29.
From the foregoing, it follows that for a particular blade material and an acceptable safe life there is an associated maximum permissible turbine entry temperature and a corresponding maximum engine power. It is not surprising, therefore, that met-allurgists and designers are constantly searching for better turbine blade materials and improved methods of blade cooling.
30.
Over a period of operational time the turbine blades slowly grow in length. This phenomenon is known as 'creep' and there is a finite useful life limit before failure occurs.
31.
The early materials used were high temperature steel forgings, but these were rapidly replaced by cast nickel base alloys which give better creep and fatigue properties.
32.
Close examination of a conventional turbine blade reveals a myriad of crystals that lie in all directions (equi-axed). Improved service life can be obtained by aligning the crystals to form columns along the blade length, produced by a method known as 'Directional Solidification'. A further advance of this technique is to make the blade out of a single
Fig. 5-13 Comparison of turbine blade life图5-13 涡轮工作叶片寿命特性的比较
properties.
时间
延伸率
等轴叶片
定向凝固叶片
断裂
单晶叶片
图5-14 陶瓷涡轮工作叶片
Fig. 5-14 Ceramic turbine blades.
crystal, Examples of these structures are shown in fig. 5-12. Each method extends the useful creep life of the blade (fig. 5-13) and in the case of the single crystal blade, the operating temperature can be sub-stantially increased.
33. A non-metal based turbine blade can be manu-factured from reinforced ceramics. Their initial production application is likely to be for small high speed turbines which have very high turbine entry 30．超过一段工作期间，涡轮工作叶片慢慢地伸长，这种现象被称为“蠕变”，而且，在损坏之前存在有限的可用寿命极限。
31．早期曾经使用高温钢锻件，但这些材料很快便由铸造镍基合金所取代。后者具有更好的蠕变和疲劳特性。
29．依据上面所述，对于某种叶片材料及允许的安全寿命，就会有一个相应的最大允许的涡轮进口温度以及相应的最大发动机功率。因此，并不奇怿，冶金学家和设计师们总是不断地在寻求更好的涡轮工作叶片材料和改善叶片的冷却方法。
temperatures. An example of a ceramic blade is shown in fig. 5-14.
BALANCING
34. The balancing of a turbine is an extremely important operation in its assembly. In view of the high rotational speeds and the mass of materials, any unbalance could seriously affect the rotating assembly bearings and engine operation. Balancing is effected on a special balancing machine, the principles of which are briefly described in Part 25. 平衡
34．涡轮的平衡是其装配中极端重要的工作。考虑到高旋转速度及材料的质量，任何不平衡都会严重影响旋转组件的轴承和发动机的工作。平衡在专用的平衡机上进行，其原理在第25章中简要叙述。
33．用强化陶瓷可以制造出非金属基的涡轮工作叶片。它们初始的生产应用似乎会用于具有极高涡轮进口温度的小型高速涡轮。图5-14示出了一种陶瓷叶片的例子。
32．对常规涡轮工作叶片的仔细研究发现，在各个方向(等轴)存在无数晶体。通过将晶粒沿叶片长度方向排成柱状可以改善使用寿命，这种方法称为“定向凝固”。这种技术的进一步改进是用一个单晶体制造叶片。这些结构的示例见图5-12。定向凝固，单晶都增长了叶片的蠕变可用寿命(图5-13)。在单晶叶片的情况下，使用温度可以大大地增高。
罗尔斯-罗伊斯公司
RB211-535E4发动机
1943年末，罗尔斯-罗伊斯公司决定研制一种涡轮螺桨发动机，用于速度为400英里／小时左右的飞机。所制成的发动机称为RB50“遄达”(Trent)，它基本上是一台“德温特”(Derwent)Ⅱ型，带有一根驱动减速器和螺旋桨的挠性的套轴。1945年9月20日，装有两台“遄达”发动机的格洛斯特公司的“流星”飞机成为全世界第一架投入飞行的以涡轮螺浆为动力的飞机。
Rolls-Royce RB50 Trent
Late in 1943 the decision was taken at Rolls-Royce to build a turbo-prop for aircraft speeds of around 400 mph. The resulting engine, known as the RB50 Trent, was basically a Derwent II with a flexible quillshaft to reduction gear and propeller. On 20 September 1945 a Gloster Meteor, fitted with two Trents, became the world's first turbo-prop powered aircraft to fly.
罗尔斯-罗伊斯公司
RB50“遄达”发动机
6: Exhaust system
第六章 排气系统
Contents Page
Introduction 59
Exhaust gas flow 61
绪言
1．航空燃气涡轮发动机有一个排气系统，它将涡轮排出的燃气以一定的速度和要求的方向排入大气，提供最后所得到的推力。在涡轮喷气发动机中排气流的速度和压力产生推力(第5段)。但在涡轮螺桨发动机中，排气流只提供少量推力，因为大部分能量已经由涡轮吸收，用来驱动螺旋桨。因此，排气采统的设计对发动机的性能有很大影响。喷管、推进喷管或排气口的面积影响到涡轮进口温度、排气流的质量流量、速度及压力。
2．依据发动机类型的不同，进入排气系统的燃气温度在550～850℃之间，采用加力燃烧室时(第11章)可达l500℃，甚至更高。所以需要采用的材料及结构形式应能够抵御挠曲和产生裂纹，并防止向飞机结构的热传导。
3.基本的排气系统示于图6-1。反推力装置(第15章)，消声器(第19章)及双位推进喷管的使用使系统的结构更为复杂，参见图6-2所示。低涵道比发动机也可以有一个混合器(图6-4)，用于促进冷热燃气流彻底的混合。 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(28)
