6．当装用收敛形喷口时，有些能量被浪费了，因为燃气离开喷口时膨胀得不够迅速，不能立即达到外界的空气压力。依据飞机的飞行计划，有些高压比发动机可以有效地利用收敛扩散喷口来回收一部分这种浪费的能量。这种喷口利用压力能进一步增加燃气的速度，从而增加推力。
6.
With the convergent type of nozzle a wastage of energy occurs, since the gases leaving the exit do not expand rapidly enough to immediately achieve outside air pressure. Depending on the aircraft flight plan, some high pressure ratio engines can with advantage use a convergent-divergent nozzle to recover some of the wasted energy This nozzle utilizes the pressure energy to obtain a further increase in gas velocity and, consequently, an increase in thrust.
7.
From the illustration (fig. 6-3), it will be seen that the convergent section exit now becomes the throat,
Exhaust system
with the exit proper now being at the end of the flared divergent section. When the gas enters the convergent section of the nozzle, the gas velocity increases with a corresponding fall in static pressure. The gas velocity at the throat corresponds to the local sonic velocity. As the gas leaves the restriction of the throat and flows into the divergent section, it progressively increases in velocity towards the exit. The reaction to this further increase in momentum is a pressure force acting on the inner wall of the nozzle. A component of this force acting parallel to the longitudinal axis of the nozzle produces the 燃气流部分内部混台
公用的或整体的排气喷管
外涵道(风扇)冷气流
高温燃气排气流
燃气流外部混合
7．由图(图6-3)可以看到，收敛段的出口目前已成为喉部，而出口本身则在喇叭形扩散段的末端。当燃气进入喷口的收敛段时，燃气速度增加，静压相应降低。喉部的燃气速度相当于此点音速。当燃气离开喉部限制区并流入扩散段时，速度不断增加，直到出口为止。这种动量进一步增加所产生的反作用是作用在喷口内壁上的压力作用力。该力作用于平行于喷管纵轴方向的分力，进一步增加了推力。
further increase in thrust.
8．推进喷管的尺寸极为重要，它的设计应当能使压力、温度和推力得到正确的均衡。小喷口使这些参数值增大，但是有可能使发动机喘振(第3章)，而大喷口使所得各数值过低。
图6-5 两种高涵道比发动机排气系统
Fig. 6-5 High by-pass ratio engine exhaust systems.
9．固定面积的推进喷管只有在很窄的发动机工作范围内有效。为了增大这个范围，可以采用可调面积的喷口。这种喷口通常是自动控制的，在设计上将各个工作状态下的压力、温度保持正确的均衡。由于性能的增益被增加的重量所抵消，实际上这种系统很少采用。但是，带加力燃烧室时，可调面积喷口是必要的，参见第16章。
Exhaust system
10．内外涵发动机有两股气流喷入大气，即低温的外涵空气流和高温的涡轮出口燃气流。
11．在低涵道比发动机中，两股气流由混合器(图6-4)掺混。混合器能使外涵的空气流入涡轮排气流之中，保证这两股气流充分混台。
12．在高涵道比发动机中，两股气流通常分别排出。高温和低温喷口是同轴线的，每个喷口的面积都设计成能获得最大效率。但是，可以加以改进，将两股燃气流结合到一个公用的即整体式喷管组件之中。这种喷管使气流在喷入大气之前先部分地混台。图6-5示出的这两种高涵道比发动机的排气系统可作为示例。
8.
The propelling nozzle size is extremely important and must be designed to obtain the correct balance of pressure, temperature and thrust. With a small nozzle these values increase, but there is a possibility of the engine surging (Part 3), whereas with a large nozzle the values obtained are too low,
9.
A fixed area propelling nozzle is only efficient over a narrow range of engine operating conditions. To increase this range, a variable area nozzle may be used. This type of nozzle is usually automatically controlled and is designed to maintain the correct balance of pressure and temperature at all operating conditions. In practice, this system is seldom used as the performance gain is offset by the increase in weight. However, with afterburning a variable area nozzle is necessary and is described in Part 16.
10.
The by-pass engine has two gas streams to eject to atmosphere, the cool by-pass airflow and the hot turbine discharge gases.
11.
In a low by-pass ratio engine, the two flows are combined by a mixer unit (fig. 6-4) which allows the by-pass air to flow into the turbine exhaust gas flow in a manner that ensures thorough mixing of the two streams.
12.
In high by-pass ratio engines, the two streams are usually exhausted separately. The hot and cold nozzles are co-axial and the area of each nozzle is designed to obtain maximum efficiency. However, an improvement can be made by combining the two gas flows within a common, or integrated, nozzle assembly. This partially mixes the gas flows prior to ejection to atmosphere. An example of both types of high by-pass exhaust system is shown in fig, 6-5.
CONSTRUCTION AND MATERIALS
13. The exhaust system must be capable of with-standing the high gas temperatures and is therefore manufactured from nickel or titanium. It is also necessary to prevent any heat being transferred to the surrounding aircraft structure. This is achieved by passing ventilating air around the jet pipe, or by lagging the section of the exhaust system with an insulating blanket (fig. 6-6). Each blanket has an inner layer of fibrous insulating material contained by 结构和材料
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