Cool Fuel

Matthew Holm
December 01, 1995

Getting a jet to go to mach 4—four times the speed of sound—is a tricky business. Powerful-enough engines would run too hot, a bigger cooling system would add too much weight. The engineers' answer? Use cool fuel.

"In the emerging generation of advanced supersonic aircraft, jet fuel is the only coolant on board," says Jun Li, a graduate student in Penn State's fuel science program.

It's not a perfect solution. "Exposing fuel to high temperatures," Li explains, "can lead to thermal degradation, or pyrolysis." Just as the oil you drain from your car is black after 5,000 miles of driving, current jet fuel breaks down, leaving a carbon-like coating on the engine's insides. "In an advanced jet," says Li, "a clogged nozzle or fuel line could be a catastrophe."

As part of a larger Penn State effort to develop thermally stable jet fuels, headed by fuel scientist Harold Schobert, Li and his adviser, assistant professor Semih Eser, are looking at the interaction between the engine surfaces and the heated fuel. Since metals can act as catalysts to speed up a reaction, Li notes, different metals may affect the rate of deposition.

Li tested nickel, copper, and stainless steel to see which reacted the most with hot fuel. To simulate engine conditions in the laboratory, he used a batch reactor, which heats the fuel evenly but does not cause it to flow as it would in an actual engine; he checked his results against flow-reactor tests and carbon-coated jet-engine parts.

Immersing metal foil samples in dodecane, a model compound found in jet fuel, Li heated them in the batch reactor at 450 degrees C. When he removed the metal foils, they were covered with what appeared to be uniform carbonaceous deposits. It wasn't until he cross-sectioned them and put them under a polarized light microscope that he could tell how each metal had actually behaved.

On nickel, for example, the carbon was deposited in two distinct layers: the one next to the metal was anisotropic (exhibiting different properties in different directions, like graphite, which is aligned into thin sheets), while the second layer was more randomly deposited. On copper, the deposit was less anisotropic than on nickel, Li found, while the coating on stainless steel appeared to be the most random. Both the copper and stainless steel deposits formed as a single, uniform layer.

Under a scanning electron microscope, further differences showed up. The deposits on nickel and copper, Li says, looked "fibrous," while those that formed on his stainless steel samples were "spherules of deposit."

"These observations indicate," Li notes, "that nickel, and to a lesser extent, copper, catalyze deposit formation. Stainless steel appears to be inactive under the experimental conditions used, even though stainless steel alloys contain significant levels of nickel."

Knowing this, jet engineers can make a cleaner supersonic engine. What they still need is a super-stable fuel.

Jun Li is pursuing a Ph.D. in materials science and engineering, College of Earth and Mineral Sciences, 209 Academic Projects Building, University Park, PA 16802; 814-865-1019. His adviser, Semih Eser, Ph.D., is assistant professor of fuel science; 863-1392. Their work is supported by the U.S. Department of Energy and Wright Laboratories.

Last Updated December 01, 1995