Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames

Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames MASAYUKI TAMURA,† PAMELA A. BERG, JOEL E. HARRINGTON, JORGE LUQUE, JAY B. JEFFRIES,* GREGORY P. SMITH, and DAVID R. CROSLEY Molecular Physics Laboratory, SRI International, Menlo Park, California 94025

Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H2O and N2. © 1998 by The Combustion Institute

INTRODUCTION Laser-induced fluorescence (LIF) has become a mature tool in combustion research. LIF is used to measure chemical kinetic reaction rates, to monitor specific species and temperatures in low pressure flames, and to make a wide variety of measurements in flames at atmospheric and higher pressures. Two-dimensional LIF images furnish a wealth of information on flame structure and behavior, while single shot point measurements are used to quantitatively understand distributions in turbulent systems. A comprehensive review of laser-based flame diagnostics of reactive intermediates, including LIF, was recently written by Kohse-Ho ¨inghaus [1]. LIF is uniquely suited to probing chemical intermediates in combustion environments: the technique is nonintrusive, extremely sensitive, highly selective, and offers excellent temporal (ca. 10 ns) and spatial (ca. 0.5–1.0 mm) resolution. The importance of measuring combustion intermediates present at ppm concentrations is two-fold. Despite their low abundance quantities, species such as CH often have a major impact on flame chemistry, including pollutant *Corresponding author. †Visiting scientist. Permanent address: Tokyo Gas Ltd., Tokyo, Japan. 0010-2180/98/$19.00 PII S0010-2180(97)00324-6

formation. In addition, correctly predicting the concentrations of radical intermediate species is a stringent test of combustion models. The relationship between the observed LIF signal and the concentration of the molecule under study requires knowledge of the fluorescence quantum yield, that is, the number of photons emitted per molecule elevated to the electronically excited state by absorption of the laser radiation. In many cases in combustion research, the LIF quantum yield is dominated by quenching, the collisional nonradiative removal of the electronically excited state. For example, in a typical atmospheric pressure flame, only about 3 out of every 1000 OH molecules excited by the laser fluoresce; the remainder of excited state OH radicals are removed by collisional quenching with the ambient gases. Several data acquisition and data analysis techniques may be used to account for the effects of collisional quenching. In low enough pressure flames, the influence of electronic quenching can be avoided by using a short, prompt electronic gate to detect fluorescence directly following the laser pulse, before significant quenching occurs [2]. At atmospheric pressure, matters are more difficult. If the electronic transition is sufficiently saturated, then stimulated emission, not quenching, dominates COMBUSTION AND FLAME 114:502–514 (1998) © 1998 by The Combustion Institute Published by Elsevier Science Inc.

HYDROCARBON FLAME COLLISIONAL QUENCHING the removal rate. However, saturated fluorescence must be carefully applied to avoid problems with an ill-defined probe volume [3]. Direct measurement of fluorescence decay time (and hence quantum yield) at atmospheric pressure has been performed [4] but requires picosecond techniques and is not presently routinely applied. Excitation of a rapidly predissociating transition where the predissociation is much faster than direct collisional removal is an elegant approach [5]. However, the signal levels can be weak, and the technique must be applied properly to avoid potential complications due to vibrational energy transfer [6]. Another approach is the direct calculation or estimation of the quenching rate at the position of measurement to determine the quantum yield. This approach requires knowledge of the quenching rate coefficients, as a function of temperature, for all colliders of interest at the point of measurement. The temperature and flame composition at this point must also be known. The total quenching rate for the excited state of species X is equal to ¥ i c i k ix ; where c i and k ix are the concentration and quenching rate coefficient, respectively, for species i. Concentrations of major species and the temperature can be simultaneously measured using singleshot Rayleigh and Raman scattering. This has been done, for example, in a turbulent flame in which OH was measured by LIF at the same point in the flame as the scattering measurements [7]. In these experiments, Rayleigh scattering is used to determine the density and hence the temperature, while each major component of the flame can be identified by its unique Raman shift; a high-resolution monochromator is used to disperse the scattered light such that the Raman spectrum can be used to measure the concentration of several flame species simultaneously. Mass spectroscopy has also been used to simultaneously measure flame composition [8]. In some cases, especially imaging experiments, simultaneous determination of flame composition is not possible but reasonable estimates of local temperature can be made. Twoline LIF temperature imaging in which the ratio of two fluorescence signals is used to infer the relative population of the two absorbing levels, and hence the Boltzmann temperature, can be

