Structure of premixed ammonia + air flames at atmospheric pressure: Laser diagnostics and kinetic modeling

Combustion and Flame 163 (2016) 370–381

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Structure of premixed ammonia + air flames at atmospheric pressure: Laser diagnostics and kinetic modeling Christian Brackmann, Vladimir A. Alekseev∗, Bo Zhou, Emil Nordström, Per-Erik Bengtsson, Zhongshan Li, Marcus Aldén, Alexander A. Konnov Division of Combustion Physics, Department of Physics, Faculty of Engineering, Lund University, P.O. Box 118, SE-221 00, Lund, Sweden

a r t i c l e

i n f o

Article history: Received 3 July 2015 Revised 9 October 2015 Accepted 12 October 2015 Available online 3 November 2015 Keywords: Ammonia Premixed combustion Laminar flames Laser Diagnostics

a b s t r a c t The structure of premixed ammonia + air flames, burning at atmospheric pressure under strain-stabilized conditions on a porous-plug burner, has been investigated using laser-diagnostic methods. Profiles of OH, NH, and NO were acquired by laser-induced fluorescence (LIF) and quantitative concentrations of OH and NO were retrieved using a concept for calibration versus absorption utilizing the LIF-signal itself, whereas NH concentrations were evaluated employing a saturated fluorescence signal. In addition, temperatures and relative oxygen concentrations were measured by rotational Coherent Anti-stokes Raman Spectroscopy (CARS). The new experimental data for flames with equivalence ratios of 0.9, 1.0, and 1.2 were used to validate and rank the performance of four contemporary detailed kinetic models. Simulations were carried out using experimental temperature profiles as well as by solving the energy equation. Two models of the same origin, developed by Mendiara and Glarborg (2009) and by Klippenstein et al. (2011), in most cases showed good agreement in terms of radical concentrations, however, the model of Mendiara and Glarborg had better prediction of temperatures and flame front positions. The model by Shmakov et al. (2010) had comparable performance concerning radical species, but significant discrepancies appeared in the prediction of flame front positions. The model of Duynslaegher et al. (2012), in addition to the flame front positions, deviated from experiments or other models in terms of NH and NO concentrations. A sensitivity analysis for the Mendiara– Glarborg mechanism indicated that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to experimental uncertainties. © 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Ammonia (NH3 ) flame structure has been the subject of many studies since combustion chemistry of ammonia is relevant to NOx formation from fuel-nitrogen [1–3], NO reduction in the thermal De-NOx process [1–3], and even in efficient operation of engines fed by ammonia-based fuels [4,5]. In many cases premixed ammonia flames have been stabilized at low pressures and studied using probe sampling and mass-spectrometry [5–7]. Probe sampling has also been employed to study nitric oxide (NO) in the product gases for atmospheric pressure flames [8–10]. The flame structure of ammonia is relatively simple compared with hydrocarbon flames, therefore all major intermediates, i.e. OH, NH2 , NH, NO, can be measured with spectroscopic methods, which have been applied in studies of H/N/O mixtures, where NH3 is a major component, at differ-

∗

Corresponding author. Fax: +46462224542. E-mail address: [email protected], [email protected] (V.A. Alekseev).

http://dx.doi.org/10.1016/j.combustflame.2015.10.012 0010-2180/© 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

ent pressures. For instance, MacLean and Wagner [11] combined mass-spectrometry with absorption measurements of OH and NH in low-pressure ammonia + oxygen flames. In later studies, absorption measurements have been extended to relative concentrations of NH2 [12] and further to NO and NH3 profiles [13,14] in addition to NH and OH [12–14] for oxygen-enriched NH3 flames. Green and Miller applied laser absorption spectroscopy to obtain relative concentrations of NH2 in low-pressure ammonia + oxygen flames [15]. Furthermore, Chou et al. [16,17] and Dean et al. [18] presented concentration profiles of OH, NH, and NH2 radicals as well as NH3 based on laser absorption measurements. Other laser-based methods have also been implemented in several ammonia flame studies, for example Raman spectroscopy for measuring NO concentrations relative to O2 [19] in NH3 +O2 flames. Laser-induced fluorescence (LIF) has been employed for measurement of NO concentration profiles in ammonia + oxygen flames [17] and to measure OH, NH, and NO in an NH3 + N2 O + Ar flame by Venizelos and Sausa [20]. These flame studies both at low and atmospheric pressures provided a solid background for understanding the nitrogen chemistry in combustion and development of detailed H/N/O reaction mechanisms [2,3,18,21].

C. Brackmann et al. / Combustion and Flame 163 (2016) 370–381 Table 1 Mixture compositions of premixed ammonia + air flames. Flame

NH3 (Ln /min)

Air (Ln /min)

N2 -flame (Ln /min)

N2 -coflow (Ln /min)

φ = 0.9 φ = 1.0 φ = 1.2

2.62 3.38 3.9

10.3 12.1 11.6

0 0.87 0

1.2 1.2 1.2

371

namic stabilization made them rather sensitive to external flow disturbances. The flames were ignited at the conditions that corresponded to a stable burner-stabilized regime, and then the co-flow was added; the flow rates were gradually adjusted to obtain the desired mixture composition and unburned gas speed, allowing some time for the stabilizer to heat up. 2.2. Laser diagnostics

