Experimental investigation of the flame structure and extinction of turbulent counterflow non-premixed flames

Extinction conditions and flame structures for methane-air turbulent non-premixed flames are investigated experimentally for a counterflow nozzle-type burner system. Extinction limits are measured by varying fuel concentrations diluted by nitrogen, mean, flow velocities of both burners, and turbulent characteristics generated by perforated plates. In particular, the flow turbulence of each burner is controlled individually. The extinction limits for the same fuel concentration can be expressed by approximately the same bulk velocity gradient. At the condition of lean fuel concentration or high intensity of flow turbulence, the flame strength is weak. It is shown that the flame strength is influenced by the turbulence of the air stream rather than that of the fuel stream. The flame shape was observed by a laser tomographic technique under various conditions. Because the methane-air counterflow non-premixed flames in the present study are formed in the air stream, the diffusion region can be visualized as the vanishing strip of seeding particles between near flames and the stagnation plane. In the case of laminar flames, there is no difference of mean flame locations, or the width of the diffusion region, for a certain range of fuel concentration. In the case of turbulent non-premixed flames, it is also observed that relative flame locations in the diffusion region are almost the same, and the width of the diffusion region is not changed for various turbulent flow conditions. It is shown that the diffusion region containing turbulent flames keeps the structure, as laminar flamelets.