Stability of CH₄-N₂/air jet diffusion flame for various burner rim thicknesses

The influence of burner rim thickness on the stabilization mechanism of methane diluted with nitrogen and air jet diffusion flame was investigated experimentally. The diffusion flame was stabilized on a two-dimensional Wolfhard-Parker burner with a variable partition wall (burner rim) thickness between the fuel and air jets. The diffusion flame on a thin burner rim showed a blowout at the stability limit, which occurred after the lifted flame behavior. For a thick burner rim, the flame base remained well anchored in the wake region, and a blowoff occurred suddenly and not via the lifted flame with an increase in the jet velocity. For moderate rim thickness, a transition from a wake-anchored flame to a lifted flame appeared (liftoff), and blowout then occurred as the jet velocity increased. The blowout limits of lifted flames were not influenced by rim thickness; however, the blowoff limits were extended with an increase in the rim thickness. Particle image velocimetry (PIV) measurements showed that unburned flow entrainment into the base of the lifted flame was of the same order of magnitude as the propagation speed of the premixed flame. Therefore, the stabilization of the lifted diffusion flame for a thin rim might be determined by the premixed propagation of the flame base. On the other hand, a flame base anchored on the thick burner rim experienced a velocity distribution similar to a counterflow. Furthermore, the stretch rates at the liftoff and blowoff limits agreed well with the extinction limits of ordinary counterflow diffusion flames. Thus, their mechanisms could be explained by flame stretched extinction.