Microgravity ignition experiment on a droplet array in high-temperature low-speed airflow

The objectives of this study were to observe ignition events and to measure ignition times of a droplet array of n-decane placed in a high-temperature low-speed airflow under microgravity field. Due to the difficulty of making droplets of the same size within a short period of time in a drop capsule, imitation droplets made of porous ceramic balls soaked with n-decane were used. Experimental conditions were a droplet diameter of 1 mm, droplet spacing within the range of 0 to 6 mm, airflow velocity of 0 to 10 cm/s, and an airflow temperature of 925 K. According to OH emission images taken by a high speed camera with an OH band-path filter, ignition occurred around the droplets simultaneously at zero airflow velocity. At higher airflow velocities of more than several centimeters per second, however, ignition was initiated in the wake flow of the droplets and the flame spreads to the forward region of droplets. A range of droplet spacing existed in which ignition times of droplet arrays were less than those of a single droplet and had a minimum ignition time at a certain spacing. The spacing of this minimum ignition time increased with an increase of airflow velocity.