Experimental Observations of the Low-Temperature Burning of Decane/Hexanol Droplets in Microgravity

This article presents the results of experiments conducted aboard the International Space Station involving the combustion of large bi-component droplets of decane and hexanol (50/50 by volume) in air ambients with ambient pressures between 0.05 and 0.30 MPa. The experiments showed the presence of sustained low-temperature or cool-flame burning following radiative extinction of large droplets at ambient pressures greater than or equal to 0.10 MPa. The droplet diameters at cool-flame extinction were larger for the decane/hexanol droplets than for pure decane droplets at atmospheric pressure, suggesting that hexanol inhibits the cool-flame burning. At 0.20 MPa large fiber-supported droplets radiatively extinguished then burned with a cool flame for a period of time before the hot flame spontaneously re-ignited. At the highest ambient pressure of approximately 0.30 MPa the droplets again radiatively extinguished and burned with a cool flame. Contrary to the 0.20 MPa tests, however, the hot flame did not spontaneously re-ignite, but the droplet burned to completion with a cool flame. Further, more detailed analyses of all camera and radiometer data suggest that the cool-flame burning at 0.30 MPa is fundamentally different than the cool-flame burning at atmospheric pressure. This result does not appear to be consistent with expectations based on currently available cool-flame chemical kinetics and may suggest the need for a different chemical-kinetic mechanism.