Unstable sound waves research could prevent jet engine damage

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Air Force Office of Scientific Research-funded scientists are studying ways to avert a combustion instability problem that has destroyed liquid and solid rockets, military jet engines, missiles and gas turbines.

The team, led by Dr. Ben Zinn, an aerospace engineering professor at Georgia Institute of Technology, may be closer to preventing the phenomenon. Dr. Zinn and his team are trying to find out under what conditions these unstable sound waves occur and how to eliminate them.

Finding a solution is challenging as each process requires an in-depth knowledge of interactions between complex fluid mechanical, combustion and acoustical processes within engines.

“In liquid rockets, the fuel and oxidizer are injected into the engine through hundreds of injectors evenly distributed on the engine’s injector plate,” Dr. Zinn said. “We believe that if the injectors are not symmetrically distributed on the injector plate, unstable and destructive sound waves will not be excited in the engine.”

He also noted that the second way the problem can be overcome is by designing better damping devices, such as baffles, that will prevent the occurrence of such unstable waves within engines.

Until recently, researchers have been unable to replicate the conditions in the lab that give rise to the spinning, destructive acoustic waves that encircle the combustion chamber because they did not possess injectors that could modify the combustion process.

With the help of Ukrainian scientist, Dr. Oleksandr Bibik, the investigators used “smart” injectors to excite these acoustic waves in laboratory combustor. Then, using a very-high-speed camera and fiber optic probes they captured the formation and movement of the sound waves within the engine. The technology successfully showed the sound waves spinning around the engine at 5,000 revolutions per second.

“If we find a means to prevent phenomena from occurring we can save the Air Force many millions of dollars that are invested to solve this problem during the development of new propulsion systems, postponement of deadlines and not meeting national defense goals in a timely fashion,” Dr. Zinn explained.

Dr. Mitat Birkan, AFOSR program manager, emphasized the importance of this research because “understanding the fundamentals of the high pressure/temperature combustion dynamics, could lead to the tools necessary to design instability free rocket engines on the computers of the future.”

Maria Callier

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