Dropping smoothly to a fiery end.

Dropping smoothly to a fiery end

How do you make a falling dropletstand still? With considerable difficulty, it turns out. Mechanical engineer Thomas Avedisian and graduate student Jiann Yang of Cornell University in Ithaca, N.Y., spent three years perfecting an apparatus to enable them to study how a droplet of fuel burns under nearweightless or "microgravity' conditions. Now they can track, on film, a burning droplet's complete life history --from ignition to extinction.

Their results, says Avedisian, willhelp test fundamental theories about how fuels burn and validate computer models of combustion. The information may also lead to a better understanding of how liquid fuels behave in spacecraft propulsion systems.

The basic idea sounds simple. It's amatter of releasing at the same time a fuel droplet and a camera focused on the droplet, allowing them to plummet 25 feet into a cushion of foam rubber chunks. Such drop-tower experiments have been done in the past, but researchers usually managed to observe less than half of a droplet's burning history. Furthermore, the sudden tug needed to release a droplet hanging from a fiber tended to give it an unpredictable initial velocity.

Avedisian and Yang overcame theseproblems by designing a special droplet generator and a precise timing circuit to control the whole experiment. Sitting in a clear plastic chamber, a tiny nozzle squirts a stream of droplets, each less than 0.5 millimeter in diameter, in a nearly vertical trajectory. The timing circuit shuts off the stream, and when the final droplet reaches the peak of its trajectory, the droplet is ignited and the whole platform on which the apparatus sits is released. At that instant, the droplet is stationary. "If the timing is correct,' says Avedisian, "the droplet looks motionless with respect to the camera.' The plunge takes about 1.2 seconds.

To make the experiment work, theresearchers had to ensure that the droplet generator yielded repeatable trajectories, even after numerous falls. In addition, they had to account for the slight delay between the time when the electromagnet holding the platform is shut off and when the platform is actually released.

These drop-tower experiments allowAvedisian and Yang to study combustion without the complicating effects of flows within droplets due to buoyancy. So far, they have done about half a dozen experiments on droplets of fuels such as heptane and hexadecane and various mixtures of the two components. Eventually, they hope to look at the effects of additives and changes in pressure.

Photo: Yang (left) and Avedisian prepare todrop their 350-pound instrument package.