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Lean premixed combustion mode has become attractive for utilization in industrial gas turbines due to its ability to meet strict emissions regulations without compromising engine efficiency. In this combustion mode, the mixing process is the key player that affect the flame structure and stability, as well as the generated emissions. Many studies have investigated the aspects that influence premixed flames, including the effects of turbulence, combustor geometry, and level of partial premixing, while mostly using conventional natural gas fuel represented by methane. Recently, ammonia, a sustainable energy source, has been considered in gas turbines due to its carbon-free fuel producing no CO2. Utilizing 100% ammonia or a blend of methane and ammonia alters the combustion performance of a premixed flame due to the variation associated with the physical and chemical properties of ammonia. Thus, investigating the coupling between blend ratios and mixing length of methane-ammonia on flame stability and emissions is an essential step toward implementing ammonia in industrial gas turbines. In this study, the influence of various methane-ammonia blends, from 0 (pure methane) to XNH3 = 75%, and mixing lengths on the flame performance were studied. The mixing length was altered by delaying the injection (i.e., partially premixing) of the ammonia while using a fixed injection location for the reference methane-air mixture. This was done by using three fuel ports located at three different heights upstream of the combustion chamber. The results showed that the flame stability is negatively influenced by increasing (decreasing) ammonia fraction (mixing length ratio) and is more sensitive to the ammonia fraction than to the mixing length. At a constant equivalence ratio, the CO and NOx performances improved positively by increasing the ammonia volume fractions (especially at XNH3 = 75% compared to XNH3 = 25% and 50%) and the mixing length.

As a fueling vector, ammonia is gaining more interest for future power generation, transportation and heating systems. Compared to typical hydrocarbons, ammonia's low reactivity characteristics and higher NOx emissions limit its practical implementation. However, the current need for decarbonization of the energy sector puts ammonia as an energy vector and carrier at the far front, making overcoming the challenges of ammonia utilization necessary. In late 2018, Kobayashi et al. presented a review article on the science and technology of ammonia combustion. In continuing their effort, this review is aimed towards giving the most recent updates in ammonia combustion. The article starts with the importance of ammonia as an energy carrier and covers the fundamentals of ammonia combustion. We then present recent activities for addressing the low ammonia reactivity and its impact on NOx emissions. In addition, the chemical kinetics of ammonia oxidation is presented through a discussion of recently proposed chemical kinetics mechanisms. This section emphasizes the role of crucial reaction differences, the updates in the NOx and De-NOx pathways, NHi recombination reactions and H2NO chemistry. Towards utilising ammonia in gas turbines and industrial applications, this review discusses recent efforts and various NO mitigation strategies. This work systematically summarizes the fundamentals of ammonia-based combustion and its application. It serves as a reference for the transition of ammonia into future practical applications.

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

This work investigates the UV and visible chemiluminescence signatures of laminar ammonia-hydrogen-air flames. A counterflow burner was used to stabilize flat and steady premixed ammonia-hydrogen-air twin-flames for wide ranges of ammonia fuel fractions (0.55 ≤ X NH 3 ≤ 0.90), equivalence ratios (0.35 ≤ ϕ ≤ 1.70), and strain rates (50 ≤ a ≤ 300 / s). The chemiluminescence spectra in the UV and visible regions were measured. The important contributors to the chemiluminescence of ammonia-hydrogen-air flames were first identified and their sensitivities to equivalence ratio and ammonia fuel fraction were then quantified. Promising ratios of chemiluminescence intensities for the non-intrusive optical sensing of equivalence ratio and ammonia fuel fraction in premixed ammonia-hydrogen-air flames were also identified. Among the 36 ratios that could be formed with the 9 contributors to chemiluminescence that were defined, 3 are particularly promising, namely, OH*/NO*, OH*/ violet (violet refers to the 350 to 400 nm range), and red/blue (blue and red refer to the 450 to 500 nm and 650 to 750 nm ranges, respectively). Ratios OH*/NO* and OH*/ violet were found to provide suitable measures of equivalence ratio for lean and rich ammonia-hydrogen-air flames, respectively, because these ratios are very sensitive to equivalence ratio, are insensitive to ammonia fuel fraction, and are marginally sensitive to strain rate. Once equivalence ratio is known, the ammonia fuel fraction can then be measured with ratio red/blue because it is sensitive to the ammonia fuel fraction and it is globally insensitive to strain rate. [ABSTRACT FROM AUTHOR]

