This study conducted microgravity experiments on flame spread over droplet arrays using n -decane as a fuel at 0.3 and 0.4 MPa and focused on the cool-flame appearance and two-stage ignition during flame spread over fuel droplets. Previous studies have reported that when a single droplet is inserted into a high temperature and pressure furnace, two-stage ignition occurs in which a hot flame appears after a cool flame. This study investigated whether two-stage ignition could occur in flame spread over fuel droplets in microgravity. The flame spread was observed by an infrared camera, and the appearance of a cool flame was identified using the Thin Filament Pyrometry (TFP) method for SiC fibers tethering the droplets. The results show that a cool flame appears in a narrow range of droplet spacing around the hot-flame-spread limit of two interactive droplets at 0.3 MPa. The two-stage ignition occurs around two interactive droplets during flame spread at 0.4 MPa. After a cool flame appears, a rapid temperature rise occurs between the two droplets, leading to the appearance of a hot flame. We further investigated whether a flammable-mixture layer around the droplet with the cool flame contributes to the occurrence of large-scale ignition observed in flame spread over the droplet cloud in microgravity aboard the International Space Station. As a result, the pre-existence of the cool-flame and longer waiting time for ignition are factors that enlarge the initial hot-flame diameter and cause large-scale ignition.
Chikami et al. (Fri,) studied this question.