A Study of the Mechanism of Lean Limit Flammability

摘要

In this study the mechanism of extinction of a lean limit flame in a premixed methane-air or propane-air system was studied in a vertical rectangular flammability tube with variable inside dimensions. Both upward and downward extinction were observed for methane while only upward extinction was studied for propane. In addition to the observations of extinction, a theoretical analysis of the flow ahead of the upward propagating flame was made which showed that at extinction the upward propagating flame is severely stretched in the Karlovitz sense. The upward propagating flame in the 50 x 50 mm tube had a stable hemispherical shape with a trailing skirt towards the walls. Extinction in this case occurred from the central holding region of the flame where the stretch was calculated to be the maximum. In larger than 50 x 50 mm tubes the flame is hydrodynamically unstable and extinction occurs on one of the instability cycles. The downward propagating flame in methane is almost flat in the tube but it propagates in an unstable manner. It was observed, both with image intensifier photographs and with a schlieren optics, that this flame extinguished by first losing heat to the walls. Thus the downward propagating flame is extinguished by heat loss mechanism coupled with a differential buoyancy driving mechanism which is sufficiently complex that it cannot be modeled using one dimensional heat loss theories.