Incident raysIncident rays

LIF & MIE spray characterisation LIF & MIE spray

characterisation

•

• Luis Le Moyne (UniversitéLuis Le Moyne (UniversitéPierre et Marie Curie)Pierre et Marie Curie)

sprays

• Multiphase flows – Droplets

• Diameters from 1 to 100µm

• Velocities from 0 to 300m/s

• Temperatures from ambient to some 10² K

– Vapour

• Fuel vapour+Ambient gas (air/nitrogen)+Combustion products

• Velocities from 0 to 10m/s

• Temperatures from ambient to 10³ K

• Scale

– Some 10^-2m in length & some 10^-3s in time

Scattering in sprays

• General expression for scattered light signal S :

(C is constant for fixed temperature and experimental parameters, expression valid for d>>λ and no Morphology Dependent Resonances)

• Light emitted by a particle in elastic scattering (λ_i=λ_e)

•Size parameter α : Raleigh(α<0.1)/MIE(0.1< α <300)/Geometrical(α >300)

• Light emitted by a particle in inelastic scattering (

λ

≠λ

)

LIF, Raman,....

2πd

α ⁼ λ

.

S = C d

Scattering modes

-2

1st order refraction

-1 1 2

-2 -1 1 2

Incident ray Reflection

2nd order refraction

3rd order

4th order 5th order

6th order

7th order 8th order

n_p n_m

n_p > n_m

The intensity of the incident ray is partly reflected and refracted.

The intensity ratio is given

by the Fresnel coefficients and depends on the incident

angle, polarization and relative refractive index.

The scattering angle is given by Snell’s law.

The phase is given by the optical path length of the ray.

Most of the intensity is contained in the first three scattering modes.

Light scattering by droplets and bubbles

-2 -1 1 2

-2 -1 1 2

Water droplet in air

Incident rays

Air bubble in water

Incident rays

-2 -1 1 2

-2 -1 1 2

Intensity of scattered light

• The scattered light intensity from the different scattering modes varies at different scattering angles.

• The scattering intensity also depends on the polarization

orientation of the incident light.

-3 -2 -1 1 2 3 4 5

-3 -2 -1 1 2

parallel 3

polarization

perpendicular polarization

1st order refraction

reflection 2nd order

refraction

Lorenz-Mie

Scattering in sprays

• For common lasers and spherical absorbing droplets

of d>1µm : ₂

MIE MIE

S = C d

MIE signal dependence on diameter

(single droplets)

Liquid : Kerosine with fluorescing components and dye for absorption

MIE signal dependence on diameter

(sprays)

• If the area observed/camera resolution compromise does not allow to distinguish individual droplets, the MIE signal depends on droplets diameter AND

number (density)

• MIE signal intensity for a group of droplets of same diameter d :

• B For

quantitative

» More information is needed : polarization, coupling with other techniques (LIF)

( , ) 2

MIE inc d

I = I ⋅ f n

θ

MIE/LIF for size measurements

• LIF signal is dependent on volume :

• MIE signal is dependent on surface :

• The ratio of the two signals in provenance of a spray is representative of SMD :

3

LIf LIF

S = C d

2

MIE MIE

S = C d

3 3

2 2 32

LIF LIF

MIE MIE

C d d

S D

S = C

∑

∑

∑ ∑

LIF specificities

• What is observed ? :

– A fluorescent molecule which is part of fuel components or a dopant added to fuel

• How it is observed ? :

• A laser light source (generally a laser sheet) induces fluorescence of molecule observed by a camera through optics (lenses, mirrors, filters, windows…)

B

AND

if fluorescence signal dependence on concentration of specific molecule is known & controlled (wavelength,

LIF specificities

• What are the differences between a LIF image of a spray and the « reality » ? :

– Dopant/Fuel miscibility & stability (at high T & P & UV)

– Atomisation & transport : droplet diameters can be very sensitive to changes on viscosity and surface tension B Dopant concentration limited

– Chemical reactions

• Parasite reactions (auto-ignition,…) – Vaporisation

• Multicomponent fuel : Only the vaporisation of fluorescent component is monitored

• Dopant added to Fuel : Only the vaporisation of dopant is monitored

LIF Techniques for sprays

• Concentration

– Vapour or Liquid

• LIF

– Liquid & Vapour

• Laser Induced Exciplex Fluorescence LIEF

• Size

• LIF/MIE ratio

• Velocities

• Fluorescence Particle Image Velocimetry

(Tracking) FPIV & FPIT

• Temperature

• Multi-Line LIF

LIF experimental set-up

Mirror 1

Mirror 2

Filter 1 Filter 2

Laser 1

CCD 1

CCD 2

• For coupled LIF/MIE images or 2

wavelengths LIF, separate Laser/Camera systems may be needed with appropriate filtering

