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In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O 2 , CO 2 , CO, NO, NO 2 , and N 2 O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O 2 concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO 2 , and N 2 O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO 2 and N 2 O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO 2 and N 2 O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities. [ABSTRACT FROM AUTHOR]

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]

Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A 2 Σ + − X 2 Π (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH 3 –H 2 –N 2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH 3 –H 2 –N 2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH 3 –H 2 –N 2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH 3 –H 2 –N 2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames. [ABSTRACT FROM AUTHOR]

This work reports on the chemiluminescence signature of premixed ammonia-methane-air swirling flames. Wide ranges of equivalence ratios (0.6 ≤ Φ ≤ 1.3), ammonia fractions in the fuel blend (0 ≤ X NH3 ≤ 0.70), and Reynolds numbers (10,000 ≤ Re ≤ 40,000) were investigated to understand effects of these parameters on the light emitted by these flames. Excited radicals contributing to chemiluminescence in the UV and visible ranges were confirmed, namely NO*, OH*, NH*, CN*, CO 2 *, CH*, and NH 2 *. With non-intrusive flame monitoring in mind, various ratios of chemiluminescence intensities were carefully studied because these allow removing effects of time-varying flame surface area that is inherent in turbulent flames. Consistent with previous findings in laminar flames, ratios CN*/OH*, CN*/NO*, and NH*/CH* were found to be promising candidates. Ratios CN*/OH* and CN*/NO* were identified as potential surrogates for equivalence ratio if X NH3 ≥ 0.20 and 0.05 ≤ X NH3 ≤ 0.50, respectively. Ratio NH*/CH* was identified as a potential surrogate for the ammonia fraction in the fuel blend provided that equivalence ratio is roughly known. Ratio Blue /NH 2 *, obtained exclusively from measurements in the visible range, is another interesting surrogate for the ammonia fraction but its sensitivity to Reynolds number may limits its range of applications. Trends of measured exhaust NO concentration with equivalence ratio and ammonia fraction were found to qualitatively match that of NO*, NH*, and CN*, implying that emissions from these excited radicals could be used to monitor the NO performance of practical ammonia-methane-air flames. [ABSTRACT FROM AUTHOR]

In this work, we propose an analytical decomposition of the flame transfer function (FTF) in the complex space. The decomposition provides new insight into the relationship between the local heat release oscillator and the global flame response of a periodically oscillating system in the linear regime. The FTF of a premixed methane-air swirling flame is decomposed using two-region and pixel-by-pixel division of the chemiluminescence images. The two-region decomposition uses horizontal or vertical dividing lines to examine the effects of dividing positions and orientations. The complex curves of the two-region division are more sensitive to the dividing line's position than its orientation. The pixel-by-pixel decomposition provides detailed distributions of the weighting factor, weighted gain, and phase of all local oscillators. The weighted gain distribution highlights the inner fluctuating region (IFR) and outer fluctuating region (OFR), as well as the node region. These regions are mainly controlled by the radial velocity and its fluctuations induced by the vortices in the inner and outer shear layers. The unwrapped phase, which gradually increases as the downstream distance from the injector increases, highlights the convective characteristics of the oscillating heat release rate. Angular wavenumbers and phase velocities are deduced from the phase distribution. The phase velocities agree well with the mean velocities measured with particle image velocimetry. Sorting all the local oscillators in the ascending phase allows constructing complex curves that favorably represent the FTF. The algebraic sum of weighted gain and the efficiency of phase interference are calculated to examine the phase interference. Phase interference plays a large role in suppressing the global fluctuating amplitude of the heat release rate. The local extrema at low and intermediate frequencies are attributed to in- or out-of-phase interference, while the reduced local response also explains why small FTF gains are observed at high frequencies. [ABSTRACT FROM AUTHOR]

This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, x NH3 , was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (x NH3 < 0.9), flames with very lean equivalence ratios (φ < 0.7) can be stabilized and these yield competitively low NO emissions (<200 ppm), regardless of pressure. For this reason, very lean swirl ammonia-hydrogen-air flames are promising candidates for micro gas turbines. However, N 2 O emissions have the potential to be unacceptably large for these operating conditions if heat loss is too large or residence time is too short. As a consequence, the post flame region must be considered carefully. Due to the lower reactivity of methane compared to that of hydrogen, very lean swirl ammonia-methane-air flames could not be stabilized and good NO performance is limited to rich equivalence ratios for ammonia-methane fuel blends. The equivalence ratio above which good NO performance depends on pressure and bulk velocity. • Increasing pressure widens the range of stability of ammonia-air flames. • Increasing pressure increases the ammonia fuel fraction for which the transition from flashback to rich blowout occurs. • NO emissions from ammonia-hydrogen/methane-air flames decrease as pressure increases. • Good NO performance is found for very lean ammonia-hydrogen-air flames under elevated pressures. [ABSTRACT FROM AUTHOR]

This paper reports on an experimental investigation of the chemical impact of nanosecond repetitively pulsed (NRP) glow discharges on a laminar methane-air flame. The chosen configuration was a lean wall stabilized flame where NRP discharges were generated across the flame front. After careful selection of the excitation lines, planar laser induced fluorescence of OH and CH was conducted. Comparisons between the OH and CH fluorescence of a base flame (without plasma actuation), and those obtained during the steady state and the transient regimes of plasma actuation, were performed. First it is shown that during the steady state regime, the intensity of OH and CH fluorescence in the flame could be increased by up to 40% and 10%, respectively. In addition, the life time of OH fluorescence in the discharge channel was estimated to be between 3 and 4.5 µs. The transient regime at the beginning of plasma actuation showed that the flame began to be affected by the discharges long before OH fluorescence could be detected in the discharge channel, upstream of the flame. After 40 ms of plasma actuation, OH intensity began to increase simultaneously in both the flame and the discharge area. Based on current knowledge of nanosecond discharge chemistry, explanations for these results are proposed. [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]

