a given stage output. For a given output the gas velocities, deflections, and hence losses, are reduced in proportion to the square of higher mean blade speeds. Stress in the turbine disc increases as the square of the speed, therefore to maintain the same stress level at higher speed the sectional thickness, hence the weight, must be increased dis-proportionately. For this reason, the final design is a compromise between efficiency and weight. Engines to weight ratio. By-pass engines have a better propulsive efficiency and thus can have a smaller turbine for a given thrust.
5. The design of the nozzle guide vane and turbine blade passages is based broadly on aerodynamic considerations, and to obtain optimum efficiency, compatible with compressor and combustion design, the nozzle guide vanes and turbine blades are of a
图5-2 一种双转子涡轮和轴的布置
Fig. 5-2 A twin turbine and shaft arrangement.
三级低压涡轮
燃烧系统安装边
低压涡轮轴
高压涡轮轴
涡轮后轴承
高压涡轮轴承
高压导向器叶片
排气装置安装边
单级高压涡轮
低压涡轮轴承
图5-3 一种三转子涡轮和轴的布置
Fig. 5-3 A triple turbine and shaft arrangement.
低压导向器叶片
涡轮有三种类型，即冲击式、反力式和这两种的组合-冲击反山式。对于冲击式涡轮，每级的总压降在固定的导向器叶片中发生。由于叶片的收敛形状，使燃气建度增加，同时降低压力。燃气被引向涡轮工作叶片，叶片承受燃气冲击在其上的冲击力。对于反力式涡轮，固定的导向器叶片设计将燃气流的方向改变，但不改变压力。收敛式工作叶片通道承受燃气膨胀和加速产生的反作用力。正常情况下，燃气涡轮发动机并不采用纯冲击或纯反力式涡轮工作叶片，而是采用冲击反力组合式(图5-5)。涡轮设计中每一种方式的比例大体上取决于装此涡轮的发动机的型别，一般来说，大约冲击式占50％反力式占50％。冲击式涡轮应用于火药和空气起动机(第11章）。
图5-4 一种典型的自由动力涡轮
Fig. 5-4 A typical free power turbine.
basic aerofoil shape. There are three types of turbine; impulse, reaction and a combination of the two known as impulse-reaction. In the impulse type the total pressure drop across each stage occurs in the fixed nozzle guide vanes which, because of their convergent shape, increase the gas velocity whilst reducing the pressure. The gas is directed onto the turbine blades which experience an impulse force caused by the impact of the gas on the blades. In the reaction type the fixed nozzle guide vanes are designed to alter the gas flow direction without changing the pressure. The converging blade passages experience a reaction force resulting from 从燃气流向涡轮的能量转移
6．由第1段所述的内容将会知道，涡轮的工作在于燃气流和涡轮之间的能量转移。由于热力和机械损失，这种转移不会达100％(第11段)。
the expansion and acceleration of the gas. Normally gas turbine engines do not use pure impulse or pure reaction turbine blades but the impulse-reaction combination (fig. 5-5). The proportion of each principle incorporated in the design of a turbine is largely dependent on the type of engine in which the turbine is to operate, but in general it is about 50 per cent impulse and 50 per cent reaction. Impulse-type turbines are used for cartridge and air starters (Part 11).
ENERGY TRANSFER FROM GAS FLOW TO TURBINE
6. From the description contained in para. 1, it will be seen that the turbine depends for its operation on the transfer of energy between the combustion
排气出口机匣
8.作用在涡轮上的扭矩或扭转动力由燃气流的流量和燃气在涡轮工作叶片的进口和出口之间的能量改变所支配。涡轮的设计应保证从燃气流动中去掉涡漩。使燃气在涡轮的出口基本上是“直流的”，保证进入排气系统的是轴向气流(第6章)。过多的残余涡漩台降低排气系统的效率，且易于导致喷管振动，它对尾锥体的支承和支柱具有有害的影响。
7．当在燃烧过程(第4章)中燃气膨胀时，它被迫进入涡轮导向器。由于导向器的收敛形状，燃气被加速到接近音速，此时在当地燃气温度下，速度约为2500英尺／秒。同时由导向器叶片将燃气流沿涡轮工作叶片的转动方向“旋转”或“打旋”。通过对叶片的冲击和随后流过叶片时的反作用力，涡轮吸收了能量，导致涡轮高速旋转，于是发出驱动涡轮轴和压气机的动力。
仅由燃气流冲击驱动的涡轮
自燃气流的冲击及通过收敛
工作叶片通道加速的反作用
驱动的涡轮
导向器
涡轮
图5-5 一种纯冲击式涡轮和一种冲击／反力式涡轮的比较
Fig. 5-5 Comparison between a pure Impulse turbine and an impulse/reaction turbine.
9．可以看到，导向器叶片和涡轮工作叶片是“扭曲”的，即叶片的安装角在叶尖处比在叶根处的大(图5-6)。扭曲的理由是使来自燃烧系统的燃气流在沿叶片长度的所有部位都做相等的功，并且保证进入排气系统的气流具有均匀的轴向速度。这就使流过涡轮的气流的速度、压力和温度发生某种改变，如(图5-7)所示。
gases and the turbine. This transfer is never 100 per cent because of thermodynamic and mechanical losses, (para. 11).
7.when the gas is expanded by the combustion process (Part 4), it forces its way into the discharge nozzles of the turbine where, because of their convergent shape, it is accelerated to about the speed of sound which, at the gas temperature, is about 2,500 feet per second. At the same time the gas flow is given a 'spin' or 'whirl' in the direction of rotation of the turbine blades by the nozzle guide vanes. On impact with the blades and during the subsequent reaction through the blades, energy is absorbed, causing the turbine to rotate at high speed and so provide the power for driving the turbine shaft and compressor.
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