July 1, 1994
Jet Engine Research Takes Off At Purdue
WEST LAFAYETTE, Ind.Working in state-of-the-art laboratories, Purdue University engineers are developing practical ways to make jet engines quieter, safer and more efficient.
"Government regulations require that in the next two years, jet engines be five times quieter than they are now, but a good part of the commercial fleet does not meet the new noise requirements," says Sanford Fleeter, professor of mechanical engineering and director of the Center for Bladed Disk Unsteady Aerodynamics Research and Technology and the Army University Research Initiative on Rotorcraft Engine Unsteady Aerodynamics, both at Purdue.
"The government wants to reduce noise, but it's up to the engine manufacturers to actually do it. We've come up with some very practical ways of handling the noise problem."
One solution developed by Fleeter and his colleagues involves active noise control, which uses noise to cancel noise at the source before it has a chance to be heard.
The Purdue researchers presented their results in a series of papers June 27-29 at the 30th Joint Propulsion Conference in Indianapolis.
When a jet engine powers up, air sucked into the engine is compressed by a series of seven to 10 compressor and fan stages. Each stage includes a disk with up to 50 blades arranged around its edge, similar to a paddlewheel. The compressed air moves through both rotating and stationary bladed disks into the combustion chamber, where it combines with sprayed jet fuel and is ignited. Most of the hot gases from the combustion provide the thrust to move the plane, but some gases spin a series of turbine blades, which in turn drive the air compressor fans.
So why are jet engines so noisy? Air flowing through the bladed disks of the engine generates sound waves. Some of these waves decay, but many others do not and escape the engine.
"These pressure waves are very small," Fleeter says. "Energy-wise, they are one-tenth of a percent or less of the energy generated by the jet engine, yet they generate all that noise. Our solution is to build computer-controlled smart airfoils, which have piezoelectric crystals of composite materials incorporated into them."
A piezoelectric crystal generates electricity when force is applied to it, and changes shape when a voltage is applied across it.
"It's really a novel approach to the problem," Fleeter says. "The piezo crystals are built right into the blade and engine materials. When we apply electricity to the piezo crystal via a computer, the crystal oscillates, generating sound waves of its own."
The sound generated by the crystals has the same frequency but is exactly opposite in amplitude to the sound generated in other parts of the engine, so the two cancel out each other.
Purdue researchers are testing these computer-controlled foils at the Purdue Large Scale Research Centrifugal Compressor Facility, which houses a six-foot diameter compressor.
"This is one of the best facilities in the country for studying air acoustics," Fleeter says. '`Using this test facility, we've shown experimentally that we can cancel the engine noise to zero. Some of the these techniques could be used on aircraft in a couple of years."
Fleeter and his colleagues also are working on ways to improve the efficiency of jet engines and to better understand how to avoid engine stalls and surges.
A surge results when the airflow through the compressor becomes unstable, causing the engine to lose power. This instability occurs when the rotational speed of the fans exceeds a critical value, or when the airflow into the engine becomes severely distorted. In a rotating stall, instabilities in the airflow generate very low amplitude pressure waves that can quickly amplify and cause the engine to lose thrust. A dramatic example of a rotating stall was illustrated in the movie "Top Gun" when the plane flown by Tom Cruise's character crashed after it flew through the wake of the plane in front of him.
"Current turbo engines avoid the onset of rotating stall and surge by operating at less than maximum efficiency," Fleeter explains. "We don't really know what causes the flow to break down, but that's one of the things we're trying to understand. If we can control the instabilities, we can boost engine performance, reduce fuel consumption and increase efficiency, as well as safety."
Fleeter has developed an active aerodynamic control system that detects pressure waves in the compressor while they are still small and corrects the distortion before it has a chance to affect the engine's performance.
This research is carried out at a new facility constructed at Purdue this spring in cooperation with Allison Gas Turbines, an Indianapolis-based developer of rotorcraft engines, the type of engines used in helicopters.
As part of a five-year, $2 million University Research Initiative grant from the U.S. Department of Defense, awarded in 1992, Allison supplies Purdue with hardware, including a compressor powered by an Allison helicopter engine.
"The Allison equipment has all the right characteristics to test our mathematical models of air flow and engine vibrations, models we developed over the past three years," Fleeter says. "There's not another facility like this in the country."
The control system consists of a series of electronic detectors and generators placed on the diffuser of the compressor. The diffuser slows the velocity of air through the compressor. The detectors, called pressure transducers and the generators are connected to a high-speed computer. When the transducers detect low amplitude pressure waves, the computer activates the generators, which produce corrective waveforms that dampen the disruptive pressure waves before they can grow.
"In effect, we've made the compressor an intelligent machine, able to dynamically adjust itself to avoid surge and operate more efficiently," Fleeter says.
Fleeter's research is supported in part by Allison Gas Turbines, Pratt & Whitney, General Electric, NASA Lewis Research Center, the Army Research Office, Wright Patterson Air Force Base and the Air Force Office of Scientific Research.
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