Filtered Rayleigh scattering Summary - cefrc.princeton.edu Lecture... · the y-z-plane is...
Transcript of Filtered Rayleigh scattering Summary - cefrc.princeton.edu Lecture... · the y-z-plane is...
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7. Rayleigh scattering
I t d ti• Introduction• Rayleigh scatteringy g g
– TheorySpecies concentration measurements– Species concentration measurements
– Temperature measurements• Filtered Rayleigh scattering• Summary• Summary
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Introduction: Why is the sky blue?
• Daytime sky looks blue on a clear dayThe sk looks red at s nset/s nrise• The sky looks red at sunset/sunrise
Why?
Rayleigh scattering!!
The scattering intensity is proportional to -4
A wavelength at 430 nm (in the blue) is thus scattered a factor of ~6 times as efficient as a wavelength of 680 nm (in the red)
g y p p
6 times as efficient as a wavelength of 680 nm (in the red).
Joakim Bood
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The physical principleElectromagnetic wave propagating along the z-axis. The polarization is g pvertical (along x-axis). The scattering in the y-z-plane is vertically polarized and of equal intensity.
Propagation
Scattering= E
When an electromagnetic wave interacts with an
; polarizability
• When an electromagnetic wave interacts with an atom/molecule/particle, the oscillating electric field creates an oscillating dipole, , when the electrons are moved back and forth.
A ill ti di l di t t th f th i id t• An oscillating dipole radiates at the same frequency as the incident radiation, E = E sin2, what is called Rayleigh scattering.
• Different molecules scatter with different efficiencies, since molecules h diff t b f l t hi h l b d i diff thave different numbers of electrons, which also are bound in different configurations.
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Diagnostic potential
• Rayleigh scattering is mostly used for
Diagnostic potential
Rayleigh scattering is mostly used for temperature measurements. 2-D measurements can be performed. Examplesmeasurements can be performed. Examples will be shown.
• The possibility to make concentration measurements is in general limited. Themeasurements is in general limited. The reason is that all molecules scatter at the same wavelength. However, when a species sa e a e e gt o e e , e a spec eswith very large cross-section (fuel) is probed, species visualization is possiblep p
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A Rayleigh scattering setup
Il
LaserIl
mixlR lNII
(d/d)eff
IR
IR = Detected Rayleigh scattering signal [J]Il = Laser pulse intensity [J]N = Number concentration [cm-3]
Detectorl = length of probe volume [cm] = Solid angle for detection [sr] Optical transmission efficiency
mix Rayleigh cross section for gas mixture [cm2/sr](d/d)eff
i
iimix X Xi = Mole fraction of species i i = Rayleigh cross section of species i [cm2/sr]
(d/d)eff (di/d)i
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Rayleigh scattering thermometry (1)
mixlR lNII (d/d)eff
The Rayleigh scattering signal is proportional to the number concentra-tion of species and the cross section of the gas mixture.
I NIf the cross section, mix, is assumed to be constant:
According to the perfect gas law:
(d/d)effI ~ N
ApN oAccording to the perfect gas law: TR
N
Since A0 and R are constants, and pressure can be considered to be constant in a combustion situation, it means that:
Rayleigh scattering signal is inversely proportional to the temperature, i.e. T/I 1 T/IR 1
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Rayleigh scattering thermometry (2)
T/IR 1
This expression can now be applied to a two-dimensional image of Rayleigh scatteringimage of Rayleigh scattering.
Example Real situation
Assume an imaging Rayleigh measurement where 1) the temperature is 300 K in measurement point A.2) the signal is a factor of five stronger in A than in B.
A BTA= 300 K T = ?) g g
Imaged Rayleigh signalThen we can calculate the temperature in point B:
KKII
TTBR
ARAB 1500
15300
,
, A BThe signal in A is a factor of 5 stronger than in B
Warning: Differences in Rayleigh cross sections for different species may give large errors!
