Investigation of sooting behavior and soot nanostructures of ethanol droplet flames in microgravity

Seul Hyun Park

doi:10.17918/etd-1801

Spherically-symmetric ethanol droplet combustion experiments were performed to investigate the influence of initial droplet diameter, ambient pressure and inert substitution on the burning and sooting behaviors. These experiments are the first to provide an analysis of sooting behavior of ethanol under microgravity conditions. Experiments were performed using the 2.2 s reduced-gravity droptower facilities at the NASA Glenn Research Center. Noting the importance of transport characteristics of heat and species and their attendant effects on flame temperature and residence time on the sooting mechanism of diffusion flames, parameter adjustments were made to vary the sooting over a wide range of conditions. In these experiments, the residence times for fuel vapor transport were varied using changes in initial droplet diameters (from 1.6 mm to 2.2 mm) and ambient pressure (from 1.6 atm to 2.4 atm) and inert substitutions (Helium, Argon, and Nitrogen). The flame temperatures and flame standoff ratios were varied using different inert substitutions. For each experiment, the soot volume fraction, droplet burning rate, sootshell and flame dynamics, flame temperatures, and flame radiative emission were measured. These measurements enabled calculation of the fuel vapor transport residence times (from droplet surface to the flame front) which provides a measure of the duration for pyrolysis reactions, soot nucleation, and soot growth. The experimental measurements demonstrate that ethanol droplets burning in Ar inert environments produced the highest soot volume fraction, followed by N2 inert environments, and He inert environments, which produced the lowest soot volume fraction. Within each inert environment, the flame temperature distribution and the flame standoff ratio were only weakly affected by changes in both initial droplet diameters and ambient pressures. However, significant increases in soot volume fraction were observed as the initial droplet diameter and ambient pressure were increased. The coupled analysis of the flame temperature and the residence time for fuel vapor transport provides correlation with the observed variations in sooting in microgravity droplet flames. Soot collected from microgravity ethanol droplet flames through thermophoretic techniques was analyzed for nanostructure properties using a fringe analysis algorithm that was developed as a part of this study. The experimental results indicate that the higher temperatures for the Ar inert experiments produce graphitic nanostructures while the lower temperatures for the He inert experiments produce amorphous nanostructures at the inner core of the soot primary particle. The variations in the initial droplet diameters which influence the residence time while maintaining constant flame temperatures create distinct soot nanostructures on the periphery of the soot particle. The higher residence times produce longer carbon lamellas with negligible curvature while the lower residence times produce shorter carbon lamellas with higher degrees of curvature. These experimental results provide important foundational understanding of the influence of time and temperature history on the soot nanostructure that has not been studied previously.

Investigation of sooting behavior and soot nanostructures of ethanol droplet flames in microgravity

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Investigation of sooting behavior and soot nanostructures of ethanol droplet flames in microgravity

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