air diffusion flames using tunable diode laser absorption spectroscopy

A Study of Carbon Monoxide in a Series of Laminar Ethylene/Air Diffusion Flames Using Tunable Diode Laser Absorption Spectroscopy R. REED SKAGGS and J. HOUSTON MILLER* Department of Chemistry, The George Washington University Washington, DC 20052

Tunable diode laser absorption spectroscopy has been used to map carbon monoxide concentrations and temperatures in a series of laminar ethylene/air, axisymmetric diffusion flames. As the quantity of soot increased, temperatures near the tip of the flames were observed to decrease. Carbon monoxide concentration profiles were found to depend on soot levels with the most dramatic differences apparent along the centerline beyond the stoichiometric surface. These measurements were combined with literature data to calculate oxidation rates for both soot and carbon monoxide. It was found that the oxidation rate for CO low in the flames was larger than that near the visible flame tip, which is attributable to both lower hydroxyl radical concentrations and temperatures at the tip. Further, soot oxidation, which is believed to form CO, occurs at a faster rate than CO oxidation processes at the visible flame tip, thus leading to carbon monoxide emission from the flame.

INTRODUCTION In this paper we present a study of carbon monoxide concentration levels in three ethylene/air, axially symmetric laminar diffusion flames near the smoke point using tunable diode laser absorption spectroscopy (TDLAS). Early studies in these flames by Santoro and coworkers have provided quantitative information on the formation, growth, and burnout of soot particles [l] which were extended to include measurements of temperature and velocity [2, 31. In a later study, temperatures were measured in the ethylene flames using Coherent Anti-Stokes Raman Spectroscopy (CARS) [4]. Both of these studies observed a decline in flame temperatures near the tip as soot loading increased. More recently, measurements of hydroxyl radical concentrations were made which allowed for the quantitative assessment of soot oxidation processes [51. It is extremely difficult to make quantitative measurements of species concentrations in the presence of soot particles. Techniques such as laser-induced fluorescence, used for the hy-

* Corresponding author. Presented at the Twenty-Fifth Symposium (International) on Combustion, Irvine, California, 31 July-5 August 1994.

droxyl radical measurements, suffer from broad-band fluorescence from polynuclear aromatic hydrocarbons, which emit light in both the ultraviolet and visible spectral regions. Probe techniques are subject to orificeclogging by particulate matter. Santoro’s group has made progress in the measurement of concentrations in sooting diffusion flames by using a probe fitted with a vibrating fiber to keep the orifice clear [6] and they have reported centerline measurements of carbon monoxide in a series of methane flames which are undiluted, mixed with butane, or mixed with butene [7]. Because soot particle absorption and scattering is substantially less in the mid-infrared than in the visible, TDLAS may be applied to the study of systems with significant particle loading [81. Carbon monoxide in hydrocarbon diffusion flames is of particular interest not only because of its toxicological impact in real fire situations [9], but also because of its interdependence with other PIC emission processes. By now, it has become well established that emissions of soot and carbon monoxide are correlated [5, 7, 10-141. Koylu and Faeth investigated the effects of flame size and residence time on soot and CO concentrations in buoyant turbulent flames. In laminar flames, it has been observed that as the concentration of COMBUSTIONAND

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Copyright 0 1995 by The Combustion Institute Published by Elsevier Science Inc.

