Do Turbulent Premixed Flame Fronts in Spark-Ignition Engines Behave Like Passive Surfaces? 2000-01-1942
A widely held belief in the combustion community is that the chemical and hydrodynamic structure of a stretched laminar premixed flame can be preserved in a turbulent flow field over a range of conditions collectively known as the flamelet regime, and the homogeneous charge spark-ignition engine combustion falls within the domain of this regime. The major assumption in the laminar flamelet concept as applied to the turbulent premixed flames is that the flame front behaves as a constant-property passive scalar surface, and an increase in the wrinkled flame surface area with increasing turbulence intensity is the dominant mechanism for the observed flame velocity enhancement. The two approaches that have been recently used for estimating a measure of the wrinkled flame surface area in spark-ignition engines and other premixed flames are the flame surface density concept and fractal geometry. A critical assessment of the experimental fractal parameters reported in literature indicated that these are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model. A similar conclusion has been reached after examining the surface density data from flames of different geometries including spark-ignition engine flame fronts. The assumption made in fractal and surface density approaches that the turbulent flame front is a passive scalar surface can not be justified, and the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.