The chemiluminescence (CL) signature of laminar H2 non-premixed flames doped with NH3 was experimentally investigated using a counterflow burner configuration. The NH3 vol fraction (XNH3) ranged from XNH3 = 0% (pure H2) up to 15%. Oxidizers included Air, O2–N2 and O2–Ar. Visible and CL images were recorded using several narrow bandpass filters (310 nm up to 660 nm). Pure H2–Air flames showed a pale blue/violet visible signature, while blending just 1% NH3 formed a distinct top (fuel side) second flame layer. As XNH3 increased, this layer shifted from bluish/whitish to the typical yellow/orange hue of NH3 combustion. The bottom layer (oxidizer side) is populated by emissions at 310 nm, 430 nm, 455 nm, and 515 nm, whereas the top layer (fuel side) is populated by emissions from 430 nm up to 660 nm. Especially, emissions centered at 568 nm, 633 nm, and 660 nm are only present in the top layer (fuel side) at high XNH3. The double-layer structure appeared regardless of oxidizer type, indicating it stems from NHi decomposition/oxidation rather than chemistry owing other components (carbon or nitrogen) in the oxidizer. Also, the gap between the top flame layer and the 310 nm peaks is on average 2.52-mm for XNH3 = 1% and decreases to 1.25-mm at XNH3 = 15% using Air or O2–N2 as oxidizer. However, it gets broader when O2–Ar is used as oxidizer, which is probably caused by higher H2 diffusion due to the higher flame temperature. This trend is confirmed by 1-D simulations using new Konnov’s mechanism that include several excited species. Novelty and significance statement Staged combustion technique is promising to reduce emissions and increase efficiency of ammonia (NH3) combustion for gas turbines. However, the secondary combustion stage remains poorly understood for NH3–staged combustion systems. This study examines the complex combustion process in the secondary-stage using a counterflow burner. Results show a consistent double-layer flame structure, when fuel is blended with NH3 (as low as 1% by vol.), regardless of the oxidizer composition. This study shows for the first time that the bottom layer (oxidizer side) is populated by emissions centered at 310 nm, 415 nm, 455 nm, and 515 nm, whereas the top layer (fuel side) is populated by emissions centered at all the VIS wavelengths tested. Emission at 568 nm, 633 nm, and 660 nm appear only on the fuel side, indicating it originates from NHi decomposition/oxidation rather than chemistry owing other components (carbon or nitrogen) in the oxidizer. These findings support the development of predictive tools for optimizing NH3–fueled staged combustion systems.

Ammonia (NH3) is being evaluated as a carbon-free energy carrier. However, combustion of NH3 leads to potentially significant amounts of NOx emissions as well as flame stabilization challenges. For both reasons, there is interest in partially cracking NH3 and combusting some blend of NH3/H2/N2. Our prior work has evaluated the minimum theoretical NOx emissions from pure NH3 combustion, which is a useful benchmark for evaluating fundamental limits, as well as to evaluate the performance of a given combustion system relative to these theoretical limits. This work is aimed to evaluate the fundamental minimum NOx emissions of partially and fully cracked NH3. Significant NOx benefits are possible with 100% cracked NH3 - i.e., H2/N2 combustion - and the optimal combustion architecture is a lean premixed strategy. However, this lean premixed strategy obviously does not work for partially cracked NH3 combustion. NOx emissions for intermediate cracking fractions exhibit both a highly nonlinear and, in certain pressure regions, a non-monotonic dependence upon cracking fraction - in other words, NOx emissions do not necessarily, linearly decrease with increased cracking. In general, partial cracking does provide NOx benefits in a manner that is highly pressure dependent; for example, minimum theoretical NO emissions decrease by around 90% and 40% between pure NH3 and 90% cracked NH3 at 1 and 20 bar for a system with 20 ms residence time, but a 2% increase in NO is observed for the same system at 4 bar. It is only at cracking levels exceeding about 99% that major NO benefits occur, with minimum NO reaching sub-30 ppm (15% O2 dry) values for all pressures. Moreover, these results show that rich-lean staged systems lead to optimal NOx emissions over cracking fractions from about 0 - 99.9%; it is only above 99.9% cracking ratio that traditional lean premixed combustion strategies show comparable results. These results indicate that only if nearly complete cracking is possible, that NH3 utilization will require retrofitting low NOx combustors from lean premixed systems to rich-lean staged systems. The sensitivity of these results to the choice of kinetic models is also addressed in this work.

