Quenching and pollutant emissions in side-wall and head-on NH3/H2/N2 premixed laminar flames

We present quenching distance, wall heat loss, and pollutant emission results from a series of simulations of NH${}_3$/H${}_2$/N${}_2$ premixed laminar flames in side-wall and head-on quenching configurations. Conditions cover lean to rich mixtures (equivalence ratio from 0.3 to 1.2) at both atmospheric and moderate (10 atm) pressures. For each set of conditions, two-dimensional “V”-flame simulations with side-wall quenching (SWQ) and with symmetric boundary conditions are compared to isolate wall heat loss from curvature effects. Simulations of one-dimensional head-on quenched (HOQ) flames covering the same range of conditions are also included for further comparison. First, a non-monotonic relationship between quenching Peclet number and equivalence ratio is found at 10 atm for both SWQ and HOQ, attributed to a significant change in flame structure at lean conditions, with the Jiang chemical kinetics mechanism. Second, wall heat loss normalized by laminar flame power is lower for HOQ flames, compared to SWQ, at lean conditions and higher at rich conditions, which is attributed to the effect of flame curvature on heat release rate in SWQ flames. Yet, normalized wall heat loss shows a similar correlation with quenching Peclet number for both pressures and quenching configurations. Third, similar to previously reported experimental results, we find that ammonia slip increases due to wall heat loss and curvature effects, while hydrogen slip departs negligibly from that of unstretched laminar flames. The contrast with ammonia slip is striking at low equivalence ratio and is attributed to the strongly diverging flux of hydrogen near the quenching point, promoting its consumption. Fourth, both wall heat loss and negative flame curvature at the wall significantly reduce NO emissions as the rates of NO forming reaction pathways are diminished. In the SWQ cases, NO consuming pathways are comparatively less inhibited, being in part fed by NO diffusing towards the wall from non-quenched regions. Finally, both negative curvature at the wall and wall heat loss increase N${}_2$O emissions. A non-monotonic relationship between N${}_2$O emissions and equivalence ratio is observed at elevated pressure, as the rate of the N${}_2$O consuming reaction

increases for equivalence ratios below 0.6.

Novelty and significance statement

We present the first flame-resolved simulations at elevated pressure of side-wall quenching (SWQ) and head-on quenching (HOQ) in NH${}_3$/H${}_2$ flames. The range of equivalence ratios covered also extends to leaner conditions than previously investigated in both HOQ and SWQ NH${}_3$/H${}_2$ flames (relevant to staged gas turbine combustors). The one-to-one comparison between 1D HOQ, 2D adiabatic, and 2D SWQ NH${}_3$/H${}_2$ flames is also novel, and reveals key insights on the effect of preferential diffusion on flame quenching and emissions. The results provide foundational insights that can be used to better understand and model flame-wall interaction in practical NH${}_3$/H${}_2$ combustion devices.