laser spectroscopic techniques for combustion diagnostics
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Laser spectroscopic techniques for combustiondiagnosticsMARCUS ALDN aa Division of Combustion Physics , Lund Institute of Technology , P.O Box 118, Lund , S-22100 , SwedenPublished online: 21 Feb 2014.
To cite this article: MARCUS ALDN (1999) Laser spectroscopic techniques for combustion diagnostics, Combustion Scienceand Technology, 149:1-6, 1-18, DOI: 10.1080/00102209908952096
To link to this article: http://dx.doi.org/10.1080/00102209908952096
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Laser spectroscopic techniquesfor combustion diagnosticsMARCUS ALDEN>
Division of Combustion Physics, Lund Institute of Technology P.D Box 118, 522100 Lund Sweden
(Received Apri/20, 1999)
During the last decade various laser spectroscopic techniques have shown great potential for com-bustion diagnostics, The largest advantages with these techniques are that they permit non-intrusivemeasurements with high spatial, temporal and spectral resolution. The most important parametersmeasurable are; species concentration, temperatureand velocity. In the present review some develop-mems and applications will be briefly described. This includes Laser-Induced Fluorescence, LIF,Coherent anti-Stokes Raman Scattering, CARS, and Polarization spectroscopy, PS.
During the last decades the awareness of our environment as well as the concernfor efficient energy production have become of utmost importance for our soci-ety. Since combustion processes constitute a major part of our energy productionas well as contribute with a considerable amount of air pollutants, e.g. NOx' par-ticulates etc, this area has become of major importance for detailed studies. Thefact that several countries are also considering to reduce or even close downnuclear power plants, puts combustion processes as even more important to
study. Another important aspect of combustion is that it plays a key role in manyindustrial devices, e.g engines, gasturbines, and boilers, which means that effi-
cient and environmentally friendly combustion is an important way for industriesto compete on an international market. A problem but also a challenge with com-bustion phenomena, is the need for interdisciplinary collaborations to create athorough understanding of these processes, thus experts e.g. in physics, chemis-
2 MARCUS ALDEN
face real practical combustion problems. During the last years two major toolshave appeared which have given the community new possibilities for detailedstudies of combustion processes: new and faster computers which are necessaryto model the processes as well as new diagnostic tools for characterization ofcombustion phenomena and validation of various combustion models. In the lat-ter field it has been shown that various measuring approaches based on lasertechniques have unique properties for diagnostics of combustion processes, seee.g. [Eckbreth (1996) and references in there. The most important features ofthese techniques are non-intrusiveness in combination with high temporal andspatial resolution. These techniques can be used for measurements of speciesconcentrations, temperatures, velocities and particle parameters (number, sizes,volume fractions).
The present paper will describe some applications of laser spectroscopic tech-niques for studies of combustion processes. Since space will neither allow a fullcoverage of the field nor any basic theory, the paper will be concentrated onexperimental work in the authors laboratory. For further details we refer toaccompanying references.
The techniques which will be covered are Laser-Induced Fluorescence (LIF),Coherent anti-Stokes Raman Scattering (CARS) and a relatively new techniquebased on polarization spectroscopy. This means that for the many techniqueswhich are not discussed, e.g. Rayleigh and Raman scattering, DFWM, REMPIetc., we refer to [Eckbreth (1996)].
LASER-INDUCED FLUORESCENCE, L1F
The laser technique which probably has received the largest attention for com-bustion diagnostics is Laser-Induced. Fluorescence, LIE In this technique thelaser beam is tuned to an atomic or molecular absorption transition and by detect-ing the atomic/molecular fluorescence emission, it is in principle possible toinfer both species concentration and temperature. For a thorough description ofthe LIF technique we refer to [Kohse-Hoinghaus (1994).
A major problem with converting LIF signals to absolute species concentrationis deexcitation due to collisions, so called quenching. Since the degree ofquenching is dependent on temperature, pressure and colliding partners, several
LASER SPECTROSCOPIC TECHNIQUES 3
rendeau, 1986], predissociation, e.g. [Andresen, et al., 1988] and time-resolveddetection, e.g. [Bergano, et aI., 1983]. May be the most important developmentof LIP is the introduction of multiple point measurements, first along a line[Alden, et aI., 1982] shortly followed by two-dimensional visualization; PlanarLaser-Induced Fluorescence, PLIF [Dyer and Crosley, 1987; Kychakoff, et al.,1982]. The PLIF technique has become a very important tool in characterizationof combustion processes, as will be exemplified below. The section of LIF willbe divided in two parts describing developments as well as applications.
From our laboratory two developments of the LIF technique will be described.
In the first experiment it has been investigated if and to what degree LIF can beused for multiple species detection. LIF has normally been used for one speciesdetection but by using aNd: YAG based laser system it has been shown that sev-eral species can be simultaneously detected utilizing spectral coincidences of oneand two-photon resonances using harmonics from the YAG/dye laser system[Westblom and Alden, 1989, 1990]. Recently, it has also been shown that instan-taneous multispecies two dimensional visualization can be made [Georgiev andAlden, 1997]. The experimental set-up is shown in Fig I. The YAG/dye laser isprimarily producing 226 nm by frequency doubling the dye laser beam followedby frequency mixing of the doubled dye and the residual IR beam from the YAGlaser. The 226 nm beam is exciting NO by a one-photon resonance in the y-bandat the same time as 0 atoms are excited by a two-photon resonance. The fluores-cence emission from NO is in the UV region between 226 and 300 nm, whereasthe 0 atom emission mainly appears at 845 nm. The frequency doubled dyebeam around 287 nm is in these experiments not dumped but used for excitationof OH in the 0-1 transition, followed by fluorescence detection around 3I0 nm.In order to capture more than one image at the same time a special designed Cas-segrainian telescope was used, which permitted several images to be capturedwith the same CCD detector. In this specific experiments, however, also a secondCCD camera with enhanced IR sensitivity was used for detection of the 0 atomfluorescence. By using appropriate spectral filters the fluorescence from eachindividual species was isolated. This means that by using one laser shot it waspossible to capture images as illustrated in Fig. 2, which shows the spatial distri-bution of NO, OH and 0 in an atmospheric pressure HzlOzlNzO flame. Thesame approach can also be used for simultaneous PLI