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH 2 O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH 4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities U j and co-flow velocities U c were examined. Data show that flames with U c = 0.6 m/s stabilize much further away from the nozzle than those with U c = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for U c = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH 4 Raman images shows that the flames with U c = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For U c = 0.6 m/s and relatively large bulk jet velocities (U j = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of U j = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and U c = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing. [ABSTRACT FROM AUTHOR]

This study considers turbulent premixed bluff-body stabilized flames at elevated pressures. Specifically, the lean blow-off (LBO) limit of such flames is determined for a range of bulk velocities (5 ≤ U ≤ 50 m/s) and operating pressures up to 3 bar. Two key observations emerge from this stability assessment. The first is that considering elevated pressure leads to two stability regimes: one at atmospheric conditions and those with elevated pressure and U ≳ 20 m/s (regime-a), and another at elevated pressures with U ≲ 20 m/s (regime-b). The second observation is that within these regimes, LBO limits are insensitive to pressure. Flames in regime-a (S-flames) are found to be more stable than those in regime-b (U-flames). Advanced image-based diagnostics were employed to understand reasons for this difference in stability. Flow field measurements indicate that U-flames are associated with an outer recirculation zone (ORZ) that formed as pressure increased but receded from the burner as U surpassed ∼ 20 m/s. PLIF images of CH 2 O and OH demonstrated that the ORZ interacts with U-flames such that their downstream regions are prevented from collapsing to the inner recirculation zone (IRZ). Furthermore, analysis of the OH-PLIF images indicate that U-flames possess larger turbulent consumption rates, helping them form large IRZs and rendering them more susceptible to influence from the ORZ. Results of high-speed OH* imaging demonstrate that LBO events differ between U- and S-flames. Namely, while S-flames collapse to their IRZs during LBO, U-flames lift off from the burner, depleting their anchoring regions of reactions and hot products. Losing back-support in this region is what ultimately reduces the stability of U-flames. Finally, the reason U-flames lift off from the burner during LBO is elucidated by joint flow-flame measurements. Specifically, the anchoring regions of U-flames reside in regions of large axial velocity, which likely stems from their enhanced burning rates. [ABSTRACT FROM AUTHOR]

Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH 4 /air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4–2%. These results confirm the potential for 1064 nm LIGS-based thermometry for high-precision temperature measurements of combustion processes. [ABSTRACT FROM AUTHOR]

Abstract Studying the lift-off behavior of non-premixed jet flames is important to understanding the flame stabilization mechanisms in practical systems, including gas flares and combustors, and to improving the safety of pressurized fuel tanks in case of fuel leaks. This is typically done with the canonical configuration of an axisymmetric fuel jet issuing into a quiescent or co-flowing oxidizer and abundant data are available in the literature. However, most of these data were collected at normal or sub-atmospheric pressure and little data are available at elevated pressure and high Reynolds numbers, conditions relevant to practical configurations. The present study fills this gap by reporting lift-off height measurements of methane and ethane non-premixed jet flames for pressures up to 7 bar and Re = 57,500 in the presence of an air co-flow. Data are interpreted using Kalghatgi's model for the dimensionless lift-off height, which was previously proven successful for sub-atmospheric to normal pressures, as well as elevated pressure but only for low turbulent Reynolds numbers and propane. In this contribution, Kalghatgi's model has been shown to accurately predict the slope of the lift-off height vs. jet velocity curves at elevated pressure and large Reynolds number for methane and ethane. A new term, a function of the stoichiometric mixture fraction, the laminar burning velocity, and a turbulent Schmidt number, is also introduced to extend the predictive capabilities of Kalghatgi's model to configurations featuring a co-flow. [ABSTRACT FROM AUTHOR]

Abstract The stability limits of a turbulent flame in a practical combustor are important characteristics that influence its performance. The mechanisms controlling the stability limits of turbulent non-premixed flames are examined here in the canonical configuration of a fuel jet in co-flow air. This study focuses on the conditions leading to the detachment of flames from the injector nozzle by means of an experimental parametric study in which pressure (1 ≤ P ≤ 10 bar), fuel (methane and ethane), nozzle wall thickness (t = 0.20 mm, 0.58 mm, and 0.89 mm), jet velocity (0.5 ≤ U j ≤ 16.5 m s−1), and co-flow velocity (U c = 0.3 m s−1, 0.6 m s−1, and 0.9 m s−1) are varied. It is shown that the mechanism leading to detachment depends on the ratio of the nozzle wall thickness to the laminar flame thickness. If this ratio is smaller than 3, the nozzle is "thin" and type I detachment occurs (flame base stability lifting). In this case, the detachment velocity decreases with pressure and is proportional to the laminar burning velocity. If the ratio is larger than 3 , the nozzle is "thick" and type II detachment occurs (local flame extinction lifting). Then, the detachment velocity is controlled by the extinction strain rate. Experiments also show that the Kolmogorov scale of turbulence regulates local flame extinction and type II detachment and a model is proposed to predict detachment for any fuel, pressure, and nozzle wall thickness using the computed extinction strain rate and the Kolmogorov time scale. Finally, the data show that elevating pressure allows stabilizing attached non-premixed jet flames with high Reynolds numbers without the need for complex stabilization strategies such as pilot flames, swirl, or oxygen/hydrogen enrichment. Pressure allows studying flame/turbulence interactions at Reynolds numbers relevant to practical applications while conserving simple configurations amenable to diagnostics and modeling. [ABSTRACT FROM AUTHOR]

