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The effects of radiation heat transfer in microgravity compared to convection heat transfer in earth gravity for opposed-flow (downward) over thermally-thick fuel using low density foam fuel were investigated. Microgravity experiments on flame spread over thermally-thick fuels were conducted using foam fuels to obtain low density and thermal conductivity, and thus large flame spread rate compared to dense fuels such as PMMA. And thereby valid microgravity results were obtained even in 2.2 second drop-tower experiments not to mention for the longer duration tests in Zero Gravity Facility. Contrast to the conventional understanding, it was found that steady flame spread can occur over thick fuels in quiescent microgravity environments, especially when radiatively-active diluent gases such as CO2 were employed. This is proposed to result from radiative heat transfer from the flame to the fuel surface, which could lead to steady spread even when the amount of the heat transfer via conduction from the flame to the fuel bed is negligible. Radiative effects are more significant at microgravity conditions because the flame is thicker and thus the volume of radiating combustion products is larger as well. These results suggested that helium may be a better inert or extinguishment agent on both a mass and a mole bases at microgravity even though CO2 is much better on a mole bases at earth gravity, and these are relevant to studies of fire safety in manned spacecraft, particularly the International Space Station that uses CO2 fire extinguishers. CO2 may not be as effective as an extinguishing agent at microgravity as it is at earth gravity in some conditions because of the differences in spread mechanisms between the two cases. In particular, the difference between conduction-dominated heat transport to the fuel bed at earth gravity and radiation-dominated heat transport at microgravity indicates that radiatively-inert diluent such as helium could be preferable in microgravity applications. Helium may be a superior fire suppression agent at microgravity on several bases. First, helium is more effective than CO2 on a mole basis (thus pressure times storage volume basis) at microgravity, meaning that the size and weight of storage bottles would be smaller for the same fire-fighting capability. Second; helium is much more effective on a mass basis (by about 11 times) at microgravity. Third; helium has no physiological activity, unlike CO2 that affects human respiration. Fourth, as compared to N2 or CO2, is not very soluble in water and thus has fewer tendencies to cause bloodstream bubble formation following rapid spacecraft cabin depressurization.

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.