Lean premixed combustion mode has become attractive for utilization in industrial gas turbines due to its ability to meet strict emissions regulations without compromising engine efficiency. In this combustion mode, the mixing process is the key player that affect the flame structure and stability, as well as the generated emissions. Many studies have investigated the aspects that influence premixed flames, including the effects of turbulence, combustor geometry, and level of partial premixing, while mostly using conventional natural gas fuel represented by methane. Recently, ammonia, a sustainable energy source, has been considered in gas turbines due to its carbon-free fuel producing no CO2. Utilizing 100% ammonia or a blend of methane and ammonia alters the combustion performance of a premixed flame due to the variation associated with the physical and chemical properties of ammonia. Thus, investigating the coupling between blend ratios and mixing length of methane-ammonia on flame stability and emissions is an essential step toward implementing ammonia in industrial gas turbines. In this study, the influence of various methane-ammonia blends, from 0 (pure methane) to XNH3 = 75%, and mixing lengths on the flame performance were studied. The mixing length was altered by delaying the injection (i.e., partially premixing) of the ammonia while using a fixed injection location for the reference methane-air mixture. This was done by using three fuel ports located at three different heights upstream of the combustion chamber. The results showed that the flame stability is negatively influenced by increasing (decreasing) ammonia fraction (mixing length ratio) and is more sensitive to the ammonia fraction than to the mixing length. At a constant equivalence ratio, the CO and NOx performances improved positively by increasing the ammonia volume fractions (especially at XNH3 = 75% compared to XNH3 = 25% and 50%) and the mixing length. [ABSTRACT FROM AUTHOR]

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions. [ABSTRACT FROM AUTHOR]

One of the main concerns regarding ammonia combustion is its tendency to yield high nitric oxide (NO) emissions. Burning ammonia under slightly rich conditions reduces the NO mole fraction to a low level, but the penalties are poor combustion efficiency and unburnt ammonia. As an alternative solution, this paper reports the experimental investigation of premixed swirl flames fueled with ammonia-hydrogen mixtures under very-lean to stoichiometric conditions. A gas analyzer was used to measure the NO mole fraction in the flame and post flame regions, and it was found that low NO emissions (as low as 100 ppm) in the exhaust were achieved under very lean conditions (ϕ ≈ 0.40). Low NO emission was also possible at higher equivalence ratios, e.g. ϕ = 0.65, for very large ammonia fuel fractions (X NH3 > 0.90). 1-D flame simulations were performed to elaborate on experimental findings and clarify the observations of the chemical kinetics. In addition, images of OH* chemiluminescence intensity were captured to identify the flame structure. It was found that, for some conditions, the OH* chemiluminescence intensity can be used as a proxy for the NO mole fraction. A monotonic relationship was discovered between OH* chemiluminescence intensities and NO mole fraction for a wide range of ammonia-hydrogen blends (0.40 < ϕ < 0.90 and 0.25 < X NH3 < 0.90), making it possible to use the low-cost OH* chemiluminescence technique to qualify NO emission of flames fueled with hydrogen-enriched ammonia blends. [ABSTRACT FROM AUTHOR]

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames' propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends. • Ammonia-hydrogen-air flames feature wider stability limits than ammonia-air or hydrogen-air flames. • Nitrogen addition to ammonia-hydrogen fuel blends does not significantly alter stability limits. • Pure ammonia-air flames feature lower NO emissions than ammonia-hydrogen-air flames. • Good NO performance is found for "very" lean ammonia-hydrogen-air flames. [ABSTRACT FROM AUTHOR]

