Chemical Kinetic Modeling and 

Mass Spectrometric and LaserInduced Fluorescence 

Measurements of NH3/N2O 

Flames 

by Demetris T. Venizelos and Rosario C. Sausa 

ARL-TR-2782 July 2002 

Approved for public release; distribution is unlimited.
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Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-2782 July 2002 

Chemical Kinetic Modeling and 

Mass Spectrometric and LaserInduced Fluorescence 

Measurements of NH3/N2O 

Flames 

Demetris T. Venizelos 

National Research Council, ARL 

Rosario C. Sausa 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
ii

Abstract 

Experimental and chemical modeling studies of a 60-Torr, NH3/N2O/Ar flame 

are performed in order to provide further testing and refinements of a detailed 

chemical mechanism developed previously in our laboratory. This mechanism, 

which is denoted as VS, consists of 87 reactions and 20 species. Flame 

temperatures are measured with a coated thin-wire thermocouple and by 

rotational analyses of OH and NH laser-induced fluorescence (LIF) spectra. 

Species concentration profiles of NH3, N2O, N2, H2O, NO, O2, NH, O, and OH are 

recorded using molecular beam-mass spectrometry, LIF, or both. The 

experimental species concentrations are compared to those obtained with both 

equilibrium and PREMIX flame code calculations. The NH3/N2O mixture 

equilibrium calculations agree very well with both the PREMIX and measured 

postflame N2 and H2O concentrations but underpredict the postflame NO 

concentration and overpredict the O2 concentration. The PREMIX calculations 

predict very well the shapes of the experimental NH3, N2O, N2, H2O, NO, OH, 

and NH profiles throughout the flame but do not adequately predict the shape of 

the O-atom profile and overpredict the O2 concentration by ~60% at 16.25 mm, 

suggesting that the VS mechanism requires some refinement. A VS-modified 

mechanism, which provides better agreement with the experimental results, is 

proposed. In this mechanism, the rate constants of the following reactions are 

altered to the limit of their experimental or calculated uncertainties: (1) N2O 

+ H = OH + N2, (2) NH2 + O = HNO + H, (3) NH2 + OH = NH + H2O, (4) NH2 

+ H = H2 + NH, (5) NH3 + OH = NH2 + H2O, and (6) NO + H + M = HNO + M. 
iii

Acknowledgments 

This work was supported by the U.S. Army Research Laboratory (ARL) Mission 

Program on Combustion and the National Research Council ARL Postdoctoral 

Fellowship Program. 
iv

INTENTIONALLY LEFT BLANK. 
v

Contents 

Acknowledgments iii

List of Figures vii

List of Tables ix

1. Introduction 1

2. Experimental/Modeling 2

3. Results/Discussion 3

4. Conclusion 12

5. References 13

Distribution List 15 

Report Documentation Page 17 
vi

INTENTIONALLY LEFT BLANK.
vii

List of Figures 

Figure 1. Experimental (....) and fitted (-) NH spectra of a 60-Torr, 

NH3/N2O/Ar flame in the region of 302.2–302.7 nm. The 

experimental data is recorded at 5.5 mm above the burner surface. ................. 4 

Figure 2. Temperature profiles of a 60-Torr, NH3/N2O/Ar flame 

obtained by OH LIF (o), NH LIF («) and a thin-wire thermocouple (C). 

The thermocouple temperatures are corrected for radiation losses 

using the LIF temperatures. ..................................................................................... 4 

Figure 3. Pathway diagram of a 60-Torr, NH3/N2O/Ar flame generated 

by integrating the net rate fluxes of the individual reactions from the 

burner surface to 30 mm (100 = 2.716 H 10-5 mol/cm2-s). For NNH 

destruction, θ + M indicates the total predissociation and collisionassisted decomposition............................................................................................. 8 

Figure 4. Normalized experimental and modeled OH and NH profiles of 

the 60-Torr, NH3/N2O flame. The OH mole fractions at 15 mm are 6.0 

H 10-3 and 3.9 H 10-3 for VS and VS-modified calculations, respectively, 

and those of NH at the peak value are 5.7 H 10-4 and 3.7 H 10-4 for the 

VS and VS-modified calculations, respectively..................................................... 9 

Figure 5. Experimental and modeled NO profiles of the 60-Torr, NH3/ 

N2O/Ar flame. The experimental profiles are obtained by MB-MS (C) 

and LIF (B)................................................................................................................. 10 

Figure 6. Experimental and modeled O2 profiles of the 60-Torr, 

NH3/N2O/Ar flame................................................................................................ 11 
viii

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ix

List of Tables 

Table 1. Sensitivity coefficients of O2 for the NH3/N2O/Ar and 

H2N2O/Ar flames using the VS mechanism at 15.0 mm above the 

burner surface. ........................................................................................................... 6 