Full profiles of formation and consumption of intermediates are available for low-pressure flames from early studies summarized by Miller et al. [21] and from recent experiments of Duynslaegher et al. [5,7], as well as for selected rich ammonia flames at atmospheric pressure [16–18]. Other flame studies at atmospheric pressure focused on radicals and fuel decay in the post-flame region since the flames stabilized too close to the burner [12–14,17,19]. To overcome this problem, Dasch and Blint [9] suspended flames on a knife-edge burner at atmospheric pressure and implemented Raman spectroscopy to measure temperature profiles from N2 spectra and O2 , NH3 , H2 , and H2 O concentration profiles relative to N2 . In all other spectroscopic and probe sampling studies described above (except for [10]), flat-flame burners were used. To facilitate flame stabilization, all studies of ammonia flame structure employed oxidizers (O2 +N2 , O2 +Ar, N2 O + Ar) with composition significantly different from that of air or had to add small amounts of H2 to stabilize the flames on the burner [5,7]. Thus, there is no flame structure data available for ammonia + air mixtures. Therefore, the goal of the present work was to acquire profiles of temperature as well as concentration of O2 and important intermediates, NH, NO, and OH, in lean, stoichiometric, and rich ammonia + air flames at atmospheric pressure using non-intrusive laserdiagnostic methods. The second aim was to provide full concentration profiles in the flame front, favorable for detailed kinetic model validation. This is a challenging task at atmospheric pressure if the flame is attached to the burner. To move the reaction zone away and be able to measure in the flame front, the flames were detached from the surface of a porous plug McKenna-type burner and stabilized by aerodynamic strain using a steel disk, which acted as a stagnation plane. In the present work, radical species were monitored by LIF and quantitative profiles of OH and NO were retrieved employing a concept of direct calibration versus absorption, utilizing the LIF-signal itself [22,23], whereas NH concentrations were evaluated employing saturated LIF. Profiles of temperature and O2 concentration were acquired using rotational Coherent Anti-Stokes Raman Spectroscopy (CARS). Experimental data are compared with predictions of several contemporary H/N/O detailed combustion mechanisms, and model behavior is analyzed. 2. Experimental 2.1. Burner and flame conditions Measurements were made above a water-cooled stainless steel porous-plug burner (Holthuis & Associates). Mixtures of ammonia (AGA Gas AB, purity 99.9%) and air (AGA Gas AB, 21% O2 , 79% N2 ), were supplied to the burner plug whereas nitrogen was fed to a surrounding shroud shielding the flames. For a stoichiometric flame, the fuel + air mixture was diluted with a small amount of nitrogen to lift the flame further from the burner. Flows of NH3 , air, and N2 are presented in Table 1. A steel disc was mounted 16.2 mm above the burner plug to obtain flames in the strain-stabilization regime lifted ∼5–6 mm above the burner surface, a flame photo is presented in the Supplementary information. The flames were completely detached from the burner, i.e. had zero temperature gradients at the burner surface, and the aerody-

2.2.1. Laser-induced fluorescence Laser-induced fluorescence (LIF) measurements were made using a Nd:YAG and dye laser system (Quanta-Ray PRO 250-10, Spectra Physics and Cobra Stretch-G-2400, Sirah) of 10 Hz repetition rate and 8 ns pulse duration. The dye laser linewidth is ∼0.1 cm−1 and a Rhodamine 610/640 mixture was used for measurements of OH whereas dye LDS698 was used for NH and NO. Frequency doubling of the dye laser fundamental allowed for excitation of the OH Q1 (6) A2  + ←X 2 (0–0) transition at 308.7 nm whereas NH was probed at the R (8) 2 A3 ←X3  − (0–0) transition at 332.7 nm. Nitric oxide was excited at the Q1 (23) A2  + ←X2 (0–0) transition at 225.5 nm, achieved by frequency-mixing between the dye laser fundamental and frequencydoubled output. The maximum output pulse energies used for excitation of OH, NH, and NO were 28 mJ, 24 mJ, and 4.5 mJ, respectively. A schematic of the experimental setup is presented in the Supplementary information. Fluorescence measurements were carried out following two approaches. The maximum available laser energy was used to approach fluorescence measurements under saturated conditions, in which the signal becomes independent of laser irradiance and impact of collisional quenching. Under these conditions the laser beams were shaped and focused into thin vertical sheets of rather uniform irradiance distribution, allowing for imaging detection of high-resolution species concentration profiles. For OH and NH the sheet-forming optics consisted of three cylindrical lenses of focal lengths f = −40 mm/f = 200 mm/f = 300 mm (OH) and f = −40 mm/f = 100 mm/f = 200 mm (NH) whereas an f = −40 mm cylindrical lens together with an f= 300 mm spherical lens were used for NO measurements. The experimental conditions result in laser spectral irradiances on the order of 108 –109 W/(cm2 ·cm−1 ) and are at least three orders of magnitude higher than saturation limits estimated and reported for the three species, see for example Eckbreth [24] and references therein. For quantification of OH and NO profiles, measurements were also made with excitation at low laser energy, in a regime where the signal is linearly dependent on laser irradiance, and in which quantitative concentrations can be retrieved by combining LIF with absorption measurements. This was made using a previously developed concept based on bi-directional excitation using co-propagating laser beams [22]. Measurements in this mode were made with unfocused laser beams with pulse energies in the range 100–250 μJ and apertures (pinholes d = 100 μm or 2 mm aperture) were used to reduce beam size. For these experiments species were measured at different heights in the flame by translation of the burner relative to the fixed laser beam. Absorption levels were sufficient for quantification of OH in the lean and stoichiometric flames and for NO in the lean flame. The peak values were used for quantification of the OH and NO fluorescence profiles measured under saturated conditions. The combined fluorescence-absorption scheme was, however, not applicable for quantification of the NH fluorescence signal into number densities as NH showed too low absorption. Instead the NH concentrations were evaluated from the saturated fluorescence signal directly. Fluorescence was detected with an intensified CCD camera (Princeton Instruments, PI-MAX2) with UV optics (f = 105 mm UV Nikkor f/4.5 or f = 100 mm B. Halle f/2) mounted. The total fluorescence emission in the excitation band was detected for all species