In a previous paper [J. Chem. Phys. 116, 7544 (2002) (Paper I)] a model, based on the energy corrected sudden approximation, was proposed for the construction of the line-mixing relaxation matrix. It was successfully tested by comparisons with measured infrared spectra of ammonia–helium mixtures. The present paper extends this preliminary study by considering mixtures of NH[sub 3] with H[sub 2] and Ar. Measurements have been made at room temperature in the regions of the ν[sub 2] and ν[sub 1] bands for pressures up to several hundred atmospheres. As in Paper I, the relaxation operator is constructed, within the impact approximation, using the ECS approximation. The data required are dynamical factors (which can be predicted from the NH[sub 3]–X potential energy surface) and a scaling length (adjusted using line broadening data). Comparisons between measured and calculated absorptions demonstrate the quality of the model which satisfactory corrects for the large deviations with respect to the purely Lorentzian behavior. Line-mixing effects for NH[sub 3]–Ar and NH[sub 3]–H[sub 2] are qualitatively similar to those observed for NH[sub 3]–He but quantitative differences exist, particularly when intra- and interbranch couplings are considered. Finally, the proposed model leads to very satisfactory results in the wings of both the purely rotational and ν[sub 2] bands of NH[sub 3] diluted in H[sub 2], opening promising perspectives for the remote sensing study of planetary atmospheres. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ammonia has the potential of being a future fuel for carbon-free combustion to reduce greenhouse gases (GHG) emission. The heat release rate (HRR) information of ammonia flames is desired if the ammonia fuel is applied in real combustion devices, but direct measurement of HRR is almost impossible. In the present work, the HRR markers of ammonia-methane-air flames were investigated through the simulation of one-dimensional freely propagating flames (FPF) with a validated chemistry mechanism. Various HRR markers were proposed. The performance of the target HRR markers was tested with the defined criteria. It was found that the profiles of NH and HCO agree well with the HRR profiles, but only NH is a suitable HRR marker because the mole fraction of HCO could be too low for laser induced fluorescence (LIF) measurement. The products of two species, such as [OH][CH 2 O] and [NO][NH 2 ], were then proposed to circumvent the low mole fraction issues. Product [OH][CH 2 O], which was usually used as HRR marker in hydrocarbon flames, was found to have the best performance in identifying the HRR location and be the only one that has a positive correlation with the amount of HRR among the proposed products. Moreover, the performance of [OH][CH 2 O] is valid over a wide range of conditions, including the very high ammonia fuel fraction (X NH3 = 95%), without suffering from the low mole fraction issues. The observations were further analyzed by looking into the reaction pathways and examining the reactions that contributes the largest amount to the total HRR. The present findings provide a meaningful reference for future indirect measurement of HRR in ammonia-methane-air flames. [ABSTRACT FROM AUTHOR]

This study reports an experimental investigation of quantitative Nitric Oxide (NO) distribution in both premixed and non-premixed NH 3 /H 2 -air flames using a counterflow burner at atmospheric pressure. One-dimensional (1D) NO laser-induced fluorescence (LIF) spectroscopy and Raman/Rayleigh spectroscopy were conducted to accurately resolve the quantitative 1D NO profile in terms of mixture fraction, temperature, and physical space. We calibrated a saturated NO-LIF model in 5 premixed lean H 2 /N 2 /NO-air flames with different seeded NO levels in a McKenna burner and validated its accuracy in three H 2 N 2 NO-air counterflow diffusion flames. The overall uncertainty of NO quantification was less than 90 ppm. Our measurements were compared with simulations using different ammonia chemical kinetic models, revealing that current models have over 30% uncertainty in predicting peak NO concentrations (mole fraction) in 1D non-premixed and premixed flames and over 100% uncertainty in lower temperature regions. In premixed flames, measured NO concentrations fell within the intermediate range of current chemical kinetic models at lean and stoichiometric conditions, but were lower than the models at rich conditions. In non-premixed flames, all models overestimated the peak NO concentrations by more than 1000 ppm. It is noted that the measured peak NO concentrations increased with higher NH 3 /H 2 ratios (from 4/6 to 8/2), strain rates (from 80 to 140 1/s), and N 2 dilution ratios in a 1:1 NH 3 /H 2 mixture (from 0 to 30%). Although most models could qualitatively predict the trends, they were inaccurate in quantifying NO. Additionally, the measured width of the NO profile in mixture fraction space expanded with increasing NH 3 /H 2 ratio, N 2 dilution ratio, and strain rate. While models could qualitatively predict this behavior, they consistently underestimated NO in the fuel-rich, lower-temperature region, resulting in a narrower NO profile width. The Manna model showed a better prediction of NO distribution in the fuel rich portion of non-premixed flames, accounting for NH 3 NO interactions at lower temperatures. These findings highlight the critical need to improve models to accurately predict NO concentrations in ammonia-containing flames and their behavior in fuel rich regions. [ABSTRACT FROM AUTHOR]