Table 2. Rate expressions of reactions altered for the VS-modified 

mechanism. The rate coefficients are in the form K = ATBe (-E/RT), 

where A and E have units of cm/mol/s/K and cal/mol, respectively............. 7 
x

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

The H2/N2O chemical system has been the subject of numerous studies because 

it is fairly simple and because it has important implications in understanding 

NOx pollutant formation and nitramine propellant combustion and 

decomposition [1–4]. The elementary reactions of the system form subsets of 

larger mechanisms needed to understand the nitrogen chemistry of more 

complex combustion systems. Recently, we reported on a combined 

experimental and modeling flame structure study of both neat and NH3-doped 

(4%) H2/N2O/Ar flames [1]. The species mole fractions were measured by 

molecular beam-mass spectrometry (MB-MS) and laser-induced fluorescence 

(LIF) and then compared to those calculated by PREMIX using both the 

Sausa, Singh, Lemire, and Anderson (SSLA) [2] and Gas Research Institute (GRI) 

2.11 mechanisms [5]. The SSLA mechanism was developed in-house from a 

critical literature review, whereas the GRI 2.11 mechanism was developed by a 

consortium of scientists for GRI for natural gas ignition and flame. Overall, the 

calculations using both mechanisms adequately predicted the profiles of the 

major species for both the neat and NH3-doped flame. However, both 

mechanisms failed to predict the postflame O2 concentration in the neat flame, 

the drop in the O2 concentration with the addition of NH3, and the NH3 decay in 

the doped flame. A modified SSLA mechanism, which provided better 

agreement with our experimental data, was proposed. This mechanism is 

denoted as “VS” in this report, and a list of the chemical reactions and their 

corresponding rate expressions is presented in Venizelos and Sausa [1] and Sausa 

et al. [2].

In this report, we provide a combined experimental and chemical modeling 

study of a stoichiometric NH3/N2O/Ar flame in order to provide further testing 

and refinements of the VS chemical mechanism. Species concentration profiles 

are recorded by MB-MS, LIF, or both, and flame temperatures are measured by 

thin-wire thermometry and by OH and NH LIF. The experimental mole 

fractions are compared to those predicted by both equilibrium calculations and 

PREMIX calculations using the VS chemical mechanism. The PREMIX results 

adequately predict all of the measured species profiles except for the O-atom and 

O2 profiles. A refined mechanism, denoted VS-modified, which better predicts 

our experimental results, is proposed. In this mechanism, six reactions are 

identified that may need new rate-constant measurements under conditions of 

temperature and pressure that are similar to our flame conditions. 
2

2. Experimental/Modeling 

The low-pressure burner assembly and the procedures for the LIF and MB-MS 

measurements are described elsewhere [1, 2]. Briefly, a 60-Torr, NH3/N2O/Ar 

flame is supported on a water-cooled, stainless steel, McKenna flat burner, which 

is mounted in a cylindrical vacuum chamber. Translation of the burner provides 

full optical access to the flame. The gas flow rates for NH3, N2O, and Ar are 1.03, 

1.59, and 0.54 slm, respectively. The experimental apparatus is configured so 

that the LIF and thermocouple measurements are recorded exactly where the 

MB-MS probe was sampling, allowing for direct comparison. 

Flame temperatures are measured with a coated, thin-wire Pt-Rh (6%)/Pt-Rh 

(30%) thermocouple and by NH and OH LIF. Measuring the flame temperatures 

as a function of height above the burner surface yields a thermocouple 

temperature profile, which is then corrected for thermocouple radiation losses by 

OH and NH LIF. The LIF temperatures are determined by a Boltzmann 

rotational population distribution analysis of the recorded spectra using a 

computer program with interactive graphics [1]. All of the temperature data are 

fitted with a sigmoid-type function and used as input in the PREMIX 

calculations. The overall uncertainty in the NH and OH LIF temperature 

measurements is ±2 and ±5% (2SD), respectively. 

The MB-MS species profiles are quantified using a calibration procedure 

previously described [1, 2]. As it is difficult to reliably introduce a calibrated 

amount of H2O vapor in the burner system, the H2O mole fraction is determined 

by balancing the N/O ratio in the premixed inlet gases to the ratio of N/O in the 

burnt gases. The resulting expression for the mole fraction ratio of H2O to Ar in 

the postflame region is 

XH2O/XAr = 6/8 (XN2

/XAr) ! 5/8 (XNO/XAr) !2 (XO2/XAr), (1) 

where the values of XN2

/XAr, XNO/XAr, and XO2/XAr are measured experimentally. It is 

assumed that the concentration of the other species in the burnt gases is much less 

than the concentrations of the species shown in equation 1; thus, they do not 

contribute to the XH2O/XAr ratio. NASA-Lewis equilibrium calculations of NH3

and N2O mixtures [6] and experimental MB-MS species observations or lack 

thereof support this assumption. The uncertainty in the MB-MS measurements is 

10%. 

