Direct numerical simulation of igniting non-premixed hydrogen combustion for the Argon Power Cycle

The Argon Power Cycle (APC) is a compression ignition combustion concept that would substantially enhance efficiency by using argon as the working fluid. When used with hydrogen and oxygen, such closed loop system would be free of emissions. Fundamental understanding on the combustion dynamics of such system is needed in order to determine the best injection strategy. A direct numerical simulation of a fully developed turbulent (Re=10000) reacting case which resembles the direct injection of H₂ has been performed. Attention was devoted to (1) understanding the influence of preferential diffusion and turbulence on the ignition behavior and development of flame kernels, (2) determining the composition space accessed by the turbulent and laminar analogue, and (3) finding the types of flamelets that could resemble such composition space. It was found that igniting kernels emerge near the stoichiometric mixture fraction in regions convex to the fuel side, and with high scalar dissipation, in contrast to what has been reported for other fuels in the literature. Furthermore, these igniting kernels can extinguish if exposed to high curvature levels due to the enhanced diffusion of radicals out of the kernel. There is good agreement between the composition space accessed by the turbulent flame and the laminar analogue, but better agreement can be reached by using strained and curved flamelets.