• Pure ammonia-air swirl flames can be stabilized for a wide range of equivalence ratios. • Ammonia addition to methane reduces the flames' propensity to flashback. • Ammonia addition to methane widens the stable range of operation. • Good NO performances are only achieved for slightly-rich equivalence ratios. Ammonia is a promising fuel for heat and power generation because it is carbon-free and it can be produced from abundant chemicals and renewable energy sources. However, ammonia-air mixtures feature a low reactivity and stabilizing turbulent ammonia-air flames is challenging. In this study, the stability limits of technically-premixed ammonia-methane-air mixtures are measured for a wide range of ammonia additions in a laboratory-scale swirl burner. Results are compared to that obtained for baseline methane-air mixtures. Data show that increasing the ammonia addition increases the equivalence ratio at the lean blowout limit but also reduces the flames' propensity to flashback. If the volume fraction of ammonia in the fuel blend exceeds a critical value, experiments also show that increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blow-out occurs instead. This significantly widens the range of equivalence ratios yielding stable flames. The critical ammonia volume fraction is a function of the Reynolds number and is x NH3 = 0.42 for Re = 7000 and x NH3 = 0.70 for Re = 3000. Because the ability to stabilize ammonia-methane-air flames is not practically relevant if NOx emissions are too large compared to that of conventional methane-air flames, exhaust NO concentrations are also measured. Consistent with previous studies, data show that, for non-marginal ammonia fuel fractions, good NO performances are not found for lean ammonia-methane-air mixtures and can only be achieved with slightly rich mixtures, which are within measured stability limits. [ABSTRACT FROM AUTHOR]

• Detailed analysis of flame structures in cracked ammonia jet flames. • Insights into N 2 O formation in ammonia flames through NH and NO multiplication. • Impact of cracking ratio on chemical reactivity, flame stability, and NO emissions. • PLIF-derived statistics contribute to validating turbulence-chemistry interactions. The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N 2 O. The effect of turbulent disturbances on N 2 O formation needs to be considered in turbulent CAJFs because N 2 O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N 2 with H 2 or NH 3. [ABSTRACT FROM AUTHOR]

The stability, local extinction, and NOx production of ammonia-hydrogen (NH 3 -H 2) flames are significantly impacted by turbulence-chemistry interactions. A powerful technique to study these interactions is planar laser-induced fluorescence (PLIF). However, multi-scalar PLIF typically requires complex and expensive systems, with multiple laser sources. This study proposes a novel PLIF technique using a single dye laser to perform simultaneous, single-shot imaging of the reactant NH 3 , radical NH, and pollutant NO in NH 3 -H 2 flames. According to the excitation scans and the emission spectra of NH 3 , NH, and NO, three wavelength couples can be used to image the three species simultaneously via two-photon excitation of NH 3 C ' − X (2,0), single-photon excitation of NH A 3 Π − X 3 Σ − (0,0) near 304 nm, and single-photon excitation of NO A 2 Σ + − X 2 Π (0,1) near 237 nm. Wavelengths near 237 nm and 304 nm are obtained simultaneously with only one dye laser by combining outputs of the frequency-doubling and frequency mixing units. NH 3 -, NH-, and NO-PLIF imaging performance is analyzed by quantifying the signal-to-noise ratios and detection limits. This technique is then used to visualize the structure of premixed and non-premixed flames over wide ranges of NH 3 and H 2 concentrations in the fuel blend. Results show that the premixed NH 3 -H 2 flames have a compact structure, while the non-premixed flames exhibit a large gap between NH 3 - and NH-PLIF layers, where ammonia undergoes significant decomposition before reaching the reaction layer. The inner edge of the NO-PLIF layer overlaps well with the reaction layer represented by the NH-PLIF layer in all flames. The fully developed turbulent structure downstream of the NH 3 -H 2 flame causes pinching-off of the premixed flame front and local extinction of the diffusion flame front. [ABSTRACT FROM AUTHOR]

Cracking NH 3 into a mixture of NH 3 /H 2 /N 2 effectively addresses its low reactivity when used as a fuel. In this study, large eddy simulation (LES) and experiments are conducted for non-premixed NH 3 /H 2 /N 2 -air jet flames with simulated cracking ratios of 14 % and 28 % at an elevated pressure of 5 bar. Detailed experimental data on the flow field are provided for assessing turbulence models. A recently proposed species-weighted flamelet/progress variable (SWF) model considering differential diffusion is adapted for simulating cracked NH 3 flames. The effects of differential diffusion on scalar structures are further analyzed. The SWF model, incorporating differential diffusion, achieves good agreement with experiments in predicting velocity, mean temperature, major species mass fractions, differential diffusion parameters, and NO formation. The model also qualitatively captures variations in localized extinction along the flame height. In contrast, the unity Lewis number flamelet/progress variable (ULF) model predicts the occurrence of localized extinction too upstream, significantly overestimating its occurrence in the near field where differential diffusion is significant. Moreover, NO formation in the cracked NH 3 flame is well predicted by the SWF model using flamelet tables, and slightly overpredicted by the ULF model without differential diffusion. [ABSTRACT FROM AUTHOR]

