![]() ![]() ![]() 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. This instability occurs when the rotational speed of the fans exceeds a critical value, or when the airflow into the engine becomes severely distorted. Some of the these techniques could be used on aircraft in a couple of years."įleeter 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.Ī surge results when the airflow through the compressor becomes unstable, causing the engine to lose power. '`Using this test facility, we've shown experimentally that we can cancel the engine noise to zero. "This is one of the best facilities in the country for studying air acoustics," Fleeter says. Purdue researchers are testing these computer-controlled foils at the Purdue Large Scale Research Centrifugal Compressor Facility, which houses a six-foot diameter compressor. 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. When we apply electricity to the piezo crystal via a computer, the crystal oscillates, generating sound waves of its own." "The piezo crystals are built right into the blade and engine materials. "It's really a novel approach to the problem," Fleeter says. Our solution is to build computer-controlled smart airfoils, which have piezoelectric crystals of composite materials incorporated into them."Ī piezoelectric crystal generates electricity when force is applied to it, and changes shape when a voltage is applied across it. "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. "These pressure waves are very small," Fleeter says. Some of these waves decay, but many others do not and escape the engine. So why are jet engines so noisy? Air flowing through the bladed disks of the engine generates sound waves. 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. 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. Each stage includes a disk with up to 50 blades arranged around its edge, similar to a paddlewheel. When a jet engine powers up, air sucked into the engine is compressed by a series of seven to 10 compressor and fan stages. The Purdue researchers presented their results in a series of papers June 27-29 at the 30th Joint Propulsion Conference in Indianapolis. 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. We've come up with some very practical ways of handling the noise problem." "The government wants to reduce noise, but it's up to the engine manufacturers to actually do it. "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. 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. ![]()
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