This study investigates simultaneous NH/NO planar laser-induced fluorescence (PLIF) measurements in plasma-assisted NH3/air and NH3/H2 (9:1 volume ratio)/air flames at equivalence ratios of cent = 0.75, cent = 0.94, and 1.1. A single dye laser system, equipped with frequency-doubling and mixing units, was employed to simultaneously generate excitation wavelengths near lambda NO = 236.214 nm and lambda NH = 303.545 nm, enabling optimized excitation of NO and NH fluorescence, respectively. Across all equivalence ratios, plasma was found to enhance NO and NH concentrations in both NH3/air and NH3/H2/air flames in the near field, although NH enhancement was less pronounced in the NH3/H2/air cases. In NH3/air flames, NO concentrations decreased faster downstream with plasma activation, whereas in NH3/H2/air flames, NO levels remained relatively unchanged regardless of plasma activation. For NH3/air flames, plasma could enhance atomic O production therefore acceleration of NH3/NH2/NH and form OH at the same time. The enhanced OH levels further promote NH production via NH2 + OH-NH + H2O in NH3/air flames, though this effect is less significant in NH3/H2/air flames owing to the contribution of H2 on radical pool buildup and less NH3 availability in NH3/H2/air mixtures. In the downstream region, the reaction NH + NO-N2H2 + H plays a key role in reducing NO emissions in NH3/air flames with plasma activation. These findings provide new insights into plasma-enhanced NH3 flame chemistry and pollutant formation pathways, contributing to the development of cleaner and more efficient NH3-based combustion technologies.

This work involves extinction strain rate (ESR) measurements and NH2 LIF and Rayleigh scattering measurements in a counterflow burner configured to burn in a diffusion mode. The burner setup was validated using ESR measurements for CH4 diffusion flames, and then further measurements were taken using NH3. For the LIF measurements, NH2 radicals were excited using a solid-state Alexandrite laser at 385.7 nm. The signal for NH2 was filtered to a range of 400 - 450 nm. The Rayleigh scattering signal was also captured simultaneously using the same laser and a bandpass filter in the range of 382 - 393 nm, allowing for 1-D axial temperature measurements.

The power generation sector has been recently moving toward decarbonization, and there is an increased interest in replacing conventional fossil fuels with fuels that produce reduced/zero carbon emissions. One such fuel is ammonia (NH3). However, ammonia is hard to ignite, has a low flame speed, and produces a significantly large amount of nitrogen oxide (NOx) emissions. Hence, using 100% ammonia as fuel in gas turbines requires significant modifications and the development of novel combustors. Blending hydrogen with ammonia, however, helps in having better control over the combustion properties. For example, a 70%/30% mixture of NH3/H-2 mixture has a flame speed comparable to natural gas. Before utilizing hydrogen-blended ammonia in an actual gas turbine combustor, thorough simulation studies are required to evaluate its performance, possible hazards, and emissions. The literature lacks well-validated chemical kinetic models for the combustion of hydrogen-blended ammonia for undiluted mixtures at gas turbine-relevant conditions (similar to 20 bar). Most models available in the literature have been developed for ammonia extremely diluted in diluents such as argon or nitrogen. Hence, in this work, we develop a detailed chemical kinetic model for hydrogen-blended ammonia combustion and validate it with a wide range of experimental data for both dilute and undiluted mixtures relevant to gas turbine operating conditions. We outline the strengths and weaknesses of the current mechanism to aid future users of our chemical kinetic mechanism. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the directed relation graph with error propagation (DRGEP) and full species sensitivity analysis. The resultant mechanism can predict a wide range of experimental results with the least cumulative error and will be a valuable tool in computational fluid dynamics (CFD) simulations that will enable the development of gas turbines for zero-carbon power generation.