CARBON MONOXIDE

IN ETHYLENE

/ AIR DIFFUSION

soot increases the concentration of CO increases, while both the OH radical concentration and temperature decrease [7]. Both CO and soot are oxidized by OH radical under flame conditions [14-161. Thus, it has been suggested that soot and carbon monoxide have a competitive relationship for OH radical in hydrocarbon diffusion flames 1.5,7, 12, 151. Puri and Santoro [7] found that increased amounts of soot result in larger concentrations of CO as well as depletion of hydroxyl radical [12] at the flame tip in a series of methane flames which were doped with increased amounts of butane or butene to increase the sooting characteristics. These flames are analogous to the series of ethylene flames studied here in that they represent flames that do not emit soot particles (nonsmoking), a flame at its smoke point, (incipient smoking), and a flame which is emitting soot particulate (smoking). Hydroxyl radical concentration measurements were also made in ethylene flames [5], which were used to compute CO and soot oxidation rates. In this study it was found that soot oxidation led to a depletion of hydroxyl radical and a decrease in the rate of CO oxidation. In the work reported here we add carbon monoxide concentrations to the database available in a series of flames which have increased quantities of soot particulate near the flame tip. For these studies we employ tunable diode laser absorption spectroscopy (TDLAS) which has been demonstrated to be a valuable probe for species’ concentrations and temperatures even when the local soot level is high [17-211. CO concentration measurements are combined with the existing data available in these flames as well as the calculated mixture fraction field to study quantitatively the correlation between CO and soot concentrations and emissions near the tips of these flames. EXPERIMENTAL Apparatus

Figure of the details scribed

1 illustrates the schematic arrangement tunable diode laser experiment. The of this arrangement have been depreviously [19], and only modifications

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431

Fig. 1. Experimental configuration for Tunable Diode Laser diagnostic measurements in an axially symmetric hydrocarbon diffusion flame.

to this earlier configuration will be described here. The spectral output of a diode laser is coarsely determined by its composition and fine tuned by its temperature. The diode laser used for this study has a spectral output between 2110 and 2190 cm-’ and was mounted in a closed-cycle helium refrigerator whose temperature is maintained with a cryogenic temperature stabilizer. Further refinements of temperature (and thus the laser’s frequency) are possible by adjusting the injection current through the diode. In a typical experiment the diode current would be ramped up slowly to produce a spectral frequency scan of = 0.75 cm-‘. A sinusoidal, lO,OOO-Hz, small-amplitude modulation was applied to the diode current while slowly scanning through an absorption feature. The emitted infrared light was collimated, directed through a mode-selection monochromator, where it was modulated using a tuning fork chopper operating at 400 Hz, passed through an axially symmetric laminar diffusion flame, and finally focused onto a liquid nitrogen-cooled InSb detector. The detector signal was split and sent to two phase-sensitive, lockin amplifiers (Stanford Research Systems (SRS) model SR850 DSP and Princeton Applied Research (PAR) model 5209). The SRS signal was sampled at twice the laser diode modulation frequency, and its output provided information on the curvature in the detector’s response as a spectral line was being scanned. The PAR lock-in was refer-

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R. R. SKAGGS AND

enced to the chopper frequency, and thus its output signal allowed for the discrimination of the laser intensity from flame radiation impinging the detector. Both lock-in amplifier signals were recorded on an IBM-PC data acquisition system. The beam size was measured by collecting a profile of the beam intensity as a razor blade was passed horizontally through the laser beam. From the resulting profile intensity, the full width at half maximum was found to be = 900 pm. To correlate the current through the diode with the relative frequency of the spectral output, scans through a Ge etalon with a free spectral range of = 0.048 cm-’ were recorded. Data collected in these etalon scans showed that a typical 0.02-A current sweep correspond to a scan of approximately 0.75 cm-‘. Flames studied in this report were supported on an axially symmetric diffusion burner, developed by Santoro et al. [l, 21. This burner consists of two concentric brass cylinders of 1.1 cm and 10.2 cm diameter. The fuel is passed through the central cylinder and air passes around the fuel tube in the outer cylinder. The air passage consists of a series of wire screens with the initial section containing glass beads to ensure uniform flow. A ceramic honeycomb section is used as a final screening section for the air flow, with the fuel tube extending 4 mm beyond the co-flow surface. The burner is supported on a two dimensional, computer controlled positioning system. In our experiments, we have reproduced three experimental conditions for the fuel flow rates given by Santoro et al. [l, 21, which are listed in Table 1. The three ethylene/air flow rates produced flames that are non-smoking (NS), in which soot was not emitted from the flame tip, incipient smoking (IS), in which soot particles were emitted from the luminous flame TABLE 1

