Reexamination of the blowoff mechanism of premixed flames—inverted flames

Abstract

As a start of a reexamination for the blowoff mechanism of premixed flames, an experimental study of a laminar, two-dimensional inverted flame established in the wake of a thin plate was performed. Streamlines and velocities in the wake were measured in detail, and they were analyzed to obtain the stretching along the flame front at the base of the flame, namely, at a “stabilization point,” and the flow velocity gradient along a streamline passing through that point. At the stabilization point, the flow velocity must be balanced by the burning velocity, and the flow velocity gradient must be smaller than the burning velocity gradient along the streamline. When these conditions are not satisfied, the blowoff will occur. The experimental results show that the stretching along the flame front at the stabilization point of inverted flames does not occur, and it is confirmed that the flame-stretch concept can not be applied to the blowoff of inverted flames. The dimensionless flow velocity gradient at the stabilization point is equivalent to the area-increase factor, which is proportional to the curvature of the flame base. The critical flow velocity gradient for the blowoff will be caused by an excessive increase in the curvature of the flame base resulting in a lowering of the burning velocity there.