Microgravity experiments on droplet motion during flame spreading along three-fuel-droplet array

Flame spreading along a fuel droplet array at microgravity has been studied as a simple model of spray combustion. A three droplet array with a pendulum suspender was employed to investigate interactions between flame spreading and droplet motion in the array direction. Initial droplet diameter was 0.8 mm and fuel was n-heptane. A silicon carbide pendulum suspender of 15 μm in diameter and 30 mm in length was used for the third droplet. The first fixed droplet was ignited by electric spark. Behavior of the flame and the third droplet was observed using a high-speed video camera. Dimensionless span, which is the averaged droplet span divided by the averaged initial diameter of the three droplets, was varied from 2.7 to 10. Large displacement of the movable droplet was observed after group flame grew around the movable droplet. As the initial dimensionless span increased, the averaged droplet speed after the occurrence of flame spreading to the movable droplet increased steeply, taking the maximum value around 5 in initial dimensionless span, and then decreased gradually. The movable droplet advanced toward the second droplet in small spans and moved away from the second droplet in large spans. The direction of the motion changed around 4.6 in initial dimensionless span. Flame spread induction time from the second to the third droplet increased exponentially as the initial dimensionless span was increased. The induction time of flame spreading to a movable droplet was longer than that of flame spreading to a fixed droplet. From calculations of flame spreading along a 20-droplet array, it was predicted that the droplet speed nearly converged after flame spread to the sixteenth droplet. The maximum speed of the nineteenth droplet appeared around 7.5 in the initial dimensionless span.