Flame Conditions Established in the Three Ethylene/Air Flames

(cm3/s)

(cm3/s)

Visible Flame Height (mm)

3.85 4.60 4.90

715.5 715.5 1068.3

88 110 = 117

Fuel Flow Rate Flame NS IS S

Air Flow Rate

J. H. MILLER

edge, and smoking (S) in which soot particles were emitted from the entire flame region. C.P. grade ethylene (C,H,) gas was purchased from Potomac Air Gas Co. The air supply was provided from an in-house compressor. The fuel flow rates were measured with a rotameter which was calibrated using a soap bubble technique. Air flow was monitored by a Hastings mass flow meter. The air flow conditions were chosen to reproduce Santoro et al.% conditions [l, 21 and aided in the stabilization of flames and isolation from laboratory air currents. Stabilization of the flames was also aided by using two concentric wire-mesh screens around the upper and lower portions of the flame. A l-cm opening between the two screens was left for passage of the infrared beam through the flame. Theoretical

Background

is a line-of-sight technique; the absorption signal observed at the detector (referred to below as projection data) is the convolution of the incremental absorptions from each spatial location in the flame. In these experiments there are two implications of this fact. First, because the field is inhomogeneous, a tomographic reconstruction technique is required to recover spatial information. Second, the incremental absorption due to any particular spatial region is small and may be difficult to detect. The second problem may be addressed by using modulation techniques to increase sensitivity [22, 231. For weak absorptions the magnitude of a signal sampled at twice the modulation frequency, x”, is a function of temperature and concentration through 124, 251 TDLAS

x ” =F*(u

- Vo)~S(T)~g(vo)~~~l~ZO,

(1)

where I0 is the incident laser intensity, S is the temperature-dependent line strength for the transition; g is the line shape factor evaluated at the line center, Pj is the partial pressure of the absorbing species, and 1 is the pathlength. Linestrength and lineshape values were obtained from the HITRAN [26] database at room temperature, and were recalculated for combustion conditions. F2 is the second

CARBON

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Fourier component of the modulated absorption coefficient [23], a function which looks qualitatively like the second derivative of the absorption feature [22]. In general, both collisional and doppler broadening effects on line shapes must be considered in quantitative spectroscopy. However, Miller et al. [19] have demonstrated that under atmospheric pressure flame conditions, a Lorentzian profile function can be used to describe the experimental lineshape, g, and its second harmonic, F2. The second experimental obstacle mentioned above is addressed by using tomographic reconstruction. Our computer code implements a three-point Abel inversion algorithm described by Dasch [27] to reconstruct the incremental absorption at each radial location in the flame from a series of projection data. Data were collected by setting the burner to a specific axial height and 12 mm (laterally) from the flame centerline relative to the laser beam, collecting a spectrum, and repeating this procedure as the burner was moved laterally in OS-mm increments through the flame, until 49 total spectra had been collected. Each projection spectrum recorded consisted of the signal

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sampled at twice the modulation frequency, x”, and the incident beam intensity, I’. Tomographic reconstruction was performed for each of the frequencies in the spectrum, and the resulting data were combined to build spectra for each flame radial location. All Jeconvolution techniques used in tomography suffer from increased noise near the centerline. Although the three point Abel inversion is less sensitive to this effect than other techniques [27], we used a Savitzky-Golay [28] digital filter before and after the inversion to smooth data. The radial (reconstructed) spectra were fit to Eq. 1 with three adjustable parameters: the of CO, the collisional partial pressure halfwidth, and the local flame temperature. Figure 2 shows a representative fit of a reconstructed spectrum 55 mm above the surface of the burner in the non-smoking flame and at the burner centerline. In this region of the infrared spectrum (near 2172 cm-‘) there are two significant absorption lines of carbon monoxide R(7) (1 +- 0) and R(15) (2 + 1). Temperature and concentration data from the fits were combined to build profiles at a number of flame heights.

