FIBER-SUPPORTED DROPLET COMBUSTION - 1

The first Fiber-Supported Droplet Combustion (FSDC-1) experiment, while motivated by scientific concerns, involves realistic fuels used in a variety of applications. A total of about 39 droplet tests in air at 300 K were run with most of them yielding publishable data; in fact these experiments were documented in a peer-reviewed article that was accepted in the most prestigious combustion symposium - the International Symposium on Combustion - held every two years. It has since served as the data against which a variety of new theoretical and numerical models has been compared in other peer-reviewed articles and conference presentations. The experiment demonstrated that fuel droplets as large as 5 mm in diameter burned in most ways as expected from droplet theory and from extrapolations of experimental data with smaller droplets. This was especially important to demonstrate the feasibility of the follow-on experiments that flew on STS-83/94. There is qualitative agreement between the theoretical and numerical predictions and experimental data on extinction diameters for methanol and methanol-water mixtures; agreement between the numerical model and the experimental data has been shown to be improved when radiative heat loss from the flame is accounted.

FIBER-SUPPORTED DROPLET COMBUSTION - 2

• We easily observed radiative vs. diffusive extinction for quiescent, isolated droplets. For the first time, we were able to re-ignite large droplets numerous times; initially they went out because of radiative extinction; eventually the initial diameter shrunk to a size where diffusive extinction took over (and the droplet burned to completion).
  
• The reburns with heptane/hexadecane produced lots of soot because of the soot producing nature of hexadecane. Being less volatile, the hexadecane was at a higher concentration in the residual droplet as compared with the initially unburned droplet.
  
• In all radiative extinction cases, an aerosol cloud was seen to form immediately after combustion ceased; this is very similar to what other investigators saw in candle experiments on Mir. Cause is speculated as: the fuel droplet continued to vaporize after the flame extinguished. The fuel vapor and the water vapor produced previously by the flame then condensed when they migrated to a (colder) position beyond where the flame once was. These serve as a reminder that the hazard associated with a spacecraft fire may not be terminated even after the flame is extinguished.
  
• We also did a few forced air flow tests which revealed a very luminous flame with a symmetric tail. Size of the flame tail was dependent, as expected, on forced air velocity. These are enabling a comparison with correlations of low-speed air flow effects that were heretofore unvalidated.
  
• Two droplet tests always showed a merged, single flame -- the two droplets were tethered 10 mm apart (center to center); the range of initial droplet diameters was from 1.5 mm to 4 mm, so a merged flame was what was expected.

FSDC Experiment Information