Detailed experiments of flame spread across deep butanol pools

Abstract

Experiments using simultaneous rainbow schlieren deflectometry, particle image velocimetry (PIV), infrated (IR) thermography, standard flame imaging, and thermocouples are conducted to reveal detailed physics of low-speed, forced-opposed-flow flame spread over liquid pools. Our effort concentrates on the effects of gravity and air flow speed on the flame-spread behavior across a deep, rectangular tray filled with 1-butanol. The microgravity (μg) part of the experiment is the first combustion experiment conducted aboard a sounding rocket. Several novel observations are made. Even with forced-air velocities of the same order of magnitude as that induced naturally by buoyancy in normal gravity, the μg flame behavior is completely different. Whereas the normal gravity (1 g) flames we studied are soot-producing, and pulsate and spread rapidly, the μg flames are free of soot and spread steadily and very slowly. The rainbow schlieren measurements show that heat penetrates far deeper into the pool in μg, suggesting that the major effect of liquid-phase buoyancy is its stratification of the temperature field in 1 g: its absence in μg may lead to a very different liquid-phase surface temperature, flow field, and flame-spread character. The IR thermography reveals hetetofore unobserved liquid surface temperature and side-flow phenomena in both 1 g and μg. PIV has been used for the first time to obtain quantitative, full-field, liquid-phase velocity fields ahead of the flame, revealing significantly more surface flow in μg than in 1 g. A state-of-the-art model's predictions are qualitatively confirmed in regard to gravitational effects on flame shape, flame extinction in μg when there is no opposed air flow, and fuel consumption rate. Disagreement is found in the flame spread character in μg, unless hot gas expansion is artificially set to zero.