Reduced and normal gravity combustion experiments were performed with fiber-supported methanol droplets with initial diameters in the 1？mm size range. Experiments were performed with air-diluent mixtures at about 0.101？MPa and 298？K, where carbon dioxide, helium, or xenon was separately used as the diluent gas. Results indicate that ambient gas transport properties play an important role in determining flammability and combustion behaviors including burning rates and radiant heat output histories of the droplets. Droplets would burn with significantly higher mole fractions of xenon than helium or carbon dioxide. In reduced gravity, droplets would burn steadily with a xenon mole fraction of 0.50 but would not burn steadily if helium or carbon dioxide mole fractions were 0.50. Comparison with previous experimental data shows that ignitability and combustion characteristics of droplets are influenced by the fuel type and also the gravitational level. Burning rates were about 40% to 70% higher in normal gravity than in reduced gravity. Methanol droplets also had burning rates that were typically larger than 1-propanol burning rates by about 20% in reduced gravity. In normal gravity, however, burning rate differences between the two fuels were significantly smaller. 1. Introduction The combustion and gasification behaviors of droplets have been studied for decades for pragmatic and scientific reasons. For example, combustion of effervescent materials can produce burning particles under reduced-gravity conditions [1], which is relevant to practical aspects of spacecraft fire safety. Studies of droplets can also increase basic understanding of phenomena such as influences of diffusion, radiant heat transfer, and chemical kinetics on combustion of condensed-phase materials. The present research is focused upon studying influences of inert gases on ignition, combustion, and gasification of methanol droplets in normal gravity and reduced gravity. The droplets were initially, that is, before ignition, in the 1？mm size range and experiments were performed where air was progressively diluted with increasing levels of inert gas until combustion was no longer apparent. The inert gases were carbon dioxide, helium, and xenon. Helium and xenon are noble gases, leading to the same adiabatic (thermodynamic) flame temperatures for the same environmental compositions. However, differences in molecular masses between these species influence energy transport and transport of oxygen to droplet flame zones from variations in ordinary and Soret mass diffusion coefficients and
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