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COMBUSTION DIAGNOSTICS – LIF. Dr. Jimmy Olofsson. Outline. Why combustion diagnostics? Molecular spectroscopy in brief Combustion LIF system Time-resolved Combustion LIF Coffee break Applications Related Techniques. Why combustion diagnostics?. Benefits of analysing combustion.

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Presentation Transcript
  • Why combustion diagnostics?
  • Molecular spectroscopy in brief
  • Combustion LIF system
  • Time-resolved Combustion LIF
  • Coffee break
  • Applications
  • Related Techniques
benefits of analysing combustion
Benefits of analysing combustion

Combustion related applications:


Electrical power production


Combustion analysis can be used for economic as well as environmental benefits by:

Optimizing fuel economy

Improving performance and reliability

Reducing pollutant emissions

benefits of using lasers for combustion diagnostics
Benefits of using lasers forcombustion diagnostics
  • Non-intrusive
  • High spatial resolution
  • High temporal resolution
  • High sensitivity
  • Species selective
  • 2D measurements

Laser-based measurements techniques can provide information on species concentrations, temperature fields, flow velocities etc. and the measurements often have the following properties:

combustion diagnostic techniques
Combustion diagnostic techniques

Combustion Radicals - LIF

Soot LII

Combined Measurements

Fuel Tracer LIF

Rayleigh Temperature

combustion species
Combustion species

Gas with chemical reactions

Production of radicals

Qualitative concentration of radical





Concentration of larger molecules/tracers




laser induced fluorescence

Excited Molecule




Laser-Induced Fluorescence

Excited State

  • Species selective measurements
  • (OH, formaldehyde, fuel tracers, etc.)

Ground State


oh absorption spectrum several absorption lines around 283 nm
OH absorption spectrumSeveral absorption lines around 283 nm

Air Wavelengths

Excitation in UV

Wavelength (Å)

oh absorption spectrum two narrow absorbtion regions within 100 nm range
OH absorption spectrumTwo narrow absorbtion regions within 100 nm range

~283 nm








Optical Density

240 260 280 300 320 340 360

Wavelength (nm)

temperature dependence
Temperature dependence

Choose a peak with for which the fluorescence is independent of temperature

in the measured temperature range

acetone absorption spectrum larger molecules have wider absorption range
Acetone absorption spectrumLarger molecules have wider absorption range







Optical Density

240 260 280 300 320 340 360

Wavelength (nm)

selection of excitation wavelength
To excite atoms or diatomic molecules the laser wavelength must be precisely tuned to match molecular energy transition.

Larger molecules, such as Acetone, 3-pentanone or Formaldehyde, have many more close-lying states, effectively making a wide continuous absorption band. Therefore, any wavelength within the absorption band can be used to excite the molecule.

Selection of excitation wavelength
laser induced fluorescence1
Laser-Induced Fluorescence





Detected LIF


Residual laser light












Normalised intensity










combustion lif system
Combustion LIF system

Image Intensifier

UV Camera Lens

Optical Filter

CCD Camera

Nd:YAG Laser


Sheet Optics

Dye Laser

standard nd yag pumped dye laser
Standard Nd:YAG pumped dye laser

1090 mm


Nd:YAG laser

250 mm

Dye laser

744 mm

Dye laser UV beams or 266nm or 355nm

250 mm

Beam combining output bench

840 mm

Nd:YAG laser

  • Single cavity 10 Hz
  • Wavelengths: 1064 nm, 532 nm, 355 nm, 266 nm
  • Pulse length ~10 ns
  • Pulse energy 400 mJ @ 532 nm

Tuneable dye laser

  • Tunability range of fundamental: 380-750 nm
  • UV extension down to 200 nm
  • Line width: 0.8 cm-1
  • Narrow band option: 0.08 cm-1
tuneable dye laser oscillator

Tuning mirror

  • Grazing incidence grating
  • Beam expander prism (NBP Option)
  • Flowing dye cell
  • High reflectivity mirror
  • Focusing lens
Tuneable dye laser oscillator

Dye Laser

species and excitation wavelengths
Species and excitation wavelengths

Our refecence species which we use during the lab training

light sheet forming optics
Light sheet forming optics

Sheet height adjuster

Beam waist adjuster

Standard mount

Holder & fixation system

  • Quartz optics for UV/visible transmission
  • Parallel light sheet
  • - Better control of reflections
  • - Enhanced energy distribution
detecting laser induced fluorescence
Detecting Laser-Induced Fluorescence
  • Sensitive, high-resolution CCD camera