LIF experimental set-up

Doc : O. Pajot PSA

• Objective: Visualize liquid and vapour phases

• Principle:

– 2 additives blended to the fuel: tracer (TMPD)+

special additive (α-methyl-naphthalene) – Excitation with UV laser light (355nm)

– Tracer fluoresces alone in vapour phase – Tracer and additive form a complex when

excited in liquid phase (Exciplex)

LIEF

Use of the optical access through piston window for:

» global UV laser lighting of the sprays

» Fluorescence collecting onto two cameras with appropriate filters

ÖSimultaneous visualization of the liquid and the vapour phase

camera

Laser

Dichroic mirrors

Filter @ 400 nm for vapor phase Or

Filter @ 532 nm for liquid phase

Exciplex technique

Photo-physics Scheme

Vapour Phase

Liquid Phase Main Relaxation Main Relaxation Subordinate

Subordinate Relaxation Relaxation

Doc : H. Zhao (1998)

Exciplex technique

A D

D₀

D₀

A

D₀

N₂

D

A_D

0 2000 4000 6000 8000 10000 12000

90% fuel 10% dopants vapour

DD

AA_DD

! requires N₂ environment to avoid

quenching by O₂ UV Laser Light

• Filtering of the fluorescence signal allows to distinguish between liquid and vapour phases

!

Strong liquid signal present in the vapour band

Ö blend optimisation via spectroscopic measurements, choice of an appropriate filter

0 2000 4000 6000 8000 10000 12000

365 423 480 538

Wavelength (nm)

90% fuel 10% dopants

DD

AA_DD

390nm 480nm

Vapour Signal Liquid Signal

Exciplex technique

Combustion chamber reflects due to Mie scattering Solution: Time-Shifted imaging:

Mie Scattering duration ~8 ns Fluorescence duration ~100 ns

•Mie Scattering •No Mie Scattering

Synchronization: Mie scattering elimination

Q-Switch

Laser Beam TriggerIntensifier

Synchronisation chart

Air Atmosphere Experience

Nitrogen Atmosphere Experience

Quenching

Doc : O. Pajot PSA

Liquid&Vapour Phases Contours Liquid Phase Contours

532nm / 10nm FWHM 400nm / 100nm FWHM

Doc : O. Pajot PSA

Polarization method

Laser

CCD

Polarizing cube

//

⊥

The ratio of // and ⊥ components depends on refraction index, incident angle and size of droplets

Polarization method

Polarization ratio versus size parameter for

Polarization method

Polarization ratio versus size parameter for different refraction index, at 84°

References

• H. Zhao and N. Ladommatos, Optical

diagnostics for in-cylinder mixture formation measurements in IC engines

• O. Pajot, mid-term report, DIME project

• L. Azizi, P. Hervé, A. Kleitz, fluvisu 1995,

polarization particle sizing.

General features of PDA

• Extension of the LDA principle

• Simultaneous measurement of velocity (up to 3 components) and size of spherical particles as well as mass flux, concentration etc.

• First publication by Durst and Zaré in 1975

• First commercial instrument in 1984

• Non-intrusive measurement (optical technique), on-line and in-situ

• Absolute measurement technique (no calibration required)

• Very high accuracy

• Very high spatial resolution (small measurement volume)

Preconditions for the application of PDA

• Optical access to the measurement area (usually from two directions)

• Sphericity of particles (droplets, bubbles, solids)

• Homogeneity of particle medium

(slight inhomogeneities may be tolerated if the concentration of the inhomogeneities is low and if the size of the

inhomogeneities is much smaller than the wavelength used)

• Refractive indices of the particle and the continuous medium must usually be known

• Particle size between ca. 0.5 µm and several millimetres

Principle set-up of PDA

X

Y

ϕ

Detector 1

Detector 2 Scattering plane

Flow

Z

θ

ψ ψ

• Beam intersection angle θ

• Scattering angle ϕ

• Elevation angle ψ

• Polarization

(parallel or perpendicular to scattering plane)

• Shape and size of detector aperture

Optical parameters of a PDA set-up:

Optical principle of PDA

• A particle scatters light from two incident laser beams

• Both scattered waves

interfere in space and create a beat signal with a frequency which is proportional to the velocity of the particle

• Two detectors receive this signal with different phases

• The phase shift between these two signals is

proportional to the diameter of the particle

Incident beams

Detector 1 Detector 2

Phase relationships

Φ = −

2

2 1 π

λ

θ ψ

θ ψ φ

d_p sin sin

( cos cos cos )

Φ = −

+ + − +

2

2 1 1 ² 2 1

π λ

θ ψ

θ ψ φ θ ψ φ

d n

n n

p rel

rel rel

sin sin

( cos cos cos ) ( ( cos cos cos )

The phase shift between two detectors is:

For reflection:

For 1st order refraction:

No calibration constant is contained in these equations.

