Autoignition and combustion of a fuel droplet in supercritical gaseous environments under microgravity

An experimental study has been made of the evaporation, autoignition, and combustion of a fuel droplet in supercritical gaseous environments for the deep understanding of the combustion processes in gas turbine, diesel engine, and rocket motor in which the operating pressure often exceeds the critical pressure of the liquid fuel of frequent utilization. The parabolic flight of aircraft provided the long period of time for the experiments that allowed observation of the processes during the entire lifetime of a fairly large droplet from the start of evaporation to the end of burning in quiescent supercritical gaseous environments under microgravity. A fuel droplet suspended at the tip of a fine quartz fiber in the cold section of the high-pressure combustion chamber was translated 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 supercritical gaseous environments. A video camera was provided to observe the behavior of the fuel droplet as well as the flame around the droplet. The fuel tested is octadecanol, which solidifies at 331 K and the critical conditions of which are 1.4 MPa and 747 K. The experiments were done in the quiescent gaseous environments at low oxygen concentration to reduce the soot produced in the flame and to make it possible to observe the backlighted image of the droplet during its entire lifetime. The ignition delay, the burning time, and the lifetime of the droplet showed minima at the ambient pressure approximately equal to the critical pressure of the liquid fuel. The burning rate constant that was determined from the time histories of the droplet diameter showed a peak at the ambient pressure 1.5 times as high as the critical pressure of the liquid fuel.