Extinction of lean near-limit methane/air flames at elevated pressures under normal- and reduced-gravity

The extinction limits of lean, near-limit, counterflowing, CH4/air twin premixed flames were studied experimentally at evaluated pressures and under normal- and micro-gravity conditions utilizing the 3.5s drop tower of the National Microgravity Laboratory of China. The results showed that under micro-gravity conditions the natural convection is minimized and the flames become more planar and symmetric compared to normal gravity. In both normal- and micro-gravity experiments and for a given strain rate and fuel concentration, the flame luminosity was found to enhance as the pressure increases. On the other hand, at a given pressure, the flame luminosity was determined to weaken as the strain rate decreases. At a given strain rate, the fuel concentration at extinction was found to vary non-monotonically with pressure, namely it first increases and subsequently decreases with pressure. The limit fuel concentration peaks around 3 and 4atm under normal- and micro-gravity, respectively. The extinction limits measured at micro-gravity were in good agreement with predictions obtained through detailed numerical simulations but they are notably lower compared to the data obtained under normal gravity. The simulations confirmed the non-monotonic variation of flammability limits with pressure, in agreement with previous studies. Sensitivity analysis showed that for pressures between one and 5atm, the near-limit flame response is dominated by the competition between the main branching, H+O2→OH+O, and the pressure sensitive termination, H+O2+M→HO2+M, reaction. However, for pressures greater than 5atm it was determined that the HO2 kinetics result in further chain branching in a way that is analogous to the third explosion limit of H2/O2 mixtures.