optical techniques introduction

8/12/2019 Optical Techniques Introduction

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www.iitk.ac.in/erl

Laser Diagnostic Techniques forEngine Research

IIT Kanpur

Kanpur, India (208016)

Dr. Avinash Kumar Agarwal,

Associate Professor,Engine Research Laboratory,

Department of Mechanical Engineering,Indian Institute of Technology Kanpur

[email protected]

Optical Diagnostics

Increasing Environmental problems - more stringent Emission Control Norms

Demand to minimize fuel consumption

Better understanding of in-cylinder processes required

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

Even the simulation results need to be verified experimentally using Optical

Diagnostic Techniques

Engine Research Laboratory, IIT Kanpur


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

thermal stresses

proper lubrication not possible, liner heating

Optical Access: Major Challenges

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

Optical access is usually obtained by:

Full Optical Access:

Transparent Piston Head, and

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 opticaldiagnostic techniques while maintaining minimum necessary operability of theengine 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 diagnostictechniques are tailored to the diagnostic demand and the restraints of engineoperation


Endoscopic access puts the emphasis on organizing and extending realisticengine operation conditions

Full Optical Access

Transparent CylinderLiner



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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

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

PIV can be Used to Study:

u y p pe ows

Free surface experiments

Sprays

Heated cavity flows


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 timedifference

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

Components

flow medium

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

pulses

Software: Calculates the velocities and makes Velocity Maps


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 orarticulate velocit vectors at man e . 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

Interrogation

region

Interrogation

region

particleparticledisplacementdisplacement

Cross-

correlation

frame 1

frame 2

Crosscorrelation

Vector fieldVector field


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Components Needed for PIV:

an illumination source

optical system for illuminating the testsection.

field

a system for image processing, particleidentification, particle tracking, andvector field cleaning

The laser is synchronized with thedi ital ima ers the laser li ht is


positioned to illuminate the test volume,the scattered light from the tracer

particles is recorded with the digitalcameras, and then image analysis isperformed.

General Aspects of PIV

Non-intrusive velocity measurementIndirect 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, MirrorArrangement

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

Types Material Mean

Diameter

(m)

Solid Polystyrene

Aluminum

Magnesium

Glass micro-balloonsGranules for synthetic

coating

10-100

2-7

2-5

30-10010-50

1-10


Liquid Different oils 50-500

Gaseous Oxygen bubbles 50-1000

Dioctylphathalate

Smoke


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PIV Recording Techniques

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

Types of CCD Camera

- Frame transfer CCD Interline transfer CCD Full-frame interline transfer CCD

POST-processing of PIV Data

Replacement of incorrect data


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 s ark-i nition SI en ines 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

erform an accelerated flame ro a ation and thereb a hi hl 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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Fluid motion within IC engine fundamentally affects

engine performance and emissions.

To analyze and optimize complex coupled processes

inside and between automotive components and

Engine Manufacturers aredeveloping more fuel

efficient, more refined andwhich produce lower amountof pollutants.

Introduction

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

Engine in-cylinder fluid

motion is known to

fundamentally affect the

combustion process.

It is important to understandcombustion phenomenon


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.

under different operatingconditions such as valve

deactivation, port injectionand variable injection timing.

Objective

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

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

Two color PIV is used to obtain full field instantaneous velocity data over planerregions within the combustion chamber with a spatial resolution of less than 1.5mm. Si oil seed burn in the flame front hence it is possible to distinguish theburnt and unburned re ion of the inc-c linder flow.


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


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Optical Engine

Bore mm 80Stroke mm 89Swept Volume cc 447 cc

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

Exhaust valve peak lift 70ABDC

ng ne pee 1000 rpm

Salient Features

Single Cylinder Optical Engine

Pent-roof combustion Chamber

Production grade, four stroke, fourvalve per cylinder, Rover K series


Fused Silica Barrel

Extended Piston incorporating Fused

Silica Piston Crown Window.

