Burning and sooting behavior of ethanol droplet combustion under microgravity conditions

The spherically-symmetric burning of an isolated droplet is a dynamic problem that involves the coupling of chemical reactions and multi-phase flow with phase change. To this end, microgravity droplet combustion serves as an ideal platform for advancing the understanding of the diffusion flame physics of liquid hydrocarbons. In an effort to reduce both gaseous and particulate pollutant formation and to extend environmental resources, renewable energy sources such as ethanol are gaining popularity. Ethanol today accounts for 1% of the highway motor fuel market in the U.S. and its usage is increasing steadily. Due to the increased importance of ethanol usage as a motor fuel, improvements in the understanding of its combustion characteristics are imperative.; The influence of initial droplet diameter, pressure, oxygen concentration and inert substitution on the sooting and burning behavior of large ethanol droplets under microgravity conditions was investigated through measurements of burning rate and soot volume fraction. The experiments were performed at the NASA Glenn Research Center (GRC) 2.2 second drop tower in Cleveland and Japan Microgravity Center (JAMIC) 10 second drop shaft. These experiments revealed that while ethanol droplets burned in 1 atmosphere air without soot formation increased sooting and formation of soot shell were observed at higher pressure and oxygen indices. The competition between the influence of residence time and flame temperature lead to an interesting behavior in which the soot volume fraction varied non-monotonically with increase in oxygen concentration at elevated pressure. Inert substitution techniques were used in order to gain additional insight on the influence of residence time and flame temperature on soot formation. Argon, helium and nitrogen were used as inerts and the oxygen concentration was varied between 21% and 50% mole fraction at 2.4 atmospheres pressure. Significant sooting was observed under some of these conditions and soot yield was highest in argon and lowest for helium. These observations were attributed to the changes in the residence time and the flame temperature. Calculations showed that soot particles in oxygen/argon inert case experience the longest residence time and the highest flame temperature.