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Differential Rayleigh cross sections
22 )1(4 nj
Differential Rayleigh cross sections for some different gases
420 Nj
sections for some different gases(for =90º at =532 nm, T=273 K, and p=1.013·105 Pa):
O2 5.08 •10-28 cm2/sr How can we measureN2 6.13 •10-28 cm2/srH2 1.34 •10-28 cm2/sr
How can we measure in a flame with
unknown CO 7.87 •10-28 cm2/srCO2 13.8 •10-28 cm2/sr
composition?
H2O 4.43 •10-28 cm2/srC3H8 79.8 •10-28 cm2/sr
Per-Erik Bengtsson
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Total Rayleigh scattering cross section in a methane/air flame
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Spatially resolved Rayleich p y ymeasurements along a line
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Example of 2D Rayleigh thermometry
yIncident planar
z
C
xlaser beam
H4+air
Airflow
Experimental setup Final 2-D temperature plot
Per-Erik Bengtsson
Experimental setup Final 2 D temperature plot
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Rayleigh thermometry in combustor
150 mmWATERCOOLEDCOMBUSTOR SHELL
CERAMICFIRETUBEGAS AND AIR
SUPPLY
AIR COOLINGFOR WINDOW
CeramicFlame Tube
Detected Area
P i d
OBSERVATION-WINDOW
OBSERVATIONDIRECTION OF
SUPPLY
PremixedUnburnt Gas
Double ConeBurner(Front Panel)
OPTICALACCESS
LIGHTSHEET
SWIRL-BURNER
Y (Front Panel)
Flame Front
Hot RecirculatingProducts
Centerline
y
900 mm
EXHAUST GAS
Z
X
Y
Centerlinex
Z
z/d = 0.1
S Kampmann T Seeger and A LeipertzS. Kampmann, T. Seeger, and A. Leipertz, Appl. Opt. 34, 2780-2786 (1995)
500 750 1000 1250 1500 1750 2000Temperature [K]
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Fuel concentration measurements
Under specific circumstances RayleighRayleigh scattering can be used for
t ticoncentration measurements.
Such a case is when there are few species with a bigspecies with a big difference in Rayleigh cross sectionsection.
C. Espey, J. E. Dec, T.A. Litzinger, D.A. Santavicca, Combustion and Flame 109: 65-86 (1997).
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Fuel concentration measurement in an engine
b)
Big hydrocarbon molecules have
Nf /N
amb
.08
07
molecules have much bigger Rayleigh scattering
ti i
atio
n (N
.07
.06
05
cross sections in comparison with small molecules
ncen
tra
.05
.04
03
such as N2.
The ratio in Rayleigh
el C
on.03
.02
y gcross section between a diesel fuel and air can be a
Fu.01and air can be a factor of 300.
C. Espey, J. E. Dec, T.A. Litzinger, D.A. Santavicca, Combustion and Flame 109: 65-86 (1997).
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DME spray imaging
Dimethyl ether (DME) as alternative to diesel fuel:• Good auto ignition characteristics• Virtually eliminates soot• Can substantially reduce NOx• Can be produced in large quantities
Aim: To investigate the mixing and ignition of DME
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DME sprays in DI Diesel engine
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Single shot images from different engine cyclesSingle-shot images from different engine cycles
Relative fuel concentrationsRelative fuel concentrations
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Rayleigh scattering: advantages
• It is an easy technique
• Arbitrary laser wavelength can be used, b h l h l dbut shorter wavelengths leads to stronger signal (the -4-dependence).
• Signal is proportional to number t ti N d/ 1/Tconcentration N and/or 1/T
Si l i ti l t l l• Signal is proportional to laser pulse energy, i.e. no quenching or saturation ff teffects.
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Limitations
• The technique is not species selective, since all t / l l / ti l tt t thatoms/molecules/particles scatter at the same
wavelength.
• For accurate thermometry, the Rayleigh cross sections for individual species must be takensections for individual species must be taken into account, which is hard work in a two-dimensional image since the mole fractiondimensional image since the mole fraction distribution must be known in every point.
It i i h t t h i• It is an incoherent technique
• Stray light from particles, optics and surfaces S ay g o pa c es, op cs a d su acescan interfere with the Rayleigh signal
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Filtered Rayleigh scattering (FRS)
• The main problem with Rayleigh scattering e a p ob e ay e g sca e gis that scattering from optics, surfaces, particles droplets interfere with theparticles, droplets interfere with the scattered light from molecules.