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done with two-laser excitation schemes. Alternatively, in a one-laser experiment, the temperature may be deduced by measuring the relative Boltzmann population of a single rotational level of a stable seeded component [9]. Armed with the temperature, often the major constituents of the flame gases can be estimated from a model, given a rudimentary knowledge of flame chemistry. H2O is a rapid quencher of most radicals of interest and is present in significant quantity from the flame front through the burnt gases. The dominant role of H2O as quencher simplifies and improves the usefulness of the estimation methods, provided quenching rate coefficient and concentration of H2O are well known. Determination of the total quenching rate requires independent collider-specific information measured for many potential flame colliders over a wide temperature range. Measurements of quenching cross sections have been made using a variety of techniques: flow tube studies, flash photolysis, laser pyrolysis, shock tube studies at high temperatures, and, in limited instances, flame measurements [10 –19]. When we first surveyed this field in 1985 [20], there were only limited measurements for radicals of combustion interest: several room temperature determinations, a few low pressure flame experiments, and one set each of high temperature measurements for OH(A), CH(A), and NH(A). Quenching of NO(A) had not been measured above room temperature. Since that time, motivated largely by needs for LIF flame diagnostics, several studies of quenching of OH, CH, and NO at elevated temperatures have been conducted. Furthermore, quenching studies of a few colliders for each radical below room temperature (down to 200 K) have provided a better understanding of the mechanism by which quenching occurs [15, 16, 21, 22]. For flame applications, we wanted to test this knowledge by comparing measured quenching rates with those predicted using precise knowledge of the collisional environment (flame composition and temperature) and our best understanding of quenching rate coefficients. Such a test was the subject of our study, which focused on the A-states of the radicals CH, OH, and NO. The quenching rate in a flame is highly dependent on the height above the burner

504 because flame composition and temperature (and therefore, quencher concentration and collision velocity) depend on the position in the flame at which the measurement is made. The spatial resolution of these excited state lifetime measurements are determined by the laser beam diameter, 0.5–1.0 mm. We measured quenching of CH(A), the radical of these three whose quenching is known least well, in five methane/O2/N2 flames. NO(A) and OH(A) quenching rates were determined in one slightly rich, methane/O2/N2 flame. We selected for each radical a set of quenching rate coefficients from those available in the literature and have formulated simple parameterizations (two or three parameters) to describe the quantitative variation as a function of temperature. A detailed model of the flame chemistry [23], based on the measured temperature profile in each flame, predicted the concentration of each quencher as a function of flame height. These concentrations, the temperature, and the quenching rate coefficients at each point were then used to calculate the quenching rate as a function of flame height for comparison with the measurements. This same approach was taken earlier in a study of quenching of NO(A) and CH(A) in a single, near stoichiometric, methane/air flame [24]. Using cross sections available at that time, quenching of NO was predicted 10% to 20% too high and quenching of CH about 30% too high. The latter issue was largely attributed to a lack of knowledge regarding quenching of CH by H2O at high temperature. Other cross sections are now improved, but there has been no further study of this important one. The lower temperature region of the flames that we studied has significant concentrations of H2O, which is produced higher in the flame, where it is hotter and diffuses backward toward the burner surface at these low pressures (25 to 30 Torr). Thus H2O quenching of each radical is important in all regions of the flames, not just in the burnt gases as would typically be the case at atmospheric pressure. However, because H2O is an important quencher for all three radicals, and because many important LIF flame measurements are made in regions where a significant amount of H2O is present, the distribution

M. TAMURA ET AL. of H2O in the low-pressure flame poses a good test of our conclusions rather than a limitation of these measurements. All of the quenching data considered in this paper are for the v 5 0 level of the electronically excited NO(A), OH(A), and CH(A). Higher vibrational levels of these radicals may have different quenching rate coefficients. Although higher vibrational levels are used for combustion diagnostics, there is not yet a sufficient data base to describe the variation of the quenching rate coefficients over the wide temperature range encountered in combustion. Similarly, other excited electronic states such as NO(B), NO(D), and CH(B) are also used for flame measurements. Again there is an insufficient data base to describe the quenching rate coefficients for these excited electronic states for the various flame constituent colliders over the necessary temperature range.