Abstract The effects of both pressure and Reynolds number on the structure of turbulent nonpremixed flames are investigated experimentally. Syngas (CO/H 2 /N 2) flames with constant composition are examined at pressures up to 12 bar and at Reynolds numbers up to 66,800. The baseline atmospheric pressure flame is identical to the "chnA" flame of the turbulent nonpremixed flames (TNF) workshop. Low-speed OH-PLIF is used to reveal the flame structure in a small region 7 diameters downstream of the nozzle. The thickness of the OH layer decreases monotonically with pressure. Increasing pressure inhibits local extinction when comparing conditions at constant Reynolds number; an effect of changing exit strain rate. Neither changes in Reynolds number nor changes in pressure affect the mean flame front location. Corrugation of the flame front is highly sensitive to changes in Reynolds number but relatively insensitive to changes in pressure. Blow-off velocity limits the highest Reynolds number achievable by the TNF jet flames. By increasing the pressure, this ceases to be the limiting parameter, meaning that we can study the effect of Reynolds number on the structure of the Sandia-ETH syngas jet flame over a wider range of turbulence, approaching what is encountered in practical combustors. [ABSTRACT FROM AUTHOR]

There exists interest in using alcohols as renewable, lower emission fuels. It has been observed that alcohol flames generally produce lower concentrations of NO emissions and the cause of these reductions is attributable to a number of mechanisms. This work therefore investigates the relative contributions to total NO formation in alcohol fueled flames, relative to comparably sized alkane flames. Measurements of quantitative NO PLIF were conducted in two common premixed configurations: a conical, Bunsen-type flames and a lower peak temperature burner-stabilized McKenna flat flame. Additionally, these flames were modeled using a detailed NO x chemical mechanism and investigated to understand the primary contribution pathways to non-thermal NO formation. From this analysis, it was observed that alcohol fueled flames produced as much as 50% less non-thermal NO than alkanes. However, under lean conditions the non-thermal contributions increased to about 80–90% of those observed in alkanes due to a greater contribution to non-thermal NO in both alcohol and alkane flames from non-hydrocarbon radical related mechanisms. [ABSTRACT FROM AUTHOR]

This paper reports on the effect of pressure on the response of methane–air and propane–air swirl flames to acoustic excitation of the flow. These effects are analyzed on the basis of the flame transfer function (FTF) formalism, experimentally determined from velocity and global OH* chemiluminescence measurements at pressures up to 5 bar. In parallel, phase-locked images of OH* chemiluminescence are collected and analyzed in order to determine the associated flame dynamics. Flame transfer functions and visual flame dynamics at atmospheric pressure are found to be similar to previous studies with comparable experimental conditions. Regardless of pressure, propane flames exhibit a much larger FTF gain than methane flames. For both fuels, the effect of pressure primarily is to modify the gain response at the local maximum of the FTF, at a Strouhal number around 0.5 (176 Hz). For methane flames, this gain maximum increases monotonically with pressure, while for propane flames it increases from 1 to 3 bar and decreases from 3 to 5 bar. At this frequency and regardless of pressure, the flame motion is driven by flame vortex roll-up, suggesting that pressure affects the FTF by modifying the interaction of the flame with the vortex detached from the injector rim during a forcing period. The complex heat transfer, fluid dynamics, and combustion coupling in this configuration does not allow keeping the vortex properties constant when pressure is increased. However, the different trends of the FTF gain observed for methane and propane fuels with increasing pressure imply that intrinsic flame properties and fuel chemistry, and their variation with pressure, play an important role in controlling the response of these flames to acoustic forcing. [ABSTRACT FROM AUTHOR]

We present an accurate measurement of the weak quadrupole S(2) 2-0 line in self-perturbed D 2 and theoretical ab initio calculations of both collisional line-shape effects and energy of this rovibrational transition. The spectra were collected at the 247–984 Torr pressure range with a frequency-stabilized cavity ring-down spectrometer linked to an optical frequency comb (OFC) referenced to a primary time standard. Our line-shape modeling employed quantum calculations of molecular scattering (the pressure broadening and shift and their speed dependencies were calculated, while the complex frequency of optical velocity-changing collisions was fitted to experimental spectra). The velocity-changing collisions are handled with the hard-sphere collisional kernel. The experimental and theoretical pressure broadening and shift are consistent within 5% and 27%, respectively (the discrepancy for shift is 8% when referred not to the speed averaged value, which is close to zero, but to the range of variability of the speed-dependent shift). We use our high pressure measurement to determine the energy, ν 0 , of the S(2) 2-0 transition. The ab initio line-shape calculations allowed us to mitigate the expected collisional systematics reaching the 410 kHz accuracy of ν 0 . We report theoretical determination of ν 0 taking into account relativistic and QED corrections up to α 5 . Our estimation of the accuracy of the theoretical ν 0 is 1.3 MHz. We observe 3.4 σ discrepancy between experimental and theoretical ν 0 . [ABSTRACT FROM AUTHOR]

Simultaneous, planar laser-induced fluorescence (PLIF) of nitric oxide (NO) and hydroxyl radical (OH) is evidenced with a single dye laser and two intensified CCD (ICCD) cameras. The technique is demonstrated on three premixed turbulent hydrogen-air flames, at atmospheric conditions, with several laser excitation wavelengths within the A 2Σ+ − X 2Π (0,0) and A 2Σ+ − X 2Π (1,0) vibrational bands of NO and OH, respectively. Via adjustments of the grating angle of the dye laser, the output wavelengths can be optimized so that both LIF signals are obtained simultaneously, or it can be tuned to reach the most common excitation wavelengths for OH-PLIF (i.e., near 283 or 284 nm) or NO-PLIF (i.e., 225 and 226 nm) alternatively. The flame visualization results, obtained with the different excitation strategies, are consistent for the three flame conditions and enable to characterize both the reaction zones and the nitric oxide formation. With appropriate selection of the wavelength pair, the technique shows its great potential to enhance the ability of existing lasers to simultaneously visualize and quantify distributions of two key species in reacting flows with only a single laser. [Display omitted] • Combined single-shot imaging of NO and OH with one dye laser in H 2 flames. • Most common excitation wavelengths also achievable alternatively. • Recommendation of optimal excitation wavelengths for future studies. • Flame structure confirmed with different excitation strategies. [ABSTRACT FROM AUTHOR]