We model the flames with the Sandia National Laboratories (SNL) flame code 

PREMIX (ver. 2.55) [7] employing the CHEMKIN-II mechanism interpreter (ver. 

3.6) and chemical kinetics libraries (ver. 4.9) [8]. The SNL thermodynamic [9] 

and transport property databases [10], our measured temperature and flow rates, 
3

and our VS chemical mechanism are used as input. Both the thermal diffusion 

and multicomponent transport package options are included in the calculations. 

The VS chemical mechanism consists of 87 reactions and 20 species [1, 2]. The 

bulk of the reactions are obtained from the benchmark review of nitrogen 

chemistry in combustion by Miller and Bowman [11]. The mechanism also 

includes the results of Hanson and coworkers for many important reactions 

involving NHx species that were poorly quantified [12, 13] and some reactions 

from a nitrogen chemistry review by Bozzelli and Dean, which relate to their 

recommended NO formation mechanism [14]. An interactive post-processing 

code written in-house is used for the rate and sensitivity analyses [15]. 

3. Results/Discussion 

The flame temperatures are measured by OH and NH LIF. Figure 1 shows the 

experimental and fitted NH LIF spectra in the region of 302.2–302.7 nm. The 

experimental spectra are recorded in a linear laser energy regime and normalized 

with respect to the laser energy. Quenching effects and laser intensity 

fluctuation errors are minimized by using a short 9-ns gated integrator and 

10-shot averaging. A multiparameter computer program based on a Boltzmann 

rotational distribution analyses fits the observed data [1]. The program utilizes 

one-photon line strengths and rotational energy levels of NH obtained from 

Brazier et al. [16], and OH from Dieke and Crosswhite [17] and Dimpfl and 

Kinsey [18]. Parameters include laser line shape, temperature, and absolute and 

relative frequency values for the data. Doppler and collisional broadening is also 

accounted by the fitting routine. The standard deviation of each parameter, as 

statistically determined from the fit, is obtained from the computer variancecovariance matrix once convergence is achieved. The best fit to the observed 

data using a Gaussian function laser line shape yields a temperature of 2012 K 

± 50 K (2SD) at 5.5 mm above the burner surface. 

Presented in Figure 2 are the flame temperatures measured by OH and NH LIF 

and a coated thin-wire thermocouple. The thermocouple temperatures are 

corrected for radiation losses using the OH LIF temperatures in the postflame 

region and NH LIF temperatures near the burner surface and in the preheat 

flame region, similar to a procedure used for a H2/N2O/Ar flame [1]. The 

corrected thermocouple temperature profiles are then fitted with a sigmoid-type 

function and used in the PREMIX calculations. As shown in Figure 2, the 

temperature profile peaks at ~2200 K near 7 mm above the burner surface. 
4

WAVELENGTH (nm)

302.2 302.3 302.4 302.5 302.6 302.7

I I I I I I I

I I I I I I

I I I I I I I

I I I I I I

I I I I I I I

I I I I I I

15 13 11

17 19 21 15 13 11 9

16 18 20 14 12 10 8

15 17 19

21

R (J)

1

R (J)

2

R (J)

3

Figure 1. Experimental (....) and fitted (-) NH spectra of a 60-Torr, NH3/N2O/Ar flame in 

the region of 302.2–302.7 nm. The experimental data is recorded at 5.5 mm 

above the burner surface.

HEIGHT ABOVE BURNER (mm)

0 5 10 15 20 25 30

TEMPERATURE (K)

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Figure 2. Temperature profiles of a 60-Torr, NH3/N2O/Ar flame obtained by OH LIF (o), 

NH LIF («), and a thin-wire thermocouple (C). The thermocouple temperatures 

are corrected for radiation losses using the LIF temperatures. 
5

The PREMIX calculations using the VS mechanism predict very well the NH3, 

N2O, N2, H2O, and NO mole fractions and the shapes of the OH and NH species 

throughout the flame. However, they overpredict the O2 mole fraction by about 

a factor of 2 in the burned region of the flame and do not adequately predict the 

O-atom profile (these results will be discussed in more detail later). These 

discrepancies are not due to temperature effects because a 5% decrease in the 

inputted temperature profile, the maximum uncertainty in our temperature 

measurements, decreases the O2 postflame mole fraction by <3% and does not 

significantly change the shape of the O-atom profile. Similar calculations 

performed on a neat and NH3-doped H2/N2O/Ar flame predict the O2 mole 

fraction in the burned region of both flames within a few percent [1]. They also 

predict that the O2 postflame mole fraction in the neat flame decreases by 84% 

when the flame is doped with 4% NH3, in excellent agreement with the observed 

90% decrease [1]. This suggests that the discrepancy between the observed and 

calculated O-atom and O2 profiles in the NH3/N2O/Ar flame is due to the model 

rather than the experiment and that the VS mechanism requires some 

refinement. 

