Laser Diagnostic Techniques for … Techniques.pdf · Laser Diagnostic Techniques for ... Better...

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IIT Kanpur Kanpur, India (208016)

www.iitk.ac.in/erl

Laser Diagnostic Techniques for Engine Research

Dr. Avinash Kumar Agarwal, Associate Professor,

Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur

[email protected]

Engine Research Laboratory, IIT Kanpur

Optical Diagnostics

Increasing Environmental problems - more stringent Emission Control Norms

Demand to minimize fuel consumption

Better understanding of in-cylinder processes required

To simulate fuel injection and combustion in the cylinder

To use optical diagnostic techniques to visualize the in-cylinder processes

Even the simulation results need to be verified experimentally using Optical

Diagnostic Techniques

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Very high non-steady pressure and temperature conditions; high mechanical and

thermal stresses

proper lubrication not possible, liner heating

fowling of optical access window; need frequent cleaning

supporting structure should not block the optical access

requirement of a ‘flat optical window’

aberration due to unwanted scattering

maintaining realistic engine geometry

Optical Access: Major Challenges


Optical Access

Optical access is usually obtained by:

Full Optical Access:

Transparent Piston Head, and

Transparent Cylinder Liner

Endoscopic Access:

Optical Fiber based Endoscopic windows

Common Materials used:

Quartz

Sapphire

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Full Optical Access vs. Endoscopic Access

Full Optical Access:

The optical access is maximized to allow application of complex optical diagnostic techniques while maintaining minimum necessary operability of the engine or engine components

Full optical access allows a wide range of diagnostics to be applied

Endoscopic Access:

Full engine operability is maintained while optical access and diagnostic techniques are tailored to the diagnostic demand and the restraints of engine operation

Endoscopic access puts the emphasis on organizing and extending realistic engine operation conditions


Full Optical Access

Transparent Cylinder Liner

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Endoscopic Access Arrangement


Optical Diagnostic Techniques

Particle Image Velocimetry (PIV)

Scattering

Mie/Raman/Rayleigh Scattering

Self Emission Spectroscopy

LASER Induced Fluorescence (LIF)

Planar LASER Induced Fluorescence (PLIF)

LASER Induced Incandescence (LII)

Laser Holography

Laser Doppler Velocimetry

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Particle Image Velocimetry (PIV) for IC Engines


Technique – Study flow of a Fluid.

The flow is illuminated with a double pulsed light – sheet and the positions of a

large number of tracer particles are recorded with a photographic camera viewing

normal to the plane of the sheet.

What is PIV?

Advantages of PIV

Non-intrusive into the flow field being studied.

2D or 3D full-field flow measurements can be made.

Instantaneous velocity fields are obtained.

Capability for studying multiphase flows.

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Single and multi-phase channel flows .

Steam bubble collapse

Flow around cylinders in a channel

Bubbly pipe flows

Free surface experiments

Sprays

Heated cavity flows

PIV can be Used to Study:


PIV measures whole velocity fields by taking two images shortly after each other

and calculating the distance individual particles travelled within this time. From

the known time difference and the measured displacement the velocity is

calculated.

Principle of PIV

3-D PIV

based on the principle of stereoscopic imaging: two cameras capture the image of

the illuminated particles from different angles and then the images are digitally

combined to obtain a 3-D images.

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Seeding: The flow medium is seeded with particles, droplets or bubbles

Double Pulsed Laser: Two laser pulses illuminate these particles with short time

difference

Light Sheet Optics: Laser light is formed into a thin light plane guided into the

flow medium

CCD Camera: A fast frame-transfer CCD captures two frames exposed by laser

pulses

Software: Calculates the velocities and makes Velocity Maps

Components


Principle of Particle Image Velocimetry (PIV)

PIV is a technique for the measurement of instantaneous planar velocity fields.

Experimental Arrangement for PIV in a Wind Tunnel

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Particle Image Velocimetry (PIV)

Definition

– An optical imaging technique to measure fluid or particulate velocity vectors at many (eg. Thousands) points in a flow field simultaneously.

– Measurements (2 or 3 components of velocity) usually made in “Planar slices” of the flow field.

Accuracy and Spatial resolution

– Comparable to LDV and HWA.


