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Combustion and Flame 142 (2005) 314–316

www.elsevier.com/locate/combustflam

Brief Communication

Laser-induced breakdown spectroscopy for in situdiagnostics of combustion parameters including

temperature

Tae-Woo Lee∗, Nikhil Hegde

Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287-6106, USA

Received 21 January 2005; received in revised form 12 May 2005; accepted 17 May 2005

Available online 28 June 2005

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1. Introduction

Laser-induced breakdown spectroscopy (LIBhas emerged as a powerful diagnostic methodmany application areas, including combustion stems. LIBS operates by a focused laser beam inding multiple ionization and dissociation of the targmolecules. The high energy state at the focal pointhe laser beam produces dissociated, excited elemwhich radiate characteristic emission bands whileturning to ground states. These emission wavelenand intensities can be used to infer the elemental cposition of the sample.

LIBS offers unique advantages as a diagnomethod for combustion systems. It is a nonintsive, optical technique with a strong signal that cbe detected in the presence of interference. Thhave been some applications of LIBS to combustprocesses, but the level of progress is not yetnificant and work remains to be done in this regaLocal fuel–air ratio has been measured by Feriolal. [1] in engine combustion chamber by using tratio of the measured carbon and oxygen peakthe laser-induced breakdown spectra. Metal emisfrom incinerators and coal-burning power plants halso been measured using LIBS[2,3]. However, noreport of measurements of temperature, an obvioimportant parameter in combustion and thermal stems, using LIBS has been made to date. Applica

* Corresponding author.E-mail address: attwl@asu.edu(T.-W. Lee).

0010-2180/$ – see front matter 2005 The Combustion Institutdoi:10.1016/j.combustflame.2005.05.003

of LIBS for temperature measurements can be qpowerful and versatile, as the signal strength is sficient to overcome many optical interferences assooting flames. The signal response is effectivelystantaneous so that it can be applied to highly inmittent turbulent flames also.

In this work, we present some results on in sLIBS measurements of temperature and other cbustion parameters in partially premixed flames unseveral varying conditions including sooting contions.

2. Experimental methods

A Nd:YAG laser with a frequency-doubled 53nm output was used as an excitation source. The bwas focused using a 100-mm-focal-length lens wan initial beam diameter of 10 mm. The resultilaser-induced breakdown signal (the radiation frthe spark) was collected using a 50-mm-diame75-mm-focal-length lens and focused to a 0.25monochromator. A PMT detector was connectedboth an oscilloscope and a boxcar averager, for simonitoring and conditioning, respectively. The monchromator was manually turned to emission peaksC, N, O, and H atomic spectra.

Two burner geometries were used to comparevalidate the LIBS-measured data. One is the cofllaminar flame burner, used by Santoro and oth[4–6]. The burner is well described in the above refences, but briefly it consists of a 11.2-mm round tu

e. Published by Elsevier Inc. All rights reserved.

T.-W. Lee, N. Hegde / Combustion and Flame 142 (2005) 314–316 315

Fig. 1. C/H and C/N ratios as functions of the equivalence ratios for methane (CH4) and ethylene (C2H4).

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surrounded by an 89-mm coflow where a constanflow of 13.3 cm/s was maintained. Through the cetral 11.2-mm tube, methane–air mixture of varyiequivalence ratio was sent. In this study, we have uequivalence ratios of 3.8 and 5 for methane–air mture during temperature measurements corresponto conditions used by Bennett et al.[6] for rapid-insertion thermocouple measurements.

The second burner was a turbulent flame buragain using varying equivalence ratios under partipremixed flame regime[7]. An internal tube of 2 mmdiameter was used along with an 8-mm outer tufor variation of the fuel–air mixing conditions. Fothese flames, the Reynolds number ranged from 1to 1630, with equivalence ratios of 1.4 and 1.7. Ttemperature measurements in these flames werepared with Rayleigh scattering measurements froprevious study[7].

In addition to the above flame geometries twere used for temperature measurements, fuel–atio and fuel type measurements were also made uethylene and methane.

3. Results and discussion

Fuel–air ratio is important as it dictates the loccondition under which chemical reactions proceWith LIBS, this is quite straightforward as set forth bprevious investigators[1]. The measurements in thlaboratory as shown inFig. 1, for example, exhibit achange in the C/N ratio as the equivalence ratiocreases. As the carbon content increases with greproportion for fuel with respect to air, the C/N ratio icreases in a nearly linear manner. The data are shfor two different fuels, methane (CH4) and ethylene(C2H4). The linearity in the data is better exhibited

ethylene, because of the fact that ethylene has a hicarbon content leading to higher signal strengththe carbon peaks in the spectra.

