緒言
燃?xì)馀艢饬?br />
結(jié)構(gòu)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.
渦輪后面級
收斂(推進(jìn))噴口
噴管
排氣錐
Exhaust system
雙位噴口
隔熱層
轉(zhuǎn)接段
反推力裝置
可調(diào)魚鱗片
消音器
渦輪后支柱
燃?xì)馀艢饬?/p>
4．從發(fā)動機(jī)渦輪流出的燃?xì)庖?50～1200英尺／秒的速度進(jìn)入排氣系統(tǒng)，由于這樣高的速度量級會產(chǎn)生很高的摩擦損失。所以氣流的速度要通過擴(kuò)散加以降低。這是通過將排氣錐和外壁之間的通道面積不斷地加大而實(shí)現(xiàn)的。如圖6-1所示。排氣錐還防止燃?xì)饬骺缭綔u輪盤后面的流動。通常，在排氣裝置出口保持速度為馬赫數(shù)0.5左右，即950英尺／秒左右。由于燃?xì)饬麟x開渦輪時存在殘余的漩渦速度，所以還產(chǎn)生附加損失。為了減步這些損失，在排氣裝置中的渦輪后部支柱設(shè)計(jì)成能將氣流在流入噴管之前先行扭直。
噴管
圖6-1 一種基本的排氣系統(tǒng)
Fig. 6-1 A basic exhaust system.
圖6-2 帶反推力裝置、消聲器和雙位推進(jìn)噴管的排氣系統(tǒng)
Fig. 6-2 Exhaust system with thrust reverser, noise suppressor and two position propelling nozzle.
5．燃?xì)馀艢饬鹘?jīng)過推進(jìn)噴管排入大氣。噴管是一個收斂形管道，它增加了燃?xì)馑俣?第2段)。對于渦輪噴氣發(fā)動機(jī)，只有在低推力狀態(tài)時排氣流的出口速度才是亞音速的。在大多數(shù)工作狀態(tài)下，出口速度達(dá)到當(dāng)時排氣流溫度下的聲速值，所以，推進(jìn)噴管認(rèn)為是“堵塞”的。也即是說，除非再增加溫度，否則速度不會再增大。當(dāng)推進(jìn)噴管堵塞時再增加上流的總壓，出口處的燃?xì)忪o壓增高到高于大氣壓力,推進(jìn)噴管前后的壓差便產(chǎn)生了所謂的“壓力推力”，它作用在噴口出口截面上。此推力附加于由燃?xì)饬鲃恿康淖兓玫降耐屏?第20章)。
外涵空氣與燃?xì)?br />
排氣流混合
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
外涵空氣
燃?xì)馀艢?br />
噴管安裝邊
排氣裝置的內(nèi)錐
混臺器斜槽
渦輪后支柱
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
圖6-4 一種低涵道比空氣混合裝置
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 靜壓
聲速
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
中國航空網(wǎng) www.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：勞斯萊斯噴氣引擎－中英(29)