Particle Image Velocimetry

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PIV - Principle

Cross-correlation

particle displacement

Interrogation region

Crosscorrelation

Vector field

Cross- correlation

frame 1

frame 2

Interrogation region

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Components Needed for PIV:

an illumination source

optical system for illuminating the test section.

digital imagers for capturing the flow field

a system for image processing, particle identification, particle tracking, and vector field cleaning

The laser is synchronized with the digital imagers, the laser light is positioned to illuminate the test volume, the scattered light from the tracer particles is recorded with the digital cameras, and then image analysis is performed.


General Aspects of PIV

Non-intrusive velocity measurement

Indirect velocity measurement

Whole field technique

Velocity lag

Illumination

Duration of illumination pulse

Time delay between illumination pulses

Distribution of tracer particles in the flow

Density of images of tracer particles on the PIV recording

Low image density (PTV)

Medium image density

High image density (LSV)

Number of illumination per recording

Number of components of the velocity vector

Extension of observation volume

Extension in time

Size of interrogation area

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Light Source (LASER): Laser Material, Pump Source, Mirror Arrangement

Schematic of a laser

Various Kinds of interactions between atoms and electromagnetic radiation


Three level laser system Four level laser system

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Particle Generation and Supply

For seeding gas flow: Air jets, condensation generator, atomizers, smoke generators, Laskin nozzle generators.

Types Material Mean Diameter (m)

Solid Polystyrene Aluminum Glass Sphere Granules for synthetic coating

10-100 2-7 10-100 10-500

Liquid Different oils 50-500

Gaseous Oxygen bubbles 50-1000

Types Material Mean Diameter (m)

Solid Polystyrene Aluminum Magnesium Glass micro-balloons Granules for synthetic coating Dioctylphathalate

10-100 2-7 2-5 30-100 10-50 1-10

Smoke <1

Liquid Different oils 0.5-10

Table: Seeding material for liquid flows Table: Seeding material for gas flows


Light Sheet Optics

Light sheet optics using three cylindrical lenses (one of them

with negative focal length)

Light sheet optics using

two spherical lenses

Light sheet optics using

three cylindrical

lenses

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PIV Recording Techniques Single frame/multi-exposure PIV Multi-frame/single exposure PIV

Types of CCD Camera Full-frame CCD Frame transfer CCD Interline transfer CCD Full-frame interline transfer CCD

POST-processing of PIV Data Replacement of incorrect data Data reduction Analysis of the information Presentation and animation of the information


Study of IC Engine Charge Motion Using PIV :

Charge motion within a IC Engine has a Significant effect on:

Power Output

Fuel Efficiency

Exhaust Emissions

The combustion behavior of internal-combustion spark-ignition (SI) engines is

strongly dependent on:

The quality of the mixture processing, which in turn is affected by the motion of

the in-cylinder flow.

Fresh charge, and residual gas resulting from the former combustion cycle, have

to form a proper mixture.

In addition, a certain level of turbulence is required at the time of ignition to

perform an accelerated flame propagation and thereby a highly efficient

combustion.

Therefore, it is highly importance to collect detailed information on the in-

cylinder flow field and its temporal development during the combustion cycle. PIV

has proven to be a helpful tool in order to analyze the air-flow in the cylinder.

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Advantages of PIV over other Techniques such as

1. Streak Photography

2. PTV ( Particle Tracing Techniques)

Advantages:

1. The identification of individual particle image, not necessary in PIV interrogation

2. PIV allows measurement of instantaneous velocity on a fine , regular measurement grid without significant interpolation.

Limitations of PIV :

1. The technique has only technological limitations to achieve a temporal resolution due to the illumination source ( lasers ) and the recording media ( CCD) frequencies which are available today .


Two-Colour Particle Image Velocimetry Analysis of the Effects of Inlet Port Deactivation on the Velocity Flow Field in a Fired

Liquid Fuelled Spark Ignition Engine

M J Haste, C P Garner*, A K Agarwal** N A Halliwell

Department of Mechanical Engineering

Loughborough University*

Loughborough, Leicestershire, England LE11 3TU

Indian Institute of Technology Kanpur **, India

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Fluid motion within IC engine fundamentally affects

engine performance and emissions.

To analyze and optimize complex coupled processes

inside and between automotive components and

structure such as the reduction of a vehicle’s interior

or outer acoustic noise, including brake noise, and

the combustion analysis for diesel and gasoline

engines to further reduce fuel consumption and

pollution.

