13．这些不同的变化受空气流过发动机时的涵道尺寸和形状的影响。在要求速度(动)能转换成压力的地方，通道呈扩张形。反之，在要求将燃气中储存的能量转换成速度能的场合，便采用收敛通道或喷管工作循环和气流(图2-3)。这些形状适用于气流速度是亚音速或音速(即当地声速)的燃气涡轮发动机。在遇到超音速的场合，如火箭的推进喷管，冲压式空气喷气发动机和某些喷气发动机(第6章)。便采用收敛-扩散喷管即文氏管(图2-4)，以便将燃气中的能量最大限度地转换成动能。
低压压气机
高压压气机
回流燃烧系统 自由动力涡轮
双轴轴流式涡轮螺桨发功机
高压压气机
低压压气机
14．通道和喷管的设计至关重要，因为能量转换时的效率就取决于它们的良好设计。对平滑气流的扰动会使效率损失，并且，由于气流涡流或紊流引起振动，还可能导致部件发生故障。
高压压气机
双级离心式涡轮螺桨发动机
低压压气机
典型单轴辅流式涡轮喷气发动机
压缩 燃烧 膨胀 排出
进气道
推进喷管
Working cycle and airflow
图2-5-1 几种气流系统
Fig. 2-5-1 Airflow systems.
双轴涡轮轴发动机(带自自动力涡轮的)
与排出的燃气流
混合的外涵道气流
低压压气机 高压压气机 外涵道气流
Working cycle and airflow
图2-5-2 几种气流系统
Fig, 2-5-2 Airflow systems.
双轴轴流式对转后风扇
(带自自功率涡轮)
对转风扇
轴流式对转螺桨风扇
(带自自功率涡轮)
压气机
对转螺桨风扇
三轴轴流式前风扇涡轮喷气发动机(高涵道比)
高压压气机
中压压气机
双轴轴流式内外涵涡轮发动机动(低涵道比)
低压压气机
AIRFLOW
15.
The path of the air through a gas turbine engine varies according to the design of the engine. A straight-through flow system (fig. 2-5) is the basic design, as it provides for an engine with a relatively small frontal area and is also suitable for use of the by-pass principle. In contrast, the reverse flow system gives an engine with greater frontal area, but with a reduced overall length. The operation, however, of all engines is similar. The variations due to the different designs are described in the subsequent paragraphs.
16.
The major difference of a turbo-propeller engine is the conversion of gas energy into mechanical power to drive the propeller. Only a small amount of 'jet thrust' is available from the exhaust system. The majority of the energy in the gas stream is absorbed by additional turbine stages, which drive the propeller through internal shafts (Part 5).
17.
As can be seen in fig. 2-5, the by-pass principle involves a division of the airflow. Conventionally, all the air taken in is given an initial low compression and a percentage is then ducted to by-pass, the remainder being delivered to the combustion system in the usual manner. As described in Part 21, this
18．内外涵发动机的一个重要设计特点是涵道比，即通过外涵道的冷空气流量与通过高压系统的空气流量之比。涵道比低，比如在1：1的量级时，这二股气流通常在从发动机排出之前混合在一起。风扇发动机可视为内外涵原理的扩展。高达5：1的高涵道比的要求大多是用双轴或三轴结构中的前风扇求满足(风扇就装于轴上，实际上成为低压压气机)。两股气流可以混合。也可以不混合。很高的涵道比(在15：1的量级)是用螺桨风扇来实现。它是涡轮螺桨理论的演变，但具有能在高飞行速度下高效率工作的先进技术螺旋桨。
19在某些前风扇发动机中，外涵道气流可以直接在风扇后边通过短涵道排出机外，也可以通过较长的涵道在发动机后部排出，并因而称为“涵道风扇”。另一种是后风扇方案，但极少采用。
气流
15．流过燃气涡轮发动机的气流通道按照该发动机的设计变化。直流气流系统(图2-5)是基本设计，因为它为发动机提供比较小的迎风面积，并且对于应用内外涵原理的气流系统也同样适用。相反，回流系统使发动机具有较大的迎风面积，但总长度较小。然而，所有发动机的工作都是类似的。下面几段介绍了不同设计带来的变化。
16．涡轮螺桨发动机的主要区别是将燃气的能量转换成机械功以驱动螺旋桨。从排气系统得到的只是少量的“喷气推力”。燃气流中的大部分能量被额外几级涡轮吸收，并通过内轴(第5章)来驱动螺旋桨。
17.正如从图2-5所能看到的那样，内外涵原理涉及到气流的分流。通常，所有吸入的气流经过最初的低压压缩，然后，一定百分比的气流流入外涵道，其余部分以常规方式流入燃烧系统。正如第21章所述，这一原理有益于改善推进效率和耗油率。
Working cycle and airflow
principle is conducive to improved propulsive efficiency and specific fuel consumption.
18.
An important design feature of the by-pass engine is the by-pass ratio; that is, the ratio of cool air by-passed through the duct to the flow of air passed through the high pressure system. With low by-pass ratios, i.e. in the order of 1:1, the two streams are usually mixed before being exhausted from the engine. The fan engine may be regarded as an extension of the by-pass principle, and the requirement for high by-pass ratios of up to 5:1 is largely met by using the front fan in a twin or triple-spool configuration (on which the fan is, in fact, the low pressure compressor) both with and without mixing of the airflows. Very high by-pass ratios, in the order of 15:1, are achieved using propfans. These are a variation on the turbo-propeller theme but with advanced technology propellers capable of operating with high efficiency at high aircraft speeds.
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