Direct numerical simulations of auto-igniting mixing layers in ammonia and ammonia-hydrogen combustion under engine-relevant conditions

Abstract

This study investigated auto-ignition characteristics of ammonia-air and ammonia-hydrogen-air laminar and turbulent mixing layers by means of direct numerical simulations (DNS) under elevated pressure conditions. The results show that elevated pressure and hydrogen addition accelerate the auto-ignition process, reducing the auto-ignition delay time. Analysis of the heat release rate revealed that the first peak of the heat release rate corresponds to the increment of appearance in temperature (induction stage) and the second peak of the heat release rate corresponds to the steady maximum temperature regime (thermal runaway stage). The results found that both induction and thermal runaway stages are affected by turbulence for pure ammonia-air mixing layers, while only the thermal runaway stage is affected by turbulence for ammonia-hydrogen-air mixing layers. The auto-ignition occurs along the most reactive mixture fraction with lower scalar dissipation rate, being further reduced by elevated pressure and hydrogen addition. Three radicals (NH2, OH, HNO) distinguish the entire auto-ignition process very well for all cases.