PMMA rod stagnation region flame blowoff limits at various radii, oxygen concentrations, and mixed stretch rates

Abstract

Normal gravity flame blowoff limits in an axisymmetric PMMA rod geometry in upward axial stagnation flow are compared with microgravity Burning and Suppression of Solids-II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but was done in normal gravity mixed convective flow. Cast polymethylmethacrylate (PMMA) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 15% to 18% by volume. The forced flow velocity where blowoff occurs was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch rate is included and the flow is corrected by considering the flow blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration and flow velocity as coordinates. We describe a new normal gravity upward blowoff test method which extrapolates this linear blowoff boundary to the zero mixed stretch rate limit to resolve microgravity flammability limits—something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain accurate but conservative estimates of material flammability in low gravity based on tests performed on Earth.