Deeper insight in modern engine combustion

concepts such as flow generation, fuel injection and

spray formation, atomization and mixing, ignition

and combustion, and formation and reduction of

pollutants.

The need for an non-intrusive measurement system.

Laser Assisted Diagnostics is an important tool for

such measurements.

Engine Manufacturers are developing more fuel efficient, more refined and which produce lower amount of pollutants.

Engine in-cylinder fluid

motion is known to

fundamentally affect the

combustion process.

It is important to understand combustion phenomenon under different operating conditions such as valve deactivation, port injection and variable injection timing.

Introduction


Objective

PIV Is used to investigate the in-cylinder fluid motions and its interaction with propagating flame in a production geometry pentroof multi-valve optical SI engine, fired using liquid fuel.

The flow structures distribution were obtained with both open and closed inlet valve injection timing under normal running and with a single inlet port deactivated.

This allows mapping of the flame position and study of the fluid motion ahead of flame front.

Two color PIV is used to obtain full field instantaneous velocity data over planer regions within the combustion chamber with a spatial resolution of less than 1.5 mm. Si oil seed burn in the flame front hence it is possible to distinguish the burnt and unburned region of the inc-cylinder flow.

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Optical Engine

Bore mm 80 Stroke mm 89 Swept Volume cc 447 cc

Compression ratio (nominal) 10 : 1 Inlet valve peak lift 70 BBDC

Exhaust valve peak lift 70 ABDC

Engine Speed 1000 rpm

Salient Features

• Single Cylinder Optical Engine

• Pent-roof combustion Chamber

• Production grade, four stroke, four valve per cylinder, Rover “K” series

• Fused Silica Barrel

•Extended Piston incorporating Fused Silica Piston Crown Window.

•Port injected with iso-octane and skip fired


Schematic of Experimental Facility

•Large Scale Motion

•Cycle to Cycle Variation

•Small Scale Motion

•Flame Convection

•Flame Geometry

Twin Oscillator, twin amplifier Nd:YAG laser, frequency doubled to generate green light at 532 nm. (Pulse duration 10µs)

Average diameter of Si oil droplets seed: 1.4μm

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Illustration of 3-D Motion Inferred from Planer Data


340-380 CAD

Normal Running Conditions

One Inlet Port Deactivated Condition

1000 RPM

Ignition timing 25 BTDC

Start of Injection 10 ATDC

Measurement Conditions

Data acquisition in

•Horizontal Plane

Horizontal light sheet located 2 mm above the piston at TDC

Camera imaging off the 45 mirror and through the piston window

•Vertical Plane

Vertical light sheet falling on 45 mirror and through the piston window

Camera imaging in horizontal plane close to piston at TDC

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Normal Operating Conditions


Normal Running 20 CAD BTDC

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Normal Running 10 CAD BTDC


Normal Running TDC

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Normal Running 10 CAD ATDC


Normal Running 10 CAD ATDC

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Inlet Port Deactivated Condition


Port Deactivated Condition 20 CAD BTDC

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Port Deactivated Condition 10 CAD BTDC


Port Deactivated Condition TDC

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Port Deactivated Condition 10 CAD ATDC


Port Deactivated Condition 20 CAD ATDC

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Tumble and Skewed Tumble Motion

Valve Jet Flow for: (1) Normal Running and (2) Valve Deactivated Conditions

Conclusions Under normal running conditions some tumble flow remains after TDC, however,

the bulk flow is highly three dimensional and exhibits a torroidal vortex-like structure.

With a single inlet port deactivated, the bulk flow shows significant differences to normal running bulk flow structure and exhibits characteristics more like axial swirl and a three dimensional helical structure.

Significant cyclic variations in large scale structure are observed, and are greatest under normal 4-valve running conditions.

Fuel injection timing was not found to significantly affect the large scale flow structure ahead of the flame around TDC.


Scattering

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Scattering

Principle: When light interacts with matter different scattering processes can happen simultaneously or exclusively depending on chemical and physical properties of the scatterer. Therefore scattered light contains information about the material, it's size and environmental conditions like temperature. Mie imaging: elastic scattering; same wavelength as the incident light; intensity is proportional to the size of the scattering particles; for particles which are ‘large’ compared to the wavelength of the incident light. Rayleigh imaging: elastic scattering; same wavelength as the incident light; intensity is proportional to the intensity of incident light, a material-dependant constant and the number density of particles; for particles are ‘small’ compared to the wavelength of the incident light. Raman imaging: inelastic scattering; shows a spectral response that is shifted from the laser line and characteristic for the Raman active molecules; do not suffer from ‘collision quenching’.


