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• 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]

• 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]

• 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]