Structure Of Flame Balls At Low Lewis-number (SOFBALL):
Summary of results from space flight experiments on STS-83 and STS-94

Paul D. Ronney, Principal Investigator
University of Southern California, Los Angeles, CA

The objective of the Structure Of Flame Balls At Low Lewis-number (SOFBALL) experiment is to study the behavior and properties of a newly discovered premixed-gas flame phenomena called “flame balls.” These spherical, stable, stationary flame structures, observed only in microgravity, provide the opportunity to study the interactions of the two most important processes necessary for combustion (chemical reaction and heat/mass transport) in the simplest possible configuration. The previously unobtainable experimental data collected during the STS-83 and STS-94 missions provide a comparison with models of flame stability and flame propagation limits that are crucial both for fire safety assessment on earth and in orbiting spacecraft as well as for the design of lean-burn combustion engines which provide improved fuel economy and lower pollutant formation.

The tests were conducted by filling a 26-liter chamber with a weakly combustible gas (hydrogen and oxygen highly diluted with an inert gas) and igniting the mixture with a spark. The flames were imaged using four video cameras. Radiometers, thermocouples and pressure transducers were used to determine the heat release from the flame balls. The experiments were performed in the Combustion Module-1 facility on the Microgravity Science Laboratory (MSL-1) Spacelab missions.

Two successful tests (out of two attempts) were conducted on STS-83 and 16 successful tests (out of 17 attempts) were conducted on STS-94. Most of the tests burned for 500 sec, until the experiment timeout extinguished the flames. Over 3 hours of test data were collected.

SOFBALL highlights:

• First premixed gas combustion experiment ever performed in space

• Weakest flames ever burned, either on the ground or in space. Flame ball powers as low as one watt were measured. By comparison, a birthday candle releases about 50 watts.

• Longest-lived gas flames ever burned in space (many mixtures burned for the entire 500 sec test duration and were still burning at the timeout.)

• Conclusive evidence of limitation of existing models of lean hydrogen-oxygen combustion chemistry, which predict many ground-based experimental results accurately but yield conflicting predictions of flame ball properties, none of which agree with the experiments. Improved models of lean combustion are currently being developed based on the SOFBALL results.

• First conclusive demonstration of the effect of reabsorption of emitted radiation on the burning characteristics of a flame, either on the ground or in space. This factor is insignificant for most laboratory-scale experiments, but of key importance in practical flames at high pressure (as in automotive and jet engines) or at large scale (as in industrial boilers).

• First conclusive demonstration of the impact of accelerations caused by Orbiter vernier thruster firings and water dumps on science data obtained during a shuttle flight.

Work supported by the SOFBALL project has led to 27 journal papers and 7 invited conference presentations by the PI and his associates since SOFBALL was approved for flight in 1991.