Effects of ambient pressure on autoignition of a fuel droplet in supercritical and microgravity environment

Abstract

An experimental study has been performed under microgravity conditions to obtain the detailed information needed to understand the combustion phenomena of a fuel droplet which autoignites in supercritical gaseous environment. A fuel droplet suspended at the tip of a fine quartz fiber in the cold section of the high-pressure combustion chamber was transferred quickly to be subjected to a hot gas in an electric furnace. This resulted in the evaporation, autoignition, and combustion of the fuel droplet in a supercritical gaseous environment. A high-speed video camera was provided to observe thebehavior of the fuel droplet and the droplet flame. In the present study, primary attention was focused on the ignition delay and the flame diameter at the initial stage of droplet burning. 1-Octadecanol and n-octadecane were selected as the test fuels. Mixtures of oxygen and carbon dioxide and of oxygen and nitrogen were used as the ambient gas. The ambient pressure was extended up to pressures around two times the critical pressure of the fuels tested. The ignition delay was found to have a minimum around the ambient pressure equal to the critical pressure of the fuel, and a maximum in the range from 1.5 to 2 times the critical pressure. Higher oxygen concentration of the ambient gas caused shorter ignition delay at ambient pressures equal to or above 1.5 times the critical pressure. Higher ambient pressure caused a smaller droplet flame at the onset of autoignition and higher flame growth rate and a larger flame diameter afterward. The flame diameter decreased with an increase in the oxygen concentration of the ambient gas.