SELECTION OF QUENCHING RATE COEFFICIENTS Single collider quenching of the radicals investigated here has been studied over a temperature range between that of cells cooled well below room temperature and that of high temperature shock tubes. OH(A) has been the most extensively studied. A nearly complete set of data for NO(A) also exists. CH(A) quenching, not as well studied, is an important aspect of this work. We discuss choices of quenching rate coefficients for each radical in turn. For OH and NO, we present two-parameter descriptions of the cross sections as functions of temperature; for CH a slightly more complex Arrhenius behavior is generally used. These fits include data over a temperature range from 200 to 3000 K and semi-empirical limits at T 5 `; we feel confident these fits provide rate coefficients as a function of temperature between 300 and 2500 K. As shown below, the selected cross sections describe OH and NO quenching quite well in the methane/N2/O2 flame, but CH quenching in methane/N2/O2 flames is not predicted as closely.

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TABLE 1 Cross Section Expressions for NO Quenching Cross Section, sQ(Å2)

Quenching Rate Coefficient (10213 cm3 s21)

sQ 300K (Å2)

5.8 1 5.3 exp(22100/T) 1 22.1 exp(24200/T) 0.88 exp(21440/T) 1 3.1 exp(24800/T) 0.001 exp(0.0028/T) 0.1 26 exp(412/T) 38 exp(157/T) 21 exp(27/T) 13 72

3.82 s Q T 0.5 3.82 s Q T 0.5 10.63 s Q T 0.5 4.50 s Q T 0.5 4.34 s Q T 0.5 3.45 s Q T 0.5 3.70 s Q T 0.5 14.8 s Q T 0.5 4.42 s Q T 0.5

5.8 0.007 0.001 0.1 102.7 64.1 23.0 13 72

Collider CO N2 H2 CH4 H2O CO2 O2 H OH

NO Quenching Recent investigations of NO-A-state quenching were conducted above room temperature in cells and shock tubes [10 –14] and at room temperature and below in a cooled cell [15]. A model to describe the temperature dependence has emerged [25] and the results of this model, parameterized with a five parameter semiempirical fit [26]. Some of the measurements disagree, in that, for a given collider, they do not conform to a smooth variation with temperature when data for the full range available (200 to 2500 K) are included. The most significant disagreements are the results for H2O and CO2. CO2 and especially H2O are responsible for a large amount of quenching in our flames. We made the following choices. The H2O cross sections were chosen from a consistent theoretical fit to the values of Zhang and Crosley [15] at low temperatures and of Paul et al. [14] and Cattolica et al. [27] at higher temperatures. Those of Drake and Ratcliffe [12] at intermediate temperatures do not match the smooth variation of these other sets. All of the other data can be described with a simple monotonic variation with temperature; we have chosen a simple two parameter description of the data. Measurements of NO quenching by CO2 include the low temperature data of Zhang and Crosley and high temperature data of Drake and Ratcliffe and Paul et al. Again, all the data cannot be simultaneously described by a single semiempirical description. In this case, the higher temperature data of Drake and

Ratcliffe are consistent with the lower temperature set, whereas those of Paul et al. are not. We thus excluded the Paul et al. data in the fit for CO2. Fits to NO quenching cross sections, both theoretically based and semiempirical, as well as an interpretation concerning well depths, are discussed in Zhang and Crosley [15]. In several cases, there is very little temperature dependence above 300 K, so the low temperature results are important in establishing the proper choices as just described. A simple two-parameter fit is used when the data warrant. This is the case for O2, H2O, and CO2, as described in [15]. Formulations are listed in Table 1. Recommendations of Paul et al. [28] are followed for the other colliders, except H2, whose expression is derived from our fit to data exhibited in Paul et al. The cross sections for the radicals H and OH are from single measurements combined with calculations by Paul et al. [28] These cross sections are thus invariant with temperature. Because the cross sections are large, and because H has a high mean relative velocity with any collision partner, these quenchers and the unmeasured temperature dependence of the rate coefficients could be important for NO quenching. OH Quenching Cross sections for OH-A-state quenching have been measured over a wide temperature range by many investigators. The experimental results were collected and summarized graphically in a report by Paul et al. [29] Also listed in this

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M. TAMURA ET AL. TABLE 2 Expressions for OH Quenching 29