Sensitivity analyses of the PREMIX results reveal which reactions in the VS 

mechanism need to be altered to better predict the experimental species profiles. 

The normalized sensitivity coefficients for O2 at 15.0 mm above the burner 

surface for the NH3/N2O/Ar flame are presented in Table 1. All of the 

sensitivity coefficients are normalized logarithmically using the maximum O2

mole fraction and then scaled to the NH + OH = HNO + H reaction (R1). Table 1 

reveals that O2 has a very strong negative sensitivity to reactions R1, R3, R4, and 

R6 and a positive sensitivity to reactions R2 and R5. Negative sensitivity values 

indicate that an increase in the reaction rate results in a decrease in the O2 mole 

fraction, whereas positive values indicate that an increase in the reaction rate 

results in an increase in the O2 mole fraction. Similar analyses for the 

H2/N2O/Ar flame reveal that O2 is also very sensitive to all of these reactions 

except reaction R3 (see Table 1). Also, reaction R4 shows a positive sensitivity 

coefficient value, different from that in the NH3/N2O/Ar flame. These 

sensitivity analyses suggest that an increase in the rate expression of both R3 and 

R4 will decrease the postflame O2 mole fraction in the NH3/N2O/Ar flame and, 

overall, slightly increase the O2 mole fraction in the H2/N2O/Ar flame. This is 

consistent with our present experimental results and those previously published 

[1]. An increase in the rate of R4 also decreases the NO postflame concentration, 

consistent with our present experimental results (sensitivity results for NO are 

not shown). However, a large increase in this reaction is not warranted because 

it would reduce the NO mole fraction more than what we observe 

experimentally. Thus, a 10% increase is proposed. This is within the 

experimental limits of the rate expression reported by Marshall et al. [19]. A very 

large increase in the rate of R3 increases the postflame NO mole fraction, 

contrary to what is observed experimentally. Thus, its rate expression is only 

increased by 50%, in line with our experimental results, the experimental
6

Table 1. Sensitivity coefficients of O2 for the NH3/N2O/Ar and H2N2O/Ar flames using 

the VS mechanism at 15.0 mm above the burner surface.

Reaction NH3/N2O/Ar H2/N2O/Ar 

 Sign % Sign % 

R1 NH + OH = HNO + H (-) 100.0 (-) 100.0 

R2 N2O + M = N2 + O + M (+) 77.7 (+) 266.4 

R3 NH2 + O = HNO + H (-) 76.0 (-) 4.0 

R4 N2O + H = OH + N2 (-) 66.0 (+) 26.7 

R5 NO + N = N2 + O (+) 58.6 (+) 85.2 

R6 NO + H = N +OH (-) 49.3 (-) 78.3 

R7 NH2 + OH = NH + H2O (+) 37.9 <3.0 

R8 NNH + O = NO +NH (+) 36.8 (+) 42.4 

R9 NH + NO = N2 = OH (+) 34.8 (+) 33.6 

R10 NH + O = NO + H (-) 30.4 (-) 39.8 

R11 NH2 + NO = NNH+OH (+) 27.8 < 3.0 

R12 NH3 + OH = NH2 + H2O (+) 26.1 < 3.0 

R13 NH2 + NO = N2 + H2O (+) 20.1 < 3.0 

R14 NH2 + H = NH + H2 (+) 19.2 < 3.0 

R15 NH + NO = N2O + H (-) 16.2 (-) 73.5 

R16 H + O2 = OH + O (+) 14.1 (+) 18.7 

R17 NO + H + M = HNO + M (+) 12.4 (+) 3.4 

R18 N2O + O = NO + NO (-) 10.8 (-) 24.2 

R19 OH + OH = H2O + O (+) 6.9 < 3.0 

R20 NH3 + H = NH2 + H2 (+) 6.1 < 3.0 

R21 HNO + OH = NO + H2O (-) 6.1 < 3.0 

R22 H2 + OH = H2O + H (-) 6.0 (-) 21.2 

R23 HNO + H = NO + H2 (-) 5.3 (-) 6.0 

Notes: The sensitivity coefficients are normalized logarithmically using 

the maximum O2 mole fraction and scaled to the NH + OH 

= HNO + H reaction. 

HN3/N2O/Ar flame: 100 units = 0.20239. 

H2/N2O/Ar flame: 100 units = 0.5508. 

uncertainty in the reported rate expression, and the uncertainty in the rate 

expression of R4. Reactions R4 and R3, along with their original and altered rate 

expressions, are presented in Table 2 as part of a refined VS-mechanism, denoted 

as VS-modified. 