Phase - diameter linearity

• A linear relationship between measured phase difference and particle diameter only exists, if the detector is positioned such that one light scattering mode dominates.

5 10 15 20 25 30

-60 -40 -20 0 20 40 60

Diameter (micron)

Phase (deg)

Air bubble in water

Water droplet in air

Scattering angle: 50°

RefractionReflection

• Simultaneous

detection of different scattering modes of comparable intensity leads to non-

linearities in the phase-diameter relationship.

2π ambiguity in a two-detector system

• The phase difference increases with increasing particle size.

• Since phase is a modulo 2π function, it cannot exceed 2π,

i.e. 360°.

• Therefore, if a particle has a size that causes the phase to go beyond a 2π jump, a two-detector PDA cannot discriminate between this size and a much smaller particle.

Φ₁ Φ₁

Φ₂

Φ₂

Φ₃ Φ₃

′ Φ₃

′ Φ₃

3-detector set-up

• Overcoming the 2

π

• Increasing the measurable size range

• Maintaining a high measurement resolution

d

Φ

Φ_1-2 Φ_1-3

360°

0 d

d

Φ ^1-2 Φ^1-3

ψ

ϕ

Detector 1 Detector 3

Detector 2

Dantec Dynamics 57X40 FiberPDA

U1

Front lens U2

Composite lens Aperture plate

Measurement volume

Multimode

fibres Detector Unit

with PMTs.

U3

• Easy set-up and alignment

• Three receivers in one probe

• Exchangeable aperture masks

• Up to three velocity components

Size range adaptation

• For a given optical configuration, the distance between the receiving apertures can be changed to adapt the size range.

• This can be achieved by exchanging the aperture mask in the receiving probe.

• The Dantec Dynamics FiberPDA has a set of three different masks:

A: small size range range B: medium size range C: large size

U1

U2 U3

A B C

• the diameter of the intersection volume of the transmitting beams

• the width of the projection of the slit shaped spatial filter which is mounted in front of the receiving fibers

The effective PDA measurement volume is much smaller than the intersection volume of the transmitting laser beams.

The effective size of the measurement volume is

determined by:

Effective PDA measurement volume

U1 U2

U3

Slit aperture Projected slit

Intersection volume

Sources for measurement uncertainties

• Oscillations in phase-diameter curve

• Low SNR due to low intensity or extinction

• Phase changes due to – surface distortions

– inhomogeneous particles – multiple scattering effects

• Gaussian intensity profile in the measurement volume

• Slit effect

Trajectory effect / Gaussian beam effect

• Depending on the trajectory of the particle, the detected

scattered light is dominated either by refraction or reflection. This is caused by the Gaussian intensity profile across the

measurement volume.

• This effect becomes noticeable for large transparent particles (d_p > ca. 50% of meas. vol. diameter)

Y Y

Z

Gaussian Intensity

Projected slit Intersection volume

Slit effect

• Due to the projection of the receiving slit aperture, the unwanted scattering mode

becomes dominating for particle trajectories at one edge of the slit projection.

Z

Projected slit Intersection volume

The DualPDA

• Measurement errors due to trajectory and slit

effects are eliminated

• Particularly optimized for applications

to sprays with

transparent droplets

• Enables improved concentration

and mass flux measurements

• Provides the ability to reject non-spherical droplets

X

Y

ϕ Z

U1

U2 V1

V2

Scattering plane

Components of the DualPDA

Planar PDA

X

Y

Z

Main Flow Direction

Receiving Apertures Transmitting

Optics

(Beams are in the y-z

plane) ϕ

Transmitting Optics

(Beams are in the x-z plane)

X

Y

Z

Main Flow Direction

Receiving Apertures

ϕ

Conventional PDA

Comparison measurements

Measurement with a DualPDA Measurement with a standard PDA

Automotive Fuel Injection

Photo: AVL, Graz, Austria

To make a successful PIV measurement:

1. Selection of appropriate tracer particle:

Particle size must be large enough to scatter sufficient light for image acquisition.

Particle size must be small enough for faithfully tracking the flow.

2. Proper seeding of tracer particles:

Homogeneous and uniform seeding No severe particle aggregation

Particle seeding concentration must be high enough for data processing and low enough for not disturbing the flow field.

IPI – Interferometric Particle Imaging

Light scattering principles

A lightwave is fully described by:

• wavelength

• intensity

• polarization

• phase

The principle of the PDA technique is the scattering of plane lightwaves by spherical particles.

Scattering is composed of:

• diffraction

• ^reflection

• ^refraction

• ^absorption

An exact description of the scattering of light by a homogeneous

sphere is given by the full solution of Maxwell’s equations formulated by Mie in 1908.

Geometric optics (Snell’s law) is a simplified way to describe light scattering.