Taylor dispersion-controlled stability of flames aligned parallel to a shear flow #

Prabakaran Rajamanickam, Joel Daou, Aiden Kelly

15:10 Monday in 4Q08.

Part of the Boundary layer flows and stability session.

Abstract #

The coupling between shear-enhanced diffusion (Taylor dispersion) and flame instabilities is a scientifically rich research topic whose investigation has only been recently initiated. The focus of the work is on the effect of Taylor-dispersion on the thermo-diffusive instabilities of both premixed and non-premixed flames, which are aligned with the flow direction. The problem is addressed analytically (only for premixed flames) and numerically (for both premixed and non-premixed flames) in the framework of a constant density two-dimensional model accounting for shear-enhanced diffusion. The analytical investigation is based on large activation energy asymptotics. A linear stability analysis is carried out leading to a dispersion relation involving two main parameters: the Lewis number Le, and the Peclet number Pe. Stability diagrams are determined in terms of these and other pertinent parameters. Particular attention is devoted to the nature of the bifurcations observed in order to identify various cellular and oscillatory flame patterns. One of the interesting results obtained is the demonstration that Taylor dispersion can significantly alter the critical conditions for the onset of the classical cellular instability expected in mixtures with sub-unity Lewis numbers, leading to its occurrence in Le >1 mixtures if the Peclet number is above a critical value. This novel result seems to provide an explanation of the unexpected appearance of cellular structures called diffusion flame streets in non-premixed micro-combustion devices. In the case of premixed flames, apart from the classical longwave stationary bifurcation (type-IIs instability), whose dynamics characterised by the Kuramoto-Sivashinsky equation, we also observe other new types of bifurcation leading to the formation of cellular structures.