Available online at www.sciencedirect.comProceedings

Proceedings of the Combustion Institute 32 (2009) 839–846

www.elsevier.com/locate/proci

of the

CombustionInstitute

TDLAS-based in situ measurement of absoluteacetylene concentrations in laminar 2D diffusion flames

Steven Wagner a, Brian T. Fisher b, James W. Fleming b, Volker Ebert a,*

a Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germanyb U.S. Naval Research Laboratory, Combustion Dynamics Section, Washington DC., USA

Abstract

We report the first quantitative and calibration-free in situ C2H2 measurement in a flame environmentusing direct Tunable Diode Laser Absorption Spectroscopy(TDLAS). Utilizing a fiber-coupled DistributedFeedback diode laser near 1535 nm we measured spatially resolved, absolute C2H2 concentration profilesin a laminar non-premixed CH4/air flame supported on a modified Wolfhard–Parker slot burner with N2

purge slots to minimize end flames. We developed a wavelength tuning scheme combining laser temperatureand current modulation to record with a single diode laser a mesh of 37 overlapping spectral windows andgenerate an�7 nm (30 cm�1) wide, high-resolution absorption spectrum centered at 1538 nm. ExperimentalC2H2 spectra in a reference cell showed excellent agreement with simulations using HITRAN2004 data. Theenhanced wavelength coverage was needed to establish correct C2H2 line identification and selection in thevery congested high temperature flame spectra and led to the P17e line as the only candidate for in situ detec-tion of C2H2 in the flame. We used highly efficient optical disturbance correction algorithms for treating trans-mission and background emission fluctuations in combination with a multiple Voigt line Levenberg–Marquardt fitting algorithm and Pt/Rh thermocouple measurements to achieve fractional optical resolutionsof up to 4 � 10�5 OD (1r) in the flame (T up to 2000 K). Temperature dependent C2H2 detection limits for theP17e line were 60 to 480 ppm. By translating the burner through the laser beam with a DC motor we obtainedspatially resolved, absolute C2H2 concentration profiles along the flame sheet with 0.5 mm spatial resolutionas measured with a knife edge technique. The TDLAS-based, transverse C2H2 concentration profiles withoutany scaling are in excellent agreement with published mass spectrometric C2H2 data for the same flame sup-ported on a similar burner, thus validating our calibration-free TDLAS measurements.Published by Elsevier Inc. on behalf of The Combustion Institute.

Keywords: Diagnostics; Acetylene; Laminar flames; Chemiluminescence; Soot formation

1. Introduction

Acetylene (C2H2, H–C„C–H) is an importantcombustion species whose flame synthesis plays akey role in the formation of soot [1,2], polycyclic

1540-7489/$ - see front matter Published by Elsevier Inc. on bdoi:10.1016/j.proci.2008.05.087

* Corresponding author. Fax: +49 0 6221 54 5050.E-mail address: volker.ebert@urz.uni-heidelberg.de

(V. Ebert).

aromatic hydrocarbons [3], nano materials [4],and chemical vapor deposition processes [5].C2H2 is also a key precursor species for under-standing, simulating and modeling chemilumines-cence emissions from flames via C2 or CH radicals[6]. Quantitative understanding of chemilumines-cence offers a promising, completely passive andinexpensive diagnostic approach for activecombustion control [7].

ehalf of The Combustion Institute.

840 S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846

Mechanistic understanding of these phenom-ena with valid predictive capabilities, however,requires precise, quantitative, in situ measurementtechniques that can be directly applicable to therelevant combustion environment. For opticalapproaches C2H2 has been a tough diagnostic tar-get under combustion conditions; mass spectro-scopic diagnostics in combination with specialsampling techniques have been used primarilyfor inflame C2H2 detection [8–11]. A drawbackis the need for careful calibration and the signifi-cant risk of physico-chemical modifications tothe flame gases by inserting sampling orifices(quartz or steel) to extract gas samples from theflame. In recent years, optical diagnostic tech-niques including CARS, Raman, and LIF havebeen investigated for C2H2 detection in combus-tion environments. However, these approachesexperience severe limitations with regard to eithersensitivity, spatial or temporal resolution, or theneed for a suitable calibration.