Collider N2 O2 H2O H2 CO2 CO CH4 H OH

sQ` from Paul et al. (Å2) 0.351 8.00 17.87 4.24 11.87 12.30 13.68 14.29 14

sQ` This Work (Å2)

e /k (K)

Quenching Rate Coefficient (10213 cm3 s21)

sQ 300 K (Å2)

0.4 8 20 4.5 11 12 11 14.5 20

624 243 434 224 488 397 320 84 384

4.47 s Q T 0.5 4.37 s Q T 0.5 4.92 s Q T 0.5 10.88 s Q T 0.5 4.16 s Q T 0.5 4.47 s Q T 0.5 5.07 s Q T 0.5 15.0 s Q T 0.5 4.99 s Q T 0.5

3.2 18.0 85.0 9.50 56.0 45.1 32.0 19.2 71.9

report are the parameters from a fit to a model developed by Paul [21]. The model concept is an electron-transfer harpoon mechanism, important at large internuclear separations and operative at high temperatures. If this was the only part of the mechanism, a nearly temperatureindependent cross section would be predicted [21], in good agreement with high temperature shock tube measurements [30]. At lower temperatures, however, a collision complex is formed owing to attractive forces between OH and the collision partner [31]. The full model in Paul [21] includes both portions of the mechanism, incorporating the collision complex part via a fit to experimental results at room temperature. For almost all colliders this model does an excellent job of describing the temperature dependence of the experimental cross sections [29]. (Some apparent inconsistencies in fitting cross sections for Kr and Xe are now resolved by the recent finding of a sharp variation in quenching with rotational level [22].) We might choose a slightly sharper temperature dependence for CO2 at low temperature, and a slightly smaller high temperature asymptote for CH4 than in Paul et al., but these make little difference to our overall results. Of particular value is the theoretical calculation of quenching due to H, for which experimental uncertainty is considerable [32]. In addition, we take the quenching cross section for OH with itself to be the same as that calculated by Paul et al. for HF, which has similar multipole moments; complex formation dominates the cross section for molecules with

large dipole moments. We thus use as the high temperature values those calculated by Paul et al. [29] (i.e., PAc0 from their Appendix A) with minor changes to the value of sQ` in cases where Paul’s value appears to us inconsistent with lower temperature data. Both Paul’s and our sQ` values are reported in columns 2 and 3, respectively, of Table 2. For the room temperature values, we chose experimental results, largely from work by our laboratory. Earlier, some uncertainty existed, due to disagreement of prior results, for quenching due to O2 and H2O, which are important at low temperature in our flames. We use the data from Wysong et al. [33], a study undertaken to resolve these and other discrepancies. The quenching rate coefficient depends on the rotational level of the OH A2¥1 [34] but this dependence has seldom been measured above room temperature. For the parameterization here, we have chosen a thermally averaged value. At higher temperatures, this rotational level dependence for H2O collider decreases [35] and the high-temperature quenching data can be considered a thermally averaged value. The temperature dependence of the cross sections for OH quenching between the highpressure limit and the room temperature measurements can be conveniently described by the empirical two-parameter expression:

s Q 5 s Q`exp~ e /kT!. This expression is appropriate to a collision dominated by attractive forces but with a non-

HYDROCARBON FLAME COLLISIONAL QUENCHING

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TABLE 3 Expressions for CH Quenching Quenching Cross Section, Å2 s Q 5 AT B exp (2C/T) Collider H2 H H2O O2 OH CH4 CO CO2 N2

A 221 221 166 8.61 e26 221 52.8 8.31 8.67e213 1.03 e29

C

Rate Coefficient (10213 cm3 s21)

sQ 300 K (Å2)

686 686 0 2867 686 84 0 2854 2192

11.02sQT0.5 15.09sQT0.5 5.30sQT0.5 4.79sQT0.5 5.36sQT0.5 5.43sQT0.5 4.88sQT0.5 4.59sQT0.5 4.88sQT0.5

