Filtered Rayleigh scattering Summary

1 7. Rayleigh scatteringIt d ti Introduction Rayleigh scatteringygg TheorySpecies concentration measurements Species concentration measurements Temperature measurements Filtered Rayleigh scattering Summary SummaryIntroduction: 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/sunriseWhy? Rayleigh scattering !!The scattering intensity is proportional to -4A wavelength at 430 nm(in the blue) is thus scattered a factor of ~6 times as efficient as a wavelength of680 nm(in the red)gypp6 times as efficient as a wavelength of 680 nm(in the red).Joakim BoodThe physical principleElectromagnetic wave propagating along the z-axis.

2 The polarization is gpvertical (along x-axis). The scattering in the y-z-plane is vertically polarized and of equal scattering = EWhen 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 tidildi tt thfth iidt 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 hdifftbf lthi h lbd i diffthave different numbers of electrons, which also are bound in different potential Rayleigh scattering is mostly used forDiagnostic potentialRayleigh scattering is mostly used for temperature measurements.

3 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 gto e e ,e a spec eswith very large cross-section (fuel) is probed, species visualization is possibleppA Rayleigh scattering setupIlLaserIlmixlRlNII (d /d )effIRIR= 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 iiimixX Xi= Mole fraction of species i i= Rayleigh cross section of speciesi [cm2/sr](d /d )eff(d i/d )iRayleigh scattering thermometry (1)mixlRlNII (d /d )

4 EffThe Rayleigh scattering signal is proportional to the number concentra-tion of species and the cross section of the gas the cross section, mix, is assumed to be constant:According to the perfect gas law:(d /d )effI ~ NApNo According to the perfect gas law: TRN Since A0and 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, T/IR1 Rayleigh scattering thermometry (2)T/IR1 This expression can now be applied to a two-dimensional image of Rayleigh scatteringimage of Rayleigh situationAssume an imaging Rayleigh measurement where 1) the temperature is 300 K in measurement point )the signal is a factor of five stronger in A than in BTA= 300 KT = ?

5 GgImaged Rayleigh signalThen we can calculate the temperature in point B:KKIITTBRARAB150015300,, A BThe signal in A is a factor of 5 stronger than in BWarning: Differences in Rayleigh cross sections for different species may give large errors!Differential Rayleigh cross sections22)1(4 nj Differential Rayleigh cross sections for some different gases420 Nj sections for some different gases(for =90 at =532 nm, T=273 K, and p= 105Pa) 10-28cm2/srHow can we measureN2 10-28cm2 10-28cm2/srHow can we measure in a flame with unknown 10-28cm2 10-28cm2/srcomposition? 10-28cm2 10-28cm2/srPer-Erik BengtssonTotal Rayleigh scattering cross section in a methane/air flameSpatially resolved Rayleich pyymeasurements along a lineExample of 2D Rayleigh thermometryyIncident planar zCxlaser beamH4+airAirflowExperimental setupFinal 2-D temperature plotPer-Erik BengtssonExperimental setupFinal 2D temperature plotRayleigh thermometry in combustor150 mmWATERCOOLEDCOMBUSTOR SHELLCERAMICFIRETUBEGAS AND AIRSUPPLYAIR COOLINGFOR WINDOWC eramicFlame TubeDetected AreaPidOBSERVATION-WINDOWOBSERVATIONDIRE CTION OFSUPPLYP remixedUnburnt GasDouble ConeBurner(Front Panel)OPTICALACCESSLIGHTSHEETSWIRL-BURNE RY(Front Panel)

6 Flame FrontHot RecirculatingProductsCenterliney900 mmEXHAUST GASZXYC enterlinexZz/d = Kampmann T Seeger and A LeipertzS. Kampmann, T. Seeger, and A. Leipertz, Appl. Opt. 34, 2780-2786 (1995)50075010001250150017502000 Temperature [K]Fuel concentration measurementsUnder specific circumstances RayleighRayleigh scattering can be used for tticoncentration measurements. Such a case is when there are few species with a bigspecies with a big difference in Rayleigh cross Espey, J. E. Dec, Litzinger, Santavicca, Combustion and Flame 109: 65-86 (1997).Fuel concentration measurement in an engineb)Big hydrocarbon molecules haveNf have much bigger Rayleigh scattering tiiation ( sections in comparison with small molecules as ratio in Rayleigh el section between a diesel fuel and air can be air can be a factor of Espey, J.

7 E. Dec, Litzinger, Santavicca, Combustion and Flame 109: 65-86 (1997).DME spray imagingDimethyl ether (DME) as alternative to diesel fuel: Good auto ignition characteristics Virtually eliminates soot Can substantially reduce NOx Can be produced in large quantitiesAim: To investigate the mixing and ignition of DMEDME sprays in DI Diesel engineSingleshot images from different engine cyclesSingle-shot images from different engine cyclesRelative fuel concentrationsRelative fuel concentrationsRayleigh scattering : advantages It is an easy technique Arbitrary laser wavelength can be used, bhl hldbut shorter wavelengths leads to stronger signal (the -4-dependence). Signal is proportional to number ttiNd/1/Tconcentration N and/or 1/TSilitilt ll Signal is proportional to laser pulse energy, no quenching or saturation ff The technique is not species selective, since all t/ll/til tttthatoms/molecules/particles scatter at the same wavelength.

8 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 iiht t h i It is an incoherent technique Stray light from particles, optics and surfaces Say go pa ces,opcsadsuacescan interfere with the Rayleigh signalFiltered Rayleigh scattering (FRS) The main problem with Rayleigh scattering e a p obeayeg sca e gis that scattering from optics, surfaces, particles droplets interfere with theparticles, droplets interfere with the scattered light from molecules.

9 This can be solved by the use of FilteredThis can be solved by the use of Filtered Rayleigh principle for FRST ransmission curve of a filter (mercury) Rayleigh scattering line shape of N2at 1 atm and 500 KScattering from particles and surfacesa filter (mercury)atm. and 500 Knb. units Transmitted molecular Rayleigh increasingnsmissionnsity / arbWith increasing temperature, the line broadens and relatively more lightTranIntenrelatively more light passes the molecular (GH)Frequency (GHz)Possible filter candidates for FRS Mercury at 254 nm - Tripled Alexandrite laser (+strong absorbing gas, +strong cross-section, (ggg, 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.

10 YAG (+ easily available laser, +high laser energy, -medium absorbing gas, - medium cross-section, -rotational finestructures)) Rubidium 780 nm Fundamental Alexandrite(bbi ilifil(+strong absorbing gas, +single isotope filter, + no molecular absorption , + high laser energy, -spectroscopic fine structures, -weaker cross-section, pp- exotic laser, - weak slope )Transmission profiles at different pT for Rubidium23 C50 C110 C140 C 0 CTransmission profile at 140 C(ll )Transmission profile at 140 C (log scale)Limited use for FRS?Experimental setup for FRSA Filtered Rayleigh scattering setup is more complex than a normal Rayleigh scattering filter for calibrationAtomic filterSpecific experimental components are shown in figure: Si ldbll Single-mode tunable laser Atomic (or molecular) gas filtersInfluence of a Hg filter on the Rayleigh signalSteel plateSteel plateFRS 2D temperature distribution in acetylene/air premixed flames.