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Combustion Imaging JOAKIM BOOD | DIV. OF COMBUSTION PHYSICS, LUND UNIVERSITY
CH CH2O OH
Combustion processes are very complex The chemistry is extremely complicated…
The most important reaction paths in acetylene oxidation is shown below
Turbulence Chemical reactions
Flow-field equations (Navier-Stokes)
Transport equations for species
Inter- action
then there is also interaction between the chemistry and the turbulent flow
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
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.
C2 470 nm
OH 308 nm
CH 432 nm
C2H2/O2 flame
Heig
ht a
bove
bur
ner (
1 m
m/d
iv)
Thermometry in sooty flames
Total signal intensity depends on both soot volume fraction and temperature. We can measure the temperature in a flame if we can detect the emission intensity as a function of wavelength.
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
nsity
(W/m
3 )
T=1600KT=2000K
Visible region
Per-Erik Bengtsson
Planck radiation The spectral shape of the emission is temperature dependent
112)( /5
2
−= kThce
hcI λλπλ
mK10898.2 3max ⋅⋅= −Tλ
4TI σ=
Planck´s law
Wien´s displacement law
Stefan-Boltzmanns law:
Photo: Per-Erik Bengtsson
CCD- Camera 2
CCD-Camera 1
Temperature map
Optical filter λ=400 nm
Optical filter λ=470 nm
The ratio between the emission signals at two wavelengths is temperature dependent.
Still there is a line-of sight limitation!
Temperature imaging using 2-D pyrometry
Introduction to laser-based combustion diagnostics
• 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 flame disturbed 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
For example • Mie/Rayleigh scattering • Laser-induced fluorescence (LIF) • Laser-induced incandescence (LII) • Laser-induced phosphorescence (LIP) • Raman scattering
Laser Lens
Spectrograph & detector
For example • Coherent anti-Stokes Raman scattering (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 a turbulent CH4/H2/N2/air flame
Rapid development of lasers and detectors over the last decades has made LIF a very powerful tool in both fundamental and
applied combustion research
125 µsec between images, Image size: 14 × 16 mm
X
v’
v’’
re’’
De’’
Pote
ntia
l ene
rgy
Internuclear distance
A
v’
X
A
v’’ = 0
v’’ = 1
J’’ = 0 J’’ = 1 J’’ = 2
J’ = 0 J’ = 1 J’ = 2
⇒ fluorescence spectrum
Laser-induced fluorescence - basics
v’
X
A
v’’ = 0
v’’ = 1
J’’ = 0 J’’ = 1 J’’ = 2
J’ = 0 J’ = 1 J’ = 2
Excitation spectrum Fluorescence spectrum
X
A
v’’ = 0
v’’ = 1
J’’ = 0 J’’ = 1 J’’ = 2
J’ = 0 J’ = 1 J’ = 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
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)
Setup for multi-species imaging
Toluene CH2O OH CH
Exc. (nm) 266 355 309 431
Det. (nm) 275-290 385-500 309±5 431±10
OH CH CH2O Toluene
20
Multi-species imaging in laminar flame
Jet speed 120m/s Jet speed 60m/s
Sjöholm et al., Proc. Combust. Inst. 34, 1475-1482 (2013).
Multi-species imaging in turbulent flames
OH CH
CH2O CH
CH2O Toluene
OH CH
CH2O CH
CH2O Toluene
• 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
pumping Lasing
(700-820 nm)
Rapid non-rad. decay
Rapid relax
Alexandrite (BeAl2O4:Cr3+) energy level scheme
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 formation Flame front marker Approach: Excitation B ← X at ~ 387 nm Emission B → X, A → X at ~430 nm Broadband excitation
Excitation scan over band head CH-PLIF
Li et al., Proc. Comb. Inst. 31, 727 (2007)
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). 25
Li et al. Comb. and Flame, 2010
Simultaneous PLIF imaging of CH and CH2O Burner 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
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.
Two-line LIF thermometry
0
1
2
λ02
λ12 F21
F20
Basic idea: To measure the relative population of two states ⇒ T from Boltzmann expression
Excitation to the same upper state ⇒ F21 and F20 are equally affected by quenching and energy transfer processes
( )C
II
FF
kEETlnln4lnln
20
21
02
12
20
21
01
+++
−=
λλ C non-dimensional system
Dependent calibration constant
PCylindricaltelescope
CCD-camera
PBurner
Interference filter ND filter
Dye cell
Quartz plate
Quartz plate
Laser systems
Power meter
Two-Line Atomic Fluorescence (TLAF) thermometry
Ultrafast imaging
• Conventional video camera Exposure Read out information Store the information
There is an electronic limitation of how fast you can read out and store images
Filming dynamics Operation of a conventional video camera
We need some kind of trick to separate the images
Speeding up the image recordings Multiple exposures
• Coding strategy Each image has a unique code The code is a stripe pattern The final photo consists of
multiple coded images
Image coding Tagging each frame with a periodic modulation
Identifying coded images The images are separated in the Fourier domain
Now let us demonstrate this on ultrafast dynamics.
FRAME Frequency Recognition Algorithm for Multiple Exposures
• Experimental details Four coded “read pulses” are arranged in a pulse train. One “pump pulse” is visualized by the “read pulses”. The part of the “read pulses” that intersects with the
“pump pulse” is detected by the camera.
A. Ehn et al., Light: Science & Applications (2017) 6, e17045; doi: 10.1038/lsa.2017.45
Experimental design
0 ps 20 ps
Experimental results Identifying movie frames
Raw data Extracted film frames Movie
A record high frame rate of 5 THz has been demonstrated. Higher frame rates can be reached with shorter laser pulses. Independent of wavelengths of the laser and detected signals. Can also be used for instantaneous 3D imaging Applicable for studies of dynamical processes in physics,
chemistry and biology over a wide range of time scales.
Experimental results Reaching frame rates of 5 THz
A. Ehn et al., Light: Science & Applications (2017) 6, e17045; doi: 10.1038/lsa.2017.45
