Spacelab Experiments on Combustion of Heptane/Hexadecane Droplets

Abstract

Results are presented from experiments on reduced-gravity combustion of individual heptane/hexadecane droplets. Initial droplet diameters ranged from 0.25 to 5.2 mm, and initial hexadecane mass fractions in the droplets were 0.058, 0.10, 0.20, and 0.40. Most droplets were burned in cabin air on Spacelab with an ambient oxygen mole fraction of about 0.21 and a pressure of about 0.1 MPa. Data were also obtained for small (0.25-mm) droplets in a ground-based apparatus that provides simulated reduced gravity environments for droplet combustion experiments. Droplets in this apparatus were burned in high-temperature gases at 0.1 MPa. Data on flame contraction (i.e., the sudden contraction of the flame from droplet heating as the surface liquid mass fraction rapidly approaches unity) were used to estimate effective liquid species diffusivities that apply to these droplets. Significant differences in effective liquid species diffusivities were observed; these differences may be related to variations in internal circulation within the droplets, as well as possible variations in droplet temperatures. The Spacelab results indicate that flame extinction can be induced by flame contraction and that the occurrence of extinction depends on the initial droplet size. It was also observed that sooting and radiant heat losses varied significantly with initial droplet size in the Spacelab experiments. Sooting decreased as initial droplet sizes increased, whereas radiant heat loss rates varied in a complex fashion with the initial droplet size.