图19-3 改变排气流形状以降低噪声
Fig. 19-3 Change of exhaust jet pattern to reduce noise level.
7.对纯喷气发动机来说，排气流噪声是如此之高，以致于除了在着陆进场工程中推力较小时，涡轮和压气机的噪声相比之下微不足道了。对于涡轮风扇发动机的而言，由于排气速度减少，排气流噪声下降，但是由于发动机内部处理的功率较大，使低压压气机和涡轮噪声加大。
Noise suppression
8.采用单级低压压气机（风扇）大大降低了压气机噪声，这是因为整个紊流和相互作用的程度都减少了。当涵道比超过约5：1时，排气流噪声降低到这样一个水平，使内部噪声源增加，而且占主导地位。低和高的涵道比发动机噪声源之间的比较见图19-4。
produced by the reaction of each blade to the passage of air over its surface, even with a smooth airstream. Turbulence in the airstream passing over the blades increases the intensity of the broadband noise and can also induce tones.
7.
With the pure jet engine the exhaust jet noise is of such a high level that the turbine and compressor noise is insignificant at all operating conditions, except low landing-approach thrusts. With the by-pass principle, the exhaust jet noise drops as the velocity of the exhaust is reduced but the low pressure compressor and turbine noise increases due to the greater internal power handling.
8.
The introduction of a single stage low pressure compressor (fan) significantly reduces the compressor noise because the overall turbulence and interaction levels are diminished. When the by-pass ratio is in excess of approximately 5 to 1, the jet exhaust noise has reduced to such a level that the increased internal noise source is predominant. A comparison between low and high by-pass engine noise sources is shown in fig. 19-4.
9. Listed amongst the several other sources of noise within the engine is the combustion chamber. It is a significant but not a predominant source, due in part to the fact that it is 'buried' in the core of the engine. Nevertheless it contributes to the broadband noise, as a result of the violent activities which occur within the combustion chamber.
METHODS OF SUPPRESSING NOISE
10. Noise suppression of internal sources is approached in two ways; by basic design to minimize noise originating within or propagating from the engine, and by the use of acoustically absorbent linings. Noise can be minimized by reducing airflow disruption which causes turbulence. This is achieved by using minimal rotational and airflow velocities and reducing the wake intensity by appropriate spacing between the blades and vanes. The ratio between the number of rotating blades and stationary vanes can also be advantageously employed to contain noise within the engine. 高涵道比
（单级风扇）
9.在发动机内部，其它几个噪声源之一是燃烧室。这是一个重要的但不是占支配地位的噪声源，其部分原因是因为，它被“埋”在发动机的核心。然而，由于在燃烧室内方式的激烈的活动，它产生宽频噪声。
图19-4 低和高涵道比发动机的噪声源比较
Fig. 19-4 Comparative noise sources of low and high by-pass engines.
11.如前所述，在纯喷气发动机和低涵道比发动机中，噪声的主要来源是尾喷气流，这可采用一迅速或较短的混合区予以降低。这样减小了低频噪声，但可能会增大高频噪声。幸而高频噪声会很快在大气中被吸收，一些传到听者的噪声已超出听觉范同，因而使人觉得这是一种噪声较小的发动机。这是在推进喷管上，采用一波纹形或瓣形的消声器(图19-5)以增大大气与排气流的接触面积来达到的。
Noise suppression
12.在波纹形喷口中，自由流的大气沿着外部的波纹流动，并进入排气流，以促进迅速混合。在瓣形喷口中，排气被分开，流过各瓣和一个小的中央喷管。这形成许多单独的喷气流并迅速与消声器瓣带来的空气混合。这种原理可以扩展到利用一系列管子，使其总面积与基术的圆形喷管相等。
13．深的波纹、瓣或多个管子使噪声降低得最多，但招致的性能损失限制了波纹或瓣的深度和管子的数目。例如，为了达到所需的喷口面积，消声器的总直径可能不得不增大，这导致阻力和重量增加过多。因此，设计师的目标是采用折衷办法，既使噪声显著降低，同时又尽可能减小发动机推力和油耗方面的损失或重量的增加。
11.
As previously described, the major source of noise on the pure jet engine and low by-pass engine is the exhaust jet, and this can be reduced by inducing a rapid or shorter mixing region. This reduces the low frequency noise but may increase the high frequency level. Fortunately, high frequencies are quickly absorbed in the atmosphere and some of the noise which does propagate to the listener is beyond the audible range, thus giving the perception of a quieter engine. This is achieved by increasing the contact area of the atmosphere with the exhaust gas stream by using a propelling nozzle incorporating a corrugated or lobe-type noise suppressor (fig. 19-5).
12.
In the corrugated nozzle, freestream atmospheric air flows down the outside corrugations and into the exhaust jet to promote rapid mixing. In the lobe-type nozzle, the exhaust gases are divided to flow through the lobes and a small central nozzle. This forms a number of separate exhaust jets that rapidly mix with the air entrained by the suppressor lobes. This principle can be extended by the use of a series of tubes to give the same overall area as the basic circular nozzle.
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