2022/07/08 by Porcarelli, Alessandro, Kruljevic, Boris, Langella, Ivan · 2 citations
#FOS: Physical sciences #Fluid Dynamics (physics.flu-dyn)
paper · doi:10.48550/arxiv.2207.03913
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 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.