On simulating the effect of gravity on concurrent flame spread over thin paper through variations in ambient pressure

Abstract

The environment inside a spacecraft, or a space habitat, can greatly differ from those on earth, affecting the burning behavior of solid fuels. 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, or a habitat in the Moon or Mars, thus making fire testing and burning predictions harder. A potential alternative approach to overcome this problem is to reduce buoyancy effects by using reduced ambient pressure in normal gravity. The objective of this work is to explore the possibility of simulating the effect of gravity, and in turn buoyancy, through changes in ambient pressure, on upward/concurrent flame spread over a thin combustible solid. Comparisons with available data at different gravity levels are used to determine the extent to which low-pressure can be used to replicate flame spread characteristics at different gravities. Experiments of the upward/concurrent flame spread over thin Kimwipe paper sheets were conducted in normal gravity inside a pressure chamber with ambient 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 paper surface, and also reduces flame intensity. Comparison with published 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.