Port injected with iso-octane and skipfired

Schematic of Experimental Facility

Average diameter of Sioil droplets seed: 1.4m

Large Scale Motion

,amplifier Nd:YAG laser,frequency doubled togenerate green light at532 nm. (Pulse duration10s)


Cycle to Cycle Variation

Small Scale Motion

Flame Convection

Flame Geometry


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Illustration of 3-D Motion Inferred from Planer Data


Measurement

Plane

340-380CAD

Normal Running Conditions

One Inlet Port Deactivated Condition

Measurement Conditions

Data acquisition in

Horizontal Plane

Horizontal light sheet located 2mm above the piston at TDC

Ignition timing 25BTDC

Start of Injection 10ATDC

amera mag ng o t e 45m rrorand through the piston window

Vertical Plane

Vertical light sheet falling on 45mirror and through the pistonwindow

Camera imaging in horizontallane close to iston 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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2

Normal Running 10 CAD ATDC


Normal Running 10 CAD ATDC



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2

Inlet Port Deactivated Condition


Port Deactivated Condition 20 CAD BTDC



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2

Port Deactivated Condition 10 CAD BTDC


Port Deactivated Condition TDC



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2

Port Deactivated Condition 10 CAD ATDC


Port Deactivated Condition 20 CAD ATDC



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2

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.


aman mag ng:

inelastic scattering; shows a spectral response that is shifted from the laser line and

characteristic for the Raman active molecules; do not suffer from collisionquenching.

Applications

Mie Scattering :

particle analysis (size, shape, distribution)

flow analysis (velocity information PIV)

spray analysis (particle size distribution and spray geometry)

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 temperaturesinvolved, 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

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 characteristicwavelengths on returning, this is the fingerprint of the species.

the emission spectrum is specific for the 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 directmeasurement of the droplet size.


LIF and Scattering Spectra

Laser Induced Fluorescence (LIF)

Molecules/atoms are excited tohigher energy states.

Intensity of fluorescence is afunction of species concentration(number density), and the gastemperature and pressure.

Selective detection of NO is possibleeven in inhomogeneous combustionenvironments like in direct injectinggasoline and diesel engines.

This technique allows the effective

Fluorescence is linearly related tonumber density.

Spectral absorption regions arediscrete.

Fluorescence occurs atwavelengths laser wavelength.

suppress on o n er er ng s gna sdue to hot oxygen and partially burnedhydrocarbons.

With this technique, influence of laserbeam attenuation is minimized.

The LIF images represent the NOconcentration present in the planedefined by the position of the laserA chemiluminescence detector (CLD)


eam w ereas t e ex aust gasmeasurements represent averagedconcentrations after homogeneouslymixing the burned gases during theexpansion and exhaust stroke.

s use o con ro e n a econcentration during calibrationmeasurements and for additionalexhaust gas NOX concentrationmeasurements for the differentoperating conditions.


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Applications of LIF:

OH, NO, O2, CnHm, H2and H2O

Pollutant formation

. .

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

Optically Accessible DIGasoline Engine

Schematic of Laser-Based Imaging Setup


mag ng measuremen s ase on n-cy n er- orme orma e y e an 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.

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 shortinjection line and an injection nozzle. The injection pressure increases from a lowlevel after start of injection.

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

Case Studies of LIF


Two different detection systems for recording 2-D images andspectroscopic data


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LIF to visualize the flash boiling effects on the development of GDIengine sprays:

LIF is used for spray characterization because of its possibility to differentiatebetween the liquid and the vapor phase.

LIF can be combined with a long distance microscope so it is possible to

Case Studies of LIF

the area nearby.

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


Planer LIF



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What is Combustion PLIF?

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

CO2 + H2O + N2OH

CH

OH

Soot (C2)


CHO

C3H8 + O2 + N2

What is Combustion PLIF?

We work with molecules, NOT particles

Species absorbs laser light, emits fluorescence

uorescence g s co ec e an ana yze

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 orconcentration


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 wavelengthof 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


e g ta mage s nterprete

Measurements with PLIF

Temperature or concentration

Heat transfer

Mass transfer

Mixing pH

Possible but not common Species measurement

Used to monitor chemical reaction intermediates


om us on, ame, an eng ne s u es 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.

ave engt

Pulse energy

Repetition rate

Image Capture Subsystem CCD Camera, Intensified CCD Camera.