Applications

Mie Scattering : particle analysis (size, shape, distribution)

flow analysis (velocity information – PIV)

spray analysis (particle size distribution and spray geometry)

general imaging tasks

Rayleigh Scattering: combustion processes

pollutant formation

total gas density

temperature fields

Raman Scattering: majority species concentrations

space and time-resolved

mixture fractions

local temperature

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Self Emission Spectroscopy

Principle:

During combustion many species get excited due to the high temperatures

involved, as their electrons come back to the ground state they emit light which

can be resolved to give the ‘fingerprint spectra’ studying which the type and

concentration of the species can be determined

The two-colour method relies on the measurement of the radiation intensity

from soot particles which are generated during combustion. The radiation

intensity can then be measured at two wavelengths.

Applications

Flame temperature, flame location & stability

Spatial Soot concentration

excited species distribution like OH*, CH*, C2*

on-set of ignition


Laser Induced Fluorescence (LIF)

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LASER Induced Fluorescence (LIF) Principle:

the species of interest is excited using a LASER light of specific wavelength.

the electrons move to a higher energy level, emits light at some characteristic wavelengths on returning, this is the ‘fingerprint’ of the species.

the emission spectrum is specific for the molecule. the incident light needs to match the energy levels of the observed molecule. LIF signal is quite strong and can be filtered from the incident laser wavelength. LIF signal is proportional to the volume of a liquid droplet, leads to a direct

measurement of the droplet size.

LIF and Scattering Spectra


Laser Induced Fluorescence (LIF)

Molecules/atoms are excited to higher energy states.

Intensity of fluorescence is a function of species concentration (number density), and the gas temperature and pressure.

Fluorescence is linearly related to number density.

Spectral absorption regions are discrete.

Fluorescence occurs at wavelengths ≥laser wavelength.

Selective detection of NO is possible even in inhomogeneous combustion environments like in direct injecting gasoline and diesel engines.

This technique allows the effective suppression of interfering LIF signals due to hot oxygen and partially burned hydrocarbons.

With this technique, influence of laser beam attenuation is minimized.

The LIF images represent the NO concentration present in the plane defined by the position of the laser beam whereas the exhaust gas measurements represent averaged concentrations after homogeneously mixing the burned gases during the expansion and exhaust stroke.

A chemiluminescence detector (CLD) is used to control the intake NO concentration during calibration measurements and for additional exhaust gas NOX concentration measurements for the different operating conditions.

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Applications of LIF:

OH, NO, O2, CnHm, H2 and H2O

Pollutant formation

rot./vib. Temperature

spray injection

liquid/gas transition

velocity fields

droplet size



PLIF: ‘Sheet’ of LASER light is used to excite; 2-D imaging

LIPF:

to avoid ‘quenching’ short lived quantum states are excited and these

'predissociative' states are so fast that no collisions occur during their

lifetime

Tracer-LIF:

a medium is ‘seeded’ with proper ‘tracer material’ to make it visible or to

observe its mixing with other medium

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Case Studies of LIF

LIF imaging measurements based on in-cylinder-formed formaldehyde and 3-

pentanone as a fuel tracer under controlled auto-igniting (CAI) conditions.

Fuel consisting of 50% n-heptane and 50% iso-octane is used to ensure stable

auto-ignition while having the reduced compression ratio and temperature

typical of most optically accessible engines.

Optically Accessible DI Gasoline Engine

Schematic of Laser-Based Imaging Setup


NO distribution in a DI diesel engine with PLN and CR injection System:

Pump-line-nozzle (PLN) system: it consists of a cam driven pump, a short injection line and an injection nozzle. The injection pressure increases from a low level after start of injection.

Common-rail (CR) system: A constant rail pressure is provided.

Two different detection systems for recording 2-D images and spectroscopic data

Case Studies of LIF

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LIF to visualize the flash boiling effects on the development of GDI engine sprays: LIF is used for spray characterization because of its possibility to differentiate

between the liquid and the vapor phase. LIF can be combined with a long distance microscope so it is possible to

analyze the spray propagation and evaporation directly at the nozzle orifice and the area nearby.

