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12/8/2015
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Combustion ApplicationsJOAKIM BOOD | DIV. OF COMBUSTION PHYSICS, LUND UNIVERSITY
CH CH2O OH
Combustion processes are very complexThe chemistry is extremely complicated…
The most important reaction paths in acetylene oxidation is shown below
Turbulence Chemical reactions
Flow‐field equations(Navier‐Stokes)
Transport equationsfor species
Inter‐action
then there is also interaction between the chemistry andthe turbulent flow
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Outline
• Multi‐spectral imaging concepts based on spontaneous flame emission
• Introduction to laser‐based combustion diagnostics
• Multi‐species imaging with planar laser‐induced fluorescence (PLIF)
• Two‐dimensional thermometry using PLIF
• High‐speed imaging
Multi‐spectral imaging concepts based on spontaneous flame emission
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Spontaneous flame emission (chemiluminescence)
Images of Bunsen‐type flames having different fuel/air‐mixtures
Flame emission spectrum recorded with spectrograph
Spectrum recorded with Ocean Optics HR2000 spectrometer. It is not corrected for the wavelength‐dependent variations in sensitivity (i.e. the intensity scale is not calibrated).
Multi‐color imaging of flame emission
Setup Result
This is a line‐of‐sight imaging technique. Three‐dimensional information requires tomographic inversion from multi‐projection recordings.
C2470 nm
OH308 nm
CH432 nm
C2H2/O2 flame
Height above burner (1 m
m/div)
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Thermometry in sooty flames
Total signal intensity depends on both soot volume fraction and temperature.
We can measure the temperature in a flame if wecan detect the emission intensity as a function ofwavelength.
How can we measure the temperature in this flame?
Photo: Per-Erik Bengtsson
0
5E+10
1E+11
1,5E+11
2E+11
2,5E+11
3E+11
3,5E+11
4E+11
4,5E+11
400 800 1200 1600 2000 2400 2800
Wavelength (nm)
Inte
nsi
ty (
W/m
3)
T=1600K
T=2000K
Visible region
Per-Erik Bengtsson
Planck radiationThe spectral shape of the emission is temperature dependent
1
12)(
/5
2
kThce
hcI
mK10898.2 3max T
4TI
Planck´s law
Wien´s displacement law
Stefan-Boltzmanns law:
Photo: Per-Erik Bengtsson
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CCD-Camera 2
CCD-Camera 1
Temperature map
Optical filter=400 nm
Optical filter=470 nm
The ratio between the emission signals at two wavelengths is temperature dependent.
Still there is aline-of sight limitation!
Temperature imaging using 2‐D pyrometry
Introduction to laser‐based combustion diagnostics
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• Nonintrusive
• High spatial resolution (<0.001 mm3)
• High temporal resolution (<10 ns)
• High spectral resolution (~MHz)
• Multiplex (multi-species, multi-point)
Why use lasers in combustion research?
Undisturbed pre‐mixed flame
Premixed flamedisturbed by a thermocouple
Photos by P.‐E. Bengtsson
Photo by H. Bladh
What can be measured with laser‐based combustion diagnostics?
• Temperatures (rotational/vibrational)
• Species concentrations (atoms, molecules, radicals)
• Velocities
• Particle number densities/diameters
• Surface characteristics
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For example
• Mie/Rayleigh scattering • Laser‐induced fluorescence (LIF)• Laser‐induced incandescence (LII)• Laser‐induced phosphorescence (LIP)• Raman scattering
LaserLens
Spectrograph &detector
For example
• Coherent anti‐Stokes Ramanscattering (CARS)
• Polarization spectroscopy (PS)• Degenerate four‐wave mixing(DFWM)
• Stimulated Emission (SE)
Laser techniques used in combustion research
Coherent techniques
Incoherent techniques
Joakim Bood
Laser‐induced fluorescence (LIF) is the most widely used laser diagnostic for combustion studies
Simultaneous OH‐LIF and PIV measurements in aturbulent CH4/H2/N2/air flame
Rapid development of lasers and detectors over the last decadeshas made LIF a very powerful tool in both fundamental and
applied combustion research
125 sec between images, Image size: 14 16 mm
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X
v’
v’’
re’’
De’’
Potential energy
Internuclear distance
A
v’
X
A
v’’ = 0
v’’ = 1
J’’ = 0J’’ = 1J’’ = 2
J’ = 0J’ = 1J’ = 2
fluorescencespectrum
Laser‐induced fluorescence ‐ basics
v’
X
A
v’’ = 0
v’’ = 1
J’’ = 0J’’ = 1J’’ = 2
J’ = 0J’ = 1J’ = 2
Excitation spectrumFluorescence spectrum
X
A
v’’ = 0
v’’ = 1
J’’ = 0J’’ = 1J’’ = 2
J’ = 0J’ = 1J’ = 2
v’
Laser tuned to a specific absorption line and the spectrometer is scanned
Laser is tuned across the various absorption lines and the total fluorescence is monitored
Fluorescence spectrum and excitation spectrum
Fluorescence spectrum
Excitation spectrum
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2‐D measurements using planar laser‐induced fluorescence (PLIF)
Sheet-forming optics
Side view
View from above
OH-PLIF image
Multi‐species imaging with planar laser‐induced fluorescence (PLIF)
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Setup for multi‐species imaging
Toluene CH2O OH CH
Exc. (nm) 266 355 309 431
Det. (nm) 275‐290 385‐500 3095 43110
OH CH CH2O Toluene
20
Multi‐species imaging in laminar flame
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Jet speed 120m/sJet speed 60m/s
Sjöholm et al., Proc. Combust. Inst. 34, 1475-1482 (2013).
