FLAME BALL EXPERIMENTS being conducted on STS107

Structure Of Flame Balls At Low Lewis-number (SOFBALL) STS-107 Science Summary

The data obtained during the mission including several totally new results, will keep combustion scientists busy for many years to come and will help lead to the development of cleaner, more fuel-efficient engines as well as improved methods for spacecraft fire safety assurance.

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January 27, 2003: The objective of Principal Investigator Paul D. Ronney's (University of Southern California) Structure Of Flame Balls At Low Lewis-number (SOFBALL) experiment was to study weakly burning flames in hydrogen-oxygen-inert and methane-oxygen-inert mixtures in a configuration called "flame balls" that were originally predicted by the Russian physicist Ya. B. Zeldovich in 1944 but not seen experimentally until 1984 in short-duration drop tower experiments conducted by Prof. Ronney. Because flame balls are steady, convection-free, spherically symmetric and occur in fuels with simple chemistry, they represent the simplest possible interaction of chemistry and transport in flames. In this sense flame balls bear a similar relationship to combustion research that the fruit fly does to genetics research.
A total of 39 tests were performed in 15 different mixtures, resulting in a total of 55 flame balls, of which 33 were named by the crew. The total burn time for all flames was 6 1/4 hours. Since flame balls are extremely sensitive to gravitational acceleration, all tests were conducted during orbiter free drift periods, time blocks where the small attitude control thrusters are not used. Microgravity levels (low frequency to quasi-DC) were measured using OARE. The quality of the microgravity was found to be excellent (average accelerations less than 1 micro-g for most tests).

Above: Image of flame balls in a hydrogen-oxygen-sulfur hexafluoride mixture at 3 atmospheres pressure.

Among the accomplishments of the experiment were:

The weakest flames ever burned, either in space or on the ground. The weakest flame balls produced about 0.5 watts of thermal power. By comparison a birthday candle produces about 50 watts of thermal power.

The leanest flames ever burned, either in space or on the ground. The leanest hydrogen-air test points burned contained about 8% of the chemically balanced mixture. By comparison, the lean limit for gasoline-air mixtures in an internal combustion engine is about 70% of the chemically balanced mixture.

The longest-lived flame ever burned in space (81 minutes).

Several totally new results were found, for example:

Oscillating flame balls that were predicted theoretically about 15 years ago by Prof. John Buckmaster at the University of Illinois and Dr. Guy Joulin of CNRS in Poitiers, France, but heretofore never observed experimentally.

Below: Radiometer signal showing flame ball oscillations.

Credit: NASA Glenn Research Center

For some tests, particularly in methane-oxygen-sulfur hexafluoride mixtures, flame ball drift not related to gravitational disturbances nor interactions with other balls or walls. This was a completely unexpected and as yet unexplained result.

The data obtained during the mission will keep combustion scientists busy for many years to come and will help lead to the development of cleaner, more fuel-efficient engines as well as improved methods for spacecraft fire safety assurance.

On behalf of the entire SOFBALL science team and the CM-2 engineering team, we would like to thank the STS-107 crew and the Code U ground support teams for the tremendous effort they put into making SOFBALL such a huge success.