LIF & MIE spray characterisation
• Luis Le Moyne (Université Pierre et Marie Curie)
An EC funded NoE on Energy Conversion in Engines
1 © 2005 ECO-Engines Partners - All rights reserved.
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
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Scattering in sprays • General expression for scattered light signal S :
S = C.d n (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 (λi=λe) •Size parameter α : Raleigh(α nm
nm
1 np
-1
1
1st order 2 refraction 4th order
5th order
-1
7th order
2nd order -2 refraction
5
Light scattering by droplets and bubbles
-2
Air bubble in water
2
2
1
1
-1
Incident rays
Incident rays
Water droplet in air
1
2
-2
-1
1
-1
-1
-2
-2
2
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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.
Lorenz-Mie
3
parallel polarization
1st order refraction
2 2nd order refraction
1
-3
-2
-1
1
2
3
4
5
-1 -2 perpendicular polarization
reflection
-3
7
Scattering in sprays
• For common lasers and spherical absorbing droplets of d>1µm : 2
S MIE = CMIE d
With twin characteristic spots on the equatorial axis 8
MIE signal dependence on diameter (single droplets)
Liquid : Kerosine with fluorescing components and dye for absorption control, Laser : Nd:Yag 538nm (Le Gal et al.) 9
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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 :
I MIE = I inc ⋅ f (n, θ ) ⋅ N d ⋅ d 2 • B For quantitative measurements of size, the spray pdf should be known….. » More information is needed : polarization, coupling with other techniques (LIF)
11
12
MIE/LIF for size measurements • LIF signal is dependent on volume :
S LIf = CLIF d 3 • MIE signal is dependent on surface :
S MIE = CMIE d
2
• The ratio of the two signals in provenance of a spray is representative of SMD : CLIF ∑ d 3
S LIF = ∝ 2 S MIE CMIE ∑ d
3 d ∑
∑d
2
= D32
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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 The observed shape can be identified to spray only if the behaviour of molecule is equivalent to fuel behaviour (atomisation, vaporisation, transport…) AND if fluorescence signal dependence on concentration of specific molecule is known & controlled (wavelength, temperature, pressure, quenching,…) 14
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 15
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
16
LIF experimental set-up • For coupled LIF/MIE images or 2 wavelengths LIF, separate Laser/Camera systems may be needed with appropriate filtering
CCD 1
Filter 1
CCD 2
Filter 2
Laser 1 Mirror 1
Mirror 2 17
LIF experimental set-up
Doc : O. Pajot PSA
18
LIEF • 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)
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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 Filter @ 400 nm for vapor phase Or Filter @ 532 nm for liquid phase
camera Dichroic mirrors
Laser
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Exciplex technique Photo-physics Scheme
Vapour Phase Main Relaxation Subordinate Relaxation
Liquid Phase Doc : H. Zhao (1998) 21
Exciplex technique A
UV Laser Light
D D0
D D0
A
AD
D0
N2
vapour
12000 10000 8000
90% fuel 10% dopants
6000
D
4000
AD
2000
! requires N2 environment to avoid quenching by O2
0 365
423 480 Wavelength (nm)
538
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Exciplex technique • Filtering of the fluorescence signal allows to distinguish between liquid and vapour phases 12000 10000 8000
90% fuel 10% dopants
6000
D
4000
AD
2000 0
423 480 Wavelength (nm)
365
538
390nm
480nm
Vapour Signal
Liquid Signal
! Strong liquid signal present in the vapour band Ö blend optimisation via spectroscopic measurements, choice of an appropriate filter
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Synchronization: Mie scattering elimination Combustion chamber reflects due to Mie scattering Solution: Time-Shifted imaging: Mie Scattering duration ~8 ns Fluorescence duration ~100 ns Synchronisation chart Q-Switch
Laser Beam
•Mie Scattering Signal in the Image
•No Mie Scattering Signal
TriggerIntensifier Intensifier Switch
Doc : O. Pajot PSA 24
Quenching
Air Atmosphere Experience
Nitrogen Atmosphere Experience Doc : O. Pajot PSA 25
Liquid Phase Contours
Liquid&Vapour Phases Contours
532nm / 10nm FWHM
400nm / 100nm FWHM
Doc : O. Pajot PSA
26
Polarization method The ratio of // and ⊥ components depends on refraction index, incident angle and size of droplets
CCD
⊥
//
Polarizing cube
Laser
27
Polarization method
Polarization ratio versus size parameter for different angles 28
Polarization method
Polarization ratio versus size parameter for different refraction index, at 84° 29
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.