1.3 1.3 9.6 1.8 1.3 2.3 8.3 0.04 0.03

B 20.5 20.5 20.5 1.64 20.5 20.5 0 3.8 2.9

zero high temperature asymptote. This form is the same as that suggested by Parmenter et al. [36] who attempted to correlate e with the well-depths of quencher-quencher interactions. This mechanistic interpretation is not successful for OH [37], particularly with respect to the temperature dependence of quenching; however the empirical form is a quite reasonable description. The values of e are fitted using sQ` and sQ (300 K), chosen as discussed above and listed in Table 2. CH Quenching Quenching rate coefficients for CH-A-State at flame temperatures are not so well established. For many important colliders found in flames, measurements were made in cooled and heated cells by Stuhl and co-workers, covering the temperature range of 240 to 420 K [16] and extended to 950 K by Heinrich and Stuhl [17]. These measurements were combined with laser pyrolysis/LIF measurements at 1300 K [18] and upper limits from flame studies [19] to obtain an overall picture of the temperature dependence [16, 17]. In addition, a number of CH quenchers were studied at 297 K by Bauer et al. [38]. Unlike OH and NO (and NH), quenching of CH does not appear to be governed by attractive forces, except for strongly polar quenchers like H2O and NH3. Rather, the rate coefficient generally increases with temperature in an Arrhenius form, faster than would be the case for a constant cross section. Indeed, a theoretical calculation [39] indicates a barrier in the poten-

tial surface for quenching by H2. Quenching by O2 has a complex dependence suggesting perhaps two mechanisms. Kenner et al. [16] contains a comprehensive discussion of theoretical formulations of quenching of several hydride radicals including CH. We examined the data listed in Kenner et al. [16] and Heinrich and Stuhl [17] to make our choices of CH quenching cross sections. The result was an amalgam from different studies. Cross sections for H2 and CO are fixed to the 1300 K values of Garland and Crosley [18] but use the E a’s from Kenner et al. [16] to form a three-parameter Arrhenius expression for the rate coefficient: k Q 5 AT nexp~ 2 E a/kT!. The cross section expression for CH quenching by CH4 is based on a 300 K measurement by Nokes and Donovan [40]. An E a value for CH4 is not available; instead we use the value of E a for C2H6. Because E a (C2H6) is small, 0.17 kcal, the exact value is not important to determining k Q. For O2, N2, and CO2, we chose the three parameter Arrhenius fits given by Heinrich and Stuhl. Because the values for N2 and CO2 have large values of the important coefficient n, uncertainty is considerable in their extrapolation to values above the highest measured temperature, viz., 1300 K. For example, Stuhl [41] indicates uncertainties on the order of 25% in the fitted values of this parameter. The expressions for the quenching cross sections and rate coefficients are in Table 3.

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M. TAMURA ET AL. TABLE 4 Flame Descriptions

Flame 1. 2. 3. 4. 5.

Standard Lean Rich Rich, Increased N2 Rich, Decreased N2

Equivalence Ratio

f 5 1.07 f 5 0.80 f 5 1.27 f 5 1.27 f 5 1.27

Flowrates (slpm)

Pressure (Torr)

Flame Front Temp (K)

25 25 30 30 30

1654 1603 1894 1896 1946

CH4 O2 N2 Total 0.45 0.84 1.93 3.22 0.34 0.84 2.22 3.40 0.78 1.22 2.78 4.78 0.78 1.22 3.21 5.21 0.78 1.22 2.20 4.20

Quenching of CH by H2O has not been measured above 415 K. Thus, the situation is no different from that in 1991 [24] and not much different from that in 1986. In the 1991 paper, the 300 K value of Heinrich, Kenner, and Stuhl [42] of 9.6Å2 was used for CH quenching by H2O at higher temperatures. In that paper, we concluded that the 9.6Å2 value might be too large and noted that the cross section for CH quenching by NH3 [24] dropped at 1300 K to two thirds of the value at 300 K. NH3 and H2O have similar multipole moments and may exhibit similar quenching characteristics. A decreased H2O quenching cross section at high temperatures would have improved our prediction in that flame. Given no new direct information, we adopt here the same cross section of 9.6Å2 at 300 K. The temperature dependence is more problematic. Although Kenner et al. [16] found an activation energy of 0.07 kcal between 240 and 415 K, it is inconsequential for extrapolation to flame temperatures. We consider two simple possibilities. The first is a constant cross section, that is, a rate coefficient that increases as the square root of temperature. The second is a constant rate coefficient, that is, a cross section that decreases as T 1/ 2 . The latter is more consistent with our previous analogy between H2O and NH3 as polar quenchers, although the actual functional form of the temperature variation remains open to question. The predictions for quenching of the standard flame are shown in Fig. 4 using both of these assumptions. The choice of constant rate coefficient is clearly in better agreement with the current experiments. We adopt this assumption in this and later calculations of CH quenching by H2O. No information exists on quenching of CH by

