The structure of triple flames stabilized on a slot burner

Abstract

A triple flame is a partially premixed flame that contains two premixed reaction zones (one fuel-lean and the other rich) that form exterior wings and a nonpremixed reaction zone that is established in between these wings. The three reaction zones merge at a “triple point.” Triple flames may play an important role in the stabilization and liftoff of laminar nonpremixed flames. They are also of fundamental importance in the reignition of turbulent mixtures. Despite their importance, many aspects of triple flames have not been adequately investigated and are, consequently, not clearly understood. Herein, laminar triple flames stabilized on a Wolfhard-Parker slot burner are investigated. The flow consists of a rich mixture of methane and air emerging from the inner slot and a lean mixture from two symmetric outer slots. In this configuration the three reaction zones that characterize a triple flame can be clearly distinguished. The loci of the “triple points” form a “triple line” in this planar configuration. The velocity field is characterized using laser Doppler velocimetry, and the temperature distribution using laser interferometric holography. In addition, C∗2-chemiluminescence images of the three reaction zones are obtained. A detailed numerical model is employed to completely characterize the flame. It is based on a 24-species and 81-reaction mechanism. The numerical results are validated through comparisons with the experimental measurements. Our results focus on the detailed structure, the interaction between the three reaction zones, the dependence of the flame structure on the initial velocities and mixture equivalence ratios, and the dominant chemical pathways. The lean premixed reaction zone (external wing) exhibits different features from the rich premixed reaction zone. In particular, it is characterized by strong HO2 formation and consumption reactions, and by relatively weak methane consumption reactions. Radical activity is higher in the nonpremixed reaction zone than in the other reaction zones. Overall, radicals from the nonpremixed reaction zone are transported to both the rich and lean premixed reaction zones where they attack the reactants. Simplifying the chemical mechanism by removing the C2-containing species produces significant differences in the predicted results only for the inner rich premixed reaction zone.