绪言
燃气排气流
结构和材料
Construction and materials 63
INTRODUCTION
1. Aero gas turbine engines have an exhaust system which passes the turbine discharge gases to atmosphere at a velocity, and in the required direction, to provide the resultant thrust. The velocity and pressure of the exhaust gases create the thrust in the turbo-jet engine (para. 5) but in the turbo-propeller engine only a small amount of thrust is contributed by the exhaust gases, because most of the energy has been absorbed by the turbine for driving the propeller. The design of the exhaust system therefore, exerts a considerable influence on the performance of the engine. The areas of the jet pipe and propelling or outlet nozzle affect the turbine entry temperature, the mass airflow and the velocity and pressure of the exhaust jet.
2.
The temperature of the gas entering the exhaust system is between 550 and 850 deg. C. according to the type of engine and with the use of afterburning (Part 16) can be 1,500 deg. C. or higher. Therefore, it is necessary to use materials and a form of con-struction that will resist distortion and cracking, and prevent heat conduction to the aircraft structure.
3.
A basic exhaust system is shown in fig. 6-1. The use of a thrust reverser (Part 15), noise suppressor (Part 19) and a two position propelling nozzle entails a more complicated system as shown in fig. 6-2. The low by-pass engine may also include a mixer unit (fig. 6-4) to encourage a thorough mixing of the hot and cold gas streams.
涡轮后面级
收敛(推进)喷口
喷管
排气锥
Exhaust system
双位喷口
隔热层
转接段
反推力装置
可调鱼鳞片
消音器
涡轮后支柱
燃气排气流
4．从发动机涡轮流出的燃气以750～1200英尺／秒的速度进入排气系统，由于这样高的速度量级会产生很高的摩擦损失。所以气流的速度要通过扩散加以降低。这是通过将排气锥和外壁之间的通道面积不断地加大而实现的。如图6-1所示。排气锥还防止燃气流跨越涡轮盘后面的流动。通常，在排气装置出口保持速度为马赫数0.5左右，即950英尺／秒左右。由于燃气流离开涡轮时存在残余的漩涡速度，所以还产生附加损失。为了减步这些损失，在排气装置中的涡轮后部支柱设计成能将气流在流入喷管之前先行扭直。
喷管
图6-1 一种基本的排气系统
Fig. 6-1 A basic exhaust system.
图6-2 带反推力装置、消声器和双位推进喷管的排气系统
Fig. 6-2 Exhaust system with thrust reverser, noise suppressor and two position propelling nozzle.
5．燃气排气流经过推进喷管排入大气。喷管是一个收敛形管道，它增加了燃气速度(第2段)。对于涡轮喷气发动机，只有在低推力状态时排气流的出口速度才是亚音速的。在大多数工作状态下，出口速度达到当时排气流温度下的声速值，所以，推进喷管认为是“堵塞”的。也即是说，除非再增加温度，否则速度不会再增大。当推进喷管堵塞时再增加上流的总压，出口处的燃气静压增高到高于大气压力,推进喷管前后的压差便产生了所谓的“压力推力”，它作用在喷口出口截面上。此推力附加于由燃气流动量的变化而得到的推力(第20章)。
外涵空气与燃气
排气流混合
Exhaust system
EXHAUST GAS FLOW
4.
Gas from the engine turbine enters the exhaust system at velocities from 750 to 1,200 feet per second, but, because velocities of this order produce high friction losses, the speed of flow is decreased by diffusion. This is accomplished by having an
外涵道
外涵空气
燃气排气
喷管安装边
排气装置的内锥
混台器斜槽
涡轮后支柱
increasing passage area between the exhaust cone and the outer wall as shown in fig. 6-1. The cone also prevents the exhaust gases from flowing across the rear face of the turbine disc. It is usual to hold the velocity at the exhaust unit outlet to a Mach number of about 0.5, i.e. approximately 950 feet per second. Additional losses occur due to the residual whirl velocity in the gas stream from the turbine. To reduce these losses, the turbine rear struts in the exhaust unit are designed to straighten out the flow before the gases pass into the jet pipe.
5.
The exhaust gases pass to atmosphere through the propelling nozzle, which is a convergent duct, thus increasing the gas velocity (Part 2). In a turbo-jet engine, the exit velocity of the exhaust gases is subsonic at low thrust conditions only. During most operating conditions, the exit velocity reaches the speed of sound in relation to the exhaust gas
分流器的整流罩
图6-4 一种低涵道比空气混合装置
Fig. 6-4 A low by-pass air mixer unit.
temperature and the propelling nozzle is then said to be 'choked'; that is, no further increase in velocity 静压
声速
速度
can be obtained unless the temperature is increased. As the upstream total pressure is increased above the value at which the propelling nozzle becomes 'choked', the static pressure of the gases at exit increases above atmospheric pressure. This pressure difference across the propelling nozzle gives what is known as 'pressure thrust' and is effective over the nozzle exit area. This is additional
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：劳斯莱斯喷气引擎－中英(29)
