6．將近百分之二十的空氣質(zhì)量流量從錐形進(jìn)口即進(jìn)氣段(圖4-2)進(jìn)來。緊靠此錐形口下游的是漩渦葉片和多孔的擴(kuò)張段，空氣從這里進(jìn)入主燃燒區(qū)。漩渦著的空氣誘導(dǎo)火焰筒中心部位的氣流向前流，促成符合愿望的再循環(huán)。未流入錐形口的空氣流入火焰筒和空氣機(jī)匣之間的環(huán)形空間。
7．在燃燒區(qū)附近的火焰筒體壁面上有選定數(shù)量的二股氣流孔，另外20％的空氣主流穿過這些孔進(jìn)入主燃區(qū)。從漩渦葉片進(jìn)來的空氣和從二股氣流孔進(jìn)來的空氣互相作用，形成一個(gè)低速回流區(qū)。它呈回旋渦流形狀，類似發(fā)煙環(huán)，起穩(wěn)定和系留火焰的作用(圖4-3)。回流燃?xì)鈱⑿聡娙氲娜加偷窝杆偌訙氐近c(diǎn)燃溫度，促進(jìn)了它們的燃燒。
聯(lián)焰管
密封環(huán)
主燃區(qū)
燃油噴嘴
錐形進(jìn)口
稀釋空氣孔
擴(kuò)張段
空氣機(jī)匣
二股空氣孔
火焰筒
漩渦葉片
Combustion chambers
圖4-1 一種早期的燃燒室
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. 燃燒過程
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
波紋形連接
8．設(shè)計(jì)中應(yīng)當(dāng)使從噴嘴呈錐形噴出的燃油與回旋渦流的中心相交。這一作用，和主燃燒區(qū)的總體紊流一起，極大地幫助了擊碎燃油并使之與正在進(jìn)入的空氣混合起來。
9．燃燒釋放的燃?xì)鉁囟却蠹s是1800到2000℃。此溫度太高，不適于進(jìn)入渦輪導(dǎo)向器葉片。因此，未用于燃燒的空氣，它占大約60％的總空氣流量，被逐漸引入火焰筒。這部分空氣大約有三分之一用來在稀釋區(qū)降低燃?xì)獾臏囟龋缓笤龠M(jìn)入渦輪，而其余的空氣則用來冷卻火焰筒的壁面。實(shí)現(xiàn)這一點(diǎn)是借助于一薄層冷卻空氣沿火焰筒壁的內(nèi)表面流動(dòng)，將火焰筒壁面與熱燃?xì)飧糸_(圖4-4)。目前的一項(xiàng)發(fā)展成就允許冷卻空氣在排出前先進(jìn)入火焰筒壁內(nèi)的一套通道，并形成一層隔熱空氣膜，這可以把要求冷卻壁面的氣流量減少50％之多。燃燒應(yīng)當(dāng)在稀釋空氣進(jìn)入火焰筒以前完成，否則，進(jìn)來的空氣會(huì)使火焰降溫，造成不完全燃燒。
10.點(diǎn)火電嘴(第11章)發(fā)出的電火花使燃燒開始然后，火焰自身維持常著不滅。
Combustion chambers
圖4-2 空氣流的分配
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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