Opposed-Flow Flame Propagation on Discrete PMMA Sheets Under Exploration Atmospheres in Microgravity

In human spaceflight, a fire on board a spacecraft is one of the most devastating scenarios. To improve fire safety on board of a spacecraft, material tests are performed to characterize the fire behavior of materials. In real-world scenarios, however, not only continuous materials may be affected, but also discrete, spatially separated fuels. To better understand the flame propagation of discrete fuels in microgravity, a series of drop tower experiments was conducted with thin poly(methyl methacrylate) sheets. Material samples were placed in a 96 mm x 96 mm x 264 mm wind tunnel and the desired flow and gas mixture was initiated using three mass flow controllers. The flame propagation was observed by two visual cameras (front and side view) and an infrared camera (front view). The gap distance between fuel strips and the atmospheric conditions were varied. The atmospheres were selected along the normoxic curve with oxygen mole fractions O2 ranging from 0.21 to 0.354, and respective pressures between 101.3 kPa and 60 kPa. The opposed flow was held constant at 150 mm/s. Depending on the gap width and atmospheric conditions, gaps either led to an increase in flame spread rates or stopped the propagation entirely. Limiting gap widths were found to lie between 5 mm for xO2 = 0.21 and close to 7 mm for xO2 = 0.354 and are approximately linearly dependent on the oxygen mole fraction in the observed parameter regime. If a gap was jumped by the flame, the apparent propagation speed was recorded to be between 1.5 and 3 times faster than on full fuel samples. With respect to the whole sample, the mean propagation rates could be seen to increase with decreasing fuel coverage.