0.010 0.008

-

0.006 -

-0.006 -0.008 t

-0.010

1

2172.60

d

2172.70

2172.80

2172.90

2173.00

2173.10

ENERGY, cm-’ Fig. 2. Fit of NS reconstructed radial data (m) at 55 mm HAB and radial synthetic spectrum is the solid line, and the residuals (experiment-calculated) dashed line. The two CO lines R(7) (1 + 0) and R(15) (2 + 1) pictured 2172-cm-’ region.

position 0 mm. The are given by the are located in the

434 In the tip regions of the flame, only a few of the projection spectra near the burner centerline revealed significant CO absorption. For these locations, tomographic reconstruction could not reliably be performed. To extract information from these data, CO concentration and temperature were assumed to be homogeneously distributed across the tip region and the projection spectra themselves were fit to Eq. 1. To accurately characterize this region, we performed the following steps. First, a profile of carbon monoxide was collected by monitoring the intensity of the R(7) (1 t 0) line across the flame tip. This profile was then fit to a Gaussian function to determine the integrated area of the absorption, after first subtracting signal from ambient CO. A pathlength was calculated which, when multiplied by the R(7) (1 +- 0) centerline intensity, produced the same area. To validate the axisymmetric flame results, spectra were recorded in an ethylene/air flame supported on a Wolfhard-Parker burner system. The Wolfhard-Parker burner is an ideal system for making absorption measurements. The burner, as previously described [19], provides a two-dimensional flame geometry, which has a relatively long homogeneous pathlength for absorption measurements. Thus, absorption signals are strong and no tomographic reconstruction is required. Because of the long path lengths in this system, the use of modulation techniques was not required.

R. R. SKAGGS AND J. H. MILLER transverse pressure gradient and longitudinal diffusion fluxes are negligible. Buoyancy forces are included in the momentum equation. Diffusivity and viscosity were calculated from the local temperature using the Sandia transport property data 1331.Temperature versus mixture fraction correlations were taken from the Ph.D. dissertation of Honnery [34]. This code has been validated by comparing temperature and velocity fields in nitrogen-diluted ethylene flames supported on axial burners similar to that used in the current study [34].

RESULTS AND DISCUSSION Temperature Figure 3 presents selected temperature profiles in the three ethylene/air flames. Shown are profiles in the nonsmoking flame as well as a profile collected at the same nondimensional height, 7, [l] for the IS and S flames. These data are qualitatively similar in appearance to previously reported temperature data by Santoro [2] using a thermocouple to measure local flame temperature and Boedeker and Dobbs [4] using a Coherent Anti-Stokes Raman Spectroscopy (CARS) optical technique. Measured peak temperatures low in the NS flame data are approximately 200 K lower than those observed by Santoro et al. [2], which is approximately the uncertainty in the TDLAS mea-

lgv-----

Flame Field Calculations

1600 1

Mixture fraction fields throughout the flame were calculated using a computer code which has been described previously [29-321. In this model, it is assumed that (a> major species concentrations are only a function of mixture fraction (and their concentrations can be found upon solution of the transport equation for mixture fraction), (b) the rates of chemical reactions that determine major species concentrations are large with respect to transport rates, and (c) all species diffusivities are the same. The transport equations for mixture fraction and momentum are solved for axisymmetric flow using a streamline coordinate transformation. The flow is parabolic and the boundary layer assumptions are made that the

1700 Y g3 s k

1600 1500

0. E

1400 1300

8

1200 L -0

8

4

-2

0

2

4

6

6

Radial Position, mm

Fig. 3. Temperature profiles of the three ethylene/air flames. The (m) are the NS data at 55 mm HAB, the (0) are the IS data at 65 mm HAB, and the ( A) are the S data at 70 mm HAB. These three heights correspond to the same nondimensional flame heights (7 = 0.222, see Ref. 1).