the radicals H and OH. For convenience, but without theoretical justification, we set these quenching cross sections the same as those for H2. EXPERIMENT The premixed, laminar flow flames were supported on a 6 cm diameter porous plug McKenna burner, housed inside a vacuum chamber designed for laser probing [43]. A concentric shroud of argon, at a flowrate of 1.0 slpm, was used to improve flame stability. The burner translates vertically to allow concentration and temperature measurements to be made as a function of distance from the burner. The flames were burned at pressures of 25 or 30 Torr; these low pressures allow much greater spatial and temporal resolution than atmospheric pressure flame measurements. In a premixed, laminar flow flame, the height above the burner is directly related to chemical reaction time; hence there is the increase in temporal resolution with increased spatial resolution. We made the NO quenching measurements in a near-stoichiometric methane/O2/N2 flame (hereafter referred to as the Standard flame). CH quenching measurements were made in five CH4/O2/N2 flames: the Standard flame, a fuellean flame, and three fuel-rich flames (the three rich flames differed only in the N2 flow rate). OH quenching measurements were made in the Standard flame. The feedstocks for each of the five flames are quantitatively described in Table 4. The table includes values of the flame front temperature; the flame front defined is the location of maximum [CH]. It is counter-intuitive that the flame front

HYDROCARBON FLAME COLLISIONAL QUENCHING temperature should increase with equivalence ratio. Adiabatic flame temperatures are greatest for stoichiometric flames, and lower for rich and lean flames. However, the oxidation of CO to CO2 is incomplete in these low-pressure flames, resulting in incomplete heat release and measured temperatures which are less than the adiabatic flame temperature. In the rich flame, the burnt gas chemistry is closer to equilibrium than in the lean or standard flames and the measured rich flame temperature is closer to the adiabatic flame temperature (i.e., hotter). The radicals were excited by a dye laser pumped at a 10 Hz repetition rate. The dye laser pulse energy was typically ,0.5 mJ/pulse. The fluorescence time decay of the entire A-X (0, 0) band was measured after excitation to a specific rotational level in the A state. The R 2 (6) transition of OH was pumped near 307 nm, the P 1e (8) transition of CH was pumped near 435.4 nm, and the Q 1 (16.5) transition of NO was pumped near 225.8 nm. These particular transitions were selected because they exhibit no spectral overlaps, and because the ground state rotational level is relatively insensitive to temperature-induced population fluctuations. The fluorescence signal was collected along an axis mutually perpendicular to the laser beam axis and the direction of gas flow; a monochromator with a 30 nm trapezoidal bandpass ensured that fluorescence from the entire band was collected with equal efficiency, regardless of wavelength. Thus, rotational energy transfer during the fluorescence decay did not affect the signal intensity. The signal from the PMT was amplified (by a factor of 10) and digitized by a transient digitizer (Transiac 2001S). The NO fluorescence decay rates were also measured with a higher resolution digital oscilloscope (Tektronix TDS 320, which samples every 2 ns, five times faster than the transient digitizer). Both the transient digitizer and oscilloscope measured the same lifetime, indicating that the transient digitizer has enough temporal resolution to accurately determine the shortest lifetimes in this study. Excited state lifetimes were determined by fitting the slope of the fluorescence decay between 10% and 90% of the maximum intensity. The spontaneous emission rates were subtracted from the measured excited state decay

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rates to yield the quenching rates. The following spontaneous emission rates were used: NO: 4.55 3 10 6 s 21 @44#, OH: 1.45 3 10 6 s 21 @45#, CH: 1.85 3 10 6 s 21 @46#. Flame temperatures as a function of height above the burner were measured for each flame by recording LIF excitation scans in a short section of the OH A-X (0, 0) band [2, 47]. Temperature data are needed as input for the model predictions of flame composition and also for data interpretation. Scans were typically averaged over 40 shots per wavelength position, with laser energy maintained at approximately 50 nJ per pulse in a 1 mm diameter beam cross section to avoid optical saturation. The fluorescence passes through a wide bandpass monochromator and onto a PMT; the signal is integrated with a boxcar averager with a short prompt gate to minimize the effects of electronic quenching. MODEL The flames were modeled using the GRIMech™ 2.11 methane combustion mechanism [23], which comprises 49 chemical species and 279 temperature and pressure dependent reactions (plus their reverse rates). This combustion mechanism serves as a database for Premix [48], which models stable, laminar, one-dimensional, premixed flames. The temperature profile of the modeled flame was constrained to the experimentally measured flame temperature profile. The model was run assuming no radial expansion of the flows and burner stabilization. The measured temperature versus height profile for the Standard flame, is shown in Fig 1a. The predicted major species concentrations in the Standard flame as a function of height are shown in Fig. 1b. The shapes of the temperature and concentration profiles are typical of hydrocarbon flames. Previous comparisons of calculations and measurements of reactive intermediates in lowpressure flames have shown the GRI-Mech mechanism, used with the Premix flame model, to be quite accurate. Agreement between mea-

