Effect of differential diffusion on head-on quenching of premixed NH3/H2/air flames within turbulent boundary layers
Cheng Chi, Chunkan Yu, Bénédicte Cuenot, Ulrich Maas, Dominique Thévenin
Abstract
Direct numerical simulations (DNS) have been done for premixed laminar and turbulent NH 3 /H 2 /air flames propagating towards a cold wall. Two turbulent channel flows (with well-characterized turbulent boundary layers) at Re τ = 280 and 313 have been simulated for the turbulent flame cases. Near-wall low temperature chemistry has been considered. Effect of differential diffusion on the flame head-on quenching characteristics (quenching distance and maximum wall heat flux) has been checked in detail. Finally, two underlying mechanisms determining differential diffusion effects have been identified: (a) preferential diffusion of H radical increases the reaction rates of the near-wall low temperature (radical recombination) reactions, resulting in larger quenching distance and wall heat flux; (b) differential diffusion of fuels results in smaller flame heat release, thus, larger quenching distance and smaller wall heat flux. The effect of H radical preferential diffusion is suppressed in rich H 2 /air turbulent flames and strengthened in lean H 2 /air turbulent flames, due to the differential diffusion focusing/defocusing effects. The effect of differential diffusion of fuels is suppressed in turbulent flames, due to the enhanced mixing process, especially at low Damköhler numbers typical for NH 3 /H 2 /air flames interacting with highly turbulent flows.