Multi-Directional Flame Spread and Flame Interaction on Thin PMMA Samples in Microgravity

Fires in human space flight must be avoided as much as possible. While materials are tested under terrestrial conditions for fire safety in space, fire behavior may be different in the absence of gravity. The usual academic approach to microgravity fire experiments is to use opposed or concurrent flow configurations. This study utilizes a more practical configuration. Samples are ignited at point sources rather than planarly, enabling the multi-directional investigation of flame spread. Furthermore, flame interaction between two ignition points is also studied. Drop tower experiments with one or two point ignition are carried out using 0.2 mm thick polymethyl methacrylate sheets with 40 mm width and 60 mm length. Pressure and flow are maintained at 75 kPa and 10 cm/s respectively, while the oxygen concentration is varied between 21 vol% and 35.4 vol%. Flame spread and sample heating are evaluated by infrared and visual image analysis. Previous experiments in opposed flow configuration are used for reference. After ignition, the flames spread in a circular manner from the ignition point, resulting in varying preheating behavior of the samples. Flame spread rates vary with time and flame size. Especially close to ignition, there is a transient phase in which the flame spread rate increases, whereas the flame spread rate approaches a steady phase with increasing flame size. Lateral flame spread is in some cases faster than opposed flame spread. Two point ignition results in the flames interacting with each other. Flame spread accelerates as the flames approach each other, especially with a flame distance smaller than 10 mm. These experiments show that non-planar ignition and multiple ignition sources can have a significant impact on multi-directional flame propagation in microgravity, which is important for real-world spacecraft fire scenarios.