Extinction studies of near-limit lean premixed syngas/air flames

• Extinction limits of near-limit syngas/air flames were measured at micro-gravity.
• Branching and terminating determine the premixed syngas/air flame extinction.
• Diffusivity is more important than chemistry in syngas/air flame extinction.
• Suppressing H2 diffusivity makes the syngas/air flame easier to extinguish.
• Suppressing H diffusivity makes the syngas/air flame slightly stronger.

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H2 mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O2 ⇔ O + OH and the main termination reaction H + O2 + M ⇔ HO2 + M in the H2/O2 chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO2 to OH + H2 ⇔ H + H2O with the increasing H2 mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H2 mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.