Influences of Water on Methanol Droplet Burning and Extinction

Studies of droplet burning and extinction have been progressing for a number of years with support from the NASA program on microgravity combustion. Besides theoretical investigations, the research involves experiments in drop towers at the NASA Lewis Research Center and in Spacelab on the Space Shuttle. The first Spacelab experiments were performed a year and a half ago in the Glovebox, a relatively inexpensive facility, on the USML-2 mission. The next Spacelab experiments are planned for the MSL-1 mission in April, after the deadline for this report but before this scheduled meeting. Besides the Glovebox experiments (the second set of fiber-supported droplet-combination experiments, FSDC-2), this April mission has a full-scale droplet-combustion experiment (DCE-1), in which heptane droplets of 2 mm to 5 mm initial diameter are burned in various helium-oxygen atmospheres. The FSDC experiments burn various alcohols and alkanes, both pure and binary mixtures, with this same range of initial diameter, but always in Spacelab cabin air. Most of the FSDC-1 data were obtained with droplets of methanol and of methanol-water mixtures. A preliminary presentation of results from these tests has been made. The results for these methanol systems motivated further theoretical studies, beyond previous work, which are reported here. Essentially, theory had predicted droplet extinction diameters that increased nearly linearly with droplet initial diameters, and the experimental results confirmed that prediction. But the values of the experimental extinction diameters exceeded those predicted theoretically. The studies reported here address the reason for this discrepancy, with special attention paid to influences of the Lewis number of the water vapor and of absorption of water form the atmosphere into the fuel prior to ignition.