An Experimental Study of Counterflow Diffusion Flames Subject to Flame Stretch Gradients.

Abstract

The flame stretch effect on counterflow diffusion flames has been experimentally investigated in the configuration of Tsuji burner in terms of the OH radical emission intensity and temperature. This is motivated by our previous study about flame base structures of turbulent lifted diffusion flames which showed lower quenching scalar dissipation rates at tips of the bases than those of the counterflow diffusion flames. Counterflow diffusion flames subject to flame stretch gradients, called edge flames, are examined to model flamelets at the lifted flame bases. Fuels are methane and propane. It is found that the increase in the scalar dissipation rate leads to the enhancement of reaction in the fully burning condition, because the OH radical emission intensity reflecting the reaction rate increases with the flame stretch until the extinction. This reveals that the effect of the finite reaction is limited to a narrow range near the quenching scalar dissipation and the Damkohler number dose not decrease monotonously. Extinctions of edge flames take place at lower flame stretches than those of the counterflow diffusion flames under the uniform flame stretch. The flame stretch gradient weekends the edge flame and quenches at a lower flame stretch. It is also found that the flame edge reacts strongly by the enhancement of the diffusion of reactants and radicals at the edge.