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
圖5-2 一種雙轉子渦輪和軸的布置
Fig. 5-2 A twin turbine and shaft arrangement.
三級低壓渦輪
燃燒系統安裝邊
低壓渦輪軸
高壓渦輪軸
渦輪后軸承
高壓渦輪軸承
高壓導向器葉片
排氣裝置安裝邊
單級高壓渦輪
低壓渦輪軸承
圖5-3 一種三轉子渦輪和軸的布置
Fig. 5-3 A triple turbine and shaft arrangement.
低壓導向器葉片
渦輪有三種類型，即沖擊式、反力式和這兩種的組合-沖擊反山式。對于沖擊式渦輪，每級的總壓降在固定的導向器葉片中發生。由于葉片的收斂形狀，使燃氣建度增加，同時降低壓力。燃氣被引向渦輪工作葉片，葉片承受燃氣沖擊在其上的沖擊力。對于反力式渦輪，固定的導向器葉片設計將燃氣流的方向改變，但不改變壓力。收斂式工作葉片通道承受燃氣膨脹和加速產生的反作用力。正常情況下，燃氣渦輪發動機并不采用純沖擊或純反力式渦輪工作葉片，而是采用沖擊反力組合式(圖5-5)。渦輪設計中每一種方式的比例大體上取決于裝此渦輪的發動機的型別，一般來說，大約沖擊式占50％反力式占50％。沖擊式渦輪應用于火藥和空氣起動機(第11章）。
圖5-4 一種典型的自由動力渦輪
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 從燃氣流向渦輪的能量轉移
6．由第1段所述的內容將會知道，渦輪的工作在于燃氣流和渦輪之間的能量轉移。由于熱力和機械損失，這種轉移不會達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
排氣出口機匣
8.作用在渦輪上的扭矩或扭轉動力由燃氣流的流量和燃氣在渦輪工作葉片的進口和出口之間的能量改變所支配。渦輪的設計應保證從燃氣流動中去掉渦漩。使燃氣在渦輪的出口基本上是“直流的”，保證進入排氣系統的是軸向氣流(第6章)。過多的殘余渦漩臺降低排氣系統的效率，且易于導致噴管振動，它對尾錐體的支承和支柱具有有害的影響。
7．當在燃燒過程(第4章)中燃氣膨脹時，它被迫進入渦輪導向器。由于導向器的收斂形狀，燃氣被加速到接近音速，此時在當地燃氣溫度下，速度約為2500英尺／秒。同時由導向器葉片將燃氣流沿渦輪工作葉片的轉動方向“旋轉”或“打旋”。通過對葉片的沖擊和隨后流過葉片時的反作用力，渦輪吸收了能量，導致渦輪高速旋轉，于是發出驅動渦輪軸和壓氣機的動力。
僅由燃氣流沖擊驅動的渦輪
自燃氣流的沖擊及通過收斂
工作葉片通道加速的反作用
驅動的渦輪
導向器
渦輪
圖5-5 一種純沖擊式渦輪和一種沖擊／反力式渦輪的比較
Fig. 5-5 Comparison between a pure Impulse turbine and an impulse/reaction turbine.
9．可以看到，導向器葉片和渦輪工作葉片是“扭曲”的，即葉片的安裝角在葉尖處比在葉根處的大(圖5-6)。扭曲的理由是使來自燃燒系統的燃氣流在沿葉片長度的所有部位都做相等的功，并且保證進入排氣系統的氣流具有均勻的軸向速度。這就使流過渦輪的氣流的速度、壓力和溫度發生某種改變，如(圖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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