• This can be solved by the use of FilteredThis can be solved by the use of Filtered Rayleigh Scattering.
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The principle for FRS
Transmission curve of a filter (mercury)
Rayleigh scattering line shape of N2 at 1 atm and 500 K
Scattering from particles and surfacesa filter (mercury) atm. and 500 K
n b. u
nits
Transmitted molecular Rayleigh scattering.
With increasingnsm
issi
on
nsity
/ ar
b
With increasing temperature, the line broadens and relatively more light
Tran
Inte
n
relatively more light passes the molecular filter.
F (GH )Frequency (GHz)
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Possible filter candidates for FRS• Mercury at 254 nm - Tripled Alexandrite laser
(+strong absorbing gas, +strong cross-section, ( g g g , g ,+single isotope filter, +no spectroscopic fine structures, - medium laser energy, - molecular absorption, - ”exotic” laser)absorption, exotic laser)
• Iodine molecules at 532 nm – Doubled Nd:YAG (+ easily available laser, +high laser energy, -medium absorbing gas, - medium cross-section, -rotational finestructures))
• Rubidium 780 nm – Fundamental Alexandrite( b bi i l i fil(+strong absorbing gas, +single isotope filter, + no molecular absorption, + high laser energy, -spectroscopic fine structures, -weaker cross-section, p p- ”exotic” laser, - weak slope )
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Transmission profiles at different pT for Rubidium
23 C50 C
110 C140 C 0 C
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Transmission profile at 140 C (l l )Transmission profile at 140 C (log scale)
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Limited use for FRS?
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Experimental setup for FRS
A filtered Rayleigh scattering setup is more complex than a normal Rayleigh scattering setup.
Atomic filter for calibration
Atomic filter
Specific experimental components are shown in figure: Si l d bl l• Single-mode tunable laser
• Atomic (or molecular) gas filters
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Influence of a Hg filter on the Rayleigh signal
Steel plate Steel plate
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FRS 2D temperature distribution in acetylene/air premixed flames. The upper picture at = 1 6 and the lower at = 2 4The upper picture at = 1.6 and the lower at = 2.4.
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Filtered Rayleigh scattering: y g gFuel/air ratio imaging
• Rayleigh scattering cross-section: fuel molecules are much larger than airmolecules are much larger than air molecules– σ /σ i ≈ 15σpropane /σair ≈ 15– σisosctane /σair ≈ 90
/ 305– σdiesel/σair ≈ 305
• Spectrally resolved molecular Rayleigh scattering from spurious scattering lightscattering from spurious scattering light
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Filtered Rayleigh scattering: lineshape (i t @20 b 800K)(isooctane@20 bar, 800K)
Hg filterHg filter transmission
Rayleigh scattering from i t disooctane and spurious scattering laser light
Transmitted FRS
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Filtered Rayleigh scattering: Fuel/air ratio imaging in a diesel engine
-3.5 CAD -2.5 CAD -1.5 CAD
Fuel injected at -10 CAD
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Simultaneous 2D FRS and PIV: Setup
Simultaneous thermometry using FRS and velocimetry using PIV. The iodine filter absorb at the wavelength 532 nm.
D. Most and A. Leipertz, App. Opt. 40, 5379 (2001).
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Simultaneous 2D FRS and PIV: Results
Instantaneous velocity and temperature field in a flame
D. Most and A. Leipertz, App. Opt. 40, 5379 (2001)
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Summary of FRS
• FRS can be used in ’dirty’ environment and close tof i th tt i f f tisurfaces, since the scattering from surfaces, optics,
droplets and particles give limited/no problem to themeasurement.
• Major species concentrations have to be known orestimated for quantitative temperature measurements,q p ,because of the different Rayleigh scattering cross-sections for different species.
• The use of FRS in IC-engines has so far only beensome initial demonstration measurements.
• FRS requires proper modeling of the lineshape,especially at high pressure and complex molecules(hydrocarbons)(hydrocarbons)