Fiber-Supported Droplet Combustion

Individual droplets with diameters ranging from about 2 mm to 5 mm were burned under microgravity conditions in air at 1 bar with an ambient temperature of 300 K. Each droplet was tethered by a silicon carbide fiber of 80 mm or 150 mm diameter to keep it in view of video recording, and, in some tests, a forced air flow was applied in a direction parallel to the fiber axis. Methanol, two methanol-water mixtures, two methanol-dodecanol mixtures, and two heptane-hexadecane mixtures were the fuels. Droplet diameters were measured as functions of time and compared with existing theoretical predictions. The prediction that methanol droplets extinguish at diameters that increase with increasing initial droplet diameter is verified by these experiments. In addition, the quasi-steady burning rate constant of the heptane-hexadecane mixtures appears to decrease with increasing droplet diameter; obscuration consistent with very heavy sooting, but without the formation of soot shells, is observed for the largest of these droplets. Forced convective flow around methanol droplets was found to increase the burning rate and to produce a ratio of downstream-to-upstream flame radius that remained constant as the droplet size decreased, a trend in agreement with earlier results obtained at higher convective velocities for smaller droplets having larger flame standoff ratios. There are a number of implications of the experimental results regarding droplet-combustion theory.