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and background contributions were monitored by measurements with the laser tuned off the probed resonance. 2.2.2. Rotational Coherent Anti-Stokes Raman Spectroscopy Flame temperature and relative oxygen concentrations, i.e., the O2 /(O2 +N2 ) ratio, were measured using an experimental setup for rotational Coherent Anti-Stokes Raman Spectroscopy (RCARS). Molecular rotational populations, primarily of N2 , are probed via a thirdorder non-linear optical interaction and the RCARS signal emerges from the probe volume as a coherent laser-like beam collected with a spectrometer [25,26]. Temperature is retrieved from relative peak intensities within the overall envelope of the acquired spectra and individual contributions from N2 and O2 allow for determination of relative oxygen concentrations. The RCARS setup is illustrated in the Supplementary information and was similar to that presented in previous work [27] with the difference that a two-beam scheme, described in more detail previously [28,29], was employed. The combined pump/Stokes beam and the probe beam of the RCARS process, generated by a dye laser (TDL 90, Quantel) and a Nd:YAG laser (YG-980, Quantel) respectively, were overlapped and focused into a ∼0.7 × 0.1 × 0.1 mm3 probe volume positioned above the center of the porous-plug burner. Since the CARS signal propagates collinearly with the probe beam when using the two-beam approach, a polarization technique, also described previously [28], was employed to suppress laser stray light. The CARS signal was recorded using an f = 1 m-spectrometer (grating 2400 grooves/mm, Newport Corporation) and a back-illuminated CCD-camera (Newton Du940N-BV, Andor Technology). Dual broadband rotational CARS enables probing the entire rotational population of the molecules in a single laser shot. Averaged spectra were acquired in the flames together with background measurements, and spectra in non-resonant argon were recorded for dyelaser-profile compensation. A scan along the central axis of the flame was performed by translating the burner vertically while keeping the RCARS probe volume at a fixed position. Temperature and relative oxygen concentrations were evaluated by a least-square fit of theoretical spectra generated using an inhouse-developed computer code [25,26] to the experimental RCARS spectra. The analysis of RCARS spectra has been described more thoroughly in [30]. In addition to the gas temperatures measured with RCARS, flame stabilizer temperatures were measured using a type K thermocouple. 2.3. Data evaluation 2.3.1. Quantification using bi-directional LIF Calibration of LIF data into quantitative species concentrations has been achieved by combined absorption and fluorescence measurements [31]. A concept developed by Versluis et al. [22], based on bi-directional LIF measurements, allows for quantification of nonuniform species distributions, and was employed for measurements of OH and NO. Data evaluation was made according to the approach presented in [22]. The fluorescence signal, Ff (x), generated along a laser beam propagating along the x-axis from position x = 0 mm, where the irradiance is I0f , can be expressed according to Eq. (1), where C represents a collection efficiency, S(x) is the Stern–Vollmer factor, σ 0 the peak absorption cross section, and N(x) the number density at coordinate x for the probed energy level. 

x Ff (x) = C · S(x) · σ0 · I0 f · e− 0 σ0 ·N(y)dy

(1)

A corresponding relation can be deduced for the signal, Fb (x), generated by a laser beam propagating along the same path in the opposite direction, from end position L to coordinate x. 

L Fb (x) = C · S(x) · σ0 · I0b · e− x σ0 ·N(y)dy

(2)

Fig. 1. OH number density (blue) evaluated at height 11 mm above the burner surface for a φ = 0.9 NH3 + air flame. The OH number density is evaluated from the derivative of the signal ratio ln(Fb /Ff ) (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Forming the ratio between the two signals gives the following expression: x

Ff (x)

I0 f e− 0 σ0 N(y)dy =  I0b e− xL σ0 N(y)dy Fb (x)

(3)

Calculating the natural logarithm results in the following expression:



ln

Ff (x)



Fb (x)



I0 f = ln I0b



= ln

I0 f I0b





−

x

0



σ0 N(y)dy +



−2

x 0



L x

σ0 N(y)dy +

σ0 N(y)dy



L 0

σ0 N(y)dy

(4)

Taking the derivative of Eq. 4 the concentration of N(x) can be retrieved as

 

1 d N(x) = ln 2σ0 dx

Fb (x) Ff (x)



(5)

This allows for retrieval of quantitative species concentrations from the LIF-data, effectively avoiding problems with unknown collisional quenching influencing the signal. Since the data is spatially resolved along the laser beam propagation path, N(x) does not have to be constant and any deviations from one-dimensionality on the edges of the flame would not affect the retrieved data, as opposed to normal line-of-sight absorption measurements. The logarithmic ratio in Eq. (4) was calculated over a 1 cm wide region of interest at the center of the flame, and fitted to a straight line with a slope that represents the derivative in Eq. (5). Figure 1 shows an example from OH data analysis for the φ = 0.9 flame at position 11 mm above the burner surface. The logarithmic signal ratio can be well fitted by a straight line, which means that the derivative in Eq. (5) and the evaluated OH concentration are rather constant at this height in the flame. The peak absorption cross section, σ 0 , needs to be evaluated by analysis of the excitation line profile. The integrated absorption cross ν¯ , can be expressed as an integral of the absorption cross section, σint section σ (ν¯ ) according to Eq. (6), where ν¯ is the wavenumber with unit cm−1 . ν¯ = σint



+∞ −∞

σ (ν) ¯ dν¯ = σ0



+∞ −∞

g(ν) ¯ dν¯

(6)

Thus, the integrated absorption cross section based on wavenumν¯ , has unit cm2 ·cm−1 and as shown in Eq. (6), it can be exbers, σint pressed as a product of the peak absorption cross section σ 0 and the integral of a lineshape function g(ν¯ ), which includes the combined

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373

lineshapes of the molecular transition and the spectral profile of the laser. The integrated absorption cross section is related to the absorption coefficient B12 f according to the following relation [32]:

σint =

hν¯ 21 f B c 12

(7)

The OH Q1 (6) line is sufficiently isolated to avoid significant line overlap and the experimental lineshape was well fitted by a Gaussian profile of FWHM 0.59 cm−1 , resulting in a lineshape integral of 0.63 cm−1 . The absorption coefficient for the Q1 (6) transition, obtained from LIFBASE [33], is 1.128·1018 m3 J−1 s−2 , resulting in a peak absorption cross section of 1.28·10−15 cm2 . A similar approach was used to fit a Voigt profile of FWHM 0.89 cm−1 to the NO Q1 (23) line resulting in a lineshape integral value of 1.14 cm−1 . The absorption coefficient provided in LIFBASE is 2.653·1017 m3 J−1 s−2 and the peak absorption cross section becomes 2.29·10−16 cm2 . The lineshape fits for OH and NO are displayed in the Supplementary information. 2.3.2. Quantification of saturated LIF The concentration profiles for NH were quantified using the saturated LIF data. Assuming a system of two energy levels, labeled “1” and “2”, a completely saturated LIF signal, Fsat , can be expressed according to Eq. (8) [24]:

