29.
If the operating conditions imposed upon the compressor blade departs too far from the design intention, breakdown of airflow and/or aerodynami-cally induced vibration will occur. These phenomena may take one of two forms; the blades may stall because the angle of incidence of the air relative to the blade is too high (positive incidence stall) or too low (negative incidence stall). The former is a front stage problem at low speeds and the latter usually affects the rear stages at high speed, either can lead to blade vibration which can induce rapid destruction. If the engine demands a pressure rise from the compressor, which is higher than the blading can sustain, 'surge' occurs. In this case there is an instan-taneous breakdown of flow through the machine and the high pressure air in the combustion system is expelled forward through the compressor with a loud 'bang' and a resultant loss of engine thrust.
定位保持螺钉
静子叶片保持环
图3-13 静子叶片在压气机机匣上的固定方法
Fig. 3-13 Methods of securing vanes to compressor casing.
空气流量控制
30.当要求在单轴上实现高增压比时，就必须在压气机设计中采用流量控制。控制形式可以是在第一级上安装可调进气导向叶片，此外，随着该轴上的压比的提高，在随后的一些级中也采用可调静子叶片(图3-15)。当压气机转速从其设计值往下降低时，这些静子叶片逐渐关小，以使空气流到后面的转子叶片上的角度合适。额外的级间放气也可设置，但是，目前它在设计中的使用通常限于在发动机加速时提供额外的裕度，因为在稳定工作状态使用效率不高，是对燃油的浪费。有二种放气系统图示如下：图3-16液压式，图3-17气压式和图3-18电子式。
Compressors
图3-14 稳定流量的限制
恒定转速线
Fig. 3-14 Limits of stable airflow.
Compressors are designed with adequate margin to ensure that this area of instability (fig. 3-14) is avoided.
AIRFLOW CONTROL
30. Where high pressure ratios on a single shaft are required it becomes necessary to introduce airflow control into the compressor design. This may take
喘振区
流量增加
工作线
增压比增加
不稳定区
安全裕度
the form of variable inlet guide vanes for the first stage plus a number of stages incorporating variable stator vanes for the succeeding stages as the shaft pressure ratio is increased (fig. 3-15). As the compressor speed is reduced from its design value these static vanes are progressively closed in order to maintain an acceptable air angle value onto the following rotor blades. Additionally interstage bleed may be provided but its use in design is now usually limited to the provision of extra margin while the engine is being accelerated, because use at steady operating conditions is inefficient and wasteful of fuel. Three types of air bleed systems are illustrated as follows: fig. 3-16 hydraulic, fig. 3-17 pneumatic and fig. 3-18 electronic.
MATERIALS
31. Materials are chosen to achieve the most cost effective design for the components in question, in practice for aero engine design this need is usually best satisfied by the lightest design that technology allows for the given loads and temperatures prevailing.
图3-15 典型的可调静子叶片
Compressors
进口导向叶片关闭（最小流量位置)
活塞
伺服节流活门
膜片
推杆
放气活门(关闭)
参考压力
感测流体
膜片
进口导向叶片打开（最大流量位置）
伺服压力
高压转速
压力信号
压气机放气
低压燃油
高压燃油
来自高压燃油泵
空气流量控制
转速信号传感器
连续放气
空气流量控制
调节器和作动筒
放气活门(打开)
配重活门
可调孔
参考压力
减压活门
来自温度计
图3-16 一种液压驱动的放气活门和进气导向叶片流量控制系统
Fig. 3-16 A hydraulically operated bleed valve and inlet guide vane airflow control system.
30
32．就机匣设计而言，要求是重量轻而刚性好的结构，能保持精确的转子叶片尖部间隙，以保证尽可能高的效率。为达到这些要求，在压缩系统的前部使用铝合金。因为压缩温度提高，后面使用合金钢。压缩系统的最后数级在温度方面提出的要求可能超过最好钢材的承受能力，故可能需要镍基合金。目前，钛合金比铝合金和钢更为人们所喜用，尤其在军用发动机中，钛合金的高的刚性密度比可以大大减轻重量。尽管初始材料成本很高,随着新的制造方式的开发，部件成本目前可以保持在较为满意的水平。
Compressors
图3-17 一种气压驱动的放气活门系统
33．静子叶片通常用钢或者镍基合金制造，主要要求是在受到吸入物击伤而出现“沟槽”时仍具有高的疲劳强度。较早的设计规定用铝合金，但是，因为它承受击伤的能力不够，其使用量已经减少。钛合金可用于低压区的静子叶片，但是，不适合压缩系统后部较小的静子叶片，因为那里的压力和温度较高。其它机械故障可能在转动和静止部件之间引起过多的摩擦，摩擦热能足以使钛合金起火，进而导致昂贵的修理费用和可能的适航性灾难。
34．在转子盘、鼓筒和叶片设计方面，离心力是主要的，要求是具有最高强度密度比的金属。这能导致可能最轻的转子组件，进而减小对发动机结构的作用力，再进一步减轻重量。为此，尽管钛合金初始成本高，但仍是人们垂青的材料，并已经取代了早期设计中喜爱采用的钢合金。随着更高温度的钛合金被研制和生产出来，它们正在逐渐取代镍基合金而用于压缩系统后部的盘和叶片上。 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(16)
