Gas Turbines - The Exhaust System — Page 289, Lesson 371

We're starting a fresh topic now: the exhaust system of gas turbines, and specifically how noise is managed. Let's pick up right where the material leads us. The key idea here is that noise from a jet engine comes from several sources, and the exhaust is just one of them. When we look at a bypass engine, the exhaust noise drops because the jet velocity is reduced. But here's the trade-off: because these engines handle a much greater power, the turbines and the low-pressure compressor actually generate a higher noise output. So you're shifting the noise problem, not eliminating it. Now, let's talk about a high ratio bypass engine, say a 5-to-1 ratio. In that case, the noise from the jet exhaust has reduced so much that the noise from the low-pressure compressor—that's the fan—and the turbine become the predominant sources. So the main source of noise has moved from the exhaust to the fan and turbine. Once manufacturers reduced noise from the main source, it was logical to concentrate on lowering the noise from the rest of the engine—the fan and the turbine. The solution was noise absorbing material, which we call acoustic-lining. This was placed in the bypass duct and the engine intake, and it was extremely efficient at reducing noise in that region. Further down the engine, in the hotter zones, slightly different materials were used to great advantage for the same purpose. But these materials have disadvantages. They add a small percentage in weight, and their skin friction is slightly higher. Together, those two factors cause a slight increase in fuel consumption. So there's a cost to noise reduction. Now, modern engines could take advantage of these new sound absorbing materials. But aircraft fitted with older pure turbojets had to find another system. You can still see aircraft with 'corrugated internal mixers' and 'lobe type nozzles' fitted to the rear of their power units. The lobe type nozzle caused the gases to flow in separate exhaust jets that rapidly mix with slower moving air trapped by the lobes. The corrugated internal mixer was the most efficient at reducing noise, but it also induced performance penalties that limited its popularity with aircraft operators. So to tie it together: exhaust noise drops with bypass ratio, fan and turbine noise rise, acoustic-lining handles the fan and turbine regions at a cost of weight and fuel, and older turbojets used mechanical mixers like corrugated internal mixers and lobe nozzles—the former being most effective but with performance penalties.