Fsat = hcυ



4π

ε lAN10

B12  1 A21 = hcυ A21 ε lAN10 B12 + B21 4π 1 + gg1

(8)

2

The quantities in Eq. 8 are the transition wavenumber ν , the collection solid angle , the efficiency of the detection system ε , the probe volume length l and area A, the population of the lower energy level N1 0 , the Einstein coefficients for absorption B12 and stimulated emission B21 , the statistical weights of the energy levels g1 and g2 , and the Einstein coefficient for spontaneous emission A21 . Using Eq. (8), N1 0 , the number density corresponding to the population of the lower energy level, can be calculated from a saturated LIF signal. However, the collection efficiency and probe volume size parameters need to be determined. The OH fluorescence signal measured under saturated conditions was used for this purpose, since the collection geometry was the same, detector response was similar for wavelengths around 308 and 333 nm, and quantitative OH number densities were retrieved from the bi-directional LIF measurements as described previously. However, for correct calibration it was necessary to compensate the OH-signal for absorption in the detection path. The transition wavenumber of the R2 (8) transition is ν = 30,050 cm−1 and the statistical weights of the laser-coupled levels are g1 = 17 and g2 = 19. Because of broadband detection of the fluorescence in the (0–0) band and with the assumption of rapid rotational energy transfer, the coefficient for spontaneous emission for the entire (0–0) vibrational band has been used in the evaluation, as suggested by Eckbreth [24]. A value of A21 = 2.2·106 s−1 , reported by Smith and Liszt [34], has been used in the evaluation. 2.3.3. Rotational populations Number densities calculated using Eqs. (5) and (8) provide values corresponding to the population of the probed rotational energy levels only. Thus, the population factor for the probed levels must be determined to retrieve total number densities for OH, NH, and NO. While population factors for OH and NO could be obtained from LIFBASE [33], values for NH were obtained from a model generated using the software PGOPHER [35] and based on molecular data presented by Dixon [36] and Brazier et al. [37]. The population fractions for the probed energy levels of OH, NH, and NO are plotted in Fig. 2. 2.3.4. Uncertainty estimations The peak absorption cross sections used in the evaluation of LIF data are directly coupled to the absorption coefficient according to Eqs. (6) and (7). From OH transition moment data presented by Luque

Fig. 2. Population fractions versus temperature for the probed rotational levels of OH (circles), NH (squares) and NO (triangles).

and Crosley [38], the uncertainty in OH absorption cross section can be estimated to ∼10%. The uncertainty in absorption coefficient /oscillator strength and accordingly the absorption cross section for NO can be estimated to 10%, from the discussion presented by Luque and Crosley [39] and references therein. In addition to the uncertainties in the values of these quantities, a 10% uncertainty is assumed for the lineshape fits. In a compilation of molecular constants for NH, Lents [40] presents values of the A-constant, used in Eq. (8), that show a spread of 7%. The temperature sensitivity of the probed transitions can be estimated from the plots of the Boltzmann factors presented in Fig. 2. The change in population fraction per 100 K for OH, NH, and NO is 2.6%, 2.1%, and 2.1%, respectively. These changes are comparatively small considering the 3% accuracy estimated for the rotational CARS temperature measurements. In addition, the accuracy in radical measurement position relative to the temperature profile will introduce temperature-related uncertainties. The uncertainty in the radical profiles’ position evaluation from the image data has been estimated to be ± 0.3 mm whereas the absolute position accuracy in the rotational CARS setup was estimated to be ± 0.5 mm. Uncertainties in positions of radical profiles and temperature profiles relative to each other will introduce uncertainties in the population factors discussed above, but mainly have impact on conversion from number-densitybased LIF signal to molar fractions. This uncertainty is most severe for the NH profiles located in the temperature gradient of the flame front region. Assuming the experimental uncertainties to be independent, evaluated total uncertainties for the OH, NH, and NO profiles are 19%, 41%, and 21%, respectively. These have been plotted as vertical error bars in the corresponding figures of Section 4. The experimental accuracy for rotational CARS thermometry was measured to be 1% for temperatures up to 800 K [25], however, data acquired at higher temperatures show an accuracy around 3%, which was also assumed for the results presented in this paper. The accuracy of oxygen concentration, evaluated as O2 /(O2 +N2 ) ratio, is 1–2% [26]. However, for O2 fractions below 0.03, a lower accuracy is expected due to limited signal-to-background ratio in the flames. 3. Modeling details 3.1. Selection of kinetic models Duynslaegher et al. [5] compared measurements in low-pressure NH3 +H2 +O2 +N2 flames with predictions of the kinetic mechanisms of Konnov and De Ruyck [41], Bian et al. [42], Lindstedt et al. [2], and GRI-mech 3.0 [43] of which only the first two showed

