Effects of H2 Addition on Flammability Dynamics and Extinction Physics of Dimethyl Ether in Laminar Spherical Diffusion Flame

Flame extinction is one of the most essential critical flame features in combustion because of its relevance to combustion safety, efficiency, and pollutant emissions. In this paper, detailed simulations were conducted to investigate the effect of H-2 addition on dimethyl ether spherical diffusion flame in microgravitational condition, in terms of flame structure, flammability, and extinction mechanism. The mole fraction of H-2 in the fuel mixture was varied from 0 to 15% by 5% in increment. The chemical explosive mode analysis (CEMA) method was employed to reveal the controlling physicochemical processes in extinction. The results show that the cool flame in microgravitational diffusive geometry had the "double-reaction-zone" structure which consisted of rich and lean reaction segments, while the hot flame featured the "single-reaction-zone" structure. We found that the existence of "double-reaction-zone" was responsible for the stable self-sustained cool flame because the lean zone merged with the rich zone when the cool flame was close to extinction. Additionally, the effect of H-2 addition on the cool flame was distinctively different from that of the hot flame. Both hot- and cool-flame flammability limits were significantly extended because of H-2 addition but for different reasons. Besides, for each H-2 addition case, the chemical explosive mode eigenvalues with the complex number appeared in the near-extinction zone, which implies the oscillation nature of flame in this zone which may induce extinction before the steady-state extinction turning point on the S-curve. Furthermore, as revealed by CEMA analysis, contributions of the most dominated species for extinction changed significantly with varying H-2 additions, while contributions of the key reactions for extinction at varying H-2 additions were basically identical.