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M. TAMURA ET AL.

Fig. 2. NO quenching rate (s21) in the Standard CH4/N2/O2 flame, as a function of height above the burner (cm). The circles are quenching data measured in the flame, and the solid line is the calculated quenching rate. The dashed line is a calculation of NO quenching due to H2O only; the dotted line is a calculation of NO quenching due to O2 only. The results of two different experiments demonstrate the reproducibility. N Fig. 1. (a) Flame temperature (K) measured as a function of height above the burner (cm) for the Standard CH4/ N2/O2 flame. The solid line is fit to the plotted data points, using an empirical equation. (b) Mole fractions of major species in the Standard CH4/N2/O2 flame, as a function of height above the burner (cm). Species concentrations were calculated using the GRI-Mech 2.11 methane combustion mechanism, constrained by the flame temperature profile plotted in a. The N2 mole fraction has been plotted as N2 3 1/3 to put it on the same scale as the other species.

sured and calculated [CH] values in the Standard flame is within 10% [49]. This is excellent agreement, especially for a species such as CH, which is typically present in quantities below 20 ppm. Excellent agreement has also been demonstrated for NO and OH measurements and calculations in low pressure methane/air flames [43]. This agreement for reactive intermediates is a stringent test of the chemical mechanism and gives us confidence that the major species are accurately predicted. To predict quenching rates in the flame, the model predictions of the concentrations of each of the species are multiplied by a temperaturedependent rate coefficient expression to calcu-

late the absolute amount of quenching by each collider as a function of height above the burner. The sums of these individual quenching contributions yield the total quenching rates, as a function of height above the burner, which are compared to experimental measurements. Given the excellent precision of our flame temperature measurements and model predictions of collider mole fractions, any discrepancy between calculated and measured quenching rates is assumed to be due, for the most part, to inaccuracies in the quenching cross sections. COMPARISON AND DISCUSSION NO NO quenching was measured and calculated as a function of height in the Standard flame. Agreement between measured and calculated quenching rates is excellent, as shown in Fig. 2. In this flame, the quenching rate of NO varies minimally as a function of flame height. At the base of the flame, O2 and H2O are the major quenchers of A-state NO, accounting for 63%

HYDROCARBON FLAME COLLISIONAL QUENCHING

Fig. 3. OH quenching rate (s21) in the Standard CH4/N2/O2 flame, as a function of height above the burner (cm). The circles are quenching data measured in the flame, and the solid line is the calculated quenching rate. Error limits are two s.

and 35% of the total quenching, respectively. O2 is consumed quickly and the O2 quenching rate drops near zero approximately 1.0 cm above the burner. Because the H2O concentration rises as the O2 concentration decreases, only a 30% drop in the total quenching rate is observed. In the burnt gases, the total quenching rate rises to 90% of its initial value as the concentration of H2O continues to increase with flame height. In the burnt gas region of the flame, H2O quenching accounts for 60% of the total NO quenching, with CO2, H, and OH quenching contributing the remainder. The excellent agreement between calculated and measured quenching rates strongly suggests that both the quenching cross sections and the model description of the flame are accurate. OH Figure 3 shows OH quenching as a function of height in the Standard flame. Major quenchers of OH in this flame are H2O, CH4, and H2. In this flame, the rate of OH quenching initially decreases as the temperature increases, due to decreasing gas density and CH4 concentration. In the burnt gas region, the quenching rate is nearly constant, because the temperature and

