_______________________________  Corresponding Author: (215)895-6984 Choi@drexel.edu , www.mem.drexel.edu ETHANOL DROPLET COMBUSTION AT ELEVATED PRESSURES AND ENHANCED OXYGEN CONCENTRATIONS Ahmet Yozgatligil and Mun Y. Choi  Department of Mechanical Engineering & Mechanics Drexel University, Philadelphia, PA 19104 Andrei Kazakov and Frederick L. Dryer Department of Mechanical and Aerospace Engineering Princeton University, Princeton, NJ 08544 Samuel L. Manzello Building and Fire Research Laboratory NIST, Gaithersburg, MD 20899 Ritsu Dobashi Department of Chemical System Engineering University of Tokyo Tokyo, Japan In an effort to gain a better understanding of ethanol combustion, isolated droplet experiments were performed by varying initial droplet diameter, oxygen concentration, and ambient pressure. Experiments were performed at the NASA Glenn 2.2 sec. drop tower and the JAMIC 10 sec. dropshaft. These experiments revealed that while ethanol droplets burned in 1 atmosphere air without soot formation, luminous radiation from soot particles at higher pressures, with increased sooting at higher oxygen indices were observed. The measurement of the burning rate, soot standoff ratio and soot volume fraction are described. These experiments provide the first measurements of the soot volume fraction for ethanol droplets burning under microgravity conditions. Introduction Ethanol is a fuel that is regaining its popularity for use in practical applications. The use of ethanol as a motor fuel dates back to early 1900’s. But, due to lower prices and wider availability, gasoline replaced ethanol as the primary motor fuel in those early years. Ethanol was reintroduced as a fuel additive in the 1970s with the advent of the oil crisis and stricter regulations on the amount of pollutants emitted from motor vehicles. Finally, the Clean Air Act Amendments of 1990 1 mandated the use of oxygenated fuels, such as ethanol and methanol, in regions of the country experiencing high levels of carbon monoxide. There are many benefits of using ethanol as a motor fuel in terms of performance and pollutant mitigation. Recent investigations 2 on the emissions from engines operated using ethanol-containing fuels show a reduction in the carbon monoxide tail-pipe emissions in the entire operating range. Nag et al. 3 found that addition of ethanol produces a reduction of 92% in the ignition time, which confirms its usefulness as an octane number enhancer in gasoline. Blending of ethanol with diesel fuel also results in a reduction of soot mass concentrations 4 . Kitamura et al. 5 developed a chemical kinetic model (662 chemical species and 3005 elementary chemical reactions) used to analyze the suppression effects of oxygenated fuel blends (such as addition of ethanol to diesel fuel) on soot formation. In their study they represented diesel fuel with n-heptane which has a very similar cetane number. Their results show that addition of oxygenated fuels suppresses soot by reducing the amount of aromatic precursors (such as acetylene) that lead to drastic suppression of PAH / soot formation. They also found that adding oxygenates so that total oxygen content of the fuel is increased to 14 % by mass yields nearly soot-free combustion, which also agrees with published experimental work 4,6 . In the present study, the burning and sooting behavior of isolated ethanol droplets in a spherically- symmetric condition was analyzed. The spherically- symmetric burning of an isolated droplet, produced under microgravity conditions, is a dynamic problem that involves the coupling of chemical reactions, multi- phase flow (liquid, gas, particulate) with phase change. To this end, microgravity droplet combustion serves as an ideal platform for advancing the understanding of the physics of diffusion flames for liquid hydrocarbon fuels and additives that are typically used in internal