Flame propagation along an array of liquid fuel droplets at zero gravity

Abstract

The transfer of flame between hydrocarbon droplets has been studied in free-fall experiments. The flame transfer time, τp (sometimes called propagation time in the literature) was measured from streak photographs for droplets with diameter d between 1.2 and 2.2 mm and spacing l from 8 to 14 mm. The principal trend of the data is that τp is proportional to l3, but at a given l, τp decreases weakly with increasing d. Streak photographs show that the ignition delay τi of the droplet is much longer than the time τs required for the flame to spread from the local ignition site over the whole droplet. This leads to a non-uniform velocity of flame propagation along an array. Schlieren motion pictures show that τi is made up of τi the time required for the thermal wave from the droplet ignited first to reach the surface of the second, plus τd the subsequent delay, which is probably mainly a heat-up time. A very simple mathematical model based on these ideas correlates the data quite well. The experiments have revealed the importance of transient processes in flame transfer between weightless droplets. At various stages of that process the same two droplets can burn singly, in two independent flames, or in a single flame.