Rich-Lean Flame Interaction in a Lamella-Type Burner

An expanded version of a practical rich-lean burner consisting of lamella geometry with lean flames in the central part and surrounded by two slits of rich flames (to prevent peripheral lean flames from lifting) is evaluated in the present study. The central part of this burner was studied in the search for an optimal geometry that maximized the stability of a methane-air lean flame, especially with regard to peripheral rich-lean flame interaction. For the burner central part, a slit thickness of 2 mm was found to be the optimum value for maximizing the overall burner stability of simple lean flames if the slit interspace was higher than 2 mm. This lean stability limit was then increased by about 9 +/- 2% when the burner was operated with peripheral rich-lean flames. Detailed experimental characterization of the rich-lean flame interaction zone was performed for a lean flame of phi(L) = 0.65, revealing the structure of a triple flame. In this rich-lean interaction, the lean flame was contaminated by diffusion of heat and radicals forcing the laminar flame speed to increase (improving overall burner stability) and, from chemiluminiscence signals, a change in the lean equivalence ratio could be quantified. An analytical expression was derived that showed how the apparent equivalence ratio of contaminated lean flames is linearly dependent on the equivalence ratio and velocity gradients as the driving forces of the mechanism of triple flame interaction. Experimental results validated the analytic expression, giving support to a holistic view of rich-lean flame interaction.