Attachment mechanisms of diffusion flames

A common view of the stabilizing mechanism of methane diffusion flames, including jet and porous flat-plate burner flames, is presented. Two-color particle-imaging velocimetry measured the velocity field in the stabilizing region of jet diffusion flames under near-lifting conditions. Computations using a time-dependent, implicit, third-order accurate numerical model, including semidetailed chemical kinetics and buoyancy effects, revealed the detailed structures of the vertical jet diffusion flames and flat-plate burner flames with different orientations of the plate surface and fuel injection. The numerical results are in a good agreement with the measurements in the flame base locations and surrounding velocity fields. In the calculations of both classes of flames, the highest reactivity spot ( reaction kernel ) with peak rates of heat release, oxygen consumption, and water vapor production, was formed in the relatively low-temperature ( 3 +O→CH 2 O+H reaction predominantly contributed to the heat-release rate peak. Heuristic correlations were found between the heat-release or oxygen-consumption rate and the local velocity over a wide range. At a high coflow air velocity in a jet diffusion flame, the flame base shifted slightly downstream before lifting, resulting in a higher reactivity and thereby withstanding at a higher local velocity. On the other hand, in a long horizontal flat-plate flame with downward fuel injection, a recirculation zone was formed ahead of the flame base, resulting in an order-of-magnitude lower local velocity and reactivity. Therefore, the reaction kernel provides a stationary ignition source and sustained stable combustion for incoming reactants, thus holding the trailing diffusion flame in the oxidizing stream.