ISS Experiments on Combustion of Heptane/Hexadecane Droplets

Abstract

Combustion experiments on heptane–hexadecane mixture droplets were conducted onboard the International Space Station. The ambient consisted of oxygen mixed with either helium or nitrogen at 1 atm. Initial droplet diameters were in the 1.7–4.8 mm range with initial heptane mass fractions of 0.95 or 0.80. Because of limitations on resources, only fiber-supported experiments were possible. Vaporization of the more volatile fuel component (heptane) was favored during the droplet combustion process, leading to flame contraction when the mass fraction of hexadecane closely approached unity at the droplet surface. Information about the onset of flame contraction was used to estimate effective liquid species diffusivities with asymptotic theory. The experiments show that droplet combustion generally became very disruptive after flame contraction as evidenced by fluctuating radiometer measurements. In some cases flame contraction was not apparent and droplets could continue burning vigorously even after visible flame extinction. Extinction was also sometimes observed to be induced by flame contraction. Additionally, it was noted that the effective liquid species diffusivities increased significantly as burning rates increased.