Extinction of interacting nonpremixed flames and existence of stationary retreating edges in twin-jet counterflow

A two-dimensional 'twin-jet counterflow' burner, in which two opposing streams from two double-slit nozzles form a counterflow, has been utilised to investigate the effect of the interaction of nonpremixed flames on extinction behavior. Results show that owing to the existence of unique petal-shaped flames, the extinction boundary for the cross-stream arrangement can be extended appreciably, as compared with that for the conventional counterflow arrangement, through the interaction of the curved sections of the interacting flames. The stationary petal-shaped flames had four flame edges, consisting of two retreating edges with negative edge speed in the direction toward the burnt gas region and two propagating edges with positive edge speed in the direction toward the unburned mixture. The OH-PLIF images of the petal-shaped flames demonstrated a strong concentration interaction between the two curved flame sections. Hysteresis in the transition between the petal-shaped flame and the curved flame having planar wing sections were observed. The representative propagation speed of the retreating edges of the petal-shaped flames correlated reasonably with maximum flame luminosity. The extinction characteristics of the retreating edges can be described in terms of the local Karlovitz number, which accounted for the local characteristic reaction time.