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reasonable agreement with the experimental data. Further analysis by Duynslaegher et al. [44] aimed at improvement of the mechanism of Konnov and De Ruyck [41] and modifications in rate constants of four reactions involving NH2 and N2 O were proposed. Based on the modified detailed mechanism, Duynslaegher et al. [44] obtained a reduced model. Kumar and Meyer [45] compared their measurements of burning velocity of NH3 +H2 +air mixtures at atmospheric pressure with predictions of the mechanisms of Konnov and De Ruyck [41], Tian et al. [46], and GRI-mech 3.0 [43] and recommended the first two. Furthermore, Shmakov et al. [47] investigated formation and consumption of NO in H2 +O2 +N2 flames doped with NO or NH3 at atmospheric pressure and proposed modification to the mechanism of Konnov and De Ruyck [41]. Therefore, in the present work we selected and tested (in comparison with the new experimental data) the reduced model of Duynslaegher et al. [44], the model of Shmakov et al. [47], and the mechanism of Mendiara and Glarborg [48], which supersedes the model of Tian et al. [46]. Even though the primary goal of Tian et al. [46] as well as Mendiara and Glarborg [48] was to study the interactions in C/H/N/O chemistry, the H/N/O subset of the mechanism was developed and validated from NH3 oxidation in a flow reactor [49]. In addition to that, Klippenstein et al. [50] developed a reaction scheme based on models of Glarborg and coworkers (e.g., [46,48,51]). The authors [50] provided new rate constants for many reactions involving NH, NH2 and NNH from high-level quantum chemistry calculations and measurements in shock tubes [50,52]. Therefore, the model of Klippenstein et al. [50] was also tested in the present work for NH3 + air flames. In the subsequent sections, the four models considered will be referred to as Model 1, standing for the scheme of Mendiara and Glarborg [48], Model 2 for Shmakov et al. [47], Model 3 for Duynslaegher et al. [44] and Model 4 for Klippenstein et al. [50]. 3.2. Simulation details and uncertainties of the initial conditions For correct reproduction of the strain-stabilized lifted flames, the stagnation flame reactor model of Chemkin-Pro [53] was chosen for simulations. This reactor model is practically identical to the widely known premixed opposed flow flame model, based on the work of Kee et al. [54], which allows simulating the opposed flow flames by reducing the three-dimensional geometry to a one-dimensional set of equations, with the assumption that radial velocities are functions of the axial coordinate only. In the stagnation flame model, the stagnation plane boundary condition appears, which assumes axial convection and diffusion velocities to zero, and the stagnation plane temperature has to be specified by user. The simulations were run both with experimental temperature profiles as input or by solving the energy equation. The GRAD parameter was set to 0.02 which resulted in grid-independent solutions with around 600–700 grid points. The experimental initial conditions, e.g., the mixture composition and inlet temperatures, possess certain uncertainties, thus the results of the modeling with nominal parameters may not correspond to the real measured flame conditions. For that reason, the influence of the uncertainties in the initial mixture parameters was estimated by performing the simulations with these parameters varied within their accuracy range. Therefore, the error bars on the modeling lines in the figures of Section 4 in fact correspond to the experimental uncertainty in the initial mixture parameters. For simulations based on experimental temperature profiles, the resulting radical concentrations would be affected by uncertainties in the equivalence ratio, inlet flow rate and experimental temperature. The mass flow controllers give an uncertainty of ± 1% of each component’s flow rate after calibration and the accuracy of the CARS temperature measurements is ± 3%, as discussed previously. Each of the flow rates and temperature were treated as independent factors that affect peak radical concentrations. The influence of each

factor was obtained by re-runs of the model with altered flow rates and temperature profiles within their accuracy range, and the total uncertainty of the predicted peak concentration was estimated as a root-mean-square error. For simulations based on solving the energy equation, the uncertainties in the mixture properties would affect the flame front position as well as the peak radical concentrations. These uncertainties were also analyzed through model re-runs. Additionally, since experimental temperature profiles were not applied, the accuracy of the inlet gas temperature was incorporated. This was estimated to be around ± 10 K based on the CARS temperatures evaluated for positions closest to the burner surface for the different flame conditions. The temperature boundary at the stagnation plane was set after measuring the temperature of the stabilizer as well as the in the upstream region in the flow close to it with a thermocouple. The values did not deviate much from the CARS measurements. Anyhow, the influence of the stagnation temperature on the flame structure in the reaction zone and its position was found insignificant compared to the other uncertainties. 4. Results and discussion 4.1. Flame structure Figure 3 shows fluorescence images of NH, OH and NO measured in a central plane across the investigated flames. The flames show similar structure with the reaction zone bent upwards towards the burner edges. As discussed in Section 2, the flame structure at the edges does not affect the LIF data quantification, and the experimental configuration would be adequately reproduced by a 1D stagnation flame model, provided that the flame area within the burner diameter is flat. However, in addition to edge effects, an indentation common for all flames can be seen to the left of the flame center and attributed to non-uniformities in the flow from the burner plug. Nevertheless, on both sides of this perturbation over a total distance of about 30 mm the flame position remains constant within ± 0.3 mm. The profiles below each set of images show number densities evaluated along the flames for regions with boundaries indicated by the horizontal dashed lines. Fluorescence signal has been converted into number densities using results evaluated from back-and-forth fluorescence (OH and NO) and saturated fluorescence (NH). The nonuniform flame structure can be observed in these profiles, however the variations are typically less than 20%, setting the uncertainties in the evaluated species profiles due to the non-uniform flame structure. Species profiles, determined for comparison with the modeling and presented later in Section 4, have been evaluated over the region to the right of the flame center, between 0 and 10 mm. The contours of the NH, OH, and NO fluorescence regions, measured at separate occasions, show good agreement for each flame confirming that the flames were reproducible. Figure 4 shows OH fluorescence profiles measured in the investigated flames at different occasions. The two solid-line profiles on each graph represent separate measurements using the previously described experimental setup, whereas the dashed-line profiles were acquired at a later stage using another laser system with OH excitation in the (1–0) band at 281 nm. These later measurements were therefore not quantified according to the previously outlined procedure but have been scaled versus the solid-line profiles for comparison. The burner setup was disassembled and relocated between these measurements. Nevertheless, the flame positions, as indicated by the sharp left-side gradients, are in agreement within 0.3 mm, i.e. of the order of the spatial resolution of the measurements. The results presented in Figs. 3 and 4 demonstrate that the investigated flames were reproducible, which was crucial for experiments since the measurements were not performed simulta-

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Fig. 3. LIF images of species distributions in NH3 + air flames (a) NH, (b) OH and (c) NO in false colors. Scale bar 5 mm. Equivalence ratio φ = 0.9 (top), φ = 1.0 (middle), and φ = 1.2 (lowest). The concentration profiles shown below the images have been evaluated between the vertical lines and averaged over the regions indicated by the horizontal dashed lines. Equivalence ratio φ = 0.9 (solid profile), φ = 1.0 (dashed profile), and φ = 1.2 (dotted profile). The number density variations are at most 20% over the evaluated region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 4. OH profiles measured in NH3 +air flames in two separate experimental setups. (a) φ = 0.9, (b) φ = 1.0 and (c) φ = 1.2. The reaction zone positions, as indicated by the sharp left OH gradient, agree within 0.3 mm for each flame.

