Directional considerations of diffusive fluxes in manifold modeling for lean ammonia/hydrogen/nitrogen-air laminar premixed flames

Ammonia is a promising hydrogen carrying fuel that, when partially cracked into mixtures of ammonia/hydrogen/nitrogen, can have similar combustion properties as methane. Containing hydrogen, under fuel-lean conditions, ammonia/hydrogen/nitrogen-air premixed flames can be thermodiffusively unstable, affecting flame propagation speeds as well as the local formation of nitrogen oxides and nitrous oxide. Understanding and accurate modeling of these pollutants are critical for the viability of ammonia as a zero-carbon fuel. Manifold-based combustion models significantly decrease computational cost by mapping the high dimensional thermochemical state to a lower dimensional manifold. Previous work evaluated a premixed manifold model in progress variable that included differential diffusion and flame curvature effects, but the manifold model did not accurately predict pollutants. Strong differential diffusion effects leading to significant diffusive transport in the direction orthogonal to the manifold coordinate were hypothesized to be responsible for this model failure. The current work investigates this hypothesis by computing the magnitude of all diffusive fluxes as well as those along the manifold coordinate and offers further insight into how these effects can be modeled.