Visualization of Counterflow Flames

Laminar counterflow flames have been used to study a variety of combustion problems, for example extinction conditions and chemical kinetic mechanisms with a review in Ref. [1], and to provide information for flamelet models of turbulent flames [2]. Recently, this flow configuration for turbulent premixed and non—premixed flames has also attracted interest [3, 4] because the velocity field provides a uniform strain rate, a quantity important for flame extinction, and because the simple geometry facilitates theoretical analyses [5, 6]. Mastorakos et al. [4] found that partial premixing improved flame stability and that a spectrum of flames from “pure” diffusion to premixed flames could be established in the counterflow geometry. The present paper is concerned with the appearance of the range of flames that can be stabilised in turbulent counterflowing streams and gives additional information on the consequences of stretching on laminar and turbulent counterflow flames with partial premixing and on the mixing between two turbulent isothermal opposed jets, information relevant to opposed jet combustion. In the next Section, the geometies and the methods used to visualise the flames are described, with the results presented and discussed in Section 3. Finally, Section 4 summarises the more important conclusions.