Optical detection of C2H2 in jet flows has beenreported by electronic resonance-enhanced CARS[12]. Farrow et al. used vibrational CARS todetect C2H2 in an ethylene flame [13]; the intensenon-resonant background, however, obscured aquantitative determination. Lucht et al. quantifiedthe C2H2 concentration in an acetylene-dopedflame using CARS and reported at a temperatureof 1000 K an estimated detection limit of2000 ppm [14]; they projected that detection limitimprovements would be possible but at theexpense of deteriorated spatial resolution andincreased measurement time. Williams and Flem-ing [15], extended the investigations of Raicheet al. [16], and measured relative acetylene concen-trations in 10 Torr low-pressure propane andmethane flames by laser-induced fluorescence.However, the lack of information on the quench-ing processes prevented an absolute C2H2 concen-tration determination. Spontaneous Ramanscattering was applied by Mokhov et al. [17]who reported a C2H2 detection limit of 500 ppmat 1750 K in a flat premixed CH4/air flame ingood agreement with values obtained by anextractive probe technique but in substantial dis-agreement with the predictions of numerical flamecalculations. The same group also reportedextractive C2H2 measurements in premixed flamesusing near infrared diode laser absorption spec-troscopy on the weaker m1 + m3 combination bandof acetylene [18], but their extractive samplingbased measurements required gas cooling andsample transport in combination with a sensorcalibration. However, such laser absorption tech-niques bear the potential for very high sensitivi-ties, especially when strong fundamental C2H2

transitions in the m3 band around 3300 cm�1areused. Using a difference frequency generated lightsource and a long pass absorption cell, Ruscianoet al. [19] demonstrated room temperature

measurement sensitivities in the low ppb rangevia an extractive sampling configuration. The onlyflame studies for C2H2 around 3300 cm�1 arereported by Scherer et al. [20] using infrared cavityring-down spectroscopy. They were only able toassign interfering lines in spectra obtained in amethane/air diffusion flame to C2H2; no attemptswere reported to derive C2H2 concentrations.

The diagnostic potential of diode laser basedabsorption for highly sensitive optical detectionof flame species and even temperature measure-ments with high spatial resolution wasdemonstrated by Miller et al. [21] in a laminar,non-premixed methane/air diffusion flame on aWolfhard–Parker slot burner. They used a leadsalt diode laser with a spectral output between2090 and 2160 cm�1 and utilized direct absorptionand wavelength modulation spectroscopy todetermine CO concentration profiles. The NISTWolfhard–Parker slot burner Miller et al. usedhas been well characterized; numerous flame spe-cies and gas temperature profiles have been mea-sured using a variety of diagnostic techniquesincluding laser induced fluorescence and massspectrometry [11]. Published flame data is avail-able on the web [22]. C2H2 profiles were generatedusing mass spectroscopy [23]. In the CO measure-ments of Ref. [21] the authors address an impor-tant aspect of the triple slot burner forabsorption measurements: the presence of ‘‘para-sitic” end flames. The end flames are orientedperpendicular to the main flame sheet and arepresent above each end of the fuel slot at thefuel–air interface. Based on computations assum-ing the presence or absence of the end flames,Miller et al. concluded that there was only a smallerror in their absorption-based CO concentrationand temperature measurements. However, theypoint out that for absorption measurements inthis burner, the presence of the end flames willimpact a given species profile greater near the bur-ner center, especially for probed transitions origi-nating in excited vibrational states or higherrotational states. In particular, temperatures nearthe burner center would be dramatically in error.

In this paper we report the first quantitativein situ optical-based, sampling and calibration freemeasurement of C2H2 using diode laser absorp-tion spectroscopy in an atmospheric pressureflame supported on a Wolfhard–Parker slot bur-ner modified to minimize end-flame effects.

2. Experimental

2.1. Details of the Wolfhard–Parker slot burner

For our studies, we constructed a Wolfhard–Parker slot burner as described in Ref. [11] follow-ing the original shop drawings of the NIST design(personal communication, Kermit Smyth). A

S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846 841

schematic for the standard Wolfhard–Parker slotburner is given in Fig. 1, top. The rectilinear bur-ner design provides a relatively homogenousabsorption pathlength of 41 mm in the burnerparallel to the flame sheet. In our flame studies,methane exited the central 8 � 41 mm fuel slotat 11 cm/s and combined with parallel air flowsfrom the outside 16 � 41 mm slots with a flowvelocity of 22 cm/s. Two identical flame sheetsare formed longitudinally along the burner.