The data inFig. 1 also yield information on thefuel type through the C/H ratio. The C/H ratio in thfuels is 1/2 and 1/4 for ethylene and methane,spectively. The C/H ratios from LIBS show roughlyfactor of two difference between these fuels, althouthe data deviate from a constant level value. ThLIBS gives some rough information on the fuel struture through the C/H ratio.

The above use of LIBS is already documentedother authors[1], and our main contribution is in thuse of LIBS for temperature measurements in flamHere, we present two sets of comparisons whLIBS-measured temperature is validated againstobtained by two alternate methods: thermocoupleRayleigh scattering. The temperature measuremwith LIBS works by the principle that the gas desity and therefore any molecular or atomic conctrations are inversely proportional to the temperatuBy tracking the concentration of a major species sas nitrogen, the temperature can be retrieved asnitrogen signal will be inversely proportional to thlocal temperature. Typically, the temperature is cbrated with known values, e.g., the flame maximtemperature near the flame tip. The LIBS signal ais proportional to the mole fraction of nitrogen, whiwas assumed to remain constant and equal to ththe known calibration conditions near the flame tipthe measurements reported.

Fig. 2is a comparison of LIBS-measured tempeture with that obtained with thermocouple by Bennet al. [6] for laminar methane–air flames at modately high equivalence ratios of 3.8 and 5. LIBS teperature has been acquired by measuring the nitroatomic emission peak at 500.5 nm. Nitrogen conc

316 T.-W. Lee, N. Hegde / Combustion and Flame 142 (2005) 314–316

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Fig. 2. Comparisons of temperature measured by LIBS with thermocouple (top) and Rayleigh scattering (bottom) d

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tration is expected to be proportional to the local dsity or inversely proportional to the local temperatuunder constant-pressure conditions. This is a utilityLIBS that hasnot been exploited thus far for combution temperature measurement. However, as notedmethod is applicable for temperature measuremfor constant-pressure conditions or when the presdata are independently available. In addition, if this any mixing between fuel-side and oxidizer-sidetrogen then selection of another species, such asbon or hydrogen, may become necessary as theperature tracer. Nitrogen atom has several emisbands, scattered through the visible range of 40746 nm wavelength range. The 500.5-nm peak is csen due to its strength and separation from emisbands of other commonly found atoms in combtion systems, such carbon, oxygen, and hydrogencan be seen inFig. 2, the agreement between LIBSderived and thermocouple data is excellent. It shobe noted that for methane–air at these equivalenctios the flames are weakly sooting.

A similar comparison can be observed betweenLIBS and the Rayleigh scattering temperature msurements, inFig. 2. The flame conditions are fopartially premixed turbulent flames using methanair mixture[7]. There is some level of departure b

tween the two sets of data, yet the level of agreemis reasonable. Indeed, LIBS exhibits the temperaprofile expected of turbulent jet flames, and Raylescattering data are typically associated with largergree of error (up to 15% simply based on the chanin the scattering cross section alone). Thus, LIBalong with other applications in thermal and floprocesses, has an important utility for measuremof temperature under various flame environments.

References

[1] F. Ferioli, P.V. Puzinauskas, S.G. Buckley, Appl. Sptros. 57 (2003) 1183.

[2] W.L. Flower, L.W. Peng, M.P. Michael, N.B. BergaH.A. Johnsen, D.K. Ottesen, R.F. Westbrook, V. Linsey, Fuel Process Technol. 39 (1994) 227.

[3] L.G. Blevins, C.R. Shaddix, S.M. Sickafoose, P.Walsh, Appl. Opt. 42 (2003) 6107.

[4] R.J. Santoro, T.T. Yeh, J.J. Horvath, H.G. SemerjiCombust. Flame 53 (1987) 89.

[5] A. Mitrovic, T.-W. Lee, Combust. Flame 123 (2000) 52[6] B.A. Bennett, C.S. McEnally, L.D. Pfefferle, M.D

Smooke, Combust. Flame 123 (2000) 522.[7] T.-W. Lee, A. Mitrovic, T. Wang, Combust. Flame 12

(2000) 378.