O2 has an appreciable positive sensitivity to several reactions in an NH3/

N2O/Ar flame which do not influence the postflame O2 concentration much in 

an H2 /N2 O/Ar flame. These reactions, which are listed in Table 1, are R7, R11, 

R12, R13, R14, and R17. Altering these reactions does not considerably influence 

the postflame NO mole fraction for both H2/N2O/Ar and NH3/N2O/Ar flames; 

thus, an increase in their rate expression would better predict our experimental 

results. Only reactions R7, R12, R14, and R17 are recommended for further study 

because reactions R11 and R3 are well established over a wide range of
7

Table 2. Rate expressions of reactions altered for the VS-modified mechanism. The rate 

coefficients are in the form K = ATBe (-E/RT), where A and E have units of 

cm/mol/s/K and cal/mol, respectively.

 VS VS-Modified 

Reaction A B E A B E Reference 

R4 N2O + H = OH + N2 2.53E10 0.0 4550.0 2.78E10 0.0 4550.0 [19] 

 2.23E14 0.0 16750.0 2.45E14 0.0 16750.0 [19] 

R3 NH2 + O = HNO + H 4.60E13 0.0 0.0 6.90E13 0.0 0.0 [20] 

R7 NH2 + OH = NH 

+ H2O 

4.00E06 2.0 1000.0 2.00E06 2.0 1000.0 [11] 

R12 NH3 + OH = NH2

+ H2O 

2.04E06 2.04 566.0 1.04E06 2.04 566.0 [11] 

R14 NH2 + H = NH + H2 4.00E13 0.0 3650.0 1.33E13 0.0 3650.0 [13] 

R17 NO + H + M = HNO 

+ M 

9.00E19 !1.30 735.0 3.00E19 !1.3

0 

735.0 [21] 

temperatures and their branching ratio is well characterized [22, 23]. Reactions 

R7, R12, R14, and R17, together with their original and altered rate expressions, 

are also presented in Table 2 as part of the VS-modified mechanism. 

PREMIX calculations using the VS-modified mechanism are also performed for 

the NH3/N2O/Ar flame in order to test the proposed mechanism. The measured 

temperature profile is used as input into the flame code; however, it is decreased 

by 5%, the experimental uncertainty. The resulting profiles are then compared to 

those obtained experimentally and to those obtained from PREMIX using the VS 

mechanism and measured temperature profile as input. The VS-modified 

calculations predict very well the NH3, N2O, H2O, and N2 species mole fraction 

throughout the flame, as do the VS calculations. At 16.25 mm above the burner 

surface, the calculations predict within 1% the observed H2O/Ar and N2/Ar 

values of 3.83 and 2.76, respectively. For comparison, NASA-Lewis equilibrium 

calculations also predict the major species mole fraction within 1% in the burned 

gas region of the flame. 

Figure 3 is a pathway diagram of the VS-modified results showing the nitrogen 

chemistry occurring in the NH3/N2O/Ar flame. The numbers in parentheses are 

the relative integrated net reaction rates from 0 to 30.0 mm for the disappearance 

of the indicated reactant by the various reactions. These integrated rates are 

normalized to 100 (2.55 H l0-5 mol/cm2-s) for the NH3 + OH = NH2 + H2O 

reaction. The N2O reactant is consumed primarily by reactions with H and M to 

form N2, whereas its companion reactant, NH3, is consumed by reactions 

involving the OH, H, and O species to form NH2. To some extent, NH3 is 

regenerated mainly from NH2 species reacting with themselves. Although the 

reactions involving N2O with H and M are the major routes for N2 formation, 

~32% of N2 is formed from the NNH, NH2, NH, NO, and N species. 
8

NH

N O

NH NH N

HNO

N

NO

N H

NNH

Fuel

Oxidizer

N(5),

NH2(6)

H(0.5)

H(0.5),O(1)

HNO(6)

O(2)

OH(2)

H(100),

NO(8), N(2)

M

NH2(2)

(62),O(1)

OH(7)

NH2(6)

OH(9)

O(23)

OH(5),

H(3)

O(1)

NO(5), NH2(2)

N

H(4), N

O(6)

NO (3)

NH(14)

NH2(5)

NH2(14)

M(14)

NH2(2)

ON

H(1)

(8), N2H2(2)

M(7)

(22)

M(4),H(9)

O(3)

OH(1)

OH(22)

H(6),O(2)

NH2(31)

NH2(31)

HNO(6),N2H2(2)

OH(100)

H(19),O(11)

NH(6)

N, H

2(8)

NH2(8), NH(3)

☺

2

2

3 2

2 2

Figure 3. Pathway diagram of a 60-Torr, NH3/N2O/Ar flame generated by integrating 

the net rate fluxes of the individual reactions from the burner surface to 

30 mm (100 = 2.716 H 10-5 mol/cm2-s). For NNH destruction, θ + M indicates 

the total predissociation and collision-assisted decomposition. 

