The effect of residence time on the ignitability of ethylene and air mixtures in a toroidal jet-stirred reactor

The ignitability of premixed ethylene and air is quantified in a toroidal jet-stirred reactor (JSR) to study the effects of fluid residence time and chemical reactivity on the forced ignition processes. Unlike ignition in a combustion bomb or flow tunnel, which exhibit zero mean flow or flow in a single direction, respectively, the JSR promotes mixing of burned products with unburned reactants and provides a defined bulk residence time, both of which are relevant to swirl-stabilized and cavity-stabilized combustors. Here ignitability, the probability of a system igniting at a given condition, is studied using forced spark-discharge ignition at mixture temperatures from 350-450 K, equivalence ratios from 0.5-1, and residence times from 15-35 ms. Ignitability is reported at each condition. Equivalence ratio at 50% ignition probability, or the minimum equivalence ratio for ignition (MER), exhibits an inverse relationship with residence time and mixture temperature. MER was compared with real engine correlations of lean light off performance by Lefebvre and Ballal and exhibited an excellent fit with the proposed model (A.H. Lefebvre, and D.R. Ballal, Gas Turbine Combustion: Alternative Fuels and Emissions, CRC Press, Boca Raton, FL, 2010). The slope of the ignitability curves tends to change with fluid exchange rate, or residence time, where the scaling is due to the interactions between fluid and chemical time scales within the JSR.