This work investigates the formation of NO in a range of laminar, premixed, burner-stabilized C1 to C3 alcohol and alkane flames, in the equivalence ratio between 0.8 and 1.2. Measurements of temperature and NO concentration were conducted, and simulations utilizing the measured temperature profile allowed for the comparison of predicted NO with experiment, as well as a detailed investigation of the contributions from a number of NO formation pathways. In the alcohol flames, reduced contributions to Prompt NO were observed along with reduced consumption of NO through the NO–HCN Reburn mechanism, demonstrating the importance of hydrocarbon radicals (CH, CH 2 , CH 3 , and HCCO) to NO formation. Additionally, significant contributions to NO through the combined NNH and N 2 O mechanism were observed, representing a greater proportion of the NO produced in the alcohol flames. [ABSTRACT FROM AUTHOR]

In this paper, a novel dual-fuel, dual-swirl burner is presented and its potential to mitigate thermoacoustic coupling for ammonia-hydrogen-air flames is explored. The key feature of this burner is that the extent of mixing between ammonia and hydrogen before injection into the combustion chamber can be tuned by recessing an inner dividing tube. Non-reacting, 1-D Raman measurements confirm that recessing the dividing tube changes the radial profile of fuel composition at the injector's outlet. Time-averaged flame imaging shows that this modifies, in turn, the flame morphology. Specifically, the more stratified is the fuel, the least compact is the flame. In the most stratified case, a "blue" flame, predominantly burning hydrogen, pilots another "orange" flame, predominantly burning ammonia, further downstream. Acoustic measurements show that the least stratified (most compact) flame exhibits intrinsic thermoacoustic modes (ITA), while these modes are not featured for the most stratified (least compact) flame. Consequently, this study demonstrates that the thermoacoustic behavior of this carbon-free burner can be altered by tailoring the injection of ammonia and hydrogen while keeping the thermal power (7.4 kW), global equivalence ratio (0.64), and fuel flow rates strictly constant. This lays the foundation for the development of future practical burners in which the extent of mixing between ammonia and hydrogen upstream of the flame would be actively controlled to mitigate thermoacoustic coupling. [ABSTRACT FROM AUTHOR]

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Topology transitions of swirl-stabilized flames in a cylindrical combustion chamber are analyzed for lean CH 4 /H 2 /air combustible mixtures. It is demonstrated that when the tip of a swirl flame is quenched at the combustor side wall, V to M flame topology transitions are triggered by flashback of the flame front along this wall. An analysis of the power spectral densities of the chemiluminescence luminosity reveals that these topological transitions are not consecutive to the appearance or disappearance of any axisymmetric or helical periodic flow structures interacting with the flame. It is shown that the appropriate space to analyze the stabilization regimes of these swirling flames corresponds to a diagram based on Karlovitz and Lewis numbers. Transitions from V to M flame shapes are found for a critical Karlovitz number below which the flame takes an M shape. The value of this critical Karlovitz number decreases when the Lewis number of the combustible mixture increases. The same type of law is found for M to V shape flame transitions, but this boundary does not coincide with the V to M shape transition boundary. In between these boundaries, the flame takes intermittently V or M shapes randomly. It is then shown that the critical Karlovitz number corresponding to the transition of V to M shape may be expressed by the product of a velocity gradient g calculated with the bulk injection velocity U and a characteristic scale of the outer recirculation zone, the laminar flame thickness α / S L , and the inverse of a vortex induced flame displacement speed S T θ . This displacement speed is related to the maximal azimuthal velocity of the swirling jet at the injector outlet and to the laminar burning velocity. Using this definition of the Karlovitz number, V to M flame topology transitions collapse roughly on the same curve for all flow conditions explored when the swirl number (0.3 ≤ S ≤ 0.68), injection bulk velocity (7 ≤ U ≤ 25 m · s − 1 ), equivalence ratio (0.5 ≤ ϕ ≤ 0.9), and hydrogen enrichment in the fuel blend ( 0 ≤ X H 2 fuel ≤ 0.37 ) are modified for rod stabilized and aerodynamically stabilized swirl flames. Results presented in this study might be used to ease predictions of the shape taken by swirl flames interacting with the chamber side wall when the flow operating conditions are modified. [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]