Figure 4 shows the experimental and modeled NH and OH profiles. The 

experimental LIF concentrations are converted to mole fractions using the 

measured flame temperatures and then normalized to the calculated mole 

fractions for comparison purposes. The OH profile is predicted extremely well 

in the region near the burner surface and the postflame region by the PREMIX 

calculations using both the VS and VS-modified mechanisms. The NH LIF 

profile reaches its peak value at 3.75 mm above the burner surface, compared to 

the VS and VS-modified calculation profiles which peak at 4 and 4.5 mm, 

respectively. A narrower width in the NH profile is also observed 

experimentally (full width at half maximum [FWHM] = 2.5 mm). In contrast, the 

VS and VS-modified calculations predict a width (FWHM) of 3 and 4 mm, 

respectively. 

Rate analysis of the VS-modified results reveals that NH is formed 

predominantly from NH2 reacting with OH, H, and NH2, and that it is consumed 

either directly to N2, N2H2, NNH, and HNO or indirectly to N by means of the 

NH intermediate (see Figure 3). A similar analysis for OH reveals that over 88% 

of this species is formed from the N2O + H = OH + N2 and OH + OH = H2O 

+ O reactions, and that more than 59% is consumed from the NH3 + OH = NH2 

+ H2O reaction. The reactions H2 + OH = H2O + H, NH2 + OH = H2O + NH, 

HNO + OH = NO + H2O, and NH + OH = HNO + H account for ~40% of its 

consumption. 
9

HEIGHT ABOVE BURNER (mm)

0 5 10 15 20

Xi (Rel.)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

NH LIF

OH LIF

VS

VS Modified

Figure 4. Normalized experimental and modeled OH and NH profiles of the 60-Torr, 

NH3/N2O flame. The OH mole fractions at 15 mm are 6.0 H 10-3 and 3.9 H 10-3

for VS and VS-modified calculations, respectively, and those of NH at the 

peak value are 5.7 H 10-4 and 3.7 H 10-4 for the VS and VS-modified 

calculations, respectively. 

Figure 5 shows the PREMIX calculated and experimental MB-MS and LIF 

profiles of NO. The NO LIF species concentration profile is converted to a 

NO/Ar mole fraction ratio using the measured temperatures and the calculated 

Ar mole fraction. Its peak concentration is normalized to the calibrated MB-MS 

peak value for comparison purposes. Figure 4 shows that the LIF and MB-MS 

profiles have the same shape. Also, the VS and VS-modified calculations predict 

very well the measured NO/Ar profiles throughout the flame, overestimating 

the value of the NO/Ar ratio at 16.25 mm only by about 7 and 10%, respectively. 

These values are within the uncertainty of the MB-MS measurements. NASALewis equilibrium calculations underpredict the NO/Ar by a factor of 10, 

indicating that NO is not in equilibrium and is preventing full energy release of 

the system. The pathway diagram presented in Figure 3 shows that NO is 

formed predominantly from the intermediates HNO, NH2, N, and NH. The N2O 

reactant is not an important source of NO in this flame as it is in the neat and 

NH3-doped H2/NO/Ar flames [1, 2]. NO is converted to N2 directly by the N, 

NH, and NH2 intermediates or indirectly by the NNH intermediate. 
10

HEIGHT ABOVE BURNER (mm)

0 5 10 15 20

XNO / XAr

0.00

0.02

0.04

0.06

0.08

0.10

0.12

VS Modified

VS

MB/MS

LIF

Figure 5. Experimental and modeled NO profiles of the 60-Torr, NH3/N2O/Ar flame. 

The experimental profiles are obtained by MB-MS (C) and LIF (B). 

A comparison between the calculated O-atom profiles with the LIF measured 

profile (profiles not shown) reveals that both VS and VS-modified profiles are 

shifted ~2 mm away from the burner surface compared to the experimental one, 

suggesting that the calculations predict a slower chemistry than what is observed 

experimentally. Also, the experimental profile peaks at a height near 5 mm 

above the burner, then decays and slowly increases in the 

postflame region. In contrast, the VS calculations show that the O-atom profile 

reaches a plateau in the postflame starting at a height near 8 mm above the 

burner surface, compared to the VS-modified calculations which predict a decay 

of 15%, similar to that observed experimentally. The VS-modified rate 

calculations show that the peak rate of production at ~7 mm above the burner 

surface corresponds to the maximum temperature rise. The net integrated rate of 

O-atom production from 0 to 30 mm is 3.26 H 10-8 mol/(cm2-s). The reaction N2O 

+ M = NO + O + M accounts for 87% of the O-atom production flux, whereas NO 

+ N = N2 + O and NNH + O = NO + NH account for 7% and 5%, respectively, of 

the O-atom production flux. The O-atoms are consumed primarily by reactions 

NH2 + O = HNO + H, OH + OH = H2O + O, NH3 + O = NH2 + OH, and H2 + O 

= H + OH reactions, with the first two reactions accounting for 62% of its 

consumption. 
11

Figure 6 shows the calculated and measured O2/Ar profiles. The shape of the 

O2/Ar profile is predicted rather well by both the VS and VS-modified PREMIX 

calculations. Both calculations predict a flat region with a slight increasing slope 

