Comparison of Carbon Dioxide and Helium as Fire Extinguishing Agents for Spacecraft

The rates of flame spread and minimum oxygen concentrations supporting flame spared over thermally thick fuel beds were determined at earth gravity and microgravity for varying concentrations of a gaseous extinguishing agent (N2, CO2, or He) added to “baseline” O2-N2 atmospheres. Drop towers were used to obtain microgravity, and foam fuels were employed to obtain sufficiently rapid flame spread in these short-duration facilities. At earth gravity, CO2 was the most effective on a molar basis at reducing spread rate whereas at microgravity, He was the most effective at reducing spread rate and causing flame extinction. These findings are proposed to result from the transition from a high-speed blow off-type limit at earth gravity to a low-speed radiative heat loss-induced limit at microgravity. CO2 is less effective as an extinguishment agent at microgravity because it reabsorbs radiation from flame and reradiates it back to the fuel bed at microgravity, whereas with N2 and He this phenomena is not evident at microgravity because these two gases are radiatively nonparticipating. He is particularly effective at microgravity because its high thermal diffusivity leads to much larger flame thicknesses resulting in much greater volume for radiative heat losses. Radiative effects are unimportant at earth gravity because convective flow is significantly faster, leading to thinner flames with much lower ratios of radiative to conductive/convective heat transfer. These results are particularly noteworthy considering that the International Space Station employs CO2 as fire extinguishers; our results suggest that helium may be a better suppressant agent on both mass and mole bases at microgravity even though CO2 is much better on a mole basis at earth gravity.