The role of a flame-induced liquid surface wave on pulsating flame spread

Abstract

The detailed temperature structure that was created during a pulsating flame spread over n -butanol was measured using holographic interferometer, shadowgraph, and IR thermograph techniques. This study found that there was a small surface wave over the previously observed cold-temperature valley located about 10 mm ahead of the spreading flame's leading edge. The newly observed surface wave was located between the spreading flame edge and the cold-temperature valley. The crest of the wave was higher than the quenching distance of the spreading flame. Due to the formation of a small gas-phase circulation cell that was observed to exist between the wave crest and the flame leading edge by laser sheet particle tracking and smoke-tracing techniques, the flammable gas mixture cannot spread beyond the wave crest; therefore, the flame's leading edge was not able to propagate beyond the wave crest. The crawling spread process was believed to be a spread process with constant speed, but this study found that it consists of a small-scale pulsation (subpulsation) with 6–12 Hz frequency. Subpulsation seems likely to have a correlation with the cyclic apperance and disappearance of a small gas-phase circulation cell that travels between the flame edge and the wave crest. When the crawling process provided sufficient heat to the liquid, the cold-temperature valley disappeared. Then the warm liquid helped to form a flammable gas layer over its surface, enabling the flame to propagate through the layer. This is called the “main” pulsating spread, a typically observed pulsating flame-spread phenomenon. This study revealed that a millimeter-order surface wave is responsible for the subpulsation that is responsible for the main pulsating spread. However, the mechanism of surface wave formation is unknown at this time.