Section B Fire and Explosion - Effect of Gravity on Flame Spread Along a Thin Combustible Solid for Different Sample Orientations in Opposed Flow

Abstract

In order to secure fire safety over the entire period of a manned space mission, gaining a systematic understanding of the effects of gravity on flame spread is important. In this study, opposed-flow flame spread along a thermally thin combustible solid for different sample orientations (−20° downward, horizontal, and +20° upward) was experimentally investigated by changing the ambient oxygen concentration and gravity level. The flame spread rate decreases with decreasing oxygen concentration under normal gravity, and its rate at 18 % oxygen concentration is equivalent to that at 21 % oxygen concentration under microgravity. The downward flame spread rate decreases with an increase in gravity. In contrast, the horizontal and the +20° upward flame spread rates clearly increase as the gravity level increases. The flame spread rate varies remarkably with sample orientation in a supergravity environment. To clarify the effect of gravity on flame spread, the relation between the non-dimensional flame spread rate and the Rayleigh number was examined. The Ra number both for horizontal and upward flame spread increases with increasing gravity, while the Ra number for downward flame spread decreases slightly with a decrease in gravity. The non-dimensional flame spread rate is almost constant under normal and supergravity conditions for Ra numbers less than 103 and is equivalent to that under microgravity. When the Ra number is greater than 103, the non-dimensional flame spread rate increases with increasing Ra number and is proportional to Ra1/3.