Microgravity flammability boundary for PMMA rods in axial stagnation flow: Experimental results and energy balance analyses

For the first time, a series of concurrent-flow rod flammability tests were conducted in microgravity aboard the International Space Station. A small flow duct was used to create 0 to 55 cm/s flows past three sizes of clear and black PMMA rods. The ambient oxygen concentration in the Microgravity Science Glovebox was varied from 13.6% to 22.2%. Oxygen, carbon dioxide, and carbon monoxide gas sensors provide initial and final readings for each test and indicate that the flames are globally stoichiometric at higher oxygen concentrations, but become more globally fuel rich as the minimum oxygen concentration is approached due to excess pyrolyzate leakage out of the open tail of the hemispherical flames. Quenching extinction occurs at very low forced flows, where the flame shrinks to a hemispherical blue flame and oscillates with increasing amplitude just before going out. Blowoff extinction is initiated by the formation of a hole in the flame sheet in the stagnation region of the flame. A critical Damkohler number formulation is applied across the flammability boundary, and the critical flame temperatures are derived. These critical flame temperatures are then used in a Nusselt number correlation to estimate the convective heat flux to the stagnation region of the rod. A model of surface energy balance is formulated that uses the critical flame temperature and convective heat flux to derive the mass burning rate along the boundary. The rod regression rates calculated from this model compare favorably with the experimental measurements. The surface energy balance reveals that along the blowoff branch, heat losses are negligible whereas in the quenching region, surface radiative loss dominates. At the bottom of the flammability map, the transition from blowoff to quenching occurs when the convective flows become the same order of magnitude as diffusive flows, shifting the critical Damkohler number from residence time limitations to diffusive time limitations.