Tangential diffusion effects in thermodiffusively unstable ammonia/hydrogen/nitrogen-air laminar premixed flames

Abstract

Partially cracked ammonia is a promising hydrogen-carrying fuel with logistical advantages compared to pure hydrogen. However, like hydrogen-air premixed flames, under fuel-lean conditions, ammonia/hydrogen/nitrogen-air premixed flames can be thermodiffusively unstable. These instabilities affect the flame propagation speeds as well as the local formation of nitrogen oxides and nitrous oxide (reactive nitrogen emissions). To assess the viability of partially cracked ammonia as a zero-carbon fuel, understanding and ultimately modeling these pollutants in thermodiffusively unstable flames is critical. In this work, detailed two-dimensional simulations of laminar premixed planar flames were conducted to understand the development of thermodiffusive instabilities in flames of ammonia/hydrogen/nitrogen mixtures and air. The degree of ammonia cracking was varied to understand the influence of fuel composition on the instability behavior and subsequent formation of nitrogen oxides and nitrous oxide. The detailed simulation results exhibit considerable differential diffusion effects and regions of increased and decreased reactive nitrogen emissions corresponding to local flame curvature. The databases from these detailed simulations were then used to evaluate a premixed manifold model. Manifold models significantly decrease computational cost by mapping the high-dimensional thermochemical state to a lower-dimensional manifold. A premixed manifold model is considered that includes differential diffusion and flame curvature. However, analysis of the databases from these detailed simulations revealed a very strong effect of transport orthogonal to the progress variable gradient, that is, tangential diffusion. Direct tangential diffusion effects are actually stronger for less cracked mixtures due to the larger flame thickness of flames with more ammonia content. For pollutants, direct tangential diffusion effects are important for all cracking ratios, and the existing formulation of the manifold model cannot accurately predict these species. Furthermore, indirect effects of tangential diffusion on pollutants through the local radical pool and equivalence ratio also influence pollutants and are apparently stronger for the higher cracking ratio. Implications for manifold modeling are discussed, and a generally applicable strategy for predicting pollutant mass fractions in partially cracked ammonia flames must directly model tangential diffusion effects rather than rely only a mixture fraction variable to account for only indirect tangential diffusion effects that are most important for fuels containing purely or mostly hydrogen.