CCD Camera

  • Image intensifier
    • Amplifies the incoming light
    • Converts UV fluorescence to visible light detectable by the CCD camera
    • Allows gated detection with very short time gates, to minimise detection of natural flame emission

Image Intensifier

UV Camera Lens

Spectral Filter

  • UV camera lens required for detection of UV fluorescence
  • Spectral filter to eliminate detection of scattered laser light and flame emission
optical filters
Interference filters are used to transmitt only in the wavelength interval of the fluorescence from the molecular species of interest, typically some few 10 nm

All other wavelengths should ideally be blocket by the filter

Optical filters
combustion lif software and timing
Combustion LIF: Software and timing
  • Synchronization unit
  • Analog Input option. Includes the A/D board and software add-on
  • Software:
    • DynamicStudio acquisition and processing software
    • Software add-ons for tracer LIF and combustion LIF
laser control from the software
Laser control from the software

Nd:YAG laser

  • Automatic detecion
  • Auto activation at Preview/Acquisition
  • Q-switch activation/de-activation during Preview/Acquisition
  • Interlock messages displayed in Log

Tuneable Dye laser

  • Wavelength set
  • Wavelength fine-tune buttons
  • Wavelength scan
  • Output wavelength calculated from fundamental depending on frequency conversion scheme
framing rate requirements
Framing rate requirements


  • Heat release event in combustion engine running at 1200 rpm.
  • The main heat release occurs within ~5CAD out of the entire 360CAD engine cycle.
  • Resolution used in the study: 0.5CAD
  • This corresponds to a 14kHz

Time-resolved Formaldehyde LIF

J.Olofsson et al SAE 2005

high speed nd ylf laser
High-speed Nd:YLF laser

Output pulse energy (527 nm) vs repetition rate (single cavity laser)

pumping of dye lasers

Pulse separation: 75 µs

Rep. Rate: 13kHz

Pumping of dye lasers

Pumping of a dye laser with high repetition rate causes two major problems:

  • Decrease in pulse energy
  • Deterioration of beam profile
tr c lif yag based pump lasers
TR C-LIF: YAG-based pump lasers

IS Series

  • Repetition rate up to 10kHz
  • Pulse length: ~10ns
  • Pulse energy @ 4kHz: 8mJ

HD Series

  • Repetition rate up to 10kHz
  • Pulse length: ~10ns
  • Pulse energy @ 10kHz: 12mJ
  • Pulse energy @ 5kHz: 20mJ
tr c lif dye laser
TR C-LIF: Dye laser


Pumping with 12W @ 1kHz => 12mJ / pulse

Dye: Rhodamine 6G (~570nm) gives 3.3mJ / pulse

Frequency doubling to ~283nm for OH LIF is estimated to give ~0.5mJ / pulse

This should be compared with the corresponding ~20mJ / pulse achieved by the standard 10Hz system!

scalar imaging applications
Scalar imaging applications

Gaseous flows

(non reactive)

Gaseous flows


Liquid flows


Mixing and heat transfer

Pre- / Post-combustion



Nd:YAG Laser

Image intensifier unit

Tuneable Dye Laser

fuel tracer lif
Fuel Tracer-LIF

Two different approaches to fuel visualization

  • ”Real” fuels
    • Real engine conditions
    • Unknown fluorescent properties (temperature, pressure, quenching etc.)
  • Non-fluorescing reference fuel with added fluorescent tracer
    • Well-known fluorescent properties
    • Allows for quantification
    • Further from real engine conditions
fluorescent tracer spectra
Acetone fluorescence spectrum

Formaldehyde fluorescence spectrum

A: in a flame

B: in an engine

Fluorescent tracer spectra
how to acheive homogeneous acetone concentration for calibration
How to acheive homogeneous Acetone concentration for calibration


Quantification of fuel vapour in constant pressure vessel using liquid fuel

“Iso-octane was used as substitute of real gasoline in PLIF experiment and 10% acetone was added in as tracer.”

“To get a homogeneous mixture, a small amount of fuel was injected into vessel. Waited about 30 seconds for vaporization, then, recorded 100 LIF signal images. After averaged the images and subtracted the background, the result gave the relationship between current equivalence ratio and the LIF signal.”