CARBON

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surements near the flame front where the concentrations of CO are low. Uncertainties in cooler and richer flame regions are less, but never less than _t75 K. To this degree of uncertainty, the temperature profiles low in each flame are equivalent. However, as the data in Fig. 3 show, higher in these flames we observe temperature differences of approximately 300 K between NS and S flames, which are statistically significant. These data confirm the observations of earlier studies [7] that measured flame tip temperature reductions with increasing soot loading due to particulate radiation. CO Concentrations At 10 mm above the burner surface in the NS flame, the carbon monoxide concentration was found to peak at 0.077 mole fraction. This value is substantially greater than the carbon monoxide concentrations found low in methane flames supported on the Wolfhard Parker flame [19] and anywhere in methane flames supported on the same axisymmetric burner [35]. There is little carbon monoxide data available in laminar diffusion flames burning fuels other than methane, presumably because of the experimental difficulties in obtaining it. There have been a number of investigations of carbon monoxide levels in turbulent diffusion flames burning with a variety of fuels [36-431 and in a laminar ethylene/air diffusion flame [44]. Carbon monoxide has also been measured in a number of premixed flames [14,20,45,46] with concentrations in excess of 10 mol.% observed in rich ethylene/air flames [47]. In our experiments in the C,H, flame supported on the Wolfhard-Parker burner, the peak CO concentration at 10 mm HAEl was 0.081 mole fraction, which compares favorably with the value reported above. Contour plots of CO concentration in the three flames are shown in Fig. 4. The data illustrate that as the fuel flow rate is increased and the flame becomes sootier, the CO levels change dramatically in appearance. The highest observed CO concentration in the NS flame (of approximately 0.11 mole fraction) occurs midway up the flame in the central region. Increasing the fuel flow rates to incipient smoking and smoking conditions (IS & S

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435

flames), the maximum CO concentration (0.11 mole fraction) near 40 mm HAJ3 reappears in the S flame. The flame structure calculations predict that the stoichiometric height in these flames ([sr = 0.064) occurs at 75, 90, and 97.5 mm HAB for the NS, IS, and S flames, respectively. Thus, the lengthening of the CO profiles which is evidenced in Fig. 4 is largely attributable to the longer flame envelopes at higher fuel flow rates. This point is made somewhat clearer when carbon monoxide concentrations are plotted as a function of mixture fraction throughout the flame. This is a somewhat risky procedure in that uncertainties exist in both the experimental concentration measurements and in the flame structure calculations. Figure 5 shows a scatter plot of this correlation for the three flames. No clear differences appear for the mixture fraction correlations in the three flames which is attributable to soot levels. This result might have been anticipated: the published data show that the difference in soot volume fraction in these flames is not observable until the oxidation region high in the flame is reached. In their study of C4-doped methane flames, Puri and Santoro [7] found that on the centerline of these flames near the stoichiometric surface, the pure methane and the methane + butane flames contained more CO in the fuel rich region than the methane + butene flame (the sootiest flame), but near the stoichiometric surface and into the fuel lean region the methane + butene flame contained more CO than the pure methane and the methane + butane flames. Figure 6 shows our carbon monoxide concentration measurements plotted as a function of mixture fraction along the centerline and near the stoichiometric surface. These data show the same behavior as was found by Puri and Santoro in the above referenced work: the sootier flame has the greater CO levels for 5 < .$sr. Oxidation Processes Both carbon monoxide and soot will react with hydroxyl radical [14-48, 491. Soot has an aromatic carbon morphology and thus is expected to react with hydroxyl radical in a similar way as do smaller aromatic molecules, which pro-

436

R. R. SKAGGS AND J. H. MILLER

80

80

80

60

60

60

40

20

Radial Position, mm Fig. 4. CO mole fraction contour maps for all three flames.