In the standard burner, end flames orientedperpendicular to the main flame sheet form atthe end of the fuel slot at the fuel/air intersection.For absorption measurements parallel to theflame sheets, end-flame effects are greatest nearthe burner centerline (along the fuel slot). Themagnitude of the end-flame effects varies with

Fig. 1. Top: Schematic of a Wolfhard–Parker Burnerused in Ref. [11,12] showing the top of the burner andplacement of flame stabilizing gulls above the burner.Bottom: Picture of the modified burner used in thecurrent work with N2 slots for end-flame suppression. Inoperation, a second copper screen covered the entireburner top.

location depending on the ground state energyof the absorption line, the vertical height abovethe burner, and the spatial profile of the probedspecies. To minimize the end-flame effects, wemodified the NIST design by adding a nitrogenpurge slot at both ends of the fuel slot followingthe approach of Ref. [24] (personal communica-tion with Chris Shaddix). The 7.5 � 16 mm end-flame N2-purge slots are indicated in Fig. 1, bottom.We found a nitrogen flow of 4.7 slpm (22 cm/s) tobe optimally effective in reducing the end-flameeffects. Effectiveness of the end-flame purge wasevidenced in the absence of the visible flames atthe fuel slot ends and the measured impact onthe temperature profiles. With no N2 purge flow,temperatures could reach peak values of up to1300 K at the ends of the fuel slot. For stabiliza-tion of the flame we used wire-screen gulls (shownin Fig. 1, top) as described in detail in Ref. [11].The entire burner assembly could be translatedlongitudinally or transversely relative to the laserbeam with step sizes between 0.25 and 0.5 mmby a software controlled DC motor.

2.2. Direct absorption spectroscopy

For the in situ gas analysis in flame environ-ments we used our variant of the ‘‘Direct Absorp-tion Spectroscopy” (DAS) technique. The DASmethod, described in detail in the literature [25–30], permits calibration-free measurements ofabsolute absorber concentrations, gas tempera-tures and even gas residence time distributions.DAS is well suited for in situ concentration mea-surements in complicated environments where cal-ibration is difficult. Compared to more sensitivedouble modulation techniques like WavelengthModulation Spectroscopy (WMS) [31], FrequencyModulation Spectroscopy (FMS) [32] or dualbeam techniques [33], DAS uses the digitized dc-coupled detector signal, including all offsets anddisturbances, in the data evaluation. Only a scan-ning modulation of the laser is needed to recoverthe complete absorption line shape; no furthermodulation of the laser wavelength is used orrequired. In the DAS setup, a diode laser beamis directed through the measurement volume ontoa photo detector. The laser is continuously andrepetitively scanned over the absorption line bylinear laser current modulation. The entiredetected signal is digitized. A careful signal correc-tion for various strong disturbances found undercombustion conditions must be made. Assuminga homogeneous medium, the resulting signal canbe described by Beer’s law [34]:

IðkÞ ¼ I0ðkÞ � exp½�SðT Þ � gðk� k0Þ � N � L�� TrðtÞ þ EðtÞ ð1Þ

where I(k) and I0 (k) are the detected and the ini-tial laser intensity, respectively. The absorption

842 S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846

signal is described by the temperature-dependentline strength S(T), the absorber number densityN, the absorption path length L, and the normal-ized line shape function g(k � k0), which is centredat wavelength k0. In addition, strong fluctuationsof the overall transmission Tr(t) may be present,caused by broadband absorption, scattering byparticles or beam steering. Background radiationE(t) can also increase the amount of signal de-tected and must be differentiated from the laserradiation.

The wavelength of the diode laser was rapidlytuned at 140 Hz to temporally isolate the molecu-lar absorption from the typically slower transmis-sion and emission variations. In the datareduction process, any signal offsets are firstremoved, followed by division of the scan signalwith a baseline function and extraction of theabsorption line area using a fitting algorithm.We calculate the absorber number density usingBeer’s law and the ideal gas relationship.

