Abstract
The formation of nitrogen oxide (NO) in wrinkled laminar NH3/H2/N2-air premixed flames is investigated utilizing two-dimensional Direct Numerical Simulation (DNS) with detailed chemical kinetics as well as one-dimensional freely propagating flame calculations. The spatial pattern of NO formation is observed to be closely linked to flame curvature and affected by thermo-diffusive effects acting on key chemical species. Preferential diffusion of H2 into convex-shaped portions of the flame front leads to a local increase in equivalence ratio. This change in local equivalence ratio is found to prominently affect the NO formation. If the fuel-oxidant mixture is globally lean, a local increase in equivalence ratio strengthens the NO formation (locally); in a globally rich fuel-oxidant mixture, conversely, the NO concentration will be reduced in correspondence of local increments of the equivalence ratio. A sensitivity analysis with respect to NO formation reveals that decomposition of NH2 is governed by two competing pathways: the decomposition via NH and N to N2 on the one hand and the oxidation via HNO to NO on the other hand. The local radical pool, which is affected by preferential diffusion of H2 and depletion of O2, and the local fuel-oxidant mixture ratio jointly strengthen further local differences between H2-depleted (concave-shaped) portions of the flame front and H2-enriched (convex-shaped) ones. This is confirmed across a wide range of equivalence ratios from lean to rich conditions.
Highlights
Hydrogen (H2) represents the simplest and one of the cleanest energy carrier for large scale thermal energy conversion and its widespread deployment in the energy sector, if realized, represents one of the most promising strategies to reduce the dependence on fossil fuels and simultaneously reduce atmospheric pollution
In our investigation, adopting the same fundamental methodological approach as in earlier studies, we provide a detailed insight into the spatial patterns of nitrogen oxide (NO) formation that take place in wrinkled laminar flames characterized by different global stoichiometric conditions and subject to various degree of strain and curvature, locally in the reaction layer
This is meant as a representative case for the process of partial cracking of ammonia that is conducted to a lower extent, thereby requiring a smaller amount of waste heat from the thermodynamic cycle
Summary
Hydrogen (H2) represents the simplest and one of the cleanest energy carrier for large scale thermal energy conversion and its widespread deployment in the energy sector, if realized, represents one of the most promising strategies to reduce the dependence on fossil fuels and simultaneously reduce atmospheric pollution. The present work does not claim to closely (and rather ambitiously) reproduce the turbulent combustion process taking place in realistic burner geometries at gas turbine conditions, DNS of 2-D geometrically simple flames provides useful insight into key, rate-limiting fundamental mechanisms that take place in more complex flame configurations These calculations are, as such, relevant to the improved understanding and the further development of industrial combustion applications. In our investigation, adopting the same fundamental methodological approach as in earlier studies, we provide a detailed insight into the spatial patterns of NO formation that take place in wrinkled laminar flames characterized by different global stoichiometric conditions and subject to various degree of strain and curvature, locally in the reaction layer This is to improve our understanding of the chemical pathways and of their. The paper is organized as follows: Section 2 describes the numerical method and the flame configurations simulated, Section 3 presents the DNS results and provides an interpretation based on chemical reaction kinetics considerations, while Section 4 summarizes the present findings and suggests topics for further work
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