duce copious quantities of carbon monoxide [50]. Thus, CO oxidation may be offset by its formation during particle oxidation in flame regions where both are occurring. Because diffusive velocities for particles are small and streamlines in these axial flames begin in lean regions and pass up through fuel rich regions [2, 5, 121, particle oxidation does not occur until the flame tip region is reached. In contrast, carbon monoxide will diffuse out from fuel rich regions where it is formed into leaner regions where it is oxidized along the full stoichiometric surface of the flame. The data collected in this study combined with soot particulate and hydroxyl radical concentrations can be used to compare the magnitude of these various oxidation rate processes: not only the rate of oxidation of carbon monoxide versus soot,

but also CO oxidation in the tip region as opposed to its oxidation lower in the flame in the absence of particle oxidation. Table 2 shows the results of the calculation of oxidation rates. For this analysis we adopted values quoted by Puri et al. [12] for the rate of reaction for CO + OH. --$ CO, + H, and for the rate of carbon monoxide production from the reaction of soot particles and hydroxyl radicals determined by Neoh et al. [161. Two important conclusions can be drawn from the data in this table. First, the rate of carbon monoxide oxidation near the stoichiometric surface is substantially slower at the flame tip than it is lower in the flame. Because the CO levels are nearly equivalent in the two locations, the slower rate can be attributed to both lower temperatures and lower hydroxyl radical

CARBON MONOXIDE

0.06

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IN ETHYLENE / AIR DIFFUSION FLAMES

0.12

0.18

0.24

b

0.000 0.04

0.30

Mixture Fraction

d

I 0.05

0.06 0.07 Mixture Fraction

0.08

Fig. 5. Plot of CO mole fraction versus mixture fraction for the three flames. The ( ??) are the NS data, the (0) are the IS data. and the (A 1 are the S data.

Fig. 6. Plot of CO centerline concentrations ture fraction near the stoichiometric surface in each flame.

concentrations near the tip of these flames. Secondly, the rate at which CO is being oxidized near the flame tip is the same order of magnitude, but slightly slower, than the rate it is being formed by soot oxidation. The difference between these rates grows as the soot loading increases (NS < IS < S>. Thus in the smoking flame, carbon monoxide is emitted from the flame tip.

differences apparent along the centerline above the stoichiometric surface. These measurements were combined with literature data to calculate oxidation rates for both particles and carbon monoxide. It was found that the oxidation rate for CO low in the flames was faster than that at the tip, attributable to both lower temperature and hydroxyl radical concentrations. Further, soot particle oxidation, which forms CO, occurs at a faster rate than CO is consumed leading to carbon monoxide emission from smoking flames.

CONCLUSIONS Tunable diode laser absorption spectroscopy has been used to map carbon monoxide concentrations and temperature in a series of axisymmetric diffusion laminar ethylene/air, flames. Temperatures at the tip of the flames were found to decrease as the quantity of soot increased. Carbon monoxide concentration profiles were also found to depend on soot levels in the flames, with the most dramatic

versus mix( tsr = 0.064)

The authors would like to thank Michael Marro, Michael Tolocka (The George Washington Unir!ersity) and Kermit C. Srnyth (NIST) for stimulating discussions and suggestions and Damon Honnery (Monash University) and John Kent (UniLlersityof Sydney) for the flame structure computer code. This work was supported by grants from the Building and Fire Research Lab-

TABLE 2 Concentrations

Used and Results

of Oxidation

Rate Calculation”

co

soot

Flame NS NS IS S

HAB/Radial Position (mm) 20/6.0 89/0.0

I IO/O.0 I IS/O.0

Temperature

(K) 2080 1510 1515 1450

Soot Number Density (particles/cm3) : 1.00 10”’ 1.00 x 10” 2.50 x 10”’

Soot Particle Diameter (nm) 0 80 80 80

a Here, soot oxidation rate is the rate of carbon monoxide formation 1 and hydroxyl radical concentration data are from Ref. 5.

CO mole fraction 6.55 3.34 4.06 5.97

x x x x

lo-’ 10-j 10-s 1O-1

from particle

OH mole fraction 3.12 3.31 5.14 3.12

x x x x

Oxidation Rate kmole/m”-s

lo-” 0 lo-” 2.58 x lo-? 1O-4 3.99 x lo-’ lo-” 6.20 x lo-’

oxidation.

