New Observations of Isolated Ethanol Droplet Flames in Microgravity Conditions

• Published in: Combustion science and technology 2008-04, Vol.180 (4), p.631-651
• Main Authors: Park, Seul-Hyun, Choi, Seuk-Cheun, Choi, Mun Young, Yozgatligil, Ahmet
• Format: Article
• Language: English
• Subjects: Applied sciences; Combustion of liquid fuels; Combustion. Flame; Diagnostics; Droplet combustion; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Microgravity; Soot; Theoretical studies. Data and constants. Metering
• ISSN: 0010-2202; 1563-521X
• DOI: 10.1080/00102200701839022

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 of ethanol droplet flames. Experiments were performed using the 2.2 sec 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 0.10 MPa to 0.24 MPa) and inert substitutions (He, Ar, and N 2 ). 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 demonstrated that ethanol droplets burning in Ar inert environments produced the highest soot volume fraction, followed by N 2 inert environments, and He inert environments, which produced the lowest soot volume fraction. For the various inert environments, 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 provided good correlation with the observed variations in sooting in microgravity droplet flames.