Estimates of Liquid Species Diffusivities in N-Propanol/Glycerol Mixture Droplets Burning in Reduced Gravity

Abstract

Results from International Space Station experiments on combustion of n-propanol/glycerol droplets are reported. The initial n-propanol mass fraction was 0.95 and droplets had initial diameters in the 2 – 5 mm range. Some droplets were fiber supported while others were free floating, and the environment was either an oxygen/nitrogen mixture at 1 atm or an oxygen/helium mixture at pressures of 1 and 3 atm. The droplets burned in a multi-stage manner where n-propanol was preferentially evaporated during the early stages of combustion. The resulting buildup of glycerol in the liquid at the droplet surface led to sudden droplet heating and flame contraction. The experimental data are evaluated to provide burning rates, radiometer outputs, and droplet diameters as functions of time. These data are used to calculate effective liquid species diffusivities, D, using asymptotic theory. The D values can be substantially larger than molecular diffusivities in some cases, indicative of the presence of strong convective mixing. It was found that support fibers can decrease D values and that high burning rates can substantially increase D. These variations are attributed to changes in droplet internal flow patterns.