This work provides a kinetic and experimental investigation of excited radical OH(2Σ+) (also referred to as OH*) in NH 3 H 2 -air flames. A counterflow burner is used to stabilize laminar diffusion flames over wide ranges of ammonia fraction in the fuel blend (0.2 ≤ x NH3 ≤ 0.8) and strain rate (40 ≤ a ≤ 200/s). Using an intensified camera or a spectrometer coupled to a Cassegrain optical system, spatially resolved and spatially integrated OH(A2Σ+-X2Π) chemiluminescence intensities are measured. These data are used to challenge a kinetic mechanism largely developed from existing literature schemes. Measurements and simulations show that two distinct OH* peaks exist in spatially resolved profiles for intermediate ammonia fractions in the fuel blend. Sensitivity analyses identified that reactions N 2 O+ H =N 2 +OH* and H + O + M =OH*+ M , respectively pertinent to NH 3 and H 2 oxidation routes, are responsible for the formation of OH*. The proposed kinetic mechanism gives a remarkable portrait of the empirical data recorded in diffusion flames as well as in premixed NH 3 H 2 -air flames from the literature. Nevertheless, the contribution of reaction N 2 O+ H =N 2 +OH* in the formation of OH* in the peak closest to the fuel side of diffusion flames is consistently overpredicted. Consequently, the frequency factor of the N 2 O+ H N 2 +OH* reaction is adjusted to A = 1.35 × 1014 cm3mol−1s−1, which significantly improves predictions of spatially integrated and spatially resolved OH* intensities for all the diffusion and premixed flames examined. [ABSTRACT FROM AUTHOR]

This work presents the first quantitative Raman measurements of temperature, mass fractions, and mixture fractions in two turbulent ammonia/hydrogen/nitrogen-air diffusion jet flames simulating 14% (CAJF14) and 28% (CAJF28) partial ammonia cracking ratio with Reynolds numbers of 11,200. The precision and accuracy of the system were validated in laminar counterflow and laminar jet flames to ensure data quality. The scalar structure in turbulent flames is examined using conditional mean and rms radial profiles, scatterplots in mixture fraction space, as well as statistics conditioned on mixture fraction and physical spaces. These results are compared with opposed diffusion flow 1D simulation with multi-component transport model and Le=1 transport model to visualize the effects of differential diffusion on the scalar structure at various axial positions. Finally, localized extinction was found in CAJF14 but not for the more stable CAJF28, and the probability of localized extinction was less than 2.5%. The dataset is provided as supplemental material and can be used for the validation of numerical models. [ABSTRACT FROM AUTHOR]

This paper reports on the chemiluminescence footprint of premixed ammonia-methane-air flames. The chemiluminescence spectrum of laminar twin-flames stabilized with a counterflow burner were measured between 200 and 457 nm for large ranges of equivalence ratio (0.60 ≤ ϕ ≤ 1.30), ammonia fraction (pure methane to pure ammonia), and strain rate (55/s ≤ a ≤ 300/s). Relevant excited radicals were identified, namely, NO*, OH*, NH*, CN*, CH*, and CO 2 * and evolutions of their chemiluminescence intensity were analyzed as a function of equivalence ratio, ammonia fraction, and strain rate. These measurements produced an unprecedented database on the chemiluminescence of ammonia-methane-air flames, which could be used in the future for model validation. A total of 15 ratios of chemiluminescence intensity were also considered and 5 ratios showing promise for the development of chemiluminescence-based flame sensors were identified. The CN*/OH* ratio is a potential surrogate for equivalence ratio even if the ammonia fuel fraction in the fuel blend is not known accurately – as long as it exceeds 0.3 by volume. The CN*/NO* ratio is another possible surrogate for equivalence ratio if the ammonia fraction in the fuel blend is below 0.5. The OH*/CH* ratio, often used to sense equivalence ratio in hydrocarbons flames, is not recommended for ammonia-methane flames. The NH*/CN* ratio is a potential surrogate for the ammonia fraction in the fuel blend if equivalence ratio is larger than ϕ = 0.7 and if the ammonia fraction in the fuel blend is below 0.4. Other ratios may be combined to provide a simultaneous measure of equivalence ratio and ammonia fraction in the fuel blend with an extended range of validity, for example NH*/OH* and CN*/NH*. [ABSTRACT FROM AUTHOR]