Dynamics of stretched flames

Recent advances in the understanding of the structure, propagation, and extinction of laminar flames under the influence of stretch, as manifested by the existence of flame curvature, flow nonuniformity, and flame motion, are reviewed. The emphasis is on premixed flames because of the richness and subtlety of the phenomena involved. The review distinguishes the influences of the tangential and normal velocity gradients on the flame response, both at the hydrodynamic scale and within the flame structure, and emphasizes the importance of the preferential diffusion nature of heat and mass transport, as well as the extent to which the flame can freely adjust its location in response to stretch in order to achieve complete reaction. It is then demonstrated that stretch has only minimal effect on an adiabatic, unrestrained, diffusionally-neutral flame with complete reaction in that the temperature, propagation rate, and thickness of the flame are invariant to stretch, and that stretch alone cannot extinguish such a flame. In the presence of preferential diffusion and/or when the flame movement is restrained, the response of the flame to stretch becomes more sensitive and extinction is also possible. The concept of flame stretch is applied to interpret such practical flame phenomena as flame stabilization and flame-front instability, determination of laminar flame speeds and flammability limits, concentration and temperature modifications in flame chemistry, and modeling of turbulent flames. The properties of stretched diffusion flames are then briefly discussed. The review closes with suggestions for further research.