NANOSTRUCTURE OF SOOT COLLECTED FROM ETHANOL DROPLET FLAMES IN MICROGRAVITY

摘要

The nanostructure of soot particles collected from spherically symmetric ethanol droplet flames were analyzed using a high resolution transmission electron microscopy (HRTEM). Nanostructure properties, including fringe length and curvature of carbon lamellae, were measured for soot particles collected in various inert environments. The sampling experiments were performed in the reduced gravity environments produced in the NASA 2.2 sec Droptower at the Glenn Research Center in Cleveland, Ohio. Microgravity droplet combustion experiments provide unique opportunities to vary the residence times over a large range and to independently vary the temperature. In this study, the time-temperature histories experienced by the soot particles were adjusted by means of inert substitutions (argon vs. helium) and variations in the initial droplet diameters (ranging from 1.6 to 2.2 mm). The variations in the initial droplet diameter were found to affect only the residence time necessary for soot inception and growth, whereas inert substitutions modified both residence time and temperature. The measurements of soot nanostructure properties indicated that the higher temperatures produced in the argon inert environment produced more graphitic nanostructures, while the lower temperatures produced in the helium inert environment produced more amorphous nanostructures at the inner core of the soot primary particle. The variations in the initial droplet diameter produced distinct soot nanostructures on the periphery of the soot particle. The higher residence times experienced for the largest initial droplet diameter experiments produced longer carbon lamellae with negligible curvature, while the lower residence times for the smallest initial droplet diameter experiments produced shorter carbon lamellae with higher degrees of curvature. These experimental results provide important foundational understanding of the influence of residence time and temperature on the soot nanostructure that has not been studied previously.