Space-based microgravity experiments on flame spread over randomly distributed n-decane-droplet clouds: Anomalous behavior in flame spread

Abstract The first combustion experiments aboard the Japanese Experiment Module “Kibo” on the International Space Station, titled “Elucidation of Flame Spread and Group Combustion Excitation Mechanism of Randomly Distributed Droplet Clouds (Group Combustion)” were conducted in 2017 in order to bridge the gap between droplet combustion and spray combustion through flame spread over droplet arrays, droplet-cloud elements and randomly distributed droplet clouds. n-Decane droplets were distributed randomly on a SiC-fiber lattice, and the flame spread and group-combustion excitation were observed. During the flame spread near the group-combustion-excitation limit, two types of anomalous behavior appeared: large-scale ignition and re-burning by a slow flame propagation in a burned area. We successfully simulated similar phenomena by a flame-spread experiment using droplet-cluster arrays. The large-scale ignition phenomenon is caused by the ignition of a flammable mixture generated by heating multiple droplets existing outside the local flame-spread limit. The re-burning phenomenon by the slow flame propagation in a burned area is caused by the ignition of a flammable mixture diluted by the combustion products after the radiative extinction of the group flame. There is the possibility of a cool-flame appearance in both phenomena.