neously. Furthermore, it was also evident that the flames were not flickering. It should be pointed out that the uniformity in radial flame shape was intentionally sacrificed to achieve lifted flames allowing to record full species profiles in the reaction zone, which would not be possible for flames stabilized on the burner, even though possessing better shape. Experimental data together with the results from the four models for the three investigated NH3 +air flames are presented and discussed in the following. 4.2. Temperature and oxygen profiles Figure 5 shows temperature and relative O2 profiles of the studied flames. The experimental uncertainties are plotted as error bars on symbols (measurement uncertainties) and modeling lines (mixture parameters uncertainties). Only Models 1 and 4 were able to reproduce the flames detached from the burner when the experimental temperature profiles are not used as an input, and these results are shown in Fig. 5. The two other models instead predicted flames stabilized on the burner, indicating an overestimated laminar flame speed

(SL ), which has to be exceeded by the inlet flow for the flame to detach from the burner. Indeed, for the lean flame (φ = 0.9), existing laminar flame speed data suggest values of 4–6 cm/s, see e.g. Refs. [55–57]. The adiabatic flame simulations yielded SL = 6.1 cm/s for Model 1 and SL = 7.4 cm/s for Model 4, compared with SL = 11.2 cm/s and SL = 12.4 cm/s for Models 2 and 3, respectively, both exceeding the experimental inlet axial velocity of 8.4 cm/s. From Fig. 5 it can be seen that the shapes of the temperature and O2 profiles are very well reproduced by Models 1 and 4 for all flames. However, in terms of absolute values and flame front position, agreement is very good for the lean and rich flames for Model 1, whereas Model 4 showed some discrepancy. For the stoichiometric flame, both models are in a fair agreement with the measurements. Comparing Models 1 and 4 with each other, Model 4 predicts flames slightly closer to the burner than Model 1 due to higher SL . In the following sections, the performance of all four models is tested in terms of the species concentrations by including experimental temperature profiles in the simulations, even though Models 2 and 3 showed a significant disagreement in SL . Such comparison can still be useful, since reactions controlling the flame structure and

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a

b

c

Fig. 5. Temperature and O2 concentration profiles for the flames with the composition corresponding to Table 1: φ = 0.9 (a), φ = 1.0 (b), φ = 1.2 (c). Symbols: experiments, lines: modeling. Colors: green – temperatures, red – O2 /(O2 + N2 ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

burning velocities can be independent from each other, and therefore deviations in SL or species predictions can be analyzed or improved separately. 4.3. Radical concentration profiles As discussed above, NH, OH and NO concentrations from the models were obtained using experimental temperature profiles as simulation input. No notable differences in predictions were observed when running Model 1 with and without the energy equation, yet the results obtained with experimental temperature profiles were closer to the experiments in all cases, except for the OH profile in the stoichiometric flame. This is shown in more detail in the Supplementary Information. The comparison between the experimental NH, OH and NO profiles and modeling is presented in Figs. 6–8. Estimated mea-

a

b

c

Fig. 6. NH concentration profiles for the flames with the composition corresponding to Table 1: φ 0.9 (a), φ = 1.0 (b), φ = 1.2 (c). Symbols: experiments, lines: modeling.

surement uncertainties in radical concentrations are shown with error bars on the symbols, and effect of the uncertainties in mixture parameters on the predicted peak or equilibrium concentrations is indicated as error bars on the modeling lines. As shown in Fig. 6, Models 1, 2 and 4 predict similar maximal NH concentrations for all three flames, whereas the values obtained with Model 3 are a factor of 2–3 lower. The experimental peak NH concentrations show good agreement with Models 1 and 4 at lean and stoichiometric conditions, but in the rich flame the experimental values become comparable to the predictions of Model 3 (about 60 ppm). To clarify the disagreement for rich conditions, Model 1 was used to simulate an oxygen-enriched NH3 flame of equivalence ratio φ = 1.28 from the work of Chou et al. [16]. The authors [16] also developed a kinetic model [18], which showed good agreement with the investigated flame [16] in terms of the peak NH levels. The performance of Model 1 for the φ = 1.28 flame [16] was found to be similar to the model presented in [18], and these results can be found in the

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a

a

b

b

c

c

377

Fig. 7. OH concentration profiles for the flames with the composition corresponding to Table 1: φ = 0.9 (a), φ = 1.0 (b), φ = 1.2 (c). Symbols: experiments, lines: modeling.

Fig. 8. NO concentration profiles for the flames with the composition corresponding to Table 1: φ = 0.9 (a), φ = 1.0 (b), φ = 1.2 (c). Symbols: experiments, lines: modeling.

Supplementary information. This comparison indicates that the experimental NH concentrations obtained in the rich flame might be underestimated. Nevertheless, the experimental error bar shows that the peak NH level could be 95 ppm, which approaches the prediction of Model 1. For OH concentration profiles, presented in Fig. 7, all models show satisfactory agreement with the experimental data, however, a tendency of under-prediction is observed for the lean flame for all models, as shown in Fig. 7a. In the rich flame, Fig. 7c, there is agreement between Models 1, 2, 4, while Model 3 predicts higher OH concentrations. For OH profiles of the stoichiometric flame, differences in peak values and positions are observed between all models. Nevertheless, all experiments and model predictions of peak OH concentrations lie within the specified error bars or very close to them (as in the lean flame). For the stoichiometric flame in Fig. 7b, the influence of the mixture parameters uncertainty on the peak OH position is also shown for Models 1 and 4 by horizontal error bars. For Mod-

els 2 and 3, as well as for all other experimental cases (Figs. 6–8), the peak radical positions were found to be insensitive to the variations in the initial mixture parameters within their accuracy range. All models predict similar NO concentrations in the product zone and show good agreement with experimental data in the lean flame (Fig. 8a). Lower NO levels are obtained for the stoichiometric case (Fig. 8b) from the measurements, for which the model predictions also differ. The lowest NO level, and closest agreement to the measurements, is observed for Model 1. In contrast to the model predictions, experimental NO concentrations in the lean and stoichiometric flames decrease in the post-flame zone. Measurements were made with laser sheet irradiance variations of about 15%, and the fluorescence profiles had a rather constant signal level across the post-flame zone. The bi-directional LIF measurements, carried out at discrete positions, resulted in number densities in the range 2.5–2.8·1016 cm−3 . Thus, mole fractions calculated using the temperature profiles from

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Table 2 Sensitivity analysis for Model 1 in the three ammonia + air flames; k = A Tn exp(−E/RT), units cal, cm3 , mol, s, K. The signs of the sensitivity coefficients are presented after the positions in the top-20 chart, “> 20” refers to a reaction outside the 20 most sensitive. No.

