Smoke visualization of the gas-phase flow during flame spread across a liquid pool

Abstract

Gas-phase flow ahead of the leading edge of a flame spreading across a pood of flammable liquid is an important heat and mass transfer mechanism that has not been explored in depth experimentally. Detailed knowledge of the flow field near the flame front is necessary for an understanding of how the flame spreads and for use by model developers as verification of predicted flame characteristics and flow patterns. In this paper, results are presented of gas-phase flow visualization obtained by releasing smoke ahead of a flame spreading across 1-butanol into a slow opposed air flow. The smoke filaments were illuminated with two orthogonal laser light sheets and show clearly for the first time the extent of the hypothesized gas-phase recirculation cell in front of the flame, and the lateral flow divergence due to thermal expansion. The cell forms during the crawling portion of the pulsation cycle in normal gravity, and in the quasi-steady regime in microgravity. Comparisons are made to a numerical model for similar cases of flame spread over alcohols. Despite significant differences between the flame spread rates and character in normal and microgravity predicted by the model and seen in the experiment, only a moderate difference between the flow patterns was seen experimentally for the two gravity levels. This is contrary to an earlier hypothesis that greater flow divergence would be measured in low gravity in the absence of a byoyant flow directed toward the flame and challenges the notion that thermal expansion should be reduced in 2-D numerical models to account for 3-D effects. Instead, buoyancy appeared to have its greatest influence on the flow pattern in the trailing portion of the flame. Corroborating previous experimental evidence from liquid-phase diagnostics, flame spread over a liquid in a narrow tray is found to be a 3-D phenomenon.