Extreme facilitation of flame propagation by turbulence through differential diffusion

Turbulent flame speed is an essential parameter in turbulent combustion. The existence of turbulence significantly enhances the flame speed of a pre-mixture, mainly through the increase in the total flame surface area and modification of the flame structure. In the present work, we report highly enhanced turbulent flame propagation with extremely weak mixtures, whose adiabatic flame temperatures are lower than 900K and beyond the flammability of sustained one-dimensional laminar flame propagation. We further identified that such a strong enhancement occurs for mixtures with either extremely small Lewis number, and that such flames exhibit different morphology from previously observed turbulent flames, as finger-shape structures are developed on the flame fronts and local extinction are frequently observed. This work demonstrates the importance of differential diffusion in intense turbulent environment, and the extension of flammability limit by turbulence, enabling sustained flame propagation with extremely low burnt gas temperature (<900K).