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
圖19-5 消聲器的類型
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．高涵道比發動機有兩個排氣流噴向大氣。然而，減小噴氣流噪聲的原理與純噴氣的或低涵道比發動機的相同。即在全面性能目標內使排氣流速度最小。高涵道比發動機的固有特點是，它比任何其它類型的燃氣渦輪具有更低的排氣速度，因而是一種噪聲較小的發動機，但人們往往希望進一步降低噪聲。用在內外涵發動機上的最成功的辦法是在發動機內將熱的和冷的排氣流混臺(圖19-5)，并通過單的噴口(第6章)排出較低速度的燃氣。
Noise suppression
15．在高涵道比發動機中，控制整個噪聲水平的主要噪聲源是風扇和渦輪。通過研究，人們對噪聲產生的機理有了很好的了解，已研究出綜合性的降低噪聲的設計方法。如前所述，這些方法是建立在需要盡量減小氣流中的紊流、減少轉動葉片和靜止葉片之間的相互作用強度和最佳使用吸聲襯墊的基礎之上的。
圖19-6 吸聲材料和位置
Fig. 19-6 Noise absorbing materials and location.
雙層孔板（鋁）
線性襯墊
（不銹鋼和鋁）
密集編織金屬絲布
堅固的底板
蜂窩結構支架
典型的帶孔襯墊
(鋁或鈦或復合材料）
帶孔面板
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．吸聲“襯墊”材料將聲能轉變成熱。一般來說，吸聲襯墊(圖19-6)由一蜂窩底板支撐的多孔面板組成，以使該面板和發動機堅固的涵道之間有必要的分隔。面板的聲學特性和襯墊的厚度要精心地與噪聲特性相匹配，以便有效地抑制噪聲。襯墊的缺點是重量以及表面摩擦稍有增加，因而油耗稍有增加。不過，它們的確是一種非常有效的抑制噪聲技術。
結構和材料
17．波紋形或瓣形消聲器構成了排氣推進噴管，它通常是一個單獨的組件，用螺栓固定到發動機噴管上。通常可以采取一些措施來調節尾噴口的面積，以對其進行精確的校準。在瓣形消聲器上裝有導流葉片，借助于引導排氣平穩地從各瓣通向大氣來防止過多的損失。消聲器是焊接結構，用耐熱合金制成。
18．在噴氣發動機上采用了各種不同的吸聲襯墊材料。它們主要分為兩類，一類是用于低溫區的輕型復合材料。另一類是用于高溫區的纖維-金屬材料。吸聲材料由一多孔金屬或復合材料面板組成，由底板上的蜂窩狀結構支撐，底板再粘接到涵道或機匣的母體金屬上去。有關這些材料的制造的詳細情況，請參閱第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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