In the context of low-carbon transition, ammonia (NH 3) is considered as a promising zero-carbon fuel owing to its high hydrogen density, well-established storage, and transportation systems. However, its low reactivity and high nitrogen content could induce flame instability and high NO emission problems during combustion, which hinders its large-scale application in industrial furnaces, gas turbines, and engines. The reactivity of NH 3 can be effectively boosted by blending it with a more reactive fuel, such as methane (CH 4). In addition, it has been demonstrated that burning NH 3 at slightly rich equivalence ratios produces low NO emissions, but air-staging is needed to oxidize the unburned fuel in a secondary lean combustion zone. This paper carries out an experimental and numerical study for NH 3 /CH 4 premixed swirling flames in a model combustor with and without air-staging. The main objective is to investigate the potential of using air-staging for controlling NO emissions from NH 3 /CH 4 flames, and to examine the influence of key air-staging parameters, such as staged air ratio (SAR), height of staged-air (H) and number of staged-air nozzle (N), on flame typology as well as NO and CO emissions. Experimental results show that globally lean condition fails to generate satisfactory NO emission for NH 3 /CH 4 mixtures under non-staging mode, and the maximum NO emission is produced by the X NH 3 = 50% mixture which exhibits the largest challenge for NO control. By initiating air-staging for X NH 3 = 50% mixture, the flame topology as well as CO and NO emissions are strongly affected by the key air-staging parameters. In relatively large SAR (> 30%) and small H (< 80 mm) conditions, the staged-air impinging effect should be taken into consideration since it could destroy the locally rich atmosphere in the primary combustion zone, creating lean burning pockets and enhancing NO formation. Increasing N plays an accelerating role on NO generation due to improved mixing homogeneity in the vicinity of staged-air, while this effect can be counteracted by mitigating the staged-air impinging effect via decreasing the staged-air injection momentum. Moreover, an improved NH 3 /CH 4 reaction mechanism based on Okafor's is proposed which shows remarkably high accuracy in predicting NO emission for NH 3 /CH 4 mixtures under both non-staging and air-staging modes. This work provides new insights towards the understanding of NH 3 /CH 4 combustion using air-staging. • NH 3 /CH 4 combustion in air-staging and non-staging modes are studied. • Effects of key staged-air parameters on NO and CO emissions are investigated. • Optimal staged-air parameters for simultaneous low NO and CO emissions are determined. • An improved NH 3 /CH 4 reaction mechanism is proposed. [ABSTRACT FROM AUTHOR]

Myocardial flow reserve (MFR) measurement is predictive of major adverse cardiovascular (CV) events (MACE) and has been shown to improve diagnostic performances of coronary artery disease (CAD) in cardiac positron emission tomography. MFR can now be assessed during dynamic myocardial perfusion imaging (MPI) in SPECT, but its correlation with the patient's risk of coronary artery disease (CAD) remains to be investigated. We evaluated the results of SPECT MFR measurement in patients referred for CAD screening. In total, 137 patients (61 male, 76 female) referred for CAD screening MPI between November 2018 and March 2020 were included in a prospective trial (CFR-OR). 10 years risk of cardiovascular death according to the European Society of Cardiology (SCORE) was calculated. SPECT data were acquired on a CZT-cardiac camera using a stress/rest one-day Tc-99m-tetrofosmin protocol. Low dose thoracic CT was used for coronary calcium score (CCS) evaluation. Invasive Coronary Angiography (ICA) was performed upon decision of the referring cardiologist. Mean SCORE was 4 ± 3.1%. Mean global MFR was 2.50 ± 0.74. There was a significant inverse correlation between MFR and SCORE (P = 0.006), gender (P = 0.019), and the number of cardiovascular risk factors (P = 0.01). MFR was significantly reduced in patients with CCS above 1 (P = 0.01). Twenty-three patients underwent ICA. Eighteen showed coronary plaques qualifying for high-risk patients; among them, 16 patients had impaired MFR (the 2 other patients had moderate stenosis, < 50%, with negative FFR), whereas only 4 had abnormal visual MPI. Based on global MFR, and with a threshold of 2, sensitivity and specificity were respectively 83.3% and 100%, and Area Under Curve (AUC) was 0.94. Global MFR measured during MPI SPECT for CAD screening may contribute to high-risk patient identification. It significantly enhances SPECT diagnostic performances and could help physician decision to perform ICA. [ABSTRACT FROM AUTHOR]

Introduction L'éosinophilie est un motif fréquent de consultation. Les étiologies sont dominées par les causes allergiques et médicamenteuses, néoplasiques et parasitaires. Lorsqu'elle est importante et durable, elle peut être symptomatique mais parfois les signes sont quasi inexistants. Cela ne dispense pas d'explorations afin de prévenir la survenue de signes systémiques. Observation Un patient de 38 ans aux antécédents de colique néphrétique consulte au service d'accueil des urgences pour douleur spontanée de l'hallux gauche d'horaire inflammatoire. La biologie de routine montre une éosinophilie à 11 G/L sans anomalie des autres lignées. Il n'y a pas d'altération de l'état général. Il a une toux chronique non productive invalidante. L'examen clinique est sans anomalie particulière. Il est convoqué pour des explorations. Dans l'intervalle, il a des douleurs du mollet gauche, sans phlébite objectivée. Le patient n'a jamais quitté la France. La NFS montre : hémoglobine 16 g/dL, plaquettes à 181 G/l, leucocytes 18,15 G/L, polynucléaires neutrophiles 2,05 G/L, polynucléaires éosinophiles 11 G/L, lymphocytes 3,65 G/L. Il n'a pas de syndrome inflammatoire. Les sérologies toxocarose, trichinellose sont négatives. Le sédiment urinaire ne montre pas de protéinurie ni d'hématurie, pas d'éosinophilie. Les ANCA, de même que les anticorps antinucléaires et le facteur rhumatoïde sont négatifs, le complément est normal. Les IgE totales sont négatives. La recherche du transcrit FIP1L1 PDGFRA (4q) par RT PCT revient positive. Sous IMATINIB, il est noté une rémission clinique avec disparition de la toux, de la douleur de l'hallux et des douleurs du mollet gauche ainsi qu'une diminution de l'éosinophilie et la négativation de la recherche du transcrit. Conclusion L'hyperéosinophilie essentielle FIP1L1/PDGFRA correspond à une délétion 4q12 responsable de la fusion entre les gènes FIP1L1 et PDGFRA. La protéine chimérique résultant de cette fusion a une activité tyrosine kinase constitutive, ce qui explique les rémissions cliniques et cytologiques des hyperéosinophilies essentielles traitées par un inhibiteur spécifique de tyrosine kinase, l'IMATINIB. Dans ce cas, les manifestations cliniques retrouvées sont dominées par des atteintes musculosquelettiques (douleur de hallux d'horaire inflammatoire, douleur du mollet sans phlébite objectivée), que l'on peut attribuer à l'éosinophilie, puisque régressives à 3 mois de l'initiation du traitement. Dans la majorité des études sur l'éosinophilie, cette atteinte n'est pas mentionnée. Elle est décrite néanmoins, dans la maladie de Shulman. Dans cette dernière, des myalgies spontanées ou provoquées sont rapportées à la phase initiale chez près de deux patients sur trois et jusqu'à 86 % des patients au moment du diagnostic. Ce cas nous amène à rechercher les atteintes musculosquelettiques dans les syndromes hyperéosinophilique, non uniquement dans la maladie de Shulman. [ABSTRACT FROM AUTHOR]

