Lean or ultra-lean stretched planar methane/air flames

Lean premixed combustion has potential advantages of reducing pollutants and improving fuel economy. In some lean engine concepts, the fuel is directly injected into the combustion chamber resulting in a distribution of lean fuel/air mixtures. In this case, very lean mixtures can burn when supported by hot products from more strongly burning flames. This study examines the downstream interaction of opposed jets of a lean-limit CH4/air mixture vs. a lean H2/air flame. The CH4 mixtures are near or below the lean flammability limit. The flame composition is measured by laser-induced Raman scattering and is compared to numerical simulations with detailed chemistry and molecular transport including the Soret effect. Several sub-limit lean CH4/air flames supported by the products from the lean H2/air flame are studied, and a small amount of CO2 product (around 1% mole fraction) is formed in a “negative flame speed” flame where the weak CH4/air mixture diffuses across the stagnation plane into the hot products from the H2/air flame. Raman scattering measurements of temperature and species concentration are compared to detailed simulations using GRI-3.0, C1, and C2 chemical kinetic mechanisms, with good agreement obtained in the lean-limit or sub-limit flames. Stronger self-propagating CH4/air mixtures result in a much higher concentration of product (around 6% CO2 mole fraction), and the simulation results are sensitive to the specific chemical mechanism. These model-data comparisons for stronger CH4/air flames improve when using either the C2 or the Williams mechanisms.