93 110

84 104 1 83 87 14 37 102 98 149

Reaction

NH2+NH=N2H2+H NH+NO=N2O+H NH+NO=N2O+H NH+NO=N2+OH NNH+O=NH+NO NH2+O=HNO+H NH2+O=NH+OH NH+OH=HNO+H NH+OH=N+H2O H+O2=O+OH NH2+H=NH+H2 NH2+OH=NH+H2O OH+H+M=H2O+M NO+H(+M)=HNO(+M) low-pressure limit: NH+H=N+H2 NH2+NO=NNH+OH NH2+NO=N2+H2O N2H2+M=NNH+H+M

A

5.00E + 13 2.90E + 14 −2.20E + 13 2.20E + 13 5.00E + 13 6.60E + 13 7.00E + 12 2.00E + 13 5.00E + 11 3.60E + 15 7.20E + 05 4.06E + 06 4.50E + 22 1.50E + 15 2.40E + 14 3.00E + 13 2.80E + 20 2.30E + 10 1.90E + 27

n

0.00 −0.40 −0.23 −0.23 0.00 0.00 0.00 0.00 0.50 −0.41 2.32 2.00 −2.00 −0.40 0.21 0.00 −2.70 0.40 −3.05

E

0 0 0 0 0 0 0 0 2000 16600 799 1000 0 0 −1550 0 1258 −814 66107

Ref.

Position in the top-20 sensitivity chart with direction NH 0.9

NH 1.0

NH 1.2

OH 0.9

OH 1.0

OH 1.2

NO 0.9

[3] [58]

2− 7−

1− 8−

1− 14−

11+ 5+

4+ 17+

6+ > 20

[46]

4−

4−

4−

18+

> 20

[3]

3−

5−

5−

> 20

[59] [49] [3] [60] [61]

6− 5+ 1+ > 20 18−

16+ 2+ 3+ > 20 12+

9− 2+ 6+ > 20 > 20

[49] [51]

14− 20−

6− 18+

[49]

> 20

17+

Fig. 5 possess decreasing trends, possibly related to effects of signal absorption. For the rich flame (Fig. 8c), further decrease in experimental NO concentration can be seen, however, values from all models are higher than the measurement results. NO profiles of Models 1, 2 and 4 are similar in shape to the experimental data and predict peak NO concentrations around a factor of 2.5 higher, Model 3 predicts NO profile very different in shape and magnitude compared to the other models as well as to the experimental data. Here it should be noted that the modification proposed by Shmakov et al. [47] to the original Konnov and De Ruyck mechanism [41], i.e. substitution of the reaction NH+H2 O=HNO+H2 by formation of the NH2 OH radical: NH+H2 O=NH2 OH, has made a major improvement for the simulation of the studied flames with Model 2, since the performance of the mechanism from [41] was found to be very close to Model 3, which does not implement this modification. The larger error bars on the modeling lines for the stoichiometric flame (Figs. 6–8bb) indicate that in this case the flame structure is more sensitive to the exact value of the equivalence ratio, compared to the lean and rich flames. For the stoichiometric flame, it is interesting to note that Model 1 predicts the peak of OH to occur further downstream compared to other models, and about 1.3 mm further compared to experiments, which is beyond the experimental uncertainty in the determination of the spatial coordinate (0.3 mm). Temperature and O2 profiles measured in the rich flame show good agreement with Model 1 (Fig. 5c), and the experimental NH, OH, and NO profiles are positioned consistently with respect to each other. However, comparing the positions of the radical profiles with the temperature data, an offset in position between experimental temperature and radicals is suggested, since the maximum radical concentrations (cf. Figs. 6–8cc) are located in the post-flame zone (cf. Fig. 5c). The radical profiles have been evaluated using the LIF images, from which the accuracy in absolute position has been estimated to be 0.3 mm. For CARS temperature measurements, not based on image data acquisition, the uncertainty in absolute probe volume position is estimated to be 0.5 mm. Shifts in positions within these uncertainties allow for a somewhat better consistency between experimental profiles of radicals and temperature. However, the consistent downstream location of the radical profiles suggests that a further offset of the experimental temperature data is possible even though modeling and CARS experiments in Fig. 5c possess a good agreement. This would in turn suggest that Model 1 also

