A Study of the Laminar Flame Tip and Implications for Premixed Turbulent Combustion

Abstract

Flame surface curvature is a significant geometrical parameter that affects the structure and propagation of premixed laminar and turbulent flames. In this study, the flame tip of a two-dimensional laminar Bunsen burner is investigated using a quasi-one dimensional model, direct numerical simulations and experimental results. The laminar flame tip is a simple prototype of curved flamelets embedded in a turbulent flow field. It is shown that two characteristic flame speeds are necessary to give a local description of a given flamelet: the consumption speed associated with the structure of the reaction zone, and the displacement speed of the flame front relative to the unburned flow. The quasi-one dimensional model shows that three different mechanisms affect the displacement speed of a curved flame in a non-uniform flow field: a chemical mechanism associated with the expansion of the reaction zone structure, a hydrodynamic mechanism due to isothermal area modification by lateral flow divergence and flame curvature, and a diffusive mechanism due to the misalignment of the diffusive and hydrodynamic processes. For unity Lewis numbers, numerical simulations of the flame tip show that the consumption speed is unaffected by curvature while the large increases in the displacement speed observed at the tip are due to the hydrodynamic and diffusive mechanisms, but not to the chemical mechanism. Based on data from experiments and numerical simulations, correlations of the flame displacement speed with flame stretch are obtained. It is shown that the linear relationship predicted by asymptotic methods for small stretch applies for a much wider range of stretch values. The slope of this function (the Markstein number) is determined and compared to analytical predictions. Implications of these results for flame/et models of premixed turbulent combustion are discussed.