Characteristics of ignition, combustion and emission formation of premixed ammonia-hydrogen blends by hydrogen-fueled pre-chamber turbulent jets

Abstract

Ammonia combustion initiated by hydrogen-fueled pre-chamber jets is a prospective combustion concept for enhancing ammonia combustion. However, the related fundamental studies on the ignition, combustion, and emissions characteristics of various ammonia + hydrogen blends by hydrogen-fueled pre-chamber jets are still inadequate. In this work, the effects of various ammonia + hydrogen blends in the main chamber with the hydrogen volumetric blending ratio ranging from 0 % to 50 % and the orientations of the orifice, i.e., inclined and straight orifice, are optically studied by the double-pass Schlieren imaging. The results indicate that increasing the hydrogen blending ratio markedly enhances the ignition characteristics of the mixture, resulting in the identification of three distinct ignition modes: re-ignition, secondary jet ignition, and jet flame ignition. The combustion regimes shift from the broken reaction zone to thin reaction zone. Increasing the hydrogen blending ratio improves the ammonia combustion efficiency from 96.9 % to 98.6 %, significantly reducing the unburned ammonia emissions. A high proportion of blended hydrogen increases the NO_x emissions, while both low and high blends cause the reduction of N₂O emissions. Additionally, the presence of straight orifice enhances the combustion of the mixture, while its emission levels remain comparable to those of the nozzle featuring all inclined orifices.