Ignition experiment on droplet matrix in microgravity

Ignition experiments on a suspended droplet matrix quickly immersed in a high-temperature ambience were conducted for hexadecane and decane in a microgravity field (10^-5 g) with a test duration of 4.5 s in the 100-meter drop-shaft of the Microgravity Laboratory of Japan (MGLAB) in Gifu, Japan. Small porous ceramic balls soaked with liquid fuel were employed as model droplets. The effect of the initial diameter and the spacing of the droplets on ignition delay time were measured. Also, by using a high-speed video camera with an image intensifier, OH emission images from the droplet matrix were observed. With a decrease in droplet spacing, in general, thermal interaction between droplets becomes significant and ignition time increases monotonically. However, in the case of small droplets, ignition time has a minimum when spacing is small, being shorter than that of a single droplet. The dependence of the vaporization time and the reaction time on the initial droplet diameter for a single droplet ignition basically elucidated the ignition time behaviour of a droplet matrix.