This study presents the first experimental measurement of NH planar laser-induced fluorescence (PLIF) in plasma-assisted ammonia/air swirling flames to investigate the effect of plasma on ammonia flame structures and kinetics. A repetitive AC-powered plasma discharge was applied to ammonia/air swirling flames with equivalence ratios (phi) ranging from 0.67 to 0.96. The flame structure was examined through flame morphology, NH layer thickness, and NH distributions, comparing cases with and without plasma. The plasma discharge effectively controlled the flame shape and stabilization location, transforming a lifted, unstable ammonia flame into a stable, fully attached flame stabilized by the inner and outer shear layers of the annulus. Flames with plasma predominantly exhibited concentrated NH radicals near the burner exit, accompanied by a reduction in flame height. The plasma kinetic effects reduced NH layer thickness by 20 % in the near field. Such reduction is owing to the early decomposition/oxidation of ammonia initiated by the plasma away from the flame front. Furthermore, plasma significantly enhanced NH LIF intensity by 30 % with plasma activation, although these effects diminished beyond 25 mm above the burner surface. The experimental results elucidate the interplay between plasma kinetics and flame dynamics, demonstrating that plasma discharge enhances NH formation and improves flame stabilization. The increase in NH radicals due to plasma is directly correlated with the observed reduction in NOx emissions reported in previous studies.

In-situ measurement of droplet size distributions is essential in understanding spray characteristics of fuel injectors at different operating conditions, as they can affect combustion efficiency and emissions. This work involves utilizing phase Doppler particle anemometry (PDPA) for measuring instantaneous drop-sizes at different spatial regions in an aviation type gas turbine combustor operating at high-pressure, high-temperature conditions. The measurements are performed in both non-reacting and reacting flow conditions to compare the effect of combustion on droplet breakup and vaporization. As opposed to the conventional PDPA configurations, the current setup involves using a mirror system to overcome the constraints of a confined pressure vessel and subsequently obtaining sufficient scattering angles for light detection. Measurements are performed over a range of conditions including changing operating pressures, preheat temperatures and fuel-air mixture ratios.

In this work, NH 2 production from plasma and NH3 flames are measured/inferred using both laser-induced fluorescence (LIF) and NH2 * chemiluminescence measurements. Non-equilibrium plasma discharge is applied to ammonia/air-swirling flames, with equivalence ratios varying from 0.1 to 0.7. The NH 2 production near the combustor nozzle exit and downstream of the plasma discharge is measured to examine the effect of plasma on ammonia flames. It can be seen that plasma can significantly promote the production of NH2/NH2 * . Direct photographs show that the activation of plasma transforms the unstable ammonia flame into a stable, fully attached ‘V’ shape flame. Further NH LIF imaging also reveals significant wrinkling of the flame surface due to plasma. Both NH and NH2 play an important role in ammonia combustion and could enhance ammonia oxidation and reduce corresponding NOx emissions. These measurements can help elucidate their detailed kinetic roles in plasma-assisted combustion systems.

With a major drive for carbon-neutral energy sources, the viability of alternative fuels such as hydrogen and ammonia are being actively investigated. One idea currently being explored is to retrofit existing natural gas power generation technologies for use with these alternative fuels. Computational models that can replicate the behavior of these types of systems can be useful in optimizing operating conditions for this type of retrofit as well as providing guidance for updated designs. In this work, a reactor network model is developed for a practical gas turbine combustion system to simulate emissions data observed in detailed laboratory experiments with highly reduced computation time. Such work can enhance both the analytical models as well as assist in validating measurements in experiments. The design methodology of such a reactor network model for a gas turbine combustor is explored, as well as the effect of key parameters in simulations on the results. This includes the complexity of the model and the sensitivity of the results to any added details in modeling. Results show that possible air leakage and radiative effects can lead to discrepancy between simulation and measured values for turbine inlet temperature. NOx values and its ability to explain differences in simulation and experiments is discussed. The implemented chemical reactor network also shows areas for improvement such as more accurate geometric calculations and implementation of “recycling” to simulate recirculating gas. The sensitivity of the results to different chemical kinetics mechanisms is also shown, indicating room for possible improvements to chemical kinetics. The conversion of NO to NO2 and the usefulness of simulating this effect is also discussed.