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Fig. 4. CH quenching rate (s21) in the Standard CH4/N2/O2 flame, as a function of height above the burner (cm). The data are plotted as circles. The two data sets were measured in independent experiments, conducted approximately one year apart. The solid line is the calculated quenching rate based on the assumption that the rate coefficient for CH quenching by H2O is constant with increasing temperature. The dashed line is the calculated quenching rate based on the assumption that the CH quenching cross section is constant with temperature. Error limits are two s.

gas composition in this region are also nearly constant. The agreement (to within 5%) between measured and calculated quenching rates is excellent. CH Measured and calculated CH quenching rates in the 5 CH4 /N2/O2 flames are shown in Figs. 4 and 5. With the exception of the lean flame, the calculated quenching rates are consistently larger than the measured rates; the discrepancy ranges from 5 to 27% and increases with increasing flame stoichiometry and height above the burner. N2 and H2O account for 30 –50 and 20 –25%, respectively, of the CH quenching. Each of the remaining species in the flame causes minimal CH quenching (,10 –15% of total) and no other single collider can be the cause of the discrepancy. The variations of quenching rate both with stoichiometry and with height above the burner provide clues regarding the differences between measured and calculated values. The rich

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Fig. 5. CH quenching rate (s21) in the Lean, Rich, Increased N2, and Decreased N2 CH4/N2/O2 flames, as a function of height above the burner (cm). Data plotted as circular symbols are quenching rates measured in the flame; data plotted as triangular symbols are calculated quenching rates. Error limits are two s.

flames, which display greater disagreement than the lean or standard flame, are 200 to 300K hotter (owing to more complete combustion, as discussed above), suggesting the calculated temperature dependence of one or more of the quenching rates may be too large. Further, the calculated quenching rate increases faster with burner height than the measured quenching. This also is due to too steep an increase in the calculated quenching rate with temperature. The N2 quenching rate coefficient increases rapidly with temperature, whereas that for H2O is much more modest (Table 3). Although H2O is at higher concentration in the richer flames and at large burner heights, its small rate coefficient temperature dependence, together with the density decrease with increasing temperature, combine to reduce its total quenching rate at large heights and under rich conditions. In fact, setting the calculated H2O quench contribution to zero reduces the overall quenching rate but does not alter the relative variation with height or stoichiometry. Thus, N2 is likely responsible for both of these trends. We decreased the temperature exponent for CH quenching by N2 from Heinrich and Stuhl’s value of T3.4 to T3 (and appropriately refit the preexponential factor). This produces a quenching rate coefficient that fits the variation with height of the lean and standard flame data exceptionally well; the T3 fit to the rich flame data shows less improvement. However, the discrepancy among all flames is reduced. At the same time, this formulation describes the Stuhl

group data (all below 1300 K) well within experimental error. The values at higher temperature are from flame measurements [19] and are only upper limits, but naturally had considerable influence on the fitted parameters of Ref. [17]. Unfortunately, the two colliders which contribute the most to CH quenching have the least well known cross sections. As discussed above, there is considerable uncertainty in the hightemperature CH quenching rate for N2, and no data at all for H2O above 415 K. High-temperature CH quenching data for these two quenchers would greatly improve the accuracy with which we are able to predict CH quenching in flames. CONCLUSION The quenching coefficients for collisions between electronically excited NO(A), OH(A), and a variety of flame constituents have been described by a simple parameterization of the published data between 300 and 2500 K. The predicted quenching of OH and NO in a slightly rich flame, using modeled flame composition and these quenching rate coefficients, is in remarkable agreement with measurement. The agreement for CH in the series of flames is between 5 and 27%, which can be considered quite good, but this is less satisfying because the calculated values are systematically high, with the exception of the lean flame. More precise high temperature measurements of CH(A)

HYDROCARBON FLAME COLLISIONAL QUENCHING quenching by N2 and H2O colliders would significantly improve our understanding of this discrepancy between measured and predicted quenching rates. For all three radicals, collisional quenching can be calculated with knowledge of gas temperature and major species corrections. This work was sponsored by the New Energy and Industrial Technology Development Organization (NEDO) as part of the New Industrial Furnaces of Higher Thermal Efficiency project via contract with Tokyo Gas Co., Ltd., and the Basic Research Group of the Gas Research Institute. The Natural Sciences and Engineering Research Council of Canada has provided a postdoctoral fellowship for Pamela A. Berg. The authors thank Dr. Phil Paul for his very helpful comments and for his careful checks of the tabulated cross section parameters.

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Received 21 May 1997; accepted 14 October 1997