• New method for populating line-by-line spectroscopic databases with beyond-Voigt lineshape parameters. • He-perturbed H2. • Ab initio quantum scattering calculations of beyond-Voigt line-shape parameters. • Simple and compact formula that allows the speed-dependence parameters to be calculated directly from the generalized spectroscopic cross sections. • Dataset compatible with the HITRAN database. • Temperature dependences of the beyond-Voigt line-shape parameters covering the range from 20 to 1000 K. We demonstrate a new method for populating line-by-line spectroscopic databases with beyond-Voigt line-shape parameters, which is based on ab initio quantum scattering calculations. We report a comprehensive dataset for the benchmark system of He-perturbed H 2 (we cover all the rovibrational bands that are present in the HITRAN spectroscopic database). We generate the entire dataset of the line-shape parameters (broadening and shift, their speed dependence, and the complex Dicke parameter) from fully ab initio quantum-scattering calculations. We extend the previous calculations by taking into account the centrifugal distortion for all the bands and by including the hot bands. The results are projected on a simple structure of the quadratic speed-dependent hard-collision profile. We report a simple and compact formula that allows the speed-dependence parameters to be calculated directly from the generalized spectroscopic cross sections. For each line and each line-shape parameter, we provide a full temperature dependence within the double-power-law (DPL) representation, which makes the dataset compatible with the HITRAN database. The temperature dependences cover the range from 20 to 1000 K, which includes the low temperatures relevant for the studies of the atmospheres of giant planets. The final outcome from our dataset is validated on highly accurate experimental spectra collected with cavity ring-down spectrometers. The methodology can be applied to many other molecular species important for atmospheric and planetary studies. [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]

• Ab initio quantum scattering calculations. • Temperature dependences of the pressure broadening, pressure shift, speed dependence and the complex Dicke parameter. • Comparison of the four-temperature-ranges (4TR) and double-power-law (DPL) temperature dependence models. • Hartmann-Tran profile (HTP) parameterization. Temperature dependences of molecular line-shape parameters are important for the spectroscopic studies of the atmospheres of the Earth and other planets. A number of analytical functions have been proposed as candidates that may approximate the actual temperature dependences of the line-shape parameters. In this article, we use our ab initio collisional line-shape calculations for several molecular systems to compare the four temperature ranges (4TR) representation, adopted in the HITRAN database [J. Quant. Spectrosc. Radiat. Transfer 2017;203:3] in 2016, with the double-power-law (DPL) representation. Besides the collisional broadening and shift parameters, we consider also the most important line-shape parameters beyond Voigt, i.e., the speed dependence of broadening and shift parameters, and real and imaginary parts of the complex Dicke parameter. We demonstrate that DPL gives better overall approximation of the temperature dependencies than 4TR. It should be emphasized that DPL requires fewer parameters and its structure is much simpler and more self-consistent than the structure of 4TR. We recommend the usage of DPL representation in HITRAN, and present DPL parametrization for Voigt and beyond-Voigt line profiles that will be adopted in the HITRAN database. We also discuss the problem of the Hartmann-Tran profile parametrization in which the correlation parameter, η , and frequency of the velocity-changing collisions parameter, ν vc , diverges to infinity when collisional shift crosses zero; we recommend a simple solution for this problem. [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]

• Piloted turbulent nonpremixed jet flames are studied at high pressure/Re. • Soot inception zone migrates upstream as pressure increases, even for constant U. • Corrugation of OH layer increases as Re increases through density. • Frequency of OH layer separation increases as pressure/Re increase. • OH layer fragments into smaller "islands" as Re increases. A recently commissioned high-pressure combustion duct is used to investigate a family of ten, piloted, sooting, turbulent nonpremixed flames over a range of pressures and Reynolds numbers. For all conditions, the central jet is composed of 35% ethylene and 65% nitrogen by volume. In one series of flames, the Reynolds number is kept constant while the pressure is increased, whereas in the other series, the bulk jet velocity is maintained. The maximum pressure, p , is 5 bar and the maximum Reynolds number, Re, is 50,000. A DSLR camera and 10-kHz OH-PLIF are used to characterized the near field. DSLR camera images show that the length of the blue region immediately downstream of the nozzle decreases as the pressure increases, rapidly in the constant Reynolds number series, and gradually in the constant velocity series. Analysis of OH-PLIF images in the near field show that corrugation of the flame front is fairly insensitive to changes in pressure if the Reynolds number is held constant. As the Reynolds number (and pressure) is increased, the flame front becomes more corrugated and the frequency of OH layer extinction increases. This leads to smaller "islands" of OH that are separated by shorter distances. However, despite increased local extinction, the flames remain attached even as the Reynolds number is increased to more than twice the maximum Reynolds number that is possible at atmospheric pressure. [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]