2.3. C2H2 spectroscopy

Acetylene absorption bands are accessible inthe 1.5 lm and the 3 lm region. Compared tothe 1.5 lm band, the 3 lm band offers 18 timesstronger absorption lines but is plagued withstrong interfering lines by H2O vapour and vari-ous hydrocarbon molecules. Furthermore thelonger wavelength is only accessible via cryogeni-cally cooled lead salt diode lasers [35] or fairlycomplicated and expensive difference frequencygeneration light sources. Unfortunately, the3 lm region is also problematic for quantum cas-cade diode lasers [36] which are not routinelyavailable at such short wavelengths. Detectionvia the m1 + m3 vibrational combination band at1.53 lm is much easier and this scheme is oftenused as a spectrometer test case. The wavelengthregion is easily accessible via inexpensive, highquality telecom type diode lasers which combinelow cost, high spectral quality, excellent availabil-ity with near perfect room temperature opera-tional fibre-coupled diode laser packages.Nevertheless, diode laser measurements at1.53 lm in the flame to our knowledge have notbeen described in the literature. This seems sur-prising as simulated flame absorption spectra cal-culated using HITRAN2004 data [37] andmodelled Voigt line shapes indicate that manyC2H2 lines around 1.53 lm are virtually free fromH2O or CO2 interferences (Fig. 5, middle panel).Several C2H2 lines should be available for in situflame measurements in this spectral region (pro-viding sufficiently high C2H2 concentrations).

2.4. Spectrometer setup

We used a tunable 1.53 lm near-infrared diodelaser (NIR-DL) as the light source. The laser was

a chip mounted butterfly type distributed feed-back (DFB) diode laser with an optical isolatorand on chip Peltier cooling. The fiber-coupledlaser had a maximum optical output of 20 mWat the fibre end. Laser temperature and currentwere stabilized by a combined precision laserand Peltier driver module. Changes in the lasertemperature yielded a modulation in the laserwavelength with a quasi-static tuning coefficientof �0.439 cm�1/K. We modulated the laser cur-rent at 140 Hz using a triangular waveform froma signal generator to produce wavelength scansover the desired absorption line. The relativelyfast scanning frequency resulted in a nonlineardynamic wavelength tuning coefficient which wasprecisely characterised with an accuracy betterthan 1% using an air-spaced reference etalonand carefully taken into account in the signal eval-uation procedure.

The laser light was collimated with a highnumerical aperture lens to form a sub-millimeterdiameter laser beam which was directed throughthe flame. The beam diameter was 0.5 mm(FWHM) as determined with the knife edgemethod. The transmitted laser light was collectedwith a standard InGaAs detector (1 mm2 detec-tion area), amplified using a low noise trans-impedance amplifier with a 500 kHz bandwidth,digitized with a 140 k Samples/s, 18-bit ADCboard, and passed to software for further com-puter based data evaluation. During the measure-ments the amplifier gain was automaticallyadjusted by Labview (National Instruments Cor-poration) based acquisition and evaluation soft-ware to keep the signal in the working range ofthe ADC [29]. After taking into account thedynamic tuning coefficient, we extracted theabsorption line area from the resulting signalusing a Levenberg–Marquardt algorithm to fit amulti-line Voigt shape [38], and a backgroundpolynomial up to 3rd order. The extended Beer’sLaw and the ideal gas law were applied togetherwith measured gas temperatures and pressures aswell as tabulated line strengths from theHITRAN2004 data base to convert measured lineareas to absolute absorber concentrations. Nofurther data treatments were used to scale theC2H2 concentration profile or to calibrate theC2H2 measurements.

The flame temperature was measured with aPt/Pt13Rh thermocouple (Type R, 203 lm diame-ter wire, 250 lm bead diameter). Measurementswere taken with thermocouple leads in an opti-mized arrangement to reduce systematic errors.Spatially resolved, radiation corrected 2D thermo-couple measurements at a height of 9 mm abovethe burner are shown in Fig. 2; clearly absent inthe plot are the high temperature end-flameregions at the end of the fuel slot. Figure 3 showsthe measured temperature profile taken in themiddle of the burner perpendicular to the fuel

Fig. 2. Temperature contour profiles from radiationcorrected thermocouple measurements in an atmo-spheric pressure, non-premixed CH4/air flame supportedon a modified Wolfhard–Parker burner with N2 purgingfor end-flame suppression.

Fig. 3. Measured transverse temperature profile 9 mmabove the Wolfhard–Parker slot burner at a lateralposition of x = 0 mm, (see Fig. 2) in a methane–air flameusing a radiation corrected thermocouple: gray line –from Ref. [11,12]; black line – this work. The inwardshift of the temperature peaks is attributed to the use ofa single screen to cover the entire burner surface versusindividual screens for each slot used in Ref. [11,12].

