13.
Deep corrugations, lobes, or multi-tubes, give the largest noise reductions, but the performance penalties incurred limit the depth of the corrugations or lobes and the number of tubes. For instance, to achieve the required nozzle area, the overall diameter of the suppressor may have to be increased by so much that excessive drag and weight results. A compromise which gives a noticeable reduction in noise level with the least sacrifice of engine thrust, fuel consumption or addition of weight is therefore the designer's aim.
14.
The high by-pass engine has two exhaust streams to eject to atmosphere. However, the principle of jet exhaust noise reduction is the same as for the pure or low by-pass engine, i.e. minimize the exhaust jet velocity within overall performance objectives. High by-pass engines inherently have a lower exhaust jet velocity than any other type of gas turbine, thus leading to a quieter engine, but further noise reduction is often desirable. The most successful method used on by-pass engines is to mix the hot and cold exhaust streams within the confines of the engine (fig. 19-5) and expel the lower velocity exhaust gas flow through a single nozzle (Part 6).
15.
In the high by-pass ratio engine the predominant sources governing the overall noise level are the fan and turbine. Research has produced
图19-5 消声器的类型
Fig. 19-5 Types of noise suppressor.
a good understanding of the mechanisms of noise generation and comprehensive noise design rules exist. As previously indicated, these are founded on the need to minimize turbulence levels in the airflow, reduce the strength of interactions between rotating blades and stationary vanes, and the optimum use of acoustically absorbent linings.
14．高涵道比发动机有两个排气流喷向大气。然而，减小喷气流噪声的原理与纯喷气的或低涵道比发动机的相同。即在全面性能目标内使排气流速度最小。高涵道比发动机的固有特点是，它比任何其它类型的燃气涡轮具有更低的排气速度，因而是一种噪声较小的发动机，但人们往往希望进一步降低噪声。用在内外涵发动机上的最成功的办法是在发动机内将热的和冷的排气流混台(图19-5)，并通过单的喷口(第6章)排出较低速度的燃气。
Noise suppression
15．在高涵道比发动机中，控制整个噪声水平的主要噪声源是风扇和涡轮。通过研究，人们对噪声产生的机理有了很好的了解，已研究出综合性的降低噪声的设计方法。如前所述，这些方法是建立在需要尽量减小气流中的紊流、减少转动叶片和静止叶片之间的相互作用强度和最佳使用吸声衬垫的基础之上的。
图19-6 吸声材料和位置
Fig. 19-6 Noise absorbing materials and location.
双层孔板（铝）
线性衬垫
（不锈钢和铝）
密集编织金属丝布
坚固的底板
蜂窝结构支架
典型的带孔衬垫
(铝或钛或复合材料）
带孔面板
16. Noise absorbing 'lining' material converts acoustic energy into heat. The absorbent linings (fig. 19-6) normally consist of a porous skin supported by a honeycomb backing, to provide the required separation between the facesheet and the solid engine duct. The acoustic properties of the skin and the liner depth are carefully matched to the character of the noise, for optimum suppression. The disad-vantage of liners is the slight increase in weight and skin friction and hence a slight increase in fuel consumption. They do however, provide a very powerful suppression technique.
CONSTRUCTION AND MATERIALS
17. The corrugated or lobe-type noise suppressor forms the exhaust propelling nozzle and is usually a separate assembly bolted to the jet pipe. Provision is usually made to adjust the nozzle area so that it can 16．吸声“衬垫”材料将声能转变成热。一般来说，吸声衬垫(图19-6)由一蜂窝底板支撑的多孔面板组成，以使该面板和发动机坚固的涵道之间有必要的分隔。面板的声学特性和衬垫的厚度要精心地与噪声特性相匹配，以便有效地抑制噪声。衬垫的缺点是重量以及表面摩擦稍有增加，因而油耗稍有增加。不过，它们的确是一种非常有效的抑制噪声技术。
结构和材料
17．波纹形或瓣形消声器构成了排气推进喷管，它通常是一个单独的组件，用螺栓固定到发动机喷管上。通常可以采取一些措施来调节尾喷口的面积，以对其进行精确的校准。在瓣形消声器上装有导流叶片，借助于引导排气平稳地从各瓣通向大气来防止过多的损失。消声器是焊接结构，用耐热合金制成。
18．在喷气发动机上采用了各种不同的吸声衬垫材料。它们主要分为两类，一类是用于低温区的轻型复合材料。另一类是用于高温区的纤维-金属材料。吸声材料由一多孔金属或复合材料面板组成，由底板上的蜂窝状结构支撑，底板再粘接到涵道或机匣的母体金属上去。有关这些材料的制造的详细情况，请参阅第22章。
Noise suppression
be accurately calibrated. Guide vanes are fitted to the lobe-type suppressor to prevent excessive losses by guiding the exhaust gas smoothly through the lobes to atmosphere. The suppressor is a fabricated welded structure and is manufactured from heat-resistant alloys.
18. Various noise absorbing lining materials are used on jet engines. They fall mainly within two categories, lightweight composite materials that are used in the lower temperature regions and fibrous-metallic materials that are used in the higher temperature regions. The noise absorbing material consists of a perforate metal or composite facing skin, supported by a honeycomb structure on a solid backing skin which is bonded to the parent metal of the duct or casing. For details of manufacture of these materials refer to Part 22.
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