Study On Flammability Limits Of Methane-Air Laminar Premixed Flames At Elevated Pressure

Understanding the lean-flammability limits and the controlling mechanisms of high-pressure mixture gases is significant in design and modeling the combustor such as gas turbine operating at high pressures. Existing researches on this subject are very limited and mainly only focus on the numerical simulation, and experiment data are nearly unavailable. In this paper, a theoretical and experimental study on the flammability of lean CH4/Air has been carried out with the recently built dropping tower located National Microgravity Laboratory of China (NMLC) operated by Chinese Academy of Sciences Institute of Mechanics. The flammability limit of CH4/Air at various pressure conditions were measured.Numerical simulation of 1-D freely propagating and opposed-jet counterflowing laminar,premixed CH4/Air flames at elevated pressures was conducted with the improved PREMIX model and OPPDIF model in CHEMKIN. The results showed that pressure possesses a significant effect on the flame's burning rate, flame structure, flame thickness, and flame speed, while has minor effect on flame temperature and radiation heat loss. Furthermore, the variation of the flammability limit with pressure is non-monotonic. When pressure increases from ambient pressure, the flammability limits increases and then decreases when it reaches a peak value. To the 1-D freely propagating flames, the peak values appear at around P=5atm, while to the opposed-jet counterflowing flames, they appear at around P=4atm. To assess this phenomenon, sensitivity analysis of the near-limit flames was conducted and the results showed that the reaction mechanisms for the strongly burning flames and the near-limit weak flames are different and for the near-limit flames at different pressure are also different. The three-body chain termination reaction was suppressed when pressure is high than a critical value.The extinction limits of opposed-jet counteflowing methane/air flames of various strain rates were measured at normal gravity and microgravity at different elevated pressures. The results shows that the microgravity data are noticeably different from the normal gravity ones, indicating that buoyancy effective is not negligible. A fairly excellent agreement was found between the microgravity experimental data and numerical simulation in the variation of flammability limits with pressure at a fixed strain rate for the counterflowing flames.