6．當裝用收斂形噴口時，有些能量被浪費了，因為燃氣離開噴口時膨脹得不夠迅速，不能立即達到外界的空氣壓力。依據飛機的飛行計劃，有些高壓比發動機可以有效地利用收斂擴散噴口來回收一部分這種浪費的能量。這種噴口利用壓力能進一步增加燃氣的速度，從而增加推力。
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 燃氣流部分內部混臺
公用的或整體的排氣噴管
外涵道(風扇)冷氣流
高溫燃氣排氣流
燃氣流外部混合
7．由圖(圖6-3)可以看到，收斂段的出口目前已成為喉部，而出口本身則在喇叭形擴散段的末端。當燃氣進入噴口的收斂段時，燃氣速度增加，靜壓相應降低。喉部的燃氣速度相當于此點音速。當燃氣離開喉部限制區并流入擴散段時，速度不斷增加，直到出口為止。這種動量進一步增加所產生的反作用是作用在噴口內壁上的壓力作用力。該力作用于平行于噴管縱軸方向的分力，進一步增加了推力。
further increase in thrust.
8．推進噴管的尺寸極為重要，它的設計應當能使壓力、溫度和推力得到正確的均衡。小噴口使這些參數值增大，但是有可能使發動機喘振(第3章)，而大噴口使所得各數值過低。
圖6-5 兩種高涵道比發動機排氣系統
Fig. 6-5 High by-pass ratio engine exhaust systems.
9．固定面積的推進噴管只有在很窄的發動機工作范圍內有效。為了增大這個范圍，可以采用可調面積的噴口。這種噴口通常是自動控制的，在設計上將各個工作狀態下的壓力、溫度保持正確的均衡。由于性能的增益被增加的重量所抵消，實際上這種系統很少采用。但是，帶加力燃燒室時，可調面積噴口是必要的，參見第16章。
Exhaust system
10．內外涵發動機有兩股氣流噴入大氣，即低溫的外涵空氣流和高溫的渦輪出口燃氣流。
11．在低涵道比發動機中，兩股氣流由混合器(圖6-4)摻混。混合器能使外涵的空氣流入渦輪排氣流之中，保證這兩股氣流充分混臺。
12．在高涵道比發動機中，兩股氣流通常分別排出。高溫和低溫噴口是同軸線的，每個噴口的面積都設計成能獲得最大效率。但是，可以加以改進，將兩股燃氣流結合到一個公用的即整體式噴管組件之中。這種噴管使氣流在噴入大氣之前先部分地混臺。圖6-5示出的這兩種高涵道比發動機的排氣系統可作為示例。
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 結構和材料 中國航空網 m.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(30)
