1.0

0.01

Fluence / J/cm2

LII dependence on the laser spatial profileLII dependence on the laser spatial profileI

Region with no vaporizationThe spatial profile of the laser vaporizationp p

drastically affects the LIIsignal.

The laser illuminatesparticles with differentenergies. This means that

Region with vaporization

contributions comes fromboth parts (edges) wherethere is no vaporization and

t ( iddl ) h thparts (middle) where thereare vaporization.

Henrik Bladh, Division of Combustion Physics

I

LII dependence on the laser spatial profileLII experimentalists often refer to the LII fluence curve. A fluence curve

LII dependence on the laser spatial profilep

is the integrated LII signal plotted as a function of the laser fluence –that is the laser pulse energy divided by the exposure area of the laserbeam. Below is shown modeled fluence curves for three differentspatial profiles.

TophatI

Gaussian sheet

Gaussian beam

Henrik Bladh, Division of Combustion PhysicsCalibration of LII by scattering/extinction

Time-resolved LII (TIRE-LII)Time resolved LII (TIRE LII)Detection of the time-resolved LII signal may yield the primaryparticle size. The decay time reflects the particle cooling andsmall particles cool faster than large ones!

a.u.

) 10000

al in

tens

ity (

1000 Larger

LII s

igna Larger

particles

0 100 200 300 400 500

100Smaller particles

Henrik Bladh, Division of Combustion PhysicsTime (ns)

0 100 200 300 400 500

Principle of 2-color LIIPrinciple of 2 color LII

• Used to minimize uncertaintiesin evaluated particle sizes fromLII signal decays as the rate ofabsorbed laser energy does notgyneed to be known.

• The temperature is calculatedusing the relative irradiance at nc

e[a

.u.]

I2

using the relative irradiance attwo wavelengths.

Irrad

ian

I1

λ1 λ2 W l th [ ]1 2 Wavelength [nm]

Courtesy: Henrik Bladh

Modelling of time-resolved LII signals

Comparison of LII pmodels between different research groups- Same experimental input- Unconstrained models- High fluence (0.7 J/cm2)

Michelsen et al Modelling ofMichelsen et al., Modelling of laser-induced incandescence of soot: A summary and comparison of LII models, p ,submitted to Applied Physics B

Experimental setup for two-color LII

Tophat spatial laser profile 0.13 J/cm2

1064 nm

Tophat profile used for particle sizing as heating to diff t t tdifferent temperatures increases the uncertainty!

From: Bladh, H., et al., Proc. Combust. Inst. 33, 641‐648 (2011)

Modelling a soot particle in LIITypical model Real-world examples

(microscopy)

ShShape described by:

• Diameter (D)• Primary particle diameter (D)• Number of primary particles

Shape described by:

Number of primary particles• Radius of gyration of aggregate• Fractal parameters (”compactness”)

t

Jonathan Johnsson, Division of Combustion Physics, Lund University

• etc.• No particle looks the same as any

other!

The effect of aggregation on LII signals

• Theory shows that LII signals should be affected by the level of soot aggregation, i.e. decay rate/shape dependent on both primaryaggregation, i.e. decay rate/shape dependent on both primary particle and aggregate size!

• First experimental evidence for this effect using LII on a cold soot source: Soot generator based on a quenched diffusion flameg

Bladh et al. Appl. Phys. B, In press

The effect of aggregation on LII signals

• First experimental evidence for this effect:

TEM data on primary particle sizes: Similar LII signals: Different decay ratesp y pprimary particle sizes!

g y

Bladh et al. Appl. Phys. B, In press

The effect of aggregation on LII signals

• Sampling using a pneumatic probe

• Large differences between TEM and LII sizes– Deviation above 10 

mm HAB may be due to aggregation

– Uncertainties in TEM Uncertainties in T Msampling procedure and analysis

Bladh, H., et al., Proc. Combust. Inst. 33, 641‐648 (2011)

Aggregation model from Liu et al. 2006, Appl. Phys. B, 83, 383‐395 

E(m) as function of height above burner (HAB)

• Evaluate difference between gas temperature and maximum soot temperature for each height!

– Gas temperature from rotational CARS

– Maximum temperature from two color pyrometryfrom two-color pyrometry

•

• Procedure as described by Snelling et al. 2004, Combust. Flame, 136, 180-190

• LII model to determine E(m)Bladh, H., et al., Proc. Combust. Inst. 33, 641‐648 (2011)

LII 2D measurementsLII 2D measurements

Beam dump

Lens Combination

M it

CombinationBurner

Monitor

CCDCamera

Premixed flame

Diffusion flame

Engine Measurementsg

Laser entrance to the engineLaser entrance to the engine

Investigation though the piston g g p

Investigation though the piston

LII signal1

Flame luminosity

2.5

3

0 7

0.8

0.9

1 5

2

0.5

0.6

0.7

1

1.5

0.2

0.3

0.4

0.5

0

0.1

D i f b l h h hDetection from below, through the piston

Left image shows soot volume fraction in parts per million, ppm.

Calibration of Laser Induced Incandescence

Th l h t i fl t d i t th lib tiThe laser sheet is reflected into the calibrationburner using high reflective mirrors, and theresulting incandescence is detected with thecamera Gain gate distance from burner lasercamera. Gain, gate, distance from burner, laserpulse energy et.c. is the same as duringmeasurements.

II. Laser-induced breakdown spectroscopy -LIBSLIBS

J. Kiefer, J.W. Tröger, T. Seeger, A. Leipertz, B Li Z S Li and M Aldén ‘Laser inducedB. Li, Z.S. Li and M. Aldén, ‘Laser-induced breakdown spectroscopy in gases using ungated detection in combination with polarization filtering and online background correction’ Measurement science andcorrection , Measurement science and technology 21, 065303 (2010).

Polarization suppression of lasersuppression of laser

light - calibration

Calibration curves

Detection limits

Argon gas with a nominal purity of 99.996% with small

t t f bcontents of carbon dioxide (<1 ppm), oxygen (<4 ppm)oxygen (<4 ppm), water (<5 ppm) and nitrogen (<10 ppm).

J. Kiefer, J.W. Tröger, T. Seeger, A. Leipertz, B. Li, Z.S. Li and M. Aldén, ‘Laser-induced breakdown spectroscopy in gases using ungated detection in combination with polarization filtering and online background correction’, Measurement science and technology 21, 065303 (2010).

Application I: FlamesppTurbulent jet flame(CH4)

Turbulent jet flame(DME)

Atom ratio H/O and breakdown threshold as a function of radial position in a non-

premixed methane flame

Application II: Measurements of N d K f l/biNa and K from coal/biomass

(a) (b) (c) (d)

15

20

m)

K_coal_1Na_coal_2K wood 1

5

10

[Na,

K] (

pp

K_wood_1Na_wood_2

00 5 10 15 20 25 30

Time (sec)

LIBS i t d i ( ) d d fl (b)Release of sodium and potassium during

LIBS is operated in (a) seeded flame (b) devolatilization (c) char (d) ash cooking

phases

devolatilization with equivalence ratio of 1.3 using LIBS.