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

TL;DR

This study numerically investigates NOx emissions in lean hydrogen flamelets under high strain rates, revealing a novel decrease in NOx emissions with increasing strain, driven by physical effects of stretch on the flame.

Contribution

First demonstration of decreasing NOx emissions in lean hydrogen flamelets with increasing strain rate, supported by detailed 1D and 2D simulations and pathway analysis.

Findings

NOx emissions decrease as strain rate increases.

No NOx production in the second dimension confirms physical effect of stretch.

Thermal NOx decreases significantly with strain, while NNH pathway remains constant.

Abstract

NO$_{\rm x}$ 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 $a=100 \, {\rm s}^{-1}$ to $a=10000 \, {\rm s}^{-1}$ and the \texttt{GRI3.0} mechanism. Following a similar setup, two-dimensional direct numerical simulations are also conducted for representative strain rates of $2000 \, {\rm s}^{-1}$ and $5000 \, {\rm s}^{-1}$. Both solutions show a decreasing NO$_{\rm x}$ 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$_{\rm x}$ 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$_{\rm x}$ formation pathways at $a=2000 \, {\rm s}^{-1}$ and $a=5000 \, {\rm s}^{-1}$ is performed. Thermal NO$_{\rm x}$ and NNH pathways are shown to both contribute significantly to the total NO$_{\rm x}$ production. While the NNH route contribution is roughly constant at different strain rates, a significant decrease is observed along the thermal NO$_{\rm x}$ route. Overall, results show that lean and highly-strained hydrogen flames experience a significant decrease of NO$_{\rm x}$. This property is discussed and analysed in the paper.