6．将近百分之二十的空气质量流量从锥形进口即进气段(图4-2)进来。紧靠此锥形口下游的是漩涡叶片和多孔的扩张段，空气从这里进入主燃烧区。漩涡着的空气诱导火焰筒中心部位的气流向前流，促成符合愿望的再循环。未流入锥形口的空气流入火焰筒和空气机匣之间的环形空间。
7．在燃烧区附近的火焰筒体壁面上有选定数量的二股气流孔，另外20％的空气主流穿过这些孔进入主燃区。从漩涡叶片进来的空气和从二股气流孔进来的空气互相作用，形成一个低速回流区。它呈回旋涡流形状，类似发烟环，起稳定和系留火焰的作用(图4-3)。回流燃气将新喷入的燃油滴迅速加温到点燃温度，促进了它们的燃烧。
联焰管
密封环
主燃区
燃油喷嘴
锥形进口
稀释空气孔
扩张段
空气机匣
二股空气孔
火焰筒
漩涡叶片
Combustion chambers
图4-1 一种早期的燃烧室
Fig. 4-1 An early combustion chamber.
COMBUSTION PROCESS
4. Air from the engine compressor enters the combustion chamber at a velocity up to 500 feet per second, but because at this velocity the air speed is far too high for combustion, the first thing that the chamber must do is to diffuse it, i.e. decelerate it and raise its static pressure. Since the speed of burning kerosine at normal mixture ratios is only a few feet per second, any fuel lit even in the diffused air stream, which now has a velocity of about 80 feet per second, would be blown away. A region of low axial velocity has therefore to be created in the chamber, so that the flame will remain alight throughout the range of engine operating conditions. 燃烧过程
5. In normal operation, the overall air/fuel ratio of a combustion chamber can vary between 45:1 and 130:1, However, kerosine will only burn efficiently at, or close to, a ratio of 15:1, so the fuel must be burned with only part of the air entering the chamber, in what is called a primary combustion zone. This is achieved by means of a flame tube (combustion liner) that has various devices for metering the airflow distribution along the chamber.
6.
Approximately 20 per cent of the air mass flow is taken in by the snout or entry section (fig. 4-2). Immediately downstream of the snout are swirl vanes and a perforated flare, through which air passes into the primary combustion zone. The swirling air induces a flow upstream of the centre of the flame tube and promotes the desired recirculation. The air not picked up by the snout flows into the annular space between the flame tube and the air casing.
7.
Through the wall of the flame tube body, adjacent to the combustion zone, are a selected number of secondary holes through which a further 20 per cent of the main flow of air passes into the primary zone. The air from the swirl vanes and that from the secondary air holes interacts and creates a region of low velocity recirculation. This takes the form of a toroidal vortex, similar to a smoke ring, which has the effect of stabilizing and anchoring the flame (fig, 4-3). The recirculating gases hasten the burning of freshly
波纹形连接
8．设计中应当使从喷嘴呈锥形喷出的燃油与回旋涡流的中心相交。这一作用，和主燃烧区的总体紊流一起，极大地帮助了击碎燃油并使之与正在进入的空气混合起来。
9．燃烧释放的燃气温度大约是1800到2000℃。此温度太高，不适于进入涡轮导向器叶片。因此，未用于燃烧的空气，它占大约60％的总空气流量，被逐渐引入火焰筒。这部分空气大约有三分之一用来在稀释区降低燃气的温度，然后再进入涡轮，而其余的空气则用来冷却火焰筒的壁面。实现这一点是借助于一薄层冷却空气沿火焰筒壁的内表面流动，将火焰筒壁面与热燃气隔开(图4-4)。目前的一项发展成就允许冷却空气在排出前先进入火焰筒壁内的一套通道，并形成一层隔热空气膜，这可以把要求冷却壁面的气流量减少50％之多。燃烧应当在稀释空气进入火焰筒以前完成，否则，进来的空气会使火焰降温，造成不完全燃烧。
10.点火电嘴(第11章)发出的电火花使燃烧开始然后，火焰自身维持常着不灭。
Combustion chambers
图4-2 空气流的分配
Fig. 4-2 Apportioning the airflow.
injected fuel droplets by rapidly bringing them to ignition temperature.
8.
It is arranged that the conical fuel spray from the nozzle intersects the recirculation vortex at its centre. This action, together with the general turbulence in the primary zone, greatly assists in breaking up the fuel and mixing it with the incoming air.
9.
The temperature of the gases released by combustion is about 1,800 to 2,000 deg. C., which is far too hot for entry to the nozzle guide vanes of the turbine. The air not used for combustion, which amounts to about 60 per cent of the total airflow, is therefore introduced progressively into the flame tube. Approximately a third of this is used to lower the gas temperature in the dilution zone before it enters
the turbine and the remainder is used for cooling the walls of the flame tube. This is achieved by a film of cooling air flowing along the inside surface of the flame tube wall, insulating it from the hot combustion gases (fig. 4-4). A recent development allows cooling air to enter a network of passages within the flame tube wall before exiting to form an insulating film of air, this can reduce the required wall cooling airflow by up to 50 per cent. Combustion should be completed before the dilution air enters the flame tube, otherwise the incoming air will cool the flame and incomplete combustion will result.
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