NOx emissions trends in hydrogen lean premixed flamelets at high strain rate

Abstract

NO${}_{\mathrm{x}}$ formation in lean premixed and highly-strained pure hydrogen-air flamelets is investigated numerically. Lean conditions are established at an equivalence ratio of 0.7. Detailed-chemistry, one-dimensional simulations are performed on a reactants-to-products counter-flow configuration with an applied strain rate ranging from $a=100{\mathrm{s}}^{-1}$ to $a=10000{\mathrm{s}}^{-1}$ and the \texttt{GRI3.0} mechanism. Following a similar setup, two-dimensional direct numerical simulations are also conducted for representative strain rates of $2000{\mathrm{s}}^{-1}$ and $5000{\mathrm{s}}^{-1}$. Both solutions show a decreasing NO${}_{\mathrm{x}}$ trend as the applied strain rate is increased. This decreasing emission outcome is highlighted for the first time in this study for lean pure-hydrogen flamelets. A deep analysis of the 2D solution underlines that there is no production of NO${}_{\mathrm{x}}$ in the second dimension, thus proving that the emission trend is not a result of a setup preconditioning, but is instead a direct physical effect of stretch on the flame. Furthermore, a detailed analysis of the NO${}_{\mathrm{x}}$ formation pathways at $a=2000{\mathrm{s}}^{-1}$ and $a=5000{\mathrm{s}}^{-1}$ is performed. Thermal NO${}_{\mathrm{x}}$ and NNH pathways are shown to both contribute significantly to the total NO${}_{\mathrm{x}}$ production. While the NNH route contribution is roughly constant at different strain rates, a significant decrease is observed along the thermal NO${}_{\mathrm{x}}$ route. Overall, results show that lean and highly-strained hydrogen flames experience a significant decrease of NO${}_{\mathrm{x}}$. This property is discussed and analysed in the paper.

Cited by in corpus (1)

• Assessment of tabulated-chemistry models for lean premixed strained hydrogen flames with low-dimensional manifolds