in the O2/Ar profile in the postflame region, a trend similar to what is observed 

experimentally. The VS calculations, however, overpredict the value of the 

O2/Ar ratio at 16.25 mm by about 61%. In contrast, the VS-modified calculations 

overpredict the O2/Ar ratio by only 13%, just within our experimental 

uncertainty. VS-modified rate analysis of O2 reveals that ~60% of this species is 

formed from the reaction H + O2 = OH + O and ~39.6% from the reaction N2O 

+ O = N2 + O2. O2 is consumed predominantly from the reactions NH + O2 

= HNO + O, NO + O = N + O2, NH2 + O2 = HNO + OH, O2 + H + M = HO2 + M, 

and NH + O2 = NO + OH, with the first two reactions accounting for ~80% of its 

consumption. The overall rate of O2 production is ~5.4 times that of its 

consumption. 

HEIGHT ABOVE BURNER (mm)

0 5 10 15 20

X

O2 / XAr

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

VS Modified

VS

O2/Ar MB/MS

Figure 6. Experimental and modeled O2 profiles of the 60-Torr, NH3/N2O/Ar flame. 

The PREMIX calculations are also performed using the GRI 3.0 chemical 

mechanism, with the supplied transport and thermodynamics databases as input 

[5]. The model calculations predict the observed NH3, N2O, H2O, and N2 species 

mole fractions within 1% at 15 mm, similar to the PREMIX calculations using 

both the VS and VS-modified mechanisms. However, they overpredict the 

observed NO mole fraction by approximately a factor of 2 at 15 mm, and they 

underpredict the observed O2 mole fraction by ~17% at its peak and by ~78% at 
12

15 mm. The model calculations also underpredict the O-atom mole fraction by 

~50% at 15 mm, when both calculated and experimental profiles are normalized 

at ~6 mm. It should be noted, however, that the GRI mechanism is optimized for 

natural gas ignition. 

4. Conclusion

A combined experimental and detailed modeling study of a NH3/N2O flame has 

been performed. The species concentration profiles of NH3, N2O, N2, H2O, NH, 

OH, O, NO, and O2 have been measured by MB-MS, LIF, or both, and the flame 

temperatures have been measured by both thin-wire thermometry and OH and 

NH LIF. The experimental species mole fractions were compared with both 

NASA-Lewis equilibrium calculations and PREMIX calculations using the VS 

mechanism. Both calculations predicted the measured major species 

concentrations in the postflame within 1%. However, the equilibrium 

calculations did not adequately predict the postflame concentrations of NO and 

O2, indicating that they were not chemically equilibrated. The PREMIX 

calculations adequately predicted the measured NO and OH profiles throughout 

the flame, but they overpredicted the O2 mole fraction ~60% in the postflame and 

did not adequately predict the shape of the O-atom profile. The experimental 

results were better predicted using a modified VS mechanism in which the rate 

expressions of the following reactions were altered to the limit of their 

experimental or calculated uncertainties: (1) N2O + H = OH + N2, (2) NH2 + O 

= HNO + H, (3) NH2 + OH = NH + H2O, (4) NH2 + H = H2 + NH, (5) NH3 + OH 

= NH2 + H2O, and (6) NO + H + M = HNO + M. Further experimental and 

modeling studies on the rate expressions of these reactions are thus 

recommended.
13

5. References 

1. Venizelos, D. T., and R. C. Sausa. Combustion and Flame. Vol. 115, pp. 313–

326, 1998, and references therein. 

2. Sausa, R. C., G. Singh, G. W. Lemire, and W. R. Anderson (SSLA). TwentySixth Symposium (International) on Combustion. The Combustion Institute, 

Pittsburgh, PA, pp. 1043, 1996, and references therein. 

3. Vandooren, J., P. J. Van Tiggelen, and J.-F. Pauwels. Combustion and Flame.

Vol. 109, pp. 647–668, 1997, and references therein. 

4. Allen, M. T., R. A. Yetter, and F. L. Dryer. Combustion and Flame. Vol. 112, 

no. 3, pp. 302–311, 1998, and references therein. 

5. Bowman, C. T., R. K. Hanson, D. F. Davidson, W. C. Gardiner, 

V. Lissianski, G. P. Smith, D. M. Golden, M. Frenklach, and M. Goldenberg. 

Gas Research Institute, Chicago, IL, <http://euler.me.berkeley.edu/gri_mech/>. 

6. Svehla, R. A., and B. J. McBride. NASA-TN-D-7057, NASA Glenn (formerly 

Lewis), Cleveland, OH, January 1973. 

