Homogeneous ignition in high-pressure combustion of methane/air over platinum: Comparison of measurements and detailed numerical predictions

The gas-phase ignition of fuel-lean methane/air premixtures over Pt was investigated experimentally and numerically in laminar channel-flow configurations at pressures of up to 10 bar. Experiments were performed in an optically accessible catalytic channel reactor established by two Pt-coated ceramic plates, 300 mm long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser-induced fluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor the onset of homogeneous (gas-phase) ignition, and thermocouples embedded beneath the catalyst provided the surface temperature distribution. Computations were carried out with a two-dimensional elliptic numerical code, which included the elementary heterogeneous (catalytic) reaction scheme for methane on Pt from Deutschmann and two different elementary homogeneous reaction schemes, Warnatz and GRI-3.0. Following homogeneous ignition, very stable V-shaped flames were established in the reactor. At pressures of up to 6 bar, the measured and predicted (Deutschmann/Warnatz schemes) flame sweep angles and OH levels were in good agreement with each other, while the homogeneous ignition distances were predicted within 10%. However, at pressures greater than or equal to 8 bar, a marked overprediction of the homogeneous ignition distances was evident (>25%). The Deutschmann/GRI-3.0 schemes yielded much shorter (∼55%-65%) homogeneous ignition distances at all pressures. Sensitivity analysis indicated that the latter discrepancies were ascribed to the homogeneous reaction pathway. GRI-3.0 yielded a much faster radical pool buildup than the scheme of Warnatz, clearly showing its inapplicability under catalytically stabilized combustion (CST) relevant conditions. The heterogeneous reactivity was enhanced with increasing pressure. Although the increase in pressure inhibited the adsorption of methane due to the resulting higher oxygen surface coverage, this effect was overtaken by the corresponding increase of the methane gas-phase concentration.