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO 2 and H 2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid. • The premixed formic acid-methane-air flames are stabilized for equivalence ratios of 0.5–0.8. • The addition of the formic acid up to mole fraction of 0.6 increases both the flashback and blowout limit. • Increasing the Reynolds number does not affect blow out limits. • The swirl number has a non-monotonous effect on the flashback and lean blowout limits. • The NO and CO emission of the lean formic acid-methane-air flames are low (<20 ppmvd). [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]

The detonation front's unstable structure leads to an unsteady and three-dimensional (3D) phenomenon that renders the study of the cell cycle challenging. Traditionally, fundamental studies are carried out in narrow channels where the detonation behavior is very peculiar (quasi two-dimensional with velocity deficit). In this study, we propose a fully experimental approach to study the cell cycle in the case of multicellular detonations. The cell cycle is characterized through three techniques: systematic and statistical analysis of soot foil, planar laser-induced fluorescence on nitric oxide, and Rayleigh scattering. These techniques provide measurements for cell size, local induction length, and local shock speed, respectively. The work is carried out in the 2 H 2 -O 2 -3.76Ar and the 2 H 2 -O 2 -3.76N 2 mixtures at 293 K, and 20 kPa and 25 kPa, respectively. These conditions ensure that the cell pattern is considered being between regular and weakly irregular, thus, a shot-to-shot reconstruction of the cell cycle is possible. The cell widths follow a normal distribution, from which a quantitative parameter (2 σ / λ) is proposed to assess the cell regularity, experimentally. The evolution of the speed and the local induction length are reconstructed along the cell cycle. The results agree with the available data for narrow channels and constitute the first of their kind for 3D detonation (i.e., multicellular in the transverse dimension). Two methods are proposed to analyze the local induction length δ i and compare it to the available literature (experimental and numerical studies). The technique can be applied to mixtures where the mean cell width is a meaningful parameter from highly regular to irregular mixtures. Novelty and Significance statement For the first time, combined soot-foils, NO-PLIF, and Rayleigh scattering measurements were used to reconstruct and characterize the cellular cycle of multicellular detonations using a 2 H 2 -O 2 -3.76Ar and 2 H 2 -O 2 -3.76N 2 mixtures at 293 K, and 20 kPa and 25 kPa, respectively. This constitutes the first combined measurements of local induction length and local front speed in a multicellular configuration, where the number of cells in the section is large, i.e., the cell width (λ) is small compared to all dimensions of the rig. This study provides two methodologies to extract an experimental Δ i , which is a quantity that can be compared with the ZND theory. Such measurements were not achievable in multicellular conditions until now. The measurement of the Δ i demonstrates that a fully experimental correlation between this experimental Δ i and λ can be obtained for the first time. These new results are important for the quantitative validation of chemical kinetic schemes for predictive simulations of detonations. [ABSTRACT FROM AUTHOR]

The present study aims to evidence the effect of the excitation line on the planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) imaging in H 2 -fueled detonation wave. We experimentally validated an updated laser-induced fluorescence (LIF) model, called KAT-LIF, which simulates spectrally-resolved fluorescence spectra, using a recently developed optical detonation duct. We numerically investigated the effects of the excitation line, the initial pressure (20–100 kPa), and the diluent (N 2 /Ar) on the fluorescence spectrum, the spectrally- and one-dimensionally-averaged LIF intensity, and the quantitative capabilities of the OH-LIF measurements for different 2H 2 -O 2 -3.76diluent detonable mixtures. The investigated excitation lines were (0,0)Q1(7), both (1,0)Q2(8) and (1,0)Q1(9), and (1,0)Q1(6), which all belong to the A 2 Σ + ← X 2 Π transition. The main findings are the following: (i) considering the commercially available OH filters, Q2(8)+Q1(9) excitation scheme has the highest LIF intensity for all the investigated H 2 -fueled detonations, while Q1(7) could provide the strongest intensity with better (custom) collection optics; (ii) the maximum LIF intensity decreases with increasing pressure for all the excitation schemes; (iii) using a single point calibration, at the fluorescence peak, it is not possible to perform quantitative measurements of OH radicals for any H 2 -fueled detonation, using the conventional excitation schemes. Finally, we experimentally evidenced more favorable excitation schemes to obtain qualitative information far behind the front, by employing the saturated regime of fluorescence or the optically thin linear regime with appropriate laser configuration. These two excitation schemes correspond to more appropriate LIF strategies that will enable better detonation flow visualization in future studies. [ABSTRACT FROM AUTHOR]

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C 2 H 4 -N 2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p 2.2. In the constant velocity series, the scaling drops to p 1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame's center and a slight increase in the mean soot volume fraction. [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]

• Experimental investigation of differential diffusion effects in NH3-H2-N2 jet flames using 1D quantitative Raman scattering measurements. • Introduction of a diff-diff parameter for ammonia combustion, providing a quantitative measure of the differential diffusion effects. • Identification of strong differential diffusion in the near field of turbulent flames, up to an axial distance of 10 diameters. • Influence of NH3 cracking ratio on the diff-diff parameter, with higher ratios leading to increased differential diffusion, particularly in the low-temperature region. • Persistence of differential diffusion effects in the far field, as reflected in NH3/H2 profiles converging to a Le = 1-like profile with an increased NH3/H2 ratio. Differential diffusion (diff-diff) effects are analyzed in both non-premixed laminar and turbulent NH 3 -H 2 -N 2 jet flames at 5 bar using 1D quantitative Raman scattering measurements. The two target flames feature fuel compositions simulating a 14% and a 28% NH 3 cracking ratio. A diff-diff parameter is introduced for ammonia combustion, analogous to the definition used for hydrocarbon flames. Measurements in laminar flames, compared to Chemkin simulation featuring multi-component transport show that the instrument is capable of measuring the mean diff-diff parameter, but that single-shot measurements are compromised by the large sensitivity of the parameter to errors in N 2 and NH 3 mass fractions. The analysis of the diff-diff parameter in turbulent flames shows that both flames have strong differential diffusion in the near field up to an axial distance of 10 diameters (Z/D = 10), then the turbulent mixing becomes dominant further downstream. The flame with a 28% NH 3 cracking ratio exhibits a higher diff-diff parameter especially in the low-temperature region (<1000 K) due to the higher H 2 content. The competition between differential diffusion and turbulent mixing in the near field is also reflected in the NH 3 , H 2 , and NH 3 /H 2 profiles, falling in between prediction for Le = 1 and multi-component transport. Interestingly the NH 3 /H 2 profiles in the far field converge to a profile matching that of Le = 1, but with an increased NH 3 /H 2 ratio, showing the persistence of the differential diffusion effects in the region dominated by turbulent mixing. [ABSTRACT FROM AUTHOR]

