The Effect of Buoyancy on Upward-Concurrent Flame Spread over Thin Paper

Understanding material flammability inside a spacecraft is important because the conditions in spacecraft environments can greatly differ from those on earth. Because in a gravity field there is a flame-induced buoyancy, it is very difficult to reproduce on Earth the environmental conditions of a spacecraft, thus making fire testing harder. To overcome this problem, alternative approaches that reduce buoyancy are required. One possibility to reduce buoyancy effects relies in using reduced ambient pressure. The objective of this work is to study the effect of pressure, and consequently buoyancy, on upward/concurrent flame spread over a thin combustible solid, and by comparison with partial gravity data, observe up to what point low-pressure can be used to replicate flame characteristics observed in different gravity levels. Experiments in normal gravity were conducted over pressures ranging between 100 and 30 kPa and a forced flow velocity of 10 cm/s. Results show that reductions of pressure slow down the flame spread over the material surface. As pressure is reduced, flame intensity is also reduced. Comparison with partial gravity data shows that as the pressure is reduced, the normal gravity flame spread rate approaches that observed at different gravity levels. The data presented is correlated in terms of a mixed convection non-dimensional number that describes the convective heat transferred from the flame to the solid, and that also describes the primary mechanism controlling the spread of the flame. The correlation provides information about the similitudes of the flame spread process in variable pressure, flow velocity and gravity environments, providing guidance for potential ground-based testing for fire safety design in spacecraft.