Ignition experiment of a blended-fuel droplet in a microgravity field

Ignition times of a fuel droplet were obtained in a microgravity field produced by a droptower of 6 m height. By use of a newly developed ignition technique under microgravity, a stationary droplet suspended by a fine silica fiber was quickly exposed to high ambient temperature and its ignition time was detected by a photodiode. n-heptane as a volatile fuel, n-hexadecane as a less volatile fuel and a combination of these two were used. As the initial droplet diameter increased from 0.75 to 1.3 mm, ignition time increased linearly for less volatile n-hexadecane. In case of n-heptane, however, ignition time slightly decreased with the initial diameter. As a result the variation of ignition time of blended fuel was found to have a maximum diameter of around 1 mm. Although these characteristic changes of ignition time were found in experiments under normal gravity and were partly suspected to be caused by natural convection, it was confirmed by the present microgravity experiment that these phenomena are based on the original properties of blended fuels composed of liquids with different volatility.