Fig. 4. New tuning procedure combining 37 consecutivestep changes in laser temperature and continuous laser

S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846 843

and air slots, i.e. lateral position of x = 0 mm (seeFig. 2). Shown for comparison is the temperatureprofile from Ref. ([11,22]) taken with a thermo-couple in a similar burner. The slight shiftbetween the profiles is attributed to differences inthe slot filling and screen configuration at the fueland air exits for the two burners.

current modulation used to generate a 7.5 nm wideoverview spectrum needed for absorption line identifi-cation. Upper panel: Eight of 37 current modulatedwavelength scans at distinct laser temperatures generat-ing absorption profiles in a 103 mm C2H2 reference cell(108 mbar). The reference cell was at room temperature.Lower panel: Multi-line Voigt fits of the C2H2 lines forthe individual current tuning sections. Thirty-sevenoverlapping sections are evaluated to yield the widebandabsorption spectra shown in Fig. 5.

3. Results and discussion

3.1. Spectral line identification

We developed a new tuning scheme for line iden-tification of the target species in the complex flameenvironment. We collected spectral data simulta-

neously in the Wolfhard–Parker burner, in aC2H2 reference cell, and in an air-spaced etalon act-ing as a relative wavelength marker. In order tomaximize the spectral coverage achievable withthe diode laser, we combined continuous, laser cur-rent induced wavelength tuning, discrete, stepwisetuning via laser temperature changes and absolutewavelength calibration. This procedure shifted thelimited wavelength window accessible by currenttuning in discrete steps (Fig. 4). The laser tempera-ture step size was chosen to ensure sufficient overlapbetween the spectral windows. With this approachwe were able to combine 37 overlapping current-scanned wavelength regions for this laser. Afterconverting the spectral absorption profiles fromthe time to the wavelength domain, the laseramplitude modulation was eliminated for eachscan. Following a multi-line Voigt fit of each sec-tion, the 37 spectral windows were connected usingthe spectral information from the reference cell andthe etalon. Figure 5 shows the resulting high-resolution absorption spectrum from 6485 cm�1

to 6515 cm�1 captured in a 103 mm long referencecell containing 108 mbar of high purity C2H2 usedto generate with a single DFB diode laser a7.5 nm wide overview spectrum needed for absorp-tion line identification. We achieved very goodagreement in line position, line width and lineheight between the low pressure cell and simulatedHITRAN2004 based spectra under the same condi-tions. Also shown in Figure 5 are a spectrum usingHITRAN2004 to simulate a flame environment of1215 K, pressure of 1 atm, [H2O] concentration of13.8 Vol.%, a [C2H2] concentration of 0.47 Vol.%,

Fig. 5. Experimental and simulated absorption spectra in the range from 6485 cm�1to 6515 cm�1 to isolate a C2H2 linesuitable for in situ flame studies: Upper panel: experimental data in a C2H2 reference cell (pathlength = 103 mm, 298 K,108 mbar C2H2) in excellent superposition with simulated Hitran2004 data. Middle panel: simulated flame absorptionspectrum using HITRAN2004 data (Assuming pathlength = 41 mm, 1 atm, 1215 K, [H2O] = 13.8 Vol.%, [C2H2] = 0.47Vol.%). Lower panel: measured absorption spectrum in the flame of the modified Wolfhard–Parker burner at a lateralposition of �3.25 mm and 9 mm above the burner (effective path length = 41 mm, p = 1 atm, T = 1215 K). Numerousunassigned non-C2H2 absorption lines mask all accessible C2H2 lines except the P17e line.

Fig. 6. Upper panel: Typical experimental in situabsorption spectrum of the P17e C2H2 line (dots) andthe multi -line model fit (line) in the flame of a modifiedWolfhard–Parker burner (9 mm above the burner exit,at a lateral position of �3.25 mm corresponding toT = 1215 K, [C2H2] = 4876 ppm). Lower panel showsthe residual between model and fit with 1r standarddeviation of 4 � 10�5 fractional absorption.