Oxidation Rate kmole/m3-s 5.58 1.9 3.60 3.35

x x x x

lo-10-I IO-’ lo-’

The soot data are from Ref.

438

R. R. SKAGGS AND J. H. MILLER

oratory /National Institute of Standards and Technology and the National Science Foundation.

REFERENCES Santoro, R. J., Semerjian, H. G., and Dobbins, R. A., (1983). Santoro, R. J., Yeh, T. T., Horvath, .I. J., and Semerjian, H. G., Cornbust. Sci. Technol., 53:89-115 (1987). Soloman, P. R., Best, P. E., Carangelo, R. M., Markham, J. R., and Chien, PO-Liang, Santoro, R. J., and Semejian, H. G., Twenty-First Symposium (Znternational) on Combustion, The Combustion Institute, Pittsburgh, 1986, pp. 1763-1771. 4. Boedeker, L. R., and Dobbs, G. M., Combust. Sci. Cornbust. Flame 51:203-218

Technol., 46:301-323

5.

(1986).

Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp.

1015-1022. 6. Rapp, D. C., and Santoro, R. J., Eastern States Section/Combustion Institute Fall Meeting, 1993, p. 370-373. 7. Puri, R., and Santoro, R. J., Fire Safety Science, Proceedings of the Third International Symposium, 1991, pp. 595-604. 8. Hanson, R. K., Appl. Opt., 19:482-484 (1980). 9. Pitts, W. M., NISTIR 89-4185 (1989). 10. Fischer, S. J., and Groshandler, W. L., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1241-1249. 11. McCaffrey, B. J., Harkleroad, M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 1555-1566. 12. Puri, R., Santoro, R. J., and Smyth, K. C., Cornbust. Flame 97:125-144 (1994). 13. Kijylu, U. O., and Faeth, G. M., Combust. Flame 87:61-76 (1991). 14. Neoh, K. G., Howard, J. B., and Sarofim, A. F., Carbon

Formation

During

Combustion

(D. C. Siegla and G. W. Smith, Eds.), Plenum, New York, 1981, p. 261. 15. Fristrom, R. M., and Westenberg, A. A., Flame Structure, McGraw-Hill, New York, 1965. 16. Neoh, K. G., Howard, J. B., and Sarofim, A. F.,

17. 18.

(1964).

29. Bilger, R. W., and Stamer, S. H., Combust. Flame 51:155 (1983). 30. Honnery, D. R., and Kent, J. H., Combust. Flame 82:426 (1990).

Puri, R., Moser, M., Santoro, R. J., and Smyth, K. C.,

Particulate

22. Arndt, R., J. Appl. Phys., 36:2522-2524 (1965). 23. Reid, J., and Labrie, D., Appl. Phys., B26:203-210 (1981). 24. Hanson, R. K., Varghese, P. L., and Schoenung, S. M., ACS Symposium Series No. 134, Laser Probes for Combustion Chemistry, 1980, pp. 413-426. 25. Hanson, R. K., and Falcone, P. K., Appl. Opt., 17:2477-2480 (1978). 26. HITRAN database of infrared transition data. United States Air Force Geophysics Laboratory, 1992 edition. 27. Dasch, C. J., Appl. Opt., 29:4884-4890 (1990). 28. Savitzky, A., and Golay, M. J. E., Anal. Chem. 36:1627

Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 951. Varghese, P. L., and Hanson, R. K., J. Quant. Spectrosco. Radia. Trans., 24:479-489 (1980). Schoenung, S. M., and Hanson, R. K., Cornbust. Sci. Technol., 24~227-237 (1981).