7. Kee, R. J., J. F. Grcar, M. D. Smooke, and J. A. Miller. SAND-85-8240, Sandia 

National Laboratories, Livermore, CA, December 1985; reprinted March 

1991. 

8. Kee, R. J., F. M. Rupley, and J. A. Miller. SAND-89-8009, Sandia National 

Laboratories, Livermore, CA, September 1989. 

9. Kee, R. J., F. M. Rupley, and J. A. Miller. SAND-87-8215, Sandia National 

Laboratories, Livermore, CA, April 1987. 

10. Kee, R. J., G. Dixon-Lewis, J. Warnatz, M. E. Coltrin, and J. A. Miller. SAND87–8246, Sandia National Laboratories, Livermore, CA, November 1988. 

11. Miller, J. A., and C. T. Bowman. Progress in Energy and Combustion Science. 

Vol. 15, pp. 287–338, 1989. 

12. Mertens, J. D., A. Y. Chang, R. K. Hanson, and C. T. Bowman. International 

Journal of Chemical Kinetics. Vol. 23, pp. 173–196, 1991. 

13. Davidson, D. F., K. K. Kohse-Höinghaus, A. Y. Chang, and R. K. Hanson. 

International Journal of Chemical Kinetics. Vol. 22, pp. 513–535, 1990, and 

references therein. 

14. Bozzelli, J. W., and A. M. Dean. International Journal of Chemical Kinetics. 

Vol. 27, pp. 1097–1109, 1995. 
14

15. Anderson, W. R., S. W. Haga, J. F. Nuzman, and A. Kotlar. U.S. Army 

Research Laboratory SREAD and PREAD computer codes, U.S. Army 

Research Laboratory, Aberdeen Proving Ground, MD. 

16. Brazier, C. R., R. S. Ram, and P. F. Bernath. Journal of Molecular Spectroscopy.

Vol. 120, no. 2, pp. 381–402, 1986. 

17. Dieke, G. H., and H. M. Crosswhite. Journal of Quantitative Spectroscopy 

Radioactive Transfer. Vol. 2, pp. 97–199, 1962. 

18. Dimpfl, W. L., and J. L. Kinsey. Journal of Quantitative Spectroscopy Radioactive 

Transfer. Vol. 21, no. 3, pp. 233–241, 1979. 

19. Marshall, P., T. Ko, and A. Fontijn. Journal of Physical Chemistry. Vol. 93, 

pp. 1922–1927, 1989. 

20. Bozzelli, J. W., and A. M. Dean. “Combustion Chemistry of Nitrogen.” 

Combustion Chemistry, 2nd ed. Edited by W. C. Gardiner, New York: Springer 

Verlag, 1979. 

21. Tsang, W., and J. T. Heron. Journal of Physical Chemical Reference Data. Vol. 

20, pp. 609–663, 1991. 

22. Votsmeier, M., S. Song, R. K. Hanson, and C. T. Bowman. Journal of Physical 

Chemistry A. Vol. 103, pp. 1566–1571, 1999. 

23. Park, J., and M. C. Lin. Journal of Physical Chemistry A. Vol. 103, pp. 8906, 

8907, 1999. 
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* Postdoctural Research Associate, National Research Council, U.S. Army Research Laboratory,

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Demetris T. Venizelos* and Rosario C. Sausa

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17

Experimental and chemical modeling studies of a 60-Torr, NH3/N2O/Ar flame are performed in order to provide further 

testing and refinements of a detailed chemical mechanism developed previously in our laboratory. This mechanism, 

which is denoted as VS, consists of 87 reactions and 20 species. Flame temperatures are measured with a coated thin-wire 

thermocouple and by rotational analyses of OH and NH laser-induced fluorescence (LIF) spectra. Species concentration 

profiles of NH3, N2O, N2, H2O, NO, O2, NH, O, and OH are recorded using molecular beam-mass spectrometry, LIF, or 

both. The experimental species concentrations are compared to those obtained with both equilibrium and PREMIX flame 

code calculations. The NH3/N2O mixture equilibrium calculations agree very well with both the PREMIX and measured 

postflame N2 and H2O concentrations but underpredict the postflame NO concentration and overpredict the O2

concentration. The PREMIX calculations predict very well the shapes of the experimental NH3, N2O, N2, H2O, NO, OH, 

and NH profiles throughout the flame but do not adequately predict the shape of the O-atom profile and overpredict the O2

concentration by ~60% at 16.25 mm, suggesting that the VS mechanism requires some refinement. A VS-modified 

mechanism, which provides better agreement with the experimental results, is proposed. In this mechanism, the rate 

constants of the following reactions are altered to the limit of their experimental or calculated uncertainties: (1) N2O 

+ H = OH + N2, (2) NH2 + O = HNO + H, (3) NH2 + OH = NH + H2O, (4) NH2 + H = H2 + NH, (5) NH3 + OH = NH2 + 

H2O, and (6) NO + H + M = HNO + M. 
18

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