緒言
燃氣排氣流
結構和材料
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)噴口
噴管
排氣錐
Exhaust system
雙位噴口
隔熱層
轉接段
反推力裝置
可調魚鱗片
消音器
渦輪后支柱
燃氣排氣流
4．從發動機渦輪流出的燃氣以750～1200英尺／秒的速度進入排氣系統，由于這樣高的速度量級會產生很高的摩擦損失。所以氣流的速度要通過擴散加以降低。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現的。如圖6-1所示。排氣錐還防止燃氣流跨越渦輪盤后面的流動。通常，在排氣裝置出口保持速度為馬赫數0.5左右，即950英尺／秒左右。由于燃氣流離開渦輪時存在殘余的漩渦速度，所以還產生附加損失。為了減步這些損失，在排氣裝置中的渦輪后部支柱設計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
Fig. 6-1 A basic exhaust system.
圖6-2 帶反推力裝置、消聲器和雙位推進噴管的排氣系統
Fig. 6-2 Exhaust system with thrust reverser, noise suppressor and two position propelling nozzle.
5．燃氣排氣流經過推進噴管排入大氣。噴管是一個收斂形管道，它增加了燃氣速度(第2段)。對于渦輪噴氣發動機，只有在低推力狀態時排氣流的出口速度才是亞音速的。在大多數工作狀態下，出口速度達到當時排氣流溫度下的聲速值，所以，推進噴管認為是“堵塞”的。也即是說，除非再增加溫度，否則速度不會再增大。當推進噴管堵塞時再增加上流的總壓，出口處的燃氣靜壓增高到高于大氣壓力,推進噴管前后的壓差便產生了所謂的“壓力推力”，它作用在噴口出口截面上。此推力附加于由燃氣流動量的變化而得到的推力(第20章)。
外涵空氣與燃氣
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
燃氣排氣
噴管安裝邊
排氣裝置的內錐
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 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)
