Flame propagation across a liquid fuel in an air stream

Abstract

The behavior of flames propagating across methanol in opposed and concurrent air streams was examined using high-speed schlieren photography, and the propagation mechanisms of these flames are discussed. For the free stream velocity U f gradually decreases as the absolute value |U| of U increases. The absolute value |U c | of the critical value U c at which V f becomes 0 is found to be much larger than the flame propagation velocity V f 0 at U=0. While for U>0, i.e., in the case of the concurrent air stream, V f remains almost constant until U becomes equal to V f 0 , and becomes nearly equal to U in the range of U>V f 0 . In the case of U f 0 , the effects of the free stream on the flow in front of the leading edge of the flame seem to be reduced to a great extent, so that the flame can propagate without a remarkable decrease in V f even in the opposed air stream of a much higher velocity than V f 0 . On the other hand in the case of U>V f 0 , the flame propagation seems to be closely related to the behavior of the hot gas overhanging the methanol surface in fron of the leading edge of the flame. The heat from the overhanging hot gas is expected to increase the methanol vapor concentration in front of the leading flame edge and ignite it. Thus, an inclined flame can follow the hot gas even when the initial methanol temperature T 1 is lower than that of the flash point. The relation (ϱu/ϱy) sc 2 =B·S L 2 ·δ 0 is found to be valid for T 1 >15°C, in which ( ϱu/ϱy) sc is the critical velocity gradient at the methanol surface in the approach flow, B a constant, S L the burning velocity at the leading flame edge, and δ 0 the flammable layer thickness at U=0.