Future Supersonic Jets Pose Problems For Jet Fuel

September 10, 1997

Las Vegas, Nev. -- The next generation of advanced supersonic aircraft will be cooled by jet fuel, but researchers must first find a way to prevent fuel degradation and carbon deposition.

"Today's aircraft are air cooled, but at the high speeds expected of advanced aircraft, air can't be used for cooling," says Dr. Semih Eser, associate professor of fuel science.

The coolant will cool the engine and also circulate through the wings and fuselage to reduce the high temperatures caused by air friction.

"These jets will not be able to carry the additional heavy weights of coolant required, as they will already carry large amounts of jet fuel," says Eser. "The solution is to use the jet fuel as a coolant before it is burned as a fuel."

Jet fuel can be used as a heat transfer liquid, but begins to break down rapidly at above 575 degrees Fahrenheit. The degradation to smaller molecules will not affect the fuel's efficiency, but eventually carbon will deposit out of the fuel onto the fuel lines. This could cause maintenance problems or worse.

"Coal-derived fuels are much more stable than standard petroleum derived jet A or the military's JP8," Eser told attendees at the fall meeting of the American Chemical Society meeting today (Sept. 10) in Las Vegas, Nev. "But carbon deposition is still a problem."

Coal-derived jet fuels are not commercially available, but Eser; Jun Li, graduate student in fuel science; and Dr. Maria Sobkowiak, post doctoral fellow in polymer science, are experimenting with these fuels to characterize the way carbon deposits on nickel, stainless steel and copper.

The research, which is sponsored by the U.S. Department of Energy and the U.S. Air Force at Wright Patterson Air Force Base, shows that there are actually two main mechanisms of carbon deposition.

"Above a certain temperature, nickel and stainless steel begin to act as a catalyst," says Eser. "Filamentous carbon deposits on the surface and particles from the metal surface actually move into the carbon layer."

This erosion and pitting of the metal surface could eventually cause pinholes in the fuel lines, while the carbon deposition could clog the fuel lines.

On smooth nickel surfaces, carbon was deposited mainly in non-fibrous layers without interaction with the surface. This deposition takes place at higher temperatures than the catalytic reaction.

"It appears that rough surfaces are more likely to act as a catalyst, while smooth surfaces simply build up layers of carbon," says Eser.

Although copper lines would probably not be used in advanced aircraft, the researchers also tested copper. Copper grows fibrous, not filamentous carbon on rough surfaces. The carbon deposited on these copper surfaces resembles minute arrows, while those on stainless steel or nickel are uniform and the size of the metal particle on which they grew.

The researchers were able to see this because they used a microscope to allow viewing of the surface morphology combined with an infrared spectrometer to characterize the deposition layer.

"If the metal is coated with silica, the catalytic type of deposition does not occur," says Eser. "It might also be possible in the future to modify the surface of fuel lines to passivate the catalytic sites so that filamentous carbon does not form.

"However, to eliminate carbon deposition completely, we need a reformulation of the fuel."

**aem**

EDITORS: Dr. Eser may be reached at (814) 863-1392.

Penn State

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