Gas Turbines - The Exhaust System — Page 289, Lesson 369

We're starting a new part of the powerplant syllabus: the exhaust system of the high ratio bypass engine. I want to walk you through how the hot core flow and the cold bypass air are handled, and then we'll get into the serious business of noise suppression. Let's begin with the exhaust arrangement. Figure 18.5 shows two methods used to exhaust the cold bypass air and the hot exhaust gases. The top illustration shows the standard method, where the hot and cold nozzles are co-axial, meaning they share the same centreline, one inside the other. In this standard arrangement, the two streams mix externally, that is, after they've both left the engine and are out in the atmosphere. Now, greater efficiency can be obtained by fitting an integrated exhaust nozzle. Within this unit, the two gas flows are partially mixed before ejection to atmosphere. So instead of letting them mix outside the engine, we bring them together inside the nozzle, partially mix them, and then eject the combined flow. That partial internal mixing is what buys us the efficiency gain. Now let's move to noise suppression, because that's a major design driver. Figure 18.6 shows relative sound levels from various sources, and some of the highest among them are aircraft engines. Although an aircraft's overall noise signature is the combination of sounds from many sources, the principal agent is the engine itself. Airport regulations and aircraft noise certifications governing the maximum noise level which aircraft are allowed to produce have forced rigorous research into ways of reducing that noise. The most significant sources of noise from the engine originate from three places: the compressor, which in high ratio bypass engines is the fan; the turbine; and the exhaust. Now, although the noises which spring from these various sources all obey slightly different laws and mechanisms of generation, they all increase with greater relative airflow velocity. That's the key common factor — velocity. Here's the important contrast. Exhaust noise is affected to a larger degree than either compressor or turbine noise by a reduction in velocity. So it's logical to expect that a reduction in exhaust jet velocity would have a stronger influence in reducing noise levels than an equivalent reduction in either compressor or turbine speeds. In other words, slowing the exhaust jet down gives you more noise reduction per unit of change than slowing the fan or turbine. Why is the exhaust so sensitive? Because of the shearing action. The relative speed difference between the exhaust jet and the atmosphere into which it is thrusting causes a shearing action, which in turn creates a violent and extremely turbulent mixing. Figure 18.7 shows the pattern formed and the zones where high and low frequency noise is generated. So the faster the jet relative to the still air, the more violent the shear, the more turbulence, the more noise. And here's the punchline for a pure jet engine. With a pure jet engine, the noise of the exhaust is of such a high level that the noise of the compressor and the turbine is insignificant, except at very low thrust conditions. So at high thrust, the exhaust dominates everything. Only when you throttle right back does the compressor and turbine noise become noticeable. Now, tie this back to the bypass engine. The whole reason a high ratio bypass engine is quieter is that the cold bypass air, moving at lower velocity, mixes with the hot jet and reduces the overall exhaust jet velocity. That's exactly the mechanism we just discussed — lower jet velocity, less shear, less noise. The figures here, Figure 18.5 for the exhaust systems and Figure 18.7 for the noise pattern, really show you the geometry and the physics working together.