Buoyancy effect on downward flame spread over PMMA cylinders

Abstract

Understanding material flammability at different gravity levels is important for fire safety applications in space facilities where the environments may include microgravity, low velocity flows, low pressure and elevated oxygen concentration. One possible approach to simulate on-earth the burning behavior inside spacecraft environments, and facilitate testing, is to reduce buoyancy effects by decreasing ambient pressure. The objective of this work is to study the effect of pressure, and consequently buoyancy and indirectly gravity, on downward flame spread rate over cylindrical samples of polymethyl-methacrylate (PMMA), and by comparison with reduced gravity data, observe up to what point low-pressure can be used to replicate flame spread in space facilities. Experiments in normal gravity are conducted using pressures ranging between 100 kPa and 30 kPa and oxygen concentrations between 19% and 23%, with a forced flow velocity of 100 mm/s. The low-pressure data is compared with microgravity data obtained aboard the International Space Station during the BASS-II experiments. Results show that reductions of ambient pressure slow down the flame spread process approaching that expected at low gravity. The normal gravity and microgravity data are correlated in terms of a mixed convection parameter that describes the main controlling mechanisms of heat transferred. Although the correlation works well for the normal gravity data it does not work as well for the microgravity data. However, it provides information about what is to be expected in environments of variable ambient pressure, oxygen concentration, and reduced gravity, providing an insight for future designs when considering fire safety in spacecrafts.