Tsinghua University

Beijing, China

formaldehyde visualization in an hcci engine
Formaldehyde visualization in anHCCI engine

Homogeneous Charge Compression Ignition Engine


  • Lower NOx levels and less soot formation compared to the Diesel engine
  • Higher part load efficiency compared to the SI engine


  • Difficult to control ignition timing

For some fuels formaldehyde is formed in the cool-flame region

J.Olofsson et al SAE 2005

formaldehyde lif in an engine
Formaldehyde LIF in an engine

High-speed laser


Wavelength: 355 nm

Fuel: N-Heptane

J.Olofsson et al SAE 2005

cycle resolved formaldehyde consumption
Cycle-resolvedFormaldehyde consumption

Single-cycle-resolved formaldehyde fluorescence imaged with a time separation of ~70 µs (0.5 CAD).

J.Olofsson et al SAE 2005

piv plif investigation of two phase vortex flame interactions
PIV/PLIF investigation of two-phase vortex-flame interactions
  • Study of two-phase vortex-flame interaction in a counterflow burner
  • Local flame extinction events
  • PIV for flow velocity field measurements giving the local strain rates
  • PLIF of CH (389.5 nm) for diffusion flame front location and flame extinction zones

Investigation done in collaboration between École Centrale Paris, France, Innovative Scientific Solutions, andWright-Patterson Air Force Base, OH, USA

simultaneous ch plif and piv
Simultaneous CH PLIF and PIV

By courtesy of

École Centrale Paris, France, Innovative Scientific Solutions, andWright-Patterson Air Force Base, OH, USA

combined oh lif fuel tracer lif and piv1
Combined OH LIF, fuel tracer LIF and PIV

OH radical

Flow velocity field

Fuel tracer / Acetone

  • Local flame extinction events
  • Create a data base of measurement data
  • Data used for model comparison

Simultaneous flow field (PIV), fuel (tracer-LIF) (blue) and OH (LIF) (green) visualisation in a turbulent atmospheric flame. Courtesy of R. Collin and P. Petersson, Division of Combustion Physics, Lund University, Sweden.







Unburnt: Methane&Air

Simultaneous PIV and TR OH LIF local flame extinction

OH: Intermediate combustion product in hydrocarbon combustion. Flame front marker.

Time-resolved OH LIF at 2.5kHz framing rate

Lund University P.Petersson and J.Olofsson

combined tr piv and tr oh lif with lund university sweden
Combined TR PIV and TR OH LIFwith Lund University, Sweden

Planar Laser-Induced Fluorescence (PLIF) system

Diode pumped Nd:YAG laser is used to pump a high repetition rate dye laser.

The emitted 283 nm laser pulses excites OH radicals in the flame –> imaged on an intensified high-speed camera.

Combined with high repetition rate Nd:YLF laser for simultaneous TR PIV.

flow field and flame front at 4 khz
Flow field and flame front at 4 kHz

Lund University P.Petersson and J.Olofsson

soot in combustion
Soot in combustion
  • Soot is a hazardous pollutant emission
  • Soot is related to incomplete combustion which has an impact on combustor performance
laser indusced incancescence

Size decreases

Laser-Indusced Incancescence
  • Soot particles are heated up by laser radiation
  • The increased particle temperature results in increased emission of Plank radiation

LII intensity (a.u.)

0 100 200 300 400 500

Time (ns)

lii measurement systems
LII measurement systems

Image Intensifier

quantitative lii
Quantitative LII

Soot-volume-fraction in a Diesel engine

  • Soot formation at different EGR rates
  • Soot formation at different piston bowl geometries

Soot volume fration (ppm)

Work done by H. Bladh et al, at Combustion Physics, Lund University, Sweden


Rayleigh Thermometry

  • The Rayleigh signal is dependent on:
    • Laser intensity
    • Scattering cross section
    • Number density
  • If species composition and pressure are known in the gas the gas temperature can be determined from imaging of the Rayleigh scattering.
results of rayleigh thermometry analysis

Mean: 1350 K

RMS: 106

Mean: 1120 K

RMS: 61,3

Mean: 295 K

RMS: 12,2

Results of Rayleigh Thermometry analysis
rayleigh thermometry results
Rayleigh Thermometry results

Takes into account:

  • Scattering cross-section
  • Pressure
  • Laser pulse energy