Multi‐species imaging in turbulent flames
OHCH
CH2OCH
CH2OToluene
OHCH
CH2OCH
CH2OToluene
• Tunable (740-790 nm)
• High pulse energy: ~400 mJ @ 776 nm, ~ 70 mJ @ 387 nm, ~10 mJ @ 259 nm
• Long pulse length: ~150 ns
• Single mode (~100 MHz linewidth)
• Multimode (~ 8 cm-1 linewidth)
• Example: 5 mJ single mode at 226 nm!
Strong potential for CH (doubling) and HCO (tripling) PLIF imaging by long pulse and broadband excitation to avoid saturation
Improved sensitivity using Alexandrite laser
1
2
3
4
pumpingLasing
(700-820 nm)
Rapid non-rad. decay
Rapid relax
Alexandrite (BeAl2O4:Cr3+)energy level scheme
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Spectral investigation of CH‐PLIF
CH visualization
Thanks to long and broad pulse ~ two orders of magn. increased sensitivity compared with conv. Nd:YAG/dye system (~25 mJ)
Co-axial jet flame
Motivation: Intermediate species in NOx formationFlame front marker
Approach: Excitation B X at ~ 387 nmEmission B X, A X at ~430 nmBroadband excitation
Excitation scan over band headCH-PLIF
Li et al., Proc. Comb. Inst. 31, 727 (2007)
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Simultaneous PLIF imaging of CH and OH
CH OH
Excitation (nm) 387 283
Detection (nm) 430 310
CH OH
Simultaneous CH/CH2O PLIF
Li et al., Combustion and Flame 157, 1087‐1096 (2010). 26
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Li et al. Comb. and Flame, 2010
Simultaneous PLIF imaging of CH and CH2OBurner Flames PLIF images (CH anf CH2O)
Phi=1.0, Ujet = 100 m/s; Ka ~90
Simultaneous imaging of CH, CH2O, and OH in a turbulent flame
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2‐D thermometry with PLIF
LIF thermometry
• Any method that reflects the distribution of population over two or more individual vibrational rotational states can in principle be used for temperature measurement. LIF is such a method.
• LIF thermometry restricted to high temperatures if molecular radicals are employed. For OH temperatures above 1500 K are needed.
• If atomic species, such as metal atoms, are used, these have to be seeded into the flame or flow.
• If LIF was used for concentration measurements it is definitely convenient to apply it for thermometry too.
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Two‐line LIF thermometry
0
1
2
02
12F21
F20
Basic idea:
To measure the relative populationof two states T from Boltzmannexpression
Excitation to the same upper state F21 and F20 are equally affected byquenching and energy transfer processes
C
II
FF
kEET
lnln4lnln20
21
02
12
20
21
01
C non-dimensional system
Dependent calibration constant
P
Cylindricaltelescope
CCD-camera
PBurner
Interference filter ND filter
Dye cell
Quartz plate
Quartz plate
Laser systems
Power meter
Two‐Line Atomic Fluorescence (TLAF) thermometry
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High‐speed imaging
• Multi YAG/framing camera approach
• kHz laser/CMOS high‐speed camera approach
t
t
Ordinary Nd:YAG laser
Nd:YAG laser cluster
Specs. max rep. rate: ~200 kHz (8 pulses), max pulse energy ~350 mJ/pulse @ 532 nm
~ 220 mJ/pulse @ 355 nm~ 70 mJ/pulse @ 266 nm
Possible to pump dye lasers and OPO units for tunable radiation Multiple dye lasers: 20–30 mJ/pulse @ 283nmOne OPO unit: ~10 mJ/pulse @ 283nm
CC
D 1
CC
D 8
Be
am s
plit
ter
Op
tion
al
imag
ein
tens
ifier
MC
P 1
MC
P 8
Fra
me
sto
re
Ma
ss s
tora
ge
Iris
Len
s m
oun
t
CC
D 2
-6
Mirror
Mirr
or
Beam splitter optics
Different approaches for high-speed visualization
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Turbulent non-premixedCH4/air flame, Re=5500
Air CH4CH4
C.F. Kaminski et al. Appl. Phys. B 68, 757 (1999) Courtesy: J. Sjöholm 2010
First objective: Temporally resolved OH visualization
More recent work: CH visualization
Δt = 100 µs
Pumping an OPO 30 mJ/pulse at 430 nm Bunsen burner flame
∆t = 125 µs
1 432
65 7 8
High-speed PLIF imaging with multi-Nd:YAG cluster
Courtesy: J. Hult, M. Richter
Fuel-tracer PLIF (fuel: iso-octane, tracer: 6% 3-pentanone)
OH-PLIF
Multi-YAG applicationsSingle-cycle-resolved engine diagnostics using PLIF
7o 7.75o 8.5o 9.25o 10o 10.75o 11.5o 12.25o
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• Information on “flame” topology
• Rapid slicing of the measurement volume
• 3-D data reconstructed from the eight resulting 2-D measurements
The multi‐YAG‐cluster also opens up for single‐shot 3‐D measurements
1 2 3 4
5 6 7 8
3‐D fuel tracer PLIF in an engine
Sheet spacing: 0.5 mm
Measurementat +6 CAD
Iso‐concentration surface
Isolated fuel islands
Nygren et al . 29th Comb Symp.
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High‐speed simultaneous CH2O/OH PLIF Experimental setup
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,0us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,20us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,40us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,60us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,80us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,100us OH&CH2O
-5 0 5
45
40
35
30
horizontal location/mm
dist
ance
abo
uve
the
nozz
le/m
m
Phi=0.6,120m/s,120us OH&CH2O
-5 0 5
45
40
35
30
High‐speed simultaneous CH2O/OH PLIF Preliminary results