Flash boiling effect causes a rapid spray breakup into a mixture of vapor and small droplets.

Case Studies of LIF


Planer LIF

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What is Combustion PLIF?

Combustion PLIF tells us how the fuel is burning, by showing the location of key species like OH, CH, NO, band CHO.

CO2 + H2O + N2

OH

CH

CHO

C3H8 + O2 + N2

OH

Soot (C2)


What is Combustion PLIF?

• We work with molecules, NOT particles • Species absorbs laser light, emits fluorescence • Fluorescence light is collected and analyzed • We examine the intensity of this light, and spatial variation – Where does it exist in space?

• It is possible to relate the image intensity to temperature or concentration

Propane Flame: Where is the OH?

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How does PLIF work ?

A sheet of laser light illuminates a plane

A target species within the plane of the sheet absorbs light at the wavelength of the laser, exciting the species to a higher energy state

The high energy state decays to a lower energy state, emitting a photon

The emitted photons are collected on a CCD array

The digital image is interpreted


Measurements with PLIF

Temperature or concentration

– Heat transfer – Mass transfer – Mixing

pH

– Possible but not common Species measurement

– Used to monitor chemical reaction intermediates – Combustion, flame, and engine studies

Pressure

– Requires calculations based on known temperature, concentration

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PLIF System Components

Illumination Subsystem - Laser (Mixture of Dye Laser & Nd:YAG 266/532 nm Lasers), Beam delivery, Light optics. – Wavelength

– Pulse energy

– Repetition rate

Image Capture Subsystem – CCD Camera, Intensified CCD Camera. – Capture the Fluorescence image and record them.

Analysis Subsystem – Calculates and Displays a two-dimensional scalar field from the fluorescence image

field.


Planer LIF

A laser source, usually pulsed and tunable in wavelength, is used to form a thin light sheet.

If the laser wavelength is resonant with an optical transition of a species, a fraction of the incident light will be absorbed.

Absorbed photons may subsequently be reemitted with a modified spectral distribution.

The emitted light, known as fluorescence, is collected and imaged onto a solid-state array camera.

The light detected by a camera depends on the concentration of the interrogated species within the corresponding measurement volume and the local flow field conditions.

This technique offers excellent temporal resolution (order of ns) and yields information along a thin (0.2 mm and better) 2-D plane.

PLIF Imaging of Self-Ignition Centers in SI Engine

Plan view into the combustion chamber, showing the self-ignition regions

Knock intensity is related to pressure traces.

The pressure recorded during knocking operation is non-uniform throughout the cylinder.

Analysis of such traces does not yield any spatial information about the self-ignition process.

Thus, optical techniques (e.g. PLIF) are used to obtain spatially and temporally resolved information.

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Experimental Engine

3D view of Cylinder Head Equipped with Optical Access

Optical Setup

Setup for PLIF imaging in the combustion chamber


PLIF Measurements in HCCI Engine

Scania D12 single cyl engine Bowditch type Scania D12 engine

Optical setup for measurements in the Scania D12 engine

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Previously, it was impossible to decide about growth of structure. Also,

measurements did not reveal if new ignition kernels appeared during the

combustion event.

High speed imaging system reveal distributed gradual consumption of fuel or

reaction fronts that spread.

The PLIF sequences shows a well-distributed gradual decay of fuel concentration

during the first stage of combustion.

During the later parts of the combustion process, the fuel concentration images

present much more structure, with distinct edges between islands of unburned

fuel and products.

Intensity histograms reveals that the transition from fuel to products in the HCCI

engine is a gradual process.

The engine configuration, laser sheet orientation and air/fuel ratio do not

influence the general results.


Laser Induced Incandescence (LII)

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LASER Induced Incandescence Principle:

intense laser light sheet is used to illuminate and slice the (reactive) particle flow at user defined locations.

the particles within the light sheet are heated up to the carbon evaporation temperature (> 4000K).

the resultant incandescence (blackbody emission) of the heated particles is detected with a fast shutter camera synchronized to the laser pulse.

appropriate filtering and time-gating of the LII emission assure accurate soot volume fraction measurements.