Capture the Fluorescence image and record them.

Analysis Subsystem


a cu ates an sp ays a two- mens ona sca ar e rom t e uorescence mage

field.

Planer LIF

A laser source, usually pulsed and tunablein wavelength, is used to form a thin lightsheet.

If the laser wavelength is resonant withan optical transition of a species, a

PLIF Imaging of Self-Ignition

Centers in SI Engine

absorbed. Absorbed photons may subsequently be

reemitted with a modified spectraldistribution.

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

The light detected by a camera dependson the concentration of the interrogated

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

Knock intensity is related to pressuretraces.

The pressure recorded during knockingoperation is non-uniform throughoutthe c linder


measurement volume and the local flowfield conditions.

This technique offers excellent temporalresolution (order of ns) and yieldsinformation along a thin (0.2 mm andbetter) 2-D plane.

. Analysis of such traces does not yield

any spatial information about the self-ignition process.

Thus, optical techniques (e.g. PLIF) areused to obtain spatially and temporallyresolved information.


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Experimental Engine Optical Setup


with Optical Access

chamber

PLIF Measurements in HCCI Engine


Scania D12 single cyl engine Bowditch type ScaniaD12 engine

Optical setup for measurementsin 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

.

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 evaporationtem erature > 000K .

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

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

Mechanism: It involves heating the particles using an intense laser pulse to theirsublimation temperature. A soot particle can absorb energy from the beam, which


. ,energy. If the energy absorption rate is sufficiently high, the temperature will riseto levels, where significant incandescence and vaporization can occur. Thesethermal radiation, when collected after an appropriate time delay is found to bedirectly proportional to the local mass concentration under specific controlledconditions.

LASER Induced Incandescence

A technique used for exhaust emissionmeasurements in engines.

Planar imaging of soot distributions insteady flames and diesel sprays.

LII gives a direct measure of the sootvolume fraction (elemental carbononly).

.However, sensitivity is limited only bythe size of the measurement volume.

Neither cooling nor dilution arerequired, and measurements can bemade either in situ or by continuoussampling through an external opticalcell.

The LII technique is capable of real-timemeasurements during transient vehicleo eration for o timizin soot emissions


performance. Also, this technique can be executed

with other laser-based techniques toobtain particle size and number densityinformation.

Ensemble-averaging for many enginecycles 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

.

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 SystemPrincipal of Holography


Optical Reconstruction System

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.


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

Li ht transmittin o tics Li ht collectin

Major Components of LDV System

optics

Photo-detectors

Signal processing electronics

External data input devices

Com uter


Software

Traversing system

Seed particles

Experimental Setup of LDVfor Diesel Spray Breakup

Length


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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 theintersection of two laser beams, the

Applications:Measurements of rotor tip vortices usingthree-component Laser DopplerVelocimetry.LDV measurements in a boundary layer.Survey of a wake field.

scattered light received from theparticles fluctuates in intensity.

The frequency of this fluctuation isequivalent to the Doppler shiftbetween the incident and scatteredlight, and is thus proportional to thecomponent of particle velocity.

The velocity direction can be fixed if

.Measurement of Diesel Spray BreakupLength

Diesel spray characteristics by laserDoppler signals:Spray tip penetration and spray breakuplength are simply obtained by measuringthe delay time of Doppler signals from


one o t e aser eams as afrequency slightly different from that

of the other. The frequency is measured usingdigital computers or photoncorrelators or spectral analyzers.

measuring point.Spray breakup length is estimated by

measuring the standard deviation of thedelay time of Doppler signals, whichindicates dispersion of the time frominjection start to Doppler signal rising.

Thermal Anemometry LDV

Comparison of Various Methods of Flow Measurements

Invasive method of measuring1, 2, or 3 components of velocity

using a heated wire or film sensor

Non-invasive method of measuring1, 2, or 3 components of velocity

using a laser technique

PIV Particle Diagnostics


Non-invasive method of measuring2 and 3 components of velocity in aplane using a double-pulsed laser

Non-invasive method of measuringParticle, droplet, or bubble sizeusing laser techniques


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optical techniques introduction

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