COMBUSTION DIAGNOSTICS – LIF

| JIMMY OLOFSSON | 2013A Nova Instruments company

COMBUSTION DIAGNOSTICS – LIFDr. 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

Combustion related applications:• Transportation• Electrical power production• Heating

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 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

Soot LII

Rayleigh Temperature

Fuel Tracer LIF

Combustion Radicals - LIF

Combined Measurements

Molecular spectroscopyin brief

Combustion species

• Gas with chemical reactions• Production of radicals• Qualitative concentration of radical

- OH- CH- NO- etc

• Concentration of larger molecules/tracers- Formaldehyde- Acetone- etc

Laser-Induced Fluorescence

Excited State

Ground State

Photon

Absorption

Excited Molecule

Emission

Fluorescence

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

Molecular energy states: Electronic

Molecular energy states:Vibrational and Rotational

OH absorption spectrumSeveral absorption lines around 283 nm

• Air Wavelengths Excitation in UV

Wavelength (Å)

OH absorption spectrumTwo narrow absorbtion regions within 100 nm range

240 260 280 300 320 340 360

Wavelength (nm)

O–H ~283 nm

Temperature dependence

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

in the measured temperature range

Acetone absorption spectrumLarger molecules have wider absorption range

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.

Laser-Induced Fluorescence

200 250 300 350 400 450 500 550

Wavelength /nm

Fluorescencespectrum

Bandpass

filter

Laserline

Absorptionspectrum

Detected LIF

Residual laser light

Combustion LIF system

CCD Camera

Burner

Nd:YAG Laser

Dye LaserSheet Optics

UV Camera Lens

Optical Filter

Image Intensifier

Standard Nd:YAG pumped dye laser

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

1090 mm

840 mm

250 mm

744 mm

250 mm

Nd:YAG laser

Dye laser

3ω/4ω 2ω

Beam combining output bench

Dye laser UV beams or 266nm or

Tuneable dye laser oscillator

Dye Laser

4. Flowing dye cell5. High reflectivity mirror6. Focusing lens

1. Tuning mirror2. Grazing incidence

grating3. Beam expander prism

(NBP Option)

Tuning curves for laser dyes

Species and excitation wavelengths

SpeciesExcitation

wavelengthLaser pulse

energyProcess Type of dye

OH 283 nm 25 mJ Doubling Rh590

CH 389 nm 28 mJ Mixing Rh610+Rh640

CO 230 nm 13 mJMixing after

doublingRh610

NO 226 nm 4.5 mJMixing after

doublingRH590+Rh610

Our refecence species which we use during the lab training

Light sheet forming optics

• Quartz optics for UV/visible transmission• Parallel light sheet

- Better control of reflections- Enhanced energy distribution

Beam waist adjusterSheet height adjuster

Holder & fixation system

Standard mount

Detecting Laser-Induced Fluorescence

Image Intensifier • 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

UV Camera Lens

• UV camera lens required for detection of UV fluorescence

Spectral Filter

• Spectral filter to eliminate detection of scattered laser light and flame emission

CCD Camera

• Sensitive, high-resolution CCD camera

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

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

Nd:YAG laser• Automatic detecion• Auto activation at

Preview/Acquisition• Q-switch activation/de-activation

during Preview/Acquisition• Interlock messages displayed in

Tuneable Dye laser• Wavelength set• Wavelength fine-tune buttons• Wavelength scan• Output wavelength calculated

from fundamental depending on frequency conversion scheme

Time-resolved Combustion LIF

Framing rate requirements

EXAMPLE

• Heat release event in combustion engine running at 1200 rpm.

• The main heat release occurs within ~5CAD out of the entire 360CAD engine cycle.

J.Olofsson et al SAE 2005

lved F

ehyde L

• Resolution used in the study: 0.5CAD

• This corresponds to a 14kHz

High-speed Nd:YLF laser

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

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

Pulse separation: 75 µs

Rep. Rate: 13kHz

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

TR C-LIF: Dye laser

Example:

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!

TR C-LIF: SpeedSense camera series

Model example SpeedSense v711

Maximum fps at full res.

7500 at1280 x 800

Resolution at 10kHz (example)

1280 x 600

Resolution at 15kHz (example)

896 x 544

TR C-LIF: Image intensifiers

Model H Series9138A1178

Maximum repetition rate

200 kHz

Minimum gate time

Diameter (input/output)

Photocathode material

Multialkali

Phosphor screen material

L Series9138A1180

100 kHz

S20(as

Multialkali)

COMBUSTION DIAGNOSTICS – APPLICATIONSDr. Jimmy Olofsson

Mixing and heat transfer Pre- / Post-combustion Combustion

Liquid flowsGaseous flows

(non reactive)

Gaseous flows

(reactive)

Image intensifier unit

Applicat

Hardwar

Tuneable Dye LaserNd:YAG Laser

Scalar imaging applications

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

Application example 1

How to acheive homogeneous Acetone concentration for calibration

Example: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 UniversityBeijing, China

Tracer-LIF calibration

Formaldehyde visualization in anHCCI engine

Homogeneous Charge Compression Ignition Engine

Advantages• Lower NOx levels and less soot formation

compared to the Diesel engine• Higher part load efficiency compared to the

SI engine

Disadvantage• Difficult to control ignition timing

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

Formaldehyde LIF in an engine

Wavelength: 355 nmFuel: N-Heptane

Field-of-view

High-speed laser

Cycle-resolvedFormaldehyde consumption

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

Fluorescence spectra diatomic radicals

• OH radical

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, and Wright-Patterson Air Force Base, OH, USA

Simultaneous CH PLIF and PIV

By courtesy of

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

Combined OH LIF, fuel tracer LIF and PIV

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.

OH radical

Fuel tracer / Acetone

Flow velocity field

• Local flame extinction events

• Create a data base of measurement data

• Data used for model comparison

Simultaneous PIV and TR OH LIF local flame extinction

BurntCoflow

Unburnt: Methane&Air

Air&Burnt

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

Multi-dye laser cluster

J.Olofsson

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.

Combined TR PIV and TR OH LIFwith Lund University, Sweden

Flow field and flame front at 4 kHz

Lund University P.Petersson and J.Olofsson

COMBUSTION DIAGNOSTICS – RELATED TECHNIQUESDr. Jimmy Olofsson

Laser-Induced Incandescence

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

• Soot particles are heated up by laser radiation• The increased particle temperature results in increased emission of

Plank radiation

Size decreases

Time (ns)

0 100 200 300 400 500

LII measurement systems

Image Intensifier

Laser diagnostics in an IC engine

Quantitative LII

Soot-volume-fraction in a Diesel engine

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

• Soot formation at different EGR rates

• Soot formation at different piston bowl geometries

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.

Required data sets forRayleigh Thermometry

Reference imageMeasurement image

Results of Rayleigh Thermometry analysis

Mean: 1120 K

RMS: 61,3

Mean: 295 K

RMS: 12,2

Mean: 1350 K

RMS: 106

Rayleigh Thermometry results

Takes into account:• Scattering cross-section• Pressure• Laser pulse energy

Thank you for your attention!

DANTECD Y N A M I C S

COMBUSTION DIAGNOSTICS – LIF

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