Fig. 3-10 Rotors of drum and disc 图3-10 鼓筒和盘式转子结构
construction.
generally the discs are assembled and welded together, close to their periphery, thus forming an integral drum.
25. Typical methods of securing rotor blades to the disc are shown in fig. 3-11, fixing may be circumfer-ential or axial to suit special requirements of the stage. In general the aim is to design a securing feature that imparts the lightest possible load on the supporting disc thus minimizing disc weight. Whilst most compressor designs have separate blades for manufacturing and maintainability requirements, it becomes more difficult on the smallest engines to design a practical fixing. However this may be overcome by producing blades integral with the disc; the so called 'blisk'.
25．转子叶片固定到盘上的典型方法示于图3-11。固定可以是沿周向或者轴向，以适应级的具体要求。一般而言，目的是设计一种固定方法，它加于支持它的盘上的载荷尽量小，从而尽量减轻盘的重量。虽然大多数压气机设计具有分开的叶片，以满足制造和可维护性要求，但是。在最小发动机上设计出可行的固定方法却变得更加困难。然而，这一困难可以用生产和盘为一整体的叶片，即所谓的 “整体式叶盘”来克服。
转子叶片
26．转子叶片呈翼型截面形状(图3-12)，通常设计成沿其长度有一压力梯度，以保证空气维持一个比较均匀的轴向速度。向叶尖方向遂渐变高的压力抵消转子作用在气流上的离心作用。为了获得这些状态，必须将叶片从叶根向尖部“扭转”，以便在每一点都具有正确的迎角。流过压气机的空气在其内外壁面处产生二个边界层，一直将气流减慢到滞止的程度。为了补偿边界层中的缓慢气流，在叶片的尖部和根部局部增加了叶片的弯度。叶片最终的形状看来象是将其每个角都扳弯形成的，因而叫做“端部弯曲”。
Compressors
静子叶片
27．静子叶片也呈翼形截面形状，固定在压气机机匣中，或者固定到静子叶片保持环中，再将这些环本身固定到机匣上(图3-13)。在前几级中，静子叶片常常成组地装配，并在其小半径一端加有凸台，以尽量减轻气流变化对较长叶片产生的振动影响。对静子叶片还必须锁定,不让它们沿机匣转动。
图3-11 叶片固定到盘上的各种方法
Fig. 3-11 Methods of securing blades to disc.
Rotor blades
26. The rotor blades are of airfoil section (fig. 3-12) and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to 'twist' the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localized increase in blade camber both at the blade tip and root has been
introduced. The blade extremities appear as if formed by bending over each corner, hence the term 'end-bend'.
Stator vanes
27. The stator vanes are again of airfoil section and are secured into the compressor casing or into stator vane retaining rings, which are themselves secured to the casing (fig. 3-13). The vanes are often assembled in segments in the front stages and may be shrouded at their inner ends to minimize the vibrational effect of flow variations on the longer vanes. It is also necessary to lock the stator vanes in such a manner that they will not rotate around the casing.
工作状态
28．多级压气机的每一级部具有一定的流量特性，而且与其相邻级的各不相同。因此，要设计一台有效工作的压气机，每级的特性都必须经过精心的匹配。这对于一套状态(设计质量流量、增压比和旋转速度)执行起来是比较简单的过程，但是，要在飞机发动机遇到的大范围状态下保持压气机工作的合理匹配就困难得多了。
29．如果加于压气机叶片的工作状态偏离设计状态过多，气流分离和／或者空气动力诱导的振动就会发生。这些现象通常具有下述两种形式之一。转子叶片可能因为空气相对叶片的迎角太高(正迎角失速)或者太低(负迎角失速)而失速。前者是前面的级在低速下发生的问题，而后者通常在高速下影响后面的级发生问题，每一种都可以导致叶片振动，振动又会诱发迅速的破坏。如果发动机要求从压气机得到的压力升高高于叶片能够保持的压力升高，“喘振”就出现了。在这种情况下，通过压气机的气流出现瞬时分离，燃烧系统中的高压空气被拥推向前而穿过压气机，并伴有“砰”的一声巨响和发动机推力的损失。压气机的设计要留有适当的裕度，以确保避免这一区域的不稳定性(图3-14)。
Compressors
Fig. 3-12 A typical rotor blade showing图3-12 表示扭转外形的典型转子叶片
twisted contour.
安装角
旋转方向
气流方向
端部弯曲
安装角
带外环的静子叶片
OPERATING CONDITIONS
28.
Each stage of a multi-stage compressor possesses certain airflow characteristics that are dissimilar from those of its neighbour; thus to design a workable and efficient compressor, the characteris-tics of each stage must be carefully matched. This is a relatively simple process to implement for one set of conditions (design mass flow, pressure ratio and rotational speed), but is much more difficult when reasonable matching is to be retained with the compressor operating over a wide range of conditions such as an aircraft engine encounters.
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