Analysis of Local Flame-Spread Characteristics of an Unevenly Arranged Droplet Cloud in Microgravity

Abstract

As a preliminary experiment of the ''Group Combustion'' experiment conducted in the Japanese Experiment Module ''Kibo'' aboard the International Space Station (ISS) in 2017, a combustion experiment of an unevenly arranged droplet cloud with 148 n-decane droplets tethered on a SiC-fiber lattice was conducted in microgravity during parabolic flight of an aircraft. In this study, three methods of identifying the local flame-spread direction were proposed to determine the local flame-spread rate between two adjacent droplets of an unevenly arranged droplet cloud considering the local flame-spread direction. The first method employed the vertical direction from the isothermal line to the unburned droplet. The isothermal line was approximated based on the luminance of SiC fibers tethering droplets. The second method considered the luminance distribution of the initial flame immediately after ignition. The third method compared the local flame-spread-rate candidates from neighboring droplets with the flame spread-rate of the linear droplet array. It was possible to identify the local flame-spread direction by any one of these methods, and the local flame-spread rate is calculated between two droplets close to this direction. The results suggest that there are many conditions under which the local flame-spread rate of an unevenly arranged droplet cloud is greater than the flame spread rate of the linear droplet array.