Fig. 3-10 Rotors of drum and disc 圖3-10 鼓筒和盤式轉子結構
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．轉子葉片固定到盤上的典型方法示于圖3-11。固定可以是沿周向或者軸向，以適應級的具體要求。一般而言，目的是設計一種固定方法，它加于支持它的盤上的載荷盡量小，從而盡量減輕盤的重量。雖然大多數壓氣機設計具有分開的葉片，以滿足制造和可維護性要求，但是。在最小發動機上設計出可行的固定方法卻變得更加困難。然而，這一困難可以用生產和盤為一整體的葉片，即所謂的 “整體式葉盤”來克服。
轉子葉片
26．轉子葉片呈翼型截面形狀(圖3-12)，通常設計成沿其長度有一壓力梯度，以保證空氣維持一個比較均勻的軸向速度。向葉尖方向遂漸變高的壓力抵消轉子作用在氣流上的離心作用。為了獲得這些狀態，必須將葉片從葉根向尖部“扭轉”，以便在每一點都具有正確的迎角。流過壓氣機的空氣在其內外壁面處產生二個邊界層，一直將氣流減慢到滯止的程度。為了補償邊界層中的緩慢氣流，在葉片的尖部和根部局部增加了葉片的彎度。葉片最終的形狀看來象是將其每個角都扳彎形成的，因而叫做“端部彎曲”。
Compressors
靜子葉片
27．靜子葉片也呈翼形截面形狀，固定在壓氣機機匣中，或者固定到靜子葉片保持環中，再將這些環本身固定到機匣上(圖3-13)。在前幾級中，靜子葉片常常成組地裝配，并在其小半徑一端加有凸臺，以盡量減輕氣流變化對較長葉片產生的振動影響。對靜子葉片還必須鎖定,不讓它們沿機匣轉動。
圖3-11 葉片固定到盤上的各種方法
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.
工作狀態
28．多級壓氣機的每一級部具有一定的流量特性，而且與其相鄰級的各不相同。因此，要設計一臺有效工作的壓氣機，每級的特性都必須經過精心的匹配。這對于一套狀態(設計質量流量、增壓比和旋轉速度)執行起來是比較簡單的過程，但是，要在飛