Initial-stage reaction of methane examined by optical measurements of weak flames in a micro flow reactor with a controlled temperature profile

To examine methane oxidation at intermediate temperatures (ca., 900–1200 K), chemiluminescence observation and laser-induced fluorescence (LIF) measurements for CH₂O and OH were conducted for methane weak flames in a micro flow reactor with a controlled temperature profile (MFR) at atmospheric and elevated pressures. Locations of CH₂O–LIF, chemiluminescence, and OH-LIF in MFR were arranged from the low temperature side at 1.0 and 5.0 bar. Spatial separation of methane oxidation was successfully demonstrated. One-dimensional computations with five detailed kinetic mechanisms were conducted. Computational profiles of CH₂O molar concentration, heat release rate (HRR), and OH molar concentration normalized by their own peak values were compared with experimentally obtained intensity profiles of the CH₂O–LIF, chemiluminescence, and OH-LIF. Computational results obtained with AramcoMech 1.3 showed better agreements with experimentally obtained results among the mechanisms employed. However, the flame position computed with AramcoMech 1.3 showed a slightly higher temperature region than the experimental flame position, indicating underprediction of methane reactivity. Sensitivity analysis identified a set of dominant reactions for weak flame positions. Rate constants of the identified reactions were modified within uncertainty to reproduce experimentally obtained weak flame positions. The modified mechanism also well predicted ignition delay times and flame speeds, and significant improvement of prediction was identified particularly for ignition delay times of lowest temperature and pressure investigated. Reaction path analysis highlighted the importance of intermediate-temperature oxidation chemistry for methane such as CH₃→CH₃O₂→CH₃O→CH₂O reactions at higher pressures. Two-stage oxidation of methane was observed by chemiluminescence observation and numerical simulations at higher pressures (6.0–10.0 bar).