A new technique for reducing the exhaust noise of jet engines for supersonic aircraft is being developed by a team at the University of California, Irvine (UCI), led by the concept's inventor, Professor Dimitri Papamoschou.
Work on the noise-suppression concept, known as the Mach Wave Eliminator, is being supported by NASA, and has attracted interest from Boeing, General Electric and McDonnell Douglas. So-called Mach waves are shock waves formed by turbulent eddies in the exhaust of engines such as those under study for the next-generation High-Speed Civil Transport (HSCT). The high-energy eddies create a ballistic effect, akin to that of gaseous bullets, which generate much of the engine noise.
Papamoschou's technique involves eliminating the wave by slowing the eddies to subsonic speeds. Two main methods are under test, involving adjustment of the "co-flow" around the core of the engine. "It is really akin to the fan stream in a turbofan engine. By selecting, or changing, the properties of the fan stream we can eliminate the Mach waves," he says.
"It is a trade-off between the speed and heat of the stream," he explains. One technique involves heating up the co-flow, either by injecting core air or by combustion in the co-flow stream itself. The other is to increase the pressure ratio of the fan, thereby increasing co-flow velocity. In either case the moving air molecules in the co-flow mix more readily with the core flow of the exhaust, , reducing shear stress between the two flows and reducing violent eddy interaction and velocities.
Preliminary results from tests on a 1/100th-scale model jet in a UCI laboratory have yielded up to 10dB noise reductions, using both techniques. "To really prove the concept, we have to move to NASA or an industrial partner and do larger-scale tests," says Papamoschou. Changes to proposed HSCT engines already in early development would be relatively modest, although an increase in pressure ratio might entail a two-stage fan.
The team stresses that the system would only be required for take-off, and would weigh far less than the large ejector/mixer designs planned as the best means of achieving Stage 3 compliance.
The penalty for fuel or hot-core air injection is expected to result in a 0.5% increase in fuel consumption averaged over the entire flight. Other possibilities include use of the system in only the lower hemisphere of the engine exhaust, or a combination of the Mach Wave Eliminator and a smaller ejector.
Source: Flight International