NO 1.0

NO 1.2

> 20 1−

8+ 1−

13+ 1−

> 20

2+

3+

2+

15−

13−

3+

2+

15+

1− 9+ 15+ 2− 3+

2− 14+ > 20 1− 3+

1− 2− 8− > 20 9+

> 20 > 20 7− 20− 14+

> 20 > 20 > 20 7− 6+

8+ 18− > 20 > 20 > 20

3− > 20

> 20 > 20

> 20 5+

5− > 20

> 20 4−

> 20 19−

> 20 3−

10+

> 20

11+

3+

> 20

15+

> 20

over-predicts the flame speed for rich NH3 flames, though less than the other models. This correlates with freely propagating flame calculations of SL = 8.9 cm/s for Model 1, compared with SL = 6– 7 cm/s at φ = 1.2 [55–57]. An offset in experimental temperature data would also explain the observed difference in position between experimental and modeled concentration profiles, since the latter were obtained using experimental temperatures as input. 4.4. Model 1 analysis Summarizing the behavior of the four models, Model 1 performs better or similar compared to the other three models for all flame conditions. Therefore, it was decided to investigate the radical concentration sensitivity for this model in attempt to analyze an impact of the uncertainty in the rate constants on its performance. The results for the three flames are presented in Table 2. Sensitivity coefficients were analyzed at the peak position of each radical profile and the sensitive reactions (in bold in Table 2) were selected by their place in the top-20 sensitivity chart for nine cases (three radicals in three flames, lower number meaning higher sensitivity). The sensitivity charts can be found in the Supplementary information. The reactions are sorted by their relative importance in all cases, and Table 2 also presents signs of the sensitivity coefficients (after the position in the chart), rate constant parameters, and sources for the individual rate constants. The notation “> 20” refers to a reaction outside the 20 most sensitive at the corresponding conditions. In addition, the branching channels of the sensitive reactions, if taken from the same sources, are given in Table 2 in regular font, some of them appeared below the main reactions in the charts. Rate constant expressions from other studies were substituted into the model one by one. For example, a 22% reduction of the pre-exponential factor for the rate constant of reaction OH+H+M=H2 O+M (14) was applied [60,62]. The rate constant of reaction NH2 +NH=N2 H2 +H (93) originated from the mechanism of Miller and Bowman [3] and was substituted by the expression from ref. [63], as recommended by Dean and Bozzelli [64]. For the reaction NH2 +O=HNO+H (84), the value from ref. [65] was introduced, matching the calculations of ref. [64]. Regarding the reactions involving NH+NO (110), the rate constants from ref. [58] were changed to expressions from Bozzelli et al. [66]. The reaction NH2 +H=NH+H2 (83) was replaced by the reverse reaction taken from ref. [67]. The theoretical value from the quantum-chemistry study of Mackie and

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a

b

b

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Fig. 9. NH concentration profiles predicted by individual rate-constant variations in the model of Mendiara and Glarborg (Model 1). The numbering of the rate constants (r) corresponds to Table 2. Symbols: experiments, solid lines: original predictions of Model 1, other: Model 1 with modified rate constants.

Fig. 10. OH concentration profiles predicted by individual rate-constant variations in the model of Mendiara and Glarborg (Model 1). The numbering of the rate constants (r) corresponds to Table 2. Symbols: experiments, solid lines: original predictions of Model 1, other: Model 1 with modified rate constants.

Bacskay [68] for reaction (87) NH2 +OH=NH+H2 O was used instead of the expression from [3]. Finally, the rate constants of reactions NO+H(+M)=HNO(+M) (37) and NH+OH=HNO+H (104) were varied within their specified (37) or assumed (104) level of uncertainty. The results of individual rate constant variation on the predicted NH, OH and NO concentrations are shown in Figs. 9–11. From Figs. 9–11 it can be seen that changes in the individual rate constants do not produce a significant impact on the performance of Model 1. For OH and NO concentrations in the stoichiometric flame, the results calculated with updated rate constants never exceeded uncertainty intervals caused by the experimental uncertainties in flows and temperature profiles. For the NO in the lean and rich flames, where the uncertainties are lower, the impact of the rate-constant change can be distinguished from the uncertainty. However, for the conditions where the discrepancy is significant (OH at φ = 0.9, NO at

φ = 1.2), individual rate-constant variation does not allow to reduce it to any considerable extent. NO profiles in the lean and stoichiometric flames were found to be more sensitive to the rate constant variations. The original Model 1 underpredicts NO in the lean flame and overpredicts the experiments in the stoichiometric flame, but, as can be seen from Table 2 and Fig. 11, the most sensitive reactions act similarly on both profiles and therefore no modifications can be proposed. For NH, the model performance could possibly be improved in the rich flame, however, no individual reaction was found to change NH concentration significantly. In addition, replacement of the rate constant of the reaction H+O2 =O+OH (1) to the widely accepted expression of Hong et al. [69] had no impact on the cases studied. It can be concluded that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to unavoidable experimental uncertainties.

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a

b

flame front position at all equivalence ratios, as well as in terms of radical concentration profiles except for NH and NO in the rich flame. In the latter case (NO at φ = 1.2), all models disagreed with the experiments. For NH at φ = 1.2, comparison with the data presented in the literature and modeling of corresponding flames possibly indicated underestimated experimental concentrations. The two mechanisms of same origin, the models of Klippenstein et al. [50] and of Mendiara and Glarborg [48], were found to be close to each other in terms of radical concentrations profiles, however, the model of Klippenstein et al. [50] showed disagreement in the flame front positions in lean and rich flames. The two other models, of Shmakov et al. [47] and Duynslaegher et al. [44], both based on [41], failed to predict the burner-detached flames. However, the model of Shmakov et al. [47] showed similar behavior regarding NH, OH, and NO concentrations in the lean and rich flames compared with the model of Mendiara and Glarborg [48], with a bigger discrepancy for the stoichiometric flame. The model of Duynslaegher et al. [44] adequately reproduced OH at all conditions and NO at φ = 0.9, but showed larger deviations from the experimental data for NO at φ = 1.0 and 1.2, and from other three models for NH profiles. Following a sensitivity analysis of the Mendiara and Glarborg mechanism it was concluded that remaining uncertainties of the rate constants implemented in the recent H/N/O models is difficult to scrutinize unambiguously due to experimental uncertainties. Model analysis should be extended to other combustion systems, such as ignition and flame propagation, combined with further experiments. Acknowledgments

c

The authors gratefully acknowledge financial support from the Swedish Research Council, the Centre for Combustion Science and Technology (CECOST), and the European Scientific Research Council (ERC) through Advanced Grant DALDECS. We also acknowledge Dr. Joakim Bood and Per Samuelsson for helpful discussions and assistance with the data evaluation. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2015.10.012. References

Fig. 11. NO concentration profiles predicted by individual rate-constant variations in the model of Mendiara and Glarborg (Model 1). The numbering of the rate constants (r) corresponds to Table 2. Symbols: experiments, solid lines: original predictions of Model 1, other: Model 1 with modified rate constants.

5. Conclusion Flame structure of ammonia + air combustion at atmospheric pressure has been investigated by laser-based diagnostic methods for lean, stoichiometric and rich mixtures in lifted flames above a porous plug burner. Concentration profiles of NH, OH, and NO in the reaction and post-flame zones have been retrieved by laserinduced fluorescence while rotational Coherent Anti-Stokes Raman Spectroscopy has been employed for thermometry and relative O2 concentration measurements. The new experimental data acquired with non-intrusive diagnostic methods were used to evaluate and rank the performance of four contemporary detailed kinetic models. Simulations were made both with experimental temperature profiles as input and with the energy equation included. The mechanism of Mendiara and Glarborg [48] was found to perform best, showing good match with the experimental results in terms of temperature and

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