Refractive index measurements are critical for characterizing the properties of hypersonic flows, but moderate- to high-pressure experiments require alternative methods to traditional interferometric fringe counting. In this work, we introduce a novel, to the best of our knowledge, multi-wavelength phase-correlation interferometric technique to estimate the refractive index changes across nearly discrete shock wave boundaries and also simultaneously capture optical dispersion and vibrational relaxation times. By comparing the interference pattern of three or more wavelengths against each other, the refractive index can be accurately determined. To demonstrate this technique, laser diodes in two wavelength combinations are tested producing refractive index resolutions on the order of 2.65 × 10-7. Results in air across a range of initial pressure conditions (P1 = 2.66 to 5.33 kPa) and incident wave speeds (Mach 2 to 5) show density changes that agree with theoretical estimates within 2%. Single-shot dispersion and vibrational relaxation measurements with this method also illustrate good agreement with other techniques.

Ammonia (NH3) has attracted substantial interest as a carbon-free, energy-dense chemical fuel for implementation in internal combustion engines, ground based and aviation gas turbines, and rocket propulsion. Understanding the detailed chemical kinetics of NH3 is essential to the development of these combustion-based applications. With the improved availability and performance of semiconductor diode lasers, especially in the near-infrared (NIR) spectral region, quantitative assessment of NH3 has been made more accessible in both laboratory research and field applications. In the present study, an absorption sensor for NH3 measurement was developed by exploiting the affordability and flexibility of an NIR fiber-coupled distributed-feedback (DFB) diode laser. The first objective of the present study was to address the incomplete spectroscopic data for the absorption feature near 2.2 μm through determination of line strengths and broadening coefficients with collisional partners including NH3, O2, N2, and Ar. Lineshape characterization both in a room-temperature absorption cell and behind shock waves was conducted to determine the temperature-dependence exponents of NH3- and air-broadening coefficients. The other objective was to demonstrate the capability of the sensor through NH3 detection in multi-component mixtures in an absorption cell, and NH3 time-history measurements in a shock tube, with the latter providing the opportunity for investigating NH3 chemical kinetics at engine and turbine relevant conditions. With the spectroscopic parameters determined and performance validated, the NH3 sensor developed in this study could serve as a cost-effective diagnostic tool for chemical kinetic measurements, combustion monitoring, and other applications where flexibility and robustness are prioritized.

The power generation sector has been recently moving towards decarbonization and there is an increased interest in replacing conventional fossil fuels with fuels that produce reduced/zero carbon emissions. One such fuel is ammonia (NH3). However, ammonia is hard to ignite, has a low flame speed, and produces a significantly large amount of nitrogen oxide (NOx) emissions. Hence, using 100% ammonia as fuel in gas turbines requires significant modifications and the development of novel combustors. Blending hydrogen with ammonia, however, helps in having better control over the combustion properties. For example, a 70%/30% mixture of NH3/H-2 mixture has a flame speed comparable to natural gas. Before utilizing hydrogen-blended ammonia in an actual gas turbine combustor, thorough simulation studies are required to evaluate its performance, possible hazards, and emissions. The literature lacks well-validated chemical kinetic models for the combustion of hydrogen-blended ammonia for undiluted mixtures at gas turbine-relevant conditions (similar to 20 bar). Most models available in the literature have been developed for ammonia extremely diluted in diluents such as argon or nitrogen. Hence, in this work, we develop a detailed chemical kinetic model for hydrogen blended ammonia combustion and validate it with a wide range of experimental data for both dilute and undiluted mixtures relevant to gas turbine operating conditions. We outline the strengths and weaknesses of the current mechanism to aid future users of our chemical kinetic mechanism. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the direct relation graph with error propagation (DRGEP) and full species sensitivity analysis. The resultant mechanism can predict a wide range of experimental results with the least cumulative error and will be a valuable tool in CFD simulations that will enable the development of gas turbines for zero-carbon power generation.

With the improved availability and performance of low-cost near-infrared diode lasers, quantitative assessment of ammonia (NH3) as a carbon-free fuel has been made more accessible and affordable in both laboratory research and field applications. In the present study, an absorption sensor for NH3 measurement was developed by exploiting the affordability and flexibility of a near-infrared fiber-coupled distributed feedback diode laser. The first objective of the present study was to address the incomplete spectroscopic data for the absorption feature near 2.2 mu m through the determination of line strengths and broadening coefficients with collisional partners including NH3, O2, N2, and Ar. Line shape characterization in both an absorption cell and a shock tube behind reflected shock waves was conducted to determine the temperature exponents of these broadening coefficients. The other objective was to demonstrate the capability of the sensor through NH3 detection in multicomponent mixtures and NH3 time-history measurements in the shock tube for investigating NH3 chemical kinetics at engine and gas-turbine-relevant conditions. The NH3 sensor developed in this study could serve as a cost-effective diagnostic tool for chemical kinetic measurements, combustion monitoring, and other applications where flexibility and robustness are prioritized.

