The Effect of Gravity on Flame Spread over PMMA Cylinders in Opposed Flow with Variable Oxygen Concentration.

Fire safety is an important concern for space travel, particularly with the operation of proposed space missions such as those of NASA’s Constellation Program. Currently there is not sufficient knowledge regarding the fire behavior of materials in environments similar to those expected in those future spacecraft (micro-gravity, low velocity flow, elevated oxygen, and reduced pressures. Flammability of solid materials is typically characterized by ignitability, flame spread rate, heat release rate, and toxicity. Although, the most effective fire safety strategy is to prevent ignition altogether, if ignition occurs the fire has to spread to present a risk. The spread of flames over cylindrical samples of cast PMMA of different diameters was investigated to better understand the effects of gravity and oxygen on the mechanisms of flame spread. The 1g experiment consisted of determining the opposed flow spread rate for 3 sample diameters (6.3, 9.5, and 12.7 mm) in gas flows with oxygen concentrations from 21% to 15% and a velocity of 25 cm/s. The micro-gravity flame-spread experiments were conducted as a part of the Burning and Suppression of Solids - II (BASS-II) campaign of micro-gravity combustion experiments conducted aboard the ISS. Opposed flame spread rates were measured for the above samples under flow velocities from 0.5 to 5 cm/s. and oxygen concentrations from 21% to 15%. It was found that for all comparable oxygen concentrations and diameters flame spread in μg was faster than in 1g conditions. A numerical modeling of flame-spread over cast PMMA rods as a function of ambient oxygen concentration and gravity was conducted using FDS. The modeling predicts the observed experimental trends qualitatively but not quantitatively. This is most likely due to the approximate description of the solid phase pyrolysis and gas phase chemistry. The work was supported by NASA Grants NNX10AE01G