An experimental and numerical investigation of the structure of steady two-dimensional partially premixed methane-air flames

Steady two-dimensional partially premixed slot-burner flames established by introducing a rich fuel-air mixture from the inner slot and air from the two outer slots are investigated. Numerical simulations are conducted using detailed chemistry, velocity measurements are made using particle image velocimetry, and images of the chemiluminescent reaction zones are obtained. Two reaction zones are evident: one in an inner rich-side premixedlike flame and the other in an outer lean-side non-premixed flame. Validation of the predictions involves a comparison of the (1) premixed and non-premixed flame heights, (2) the double-flame structure and (3) velocity vectors. The measured and predicted velocity vectors are in good agreement and show that the flame interface separates smaller velocity magnitudes on the reactant side from large values on the (partially) burned side. The outer flame temperature is higher than that of the inner premixed flame. A substantial amount of methane leaks past the inner flame and reacts in the outer non-remixed zone. The inner flame produces partially oxidized products such as H2 and CO, which provide the fuel for the non-premixed flame. The initiation reaction CH4+H⇔CH3+H2 proceeds strongly at the base of the flame where both the inner and outer flames are connected and at the tip of the inner flame, and it is weak along the sides of both inner and outer flames in accord with the chemiluminescent images. Carbon dioxide formation through the reaction CO+OH⇔CO2+H is more diffuse than methane consumption in the outer flame, because the availability of hydroxyl radicals in that region is limited through oxidizer transport.