Mechanism: It involves heating the particles using an intense laser pulse to their sublimation temperature. A soot particle can absorb energy from the beam, which causes the particle’s temperature to increase. At the same time, the soot can loose energy. If the energy absorption rate is sufficiently high, the temperature will rise to levels, where significant incandescence and vaporization can occur. These thermal radiation, when collected after an appropriate time delay is found to be directly proportional to the local mass concentration under specific controlled conditions.


LASER Induced Incandescence A technique used for exhaust emission

measurements in engines. Planar imaging of soot distributions in

steady flames and diesel sprays. LII gives a direct measure of the soot

volume fraction (elemental carbon only).

It is insensitive to other species. However, sensitivity is limited only by the size of the measurement volume.

Neither cooling nor dilution are required, and measurements can be made either in situ or by continuous sampling through an external optical cell.

The LII technique is capable of real-time measurements during transient vehicle operation for optimizing soot emissions performance.

Also, this technique can be executed with other laser-based techniques to obtain particle size and number density information.

Ensemble-averaging for many engine cycles can be used to reconstruct cycle-resolved transient behavior. Sectional top view of optical cell for LII measurements

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Laser Holography

Sectional top view of optical cell for LII measurements


Principle of Holography Method

An object beam and a reference beam are required during recording.

In-line method: The object beam also serves as the reference beam. It is used for

droplet measurement. However, it is difficult to obtain a clear image in an area

where ambient density or droplet density is high.

Off-axis method: The reference beam and object beam take different optical

path.

Interference fringes on the holographic plate are recorded by adjusting the

difference of the optical path of object beam and reference beam. Reconstruction

beam is incident on the holographic plate to reproduce the spray image in the

space. Enlarged photograph is taken using CCD camera and diameter of each

droplet is measured. Then the 3-Dimensional structure of the droplet is obtained.

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Experimental Setup

Optical Recording System

Optical Reconstruction System

Principal of Holography


Laser Holography To measure atomization: Laser holography method, Direct recording method,

The PDPA method, and the Fraunhofer diffraction method.

The holography method is a 3-D measuring method, which utilizes the

interference of light. The Phase Doppler Particle Anemometry (PDPA) method

utilizes the Doppler signal of the droplets. The Fraunhofer diffraction method

obtains the distribution of the droplet diameter from the distribution of

diffracted light.

Laser holography method can

record the shape of each droplet in

the entire spray area and the spatial

distribution with one recording.

Drawbacks: Droplet measurement

in high-density fields, long

analyzing period.

Measuring Methods of Spray Droplets

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Laser Doppler Velocimetry (LDV)


Laser light source

Light separation optics

Light transmitting optics/ Light collecting

optics

Photo-detectors

Signal processing electronics

External data input devices

Computer

Software

Traversing system

Seed particles

Experimental Setup of LDV for Diesel Spray Breakup

Length

Major Components of LDV System

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Laser Doppler Velocimetry (LDV) Data at a single point. Offers flexibility. It works in air or water. As micron-sized particles entrained

in a fluid pass through the intersection of two laser beams, the scattered light received from the particles fluctuates in intensity.

The frequency of this fluctuation is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity.

The velocity direction can be fixed if one of the laser beams has a frequency slightly different from that of the other.

The frequency is measured using digital computers or photon correlators or spectral analyzers.

Applications: Measurements of rotor tip vortices using three-component Laser Doppler Velocimetry. LDV measurements in a boundary layer. Survey of a wake field. Measurements in a shock tube flow. Measurement of Diesel Spray Breakup Length

Diesel spray characteristics by laser Doppler signals: Spray tip penetration and spray breakup length are simply obtained by measuring the delay time of Doppler signals from injection start to spray tip arrival at each measuring point. Spray breakup length is estimated by measuring the standard deviation of the delay time of Doppler signals, which indicates dispersion of the time from injection start to Doppler signal rising.


Thermal Anemometry

Invasive method of measuring 1, 2, or 3 components of velocity

using a heated wire or film sensor

LDV

Non-invasive method of measuring 1, 2, or 3 components of velocity

using a laser technique

PIV

Non-invasive method of measuring 2 and 3 components of velocity in a plane using a double-pulsed laser

Particle Diagnostics

Non-invasive method of measuring Particle, droplet, or bubble size using laser techniques

Comparison of Various Methods of Flow Measurements

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