Ammonia (NH3) is a carbon-free fuel and hydrogen carrier; however, the presence of fuel-bound nitrogen has the potential to produce substantial nitrous oxide (NOx) emissions. Staged combustion, starting with a rich fuel reaction/decomposition zone, has the potential for low NO levels but can also lead to very significant NO levels if not designed correctly. This staged strategy takes advantage of the fact that equilibrium NO emissions from rich NH3 combustion are quite low. However, significant NO levels are still produced in rich flames and require a specific relaxation time, which can be quite long, to drop down to equilibrium levels. Thus, a key design attribute for ammonia combustors is sufficient residence time for NO relaxation. Given an overall combustor length, the NO relaxation zone is a direct function of the rich flame length, which can be quite long. Moreover, the flame length is a strong function of the flame stabilization location, e.g., the flame length in an annular swirling flow can easily vary by a factor of two depending upon the flame stabilization location. Moreover, swirl flames exhibit very long “tornado” configurations in certain conditions. For these reasons, even under conditions where the flame is stabilized, its configuration will dramatically influence the amount of time for the NO relaxation process. This paper introduces a test rig to explore flame configurations and post-flame NO relaxation for ammonia combustion. The rig features a modular swirler, a long primary stage, and a traversing emissions probe. The paper presents images of flame configurations over a range of conditions and maps different bifurcations in flame configuration.

This work presents the NH/NH2 laser-induced fluorescence (LIF) measurements conducted in plasma-assisted ammonia combustion in a model gas turbine combustor. NH2 LIF was excited at 385.7 nm using the second harmonic of a solid-state Alexandrite laser, and NH LIF was probed at 303.545 nm. Both 1D and 2D LIF imaging techniques were examined in laminar flames, the detection limits were analyzed. The NH PLIF has a detection limit of 100 ppm in a 60*10 mm field of view. The detection limit is down to 800 ppm for 2D NH2 LIF imaging; instead, 1D NH2 LIF imaging is preferred with a 200 ppm detection limit achieved. The detailed NH/NH2 distribution in a model gas turbine combustor was analyzed in the presence and absence of plasma. The results show that the peak intensity of NH/NH2 was increased by the application of plasma, potentially aiding in the DeNOX process within the fuel-nitrogen combustion chemistry.

This work presents the NH/NH2 laser-induced fluorescence (LIF) measurements conducted in plasma-assisted ammonia combustion in a model gas turbine combustor. NH2 LIF was excited at 385.7 nm using the second harmonic of a solid-state Alexandrite laser, and the emitted signal was collected within the range of 400 to 450 nm. NH LIF was probed at 303.545 nm. Both 1D and 2D LIF imaging techniques were examined in laminar flames, the detection limits were analyzed. The NH PLIF has a detection limit of 100 ppm in a 60*10 mm field of view. The detection limit is down to 800 ppm for 2D NH2 LIF imaging; instead, 1D NH2 LIF imaging is preferred with a 200 ppm detection limit achieved. The detailed NH/NH2 distribution in a model gas turbine combustor was analyzed in the presence and absence of plasma. The results show that the peak intensity of NH/NH2 was increased by the application of plasma, potentially aiding in the DeNOX process within the fuel-nitrogen combustion chemistry.

This study reports an experimental investigation of quantitative Nitric Oxide (NO) distribution in both premixed and non-premixed NH3/H2-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 H2/N2/NO-air flames with different seeded NO levels in a McKenna burner and validated its accuracy in three H2N2NO-air counterflow diffusion flames. The overall uncertainty of NO quantification was less than90 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 NH3/H2 ratios (from 4/6 to 8/2), strain rates (from 80 to 140 1/s), and N2 dilution ratios in a 1:1 NH3/H2 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 NH3/H2 ratio, N2 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 NH3NO 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.