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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)
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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 • Max. particle number concentration is limited 32
Principle set-up of PDA Optical parameters of a PDA set-up:
• • • •
X
Beam intersection angle θ
θ
Scattering angle ϕ
ψ
ϕ
Elevation angle ψ
Z
Polarization (parallel or perpendicular to scattering plane)
•
Detector 1
Flow
Shape and size of detector aperture
ψ
Y
Scattering plane
Detector 2
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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
or 1 t c e Det
Detec tor 2
Incident beams
•
34
Phase relationships The phase shift between two detectors is: For reflection: Φ=
2π dp
λ
sin θ sin ψ 2 (1 − cos θ cos ψ cos φ )
For 1st order refraction: Φ=
−2 π d p
λ
n rel sin θ sin ψ 2 2 (1 + cos θ cos ψ cos φ ) (1 + n rel − n rel
2 (1 + cos θ cos ψ cos φ )
No calibration constant is contained in these equations.
35
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. • Simultaneous Scattering angle: 50° Air bubble in water
20 0
5
10
15
20
-20 -40
Water droplet in air
-60
Diameter (micron)
25
30
Refraction
40
Reflection
60
Phase (deg)
detection of different scattering modes of comparable intensity leads to nonlinearities in the phase-diameter relationship.
36
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
Φ 3′
Φ3
Φ3
Φ 3′
37
3-detector set-up • Overcoming the 2π ambiguity • Increasing the measurable size range • Maintaining a high measurement resolution Φ
ψ 360°
Φ
Detector 3
ϕ
3
Φ 1-
1-2
Detector 1
Φ1-3 Φ1-2
Detector 2 0
dmeas.
dmax
d
38
Dantec Dynamics 57X40 FiberPDA Measurement volume Aperture plate Composite lens
Front lens
• • • •
Easy set-up and alignment Three receivers in one probe
U1 U3 U2
Multimode fibres
Detector Unit with PMTs.
Exchangeable aperture masks Up to three velocity components
39
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 B
A
C: large size
C
U1 U3 U2
40
Effective PDA measurement volume Intersection volume
The effective size of the measurement volume is determined by:
• •
Projected slit
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
Slit aperture
U U3 1 U 2
The effective PDA measurement volume is much smaller than the intersection volume of the transmitting laser beams.
41
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
42
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 (dp > ca. 50% of meas. vol. diameter)
Gaussian Intensity Projected slit
Intersection volume
Z Y
Y 43
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.
Projected slit
Intersection volume
Z
Y 44
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 U1
Z
ϕ V2
Y
Scattering plane
V1 U2
45
Components of the DualPDA Planar PDA
Conventional PDA X
X
Main Flow Direction Transmitting Optics (Beams are in the x-z plane)
Y
Main Flow Direction
ϕ Z
Receiving Apertures
Transmitting Optics (Beams are in the y-z plane)
Y
ϕ
Z
Receiving Apertures
46
Comparison measurements Measurement with a standard PDA
Measurement with a DualPDA
47
Automotive Fuel Injection
Photo: AVL, Graz, Austria
48
49
50
51
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.
52
53
54
55
IPI – Interferometric Particle Imaging
56
57
58
59
Light scattering principles The principle of the PDA technique is the scattering of plane lightwaves by spherical particles.
A lightwave is fully described by: • wavelength • intensity • polarization • phase
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.
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