844 S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846

and a path length of 41 mm (middle panel) and theTDLAS measured flame spectrum at 9 mm abovethe burner and a lateral position of �3.25 mm(lower panel). There are a surprisingly large num-ber of interfering lines in the flame spectrum whichare not included in the HITRAN database. Thehigh-resolution, broad tuning scheme provides aspectral overview of about 7 nm (30 cm�1) with asingle diode laser which permits an exact identifica-tion of the designated absorption line under theflame conditions. Using this method we were ableto identify the P17e line at 6512.99 cm�1 as the onlycandidate in the laser tuning range for flame con-centration measurements because of its isolationfrom neighbouring high-temperature non-C2H2

absorption lines.We applied the 37-stepwise spectral tuning

scheme for each laser at a dense mesh of spatiallocations across the flame by translating the bur-ner from x = � 10 mm to +10 mm with a spatialresolution of 0.25 mm. This way we generated aspatially resolved set of 7 nm wide, continuous,high resolution absorption profiles which wereused to derive spatial species profiles from withinthe flame.

3.2. Flame measurements

Figure 6 shows a typical narrow-band absorp-tion scan in the Wolfhard–Parker methane–airflame of the C2H2 P17e line using the 1.5 lmdirect TDLAS spectrometer after emission andtransmission correction. The spectrum is an aver-age of 100 wavelength scans captured with a rep-etition rate of 140 Hz. From the fit residual of theabsorption line scan in Fig. 6 with a peak frac-

tional absorption of 3.8 � 10�4 OD we derivedan optical resolution (i.e. one standard deviation,1r, of the residual) of ±4.0 � 10�5 OD. This cor-responds to a peak C2H2 concentration of4763 ppm at a temperature of 1380 K and to a1r detection limit of 480 ppm C2H2 or a normal-ized limit of detection of 20 ppm m. The lowestdetection limit of 63 or 2.6 ppm m was reachedat a temperature of 720 K.

Transverse spatial concentration profiles ofacetylene in the flame derived from the spectraldata sets are shown in Fig. 7 and compared witha reference profile taken with a mass spectrometer

Fig. 7. Experimental C2H2 concentration profiles (at9 mm height) in the non-premixed, atmospheric pressureCH4/air flame of a Wolfhard–Parker slot burner: Grayline - mass spectrometric local measurements from Ref.[11,12] in a burner without N2 purging. Black line –TDLAS-based, spatially integrated, calibration-freeabsolute C2H2 profile in a N2-purged burner with end-flame suppression (this work). (Note: No scaling wasused to match the two curves; The inward shift of thepeak position is attributed to the different wire screencovers – see also Fig. 3).

S. Wagner et al. / Proceedings of the Combustion Institute 32 (2009) 839–846 845

[11,22] in a similar Wolfhard–Parker burner (butwithout end-flame purging). There are slightasymmetries in the profiles which can be attrib-uted to inhomogeneities in the burner slot fillingand wire screen configuration at the slot exits.The same trends are also observable in the tem-perature profile measurements. There are alsoslight differences in the absolute concentrationvalues between the two flame sheets that likelyreflect the temperature dependency of the linestrength, partition sum at the elevated tempera-ture and possible inhomogeneity along theabsorption path. Overall, there is very good qual-itative and quantitative agreement with the refer-ence data.

4. Conclusions

We report the first absorption based in situC2H2 measurements in a flame environment at1.535 lm. We obtained – without the need of aseparate calibration – absolute concentrations ofproduct C2H2 in a non-premixed atmosphericpressure methane–air flame using direct tunablediode laser absorption spectroscopy. We acquireda 7 nm (30 cm�1) wide, highly resolved C2H2 spec-trum centered at 1.535 lm using a single DFBlaser that led to the identification of the only inter-ference-free C2H2 line (P17e) in this wavelengthregion for the methane flame environment. TheDAS method along with the combined laser tem-perature and current modulation wavelength-tun-ing scheme are applicable to quantifying C2H2 aswell as other species in combustion systems.

Using the same approach outlined here forC2H2, we also successfully measured methane at1.65 lm as well as OH radical concentration pro-files at 1.5 lm in the Wolfhard–Parker burner.These data are now under review and will be pub-lished in the near future. Combining the absolutespecies profile measurements with detailed chemi-cal flame simulations will help to validate newflame models and improve the quantitative under-standing of chemiluminescence (CL) radiation inflames. This understanding will contribute to thedevelopment of next generation, highly dynamicchemiluminescence-based heat-release sensorsand their application to active combustion control.

Acknowledgments

Funding for this research was from the Office ofNaval Research (ONR) through the Naval Re-search Laboratory core funding and from TheDeutsche Forschungsgemeinschaft (German Re-search Foundation) with travel support for VEand SW from the Deutscher Akademischer Aust-auschdienst (German Academic Exchange Service).

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