19. Miller, J. H., Elreedy, S., Ahvazi, B., Woldu, F., and Hassanzadeh, P., Appl. Opt., 32:6082-6089 (1993). 20. Nguyen, Q. V., Hofman, D., Kampmann, S., and Dibble, R. W., Western States Section/Combustion Institute Fall Meeting, 1992, pp. 92-116. 21. Ouyang, X., Varghese, P. L., and Howell, J. R., SPIE Conference on Environmental and Process Monitoring Technologies, paper 1637-20 (1992).

31.

Kent, J. H., and Honnery, D. R., Combust. Sci. Technol., 54383

32.

(1987).

Miller, J. H., Honnery,

D. R., and Kent, J. H.,

Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, 1992, pp. 1031-1039.

33. 34. 35.

36. 37.

Kee, R. J., Miller, J. A., and Jefferson, T. A., Sandia National Labs, Report SAND80-8003, (1980). Honnery, D. R., Ph.D. thesis, The University of Sydney, 1991. Skaggs, R. Reed, Le, Khanh N., Miller, J. H., Honnery, D. R., and Kent, J. H., Combustion Fundamentals and Applications, 1993, pp. 167-170. Gore, J. P., Ip, U.-S., Sivathanu, Y. R., J. Heat Transf 114:487-493 (1992). Kijylu, U. O., Sivathanu, Y. R., and Faeth, G. M., Fire Safety Science, Proceedings of the Third International Symposium, 1991, 625-634,

Sivathanu, Y. R., and Faeth, G. M., Cornbust. Flame 81:133-149 (1990). 39. Gore, J. P., Faeth, G. M., Evans, D., and Pfenning, D. B., Fire Mater. 10:161-169 (1986). 40. Gore, J. P. Ph.D. thesis, The Pennsylvania State University, University Park, PA, 1986. 41. Gore, J. P., and Faeth, G. M., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1986, p. 1521. 42. Gore, J. P., and Faeth, G. M., J. Heat Transj 38.

110:173-181

43. 44.

(1988).

Sivathanu, Y. R., and Faeth, G. M., Combust. Flame 82:211-230 (1990). Nishida, O., and Mukoharta, S., Combust. Sci. Technol. 35:157-173

(1983).

45. Aron, K., Harris, L. E., and Fendell, J., Appl. Opt. 22:3604-3611 (1983). 46. Farrow, R. L., Lucht, R. P., Clark, G. L., and Palmer, R. E., Appl. Opt. 24:2241-2251 (1985). 47. Wagner, H. G., Particulate Carbon Formation During Combustion (D. C. Siegla and G. W. Smith, Eds.), Plenum, New York, 1981, pp. l-29. J. T., Jr., Ph.D. dissertation, Mas48. M&&non, sachusettes Institute of Technology, 1989. 49. Bradley, D., Dixon-Lewis, G., Habik, E. E., and Mushi,

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FLAMES

E. M. J., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985,

50.

p. 931.

Received I December 1993; reeked 20 April 1994

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Brezinsky, K., Prog. Ener. Combust. Sci. 12:l (1986).

Comments G. Fisk, Sandia National Laboratories, USA. Is the laser beam steered significantly by the flame? If so, how does this affect the interpretation of the data? Authors’ Reply. We do not believe

that the results presented in this paper have been dramatically affected by beam steering for several reasons. First, we align our infrared laser beam with a visible (red) beam from a HeNe laser which follows the same optical path around our optical table, through the flame, and onto the InSb detector. We have observed great sensitivity of the detector’s signal to optical alignment: a slight misalignment leads to total loss of signal. Alignment of either the HeNe or

infrared beams is not affected by the position of the beam through the flame or even by the presence of any flame in the beam path. It is a difficult optics problem to provide a less-empirical answer to this question. We have performed a simple calculation of the refraction that would accompany passing an infrared beam through an = 1 cm diameter cylinder of 2000 K air. Even for the worst case, in which the beam crosses nearly normal and just inside the cylinder’s edge, the beam would exit the flame only a few pm from where it would have in the absence of refraction. Because our beam is much larger in diameter than this deviation, it is unlikely that we could observe such a small deflection.