We report revisited experimentally retrieved and theoretically calculated He-broadened Lorentz half-width coefficients and He- pressure-shift coefficients of 45 carbon monoxide transitions in the 1←0 band. The spectra analyzed in this study were recorded over a range of temperatures between 79 and 296 K. The He-broadened line parameters and their temperature dependences were retrieved using a multispectrum nonlinear least squares analysis program. The line shape models used in this study include Voigt, speed dependent Voigt, Rautian (to take into account confinement narrowing) and Rautian with speed dependence, all with an asymmetric component added to account for weak line mixing effects. We were unable to retrieve the temperature dependence of line mixing coefficients. A classical method was used to determine the He-narrowing parameters while quantum dynamical calculations were performed to determine He-broadening and He-pressure shifts coefficients at different temperatures. The line mixing coefficients were also derived from the exponential power gap law and the energy corrected sudden approximation. The current measurements and theoretical results are compared with other published results, where appropriate. [ABSTRACT FROM AUTHOR]

• MILD combustion of CH 4 is experimentally studied in model combustor. • Inverse-diffusion flame configuration improves flame stability in MILD combustion. • MILD combustion is not sustainable in smaller loads. • Flame response is more sensitive to operating parameters in swirling combustion. MILD (moderate or intense low-oxygen dilution) combustion is a promising technology for mitigating NOx emission from fuel combustion, but the combustion stability becomes an issue when using low-preheat air due to the slower reaction at the burner exit. In this study, an inverse-diffusion flame (IDF) burner configuration is proposed to enhance the combustion stability of MILD regime. The burner features an earlier preheat of fuel stream instead of usually adopted air stream, and can be also operated in traditional diffusion swirling combustion mode. Experiments are conducted in a model combustor to examine the effect of varying burner load (P) and equivalence ratio (φ) on flame topology, pollutant (CO and NO) emission performance, as well as flame emitting spectrum for this burner. It shows that increasing P can enhance the combustion stabilities of both swirling mode and MILD mode, and MILD combustion is found unable to be sustained when P is lower than 2 kW. Increasing φ enlarges the flame liftoff distance under swirling combustion, but has a much smaller effect on MILD combustion. Emission data shows a nearly 50 % reduction of NO by MILD combustion comparing to swirling combustion in all operating cases. Spectra analysis suggests a higher sensitivity of the flame response to the operating parameters under swirling combustion in respect to MILD combustion. A longer combustion chamber is found to produce a lower CO emission but has negligible effect on NO emission. Flow field and temperature distributions by further numerical simulation clearly demonstrate the effectiveness of fuel preheat by adopting IDF burner configuration for enhancing the flame stability when adopting normal temperature air in MILD combustion. [ABSTRACT FROM AUTHOR]

This study describes a technique that utilizes a single, tunable, pulsed dye laser and two intensified CCD cameras to image NH and NO simultaneously in turbulent ammonia-hydrogen-nitrogen jet flames. The NO radical is excited at 236.214 nm in its (0,1) band, while NH is excited in its A3Π-X3Σ–(1,0) band using the residual energy of the beam at 303.545 nm, necessary to yield 236.214 nm via mixing with the fundamental of the pump laser at 1064.ßnm. Data show that it is possible to image the instantaneous structure of these turbulent flames, specifically, NH delineates the reaction zone while NO also marks the location of burnt products. [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]

The external- and self-excited premixed swirling flames are experimentally studied to understand the thermoacoustic instabilities under different injector sizes. We use high-speed CH* chemiluminescence, formaldehyde laser-induced fluorescence and particle image velocimetry to measure the dynamic characteristics of the flame surface and flow field. Reducing the size of the injector from 20 mm to 16 mm changes the thermoacoustic system's stability. The flame transfer function shows that reducing the size of the injector significantly reduces the phase delay, enhances the response amplitude, and annihilates the local gain extrema. The low-order network model analysis is performed, and it is found that the phase variation caused by changing injector size is the main reason for the induced thermoacoustic instability. The pixel-by-pixel decomposition of CH* images is used to extract the weighted gain and phase maps of local heat release oscillation. The swirling flame with a small injector size of 16 mm has a short time delay and a slow phase velocity, which results in strong in-phase interference of the local heat release oscillators and enhanced gains. Reducing the size of the injector strengthens the inner and outer shear layers, thereby suppressing the heat release fluctuation of the flame base. The enhanced vorticity supply of the shear layer increases the size and strength of the outer vortex ring, resulting in strong heat release fluctuations at the flame tip, which is responsible for its large unadulterated fluctuation intensity. These changes in the flow field change the relative heat release intensity and fluctuation amplitude between the flame base and tip. The out-of-phase interference among multiple vortices causes weak flame response at high frequencies. [ABSTRACT FROM AUTHOR]