Studies of curvature, strain and unsteady effects on premixed flames

The effects of curvature, strain and unsteadiness on premixed flames are studied analytically, numerically and experimentally. The approach includes the development of simplified problem formulations and the study of flames in which the various effects may be isolated.; The steady and unsteady (periodic and transient) response to strain are investigated using the one-dimensional strained flame. A simplified reaction rate model is proposed to eliminate the non-linearity associated with Arrhenius models. Solutions using this model agree very well with numerical results based on the Arrhenius model. The analytical treatment, however, is much simpler.; The effect of flame curvature on stationary cylindrical flames (sink and source configurations) are studied analytically using the simplified reaction rate model. Analytical solutions show that source and sink effects on the flame propagation rates are negligible except when the flame is near the source or sink despite the strong dependence of the preheat zone structure on flame curvature. Consequently, the flame may be considered locally planar.; A differential quasi-one dimensional flame model which accounts for flame curvature, lateral flow expansion and cross-stream diffusion is formulated. It is used to identify the mechanisms that enhance the displacement speed which denotes the velocity of the unburnt gas relative to the flame. Computations of a quasi-one dimensional flame show that the contribution of cross-stream diffusion to displacement speed enhancement may be significant. The results also show that, for a neutrally diffusive mixture, there is a surface in the flame where the mass flux is invariant to lateral flow expansion.; The effect of curvature and stretch on the displacement speed are studied experimentally and numerically using the flame tip of a slot burner. The study is limited to a diffusionally neutral mixture to isolate hydrodynamic effects; this is verified by temperature measurements using Rayleigh scattering. The results show that the displacement speed correlates linearly with stretch. The same correlations give a nonlinear dependence on curvature which suggests the existence of a minimum radius of curvature associated with limitations on diffusional transport in highly curved flames.