The effect of crossflow perturbations on streaks in laminar boundary layer flames

Abstract

This study analyses the dynamics of streaks in laminar boundary layer flames by direct numerical simulation, under three crossflow perturbation states: unperturbed, perturbed by freestream turbulence $(0.03\text{\%}\le TI\le 1\text{\%})$, and perturbed by a steady sinusoidal mimicking manufactured streaks. Simulations reveal that streak behaviour is highly dependent on the perturbation state. Unperturbed crossflows result in delayed streak onset and produce smaller meandering streaks that remain unsteady. Heat flux to the surface below the flame remains uniform in these conditions, due to the small streak amplitude. Freestream turbulence induces earlier onset, increases streak wavenumber, and results in larger flame surface perturbation amplitudes. Streak growth rates are consistent with quadratic scaling typical of Rayleigh–Taylor instability for $TI\ge 0.6\text{\%}$, but, for lower $TI$, the instability amplitude remains in the regime where growth is governed by Linear Stability Theory scaling. Steady sinusoidal perturbations can be used to control the number of streaks generated, which align with crossflow velocity valleys. For low wavenumber sinusoidal perturbations $(k<4)$, larger quasi-steady streaks are overlapped by smaller unsteady meandering streaks. Larger sinusoidal perturbation wavenumbers $(k\ge 4)$ induce large steady streaks that separate into finger-like structures, inside which fuel packets are carried downstream and combustion continues. This has a strong influence on estimated heat flux to the bottom surface, which is initially stronger below troughs, due to the lower flame standoff distance, but the contrary is true once the finger-like structures develop.

Novelty and significance statement

Boundary layer flames are a largely unexplored setup of reactive flows, especially in the laminar regime. This study features the first simulations of laminar boundary layer flames, addressing the role of crossflow perturbation state on streak development and growth, which is a major gap in the literature left by previous experimental studies. Results confirm the presence of Rayleigh–Taylor instabilities, and show that the perturbation introduced determines perturbation growth and wavenumber, and greatly influences heat flux to nearby surfaces.

The fundamental physical insights revealed provide key improvements in the understanding of fuel consumption and heating of nearby surfaces. Practical applications that rely on this include wildfire propagation in wind-driven scenarios, and diffusion flame engineering setups.