Soot formation during combustion of unsupported methanol/toluene mixture droplets in microgravity

Abstract

An experimental study is reported concerning the influence of liquid composition on soot formation and burning rate of a droplet composed of a binary miscible mixture of liquids. The mixture components represented a highly sooting fuel, toluene, and a non-sooting fuel, methanol. The experimental observations were made in a microgravity environment to create near spherically symmetric burning of the droplets. Mixtures of 5%, 25% and 50% by volume toluene in methanol were burned in room temperature air. Initial droplet diameters ranged from 0.47 mm to 0.60 mm, and the available experimental time was sufficient to record the complete droplet burning history. Toluene concentration in methanol was shown to dramatically influence flame luminosity and soot production. However, neither burning rates nor propensity for flame extinction appeared to be significantly affected by toluene mixture fractions. 5% toluene mixture droplets behaved like pure methanol droplets in terms of burning rate, lack of flame luminosity, and extinction. Increasing the toluene concentration in the droplets to 25% increased flame luminosity, yet no visible soot agglomerates were observed. The 50% mixture droplets, however, burned with highly luminous flames and large amounts of soot agglomerates collecting inside the flame. None the less, all the mixture droplets showed similar burning rates to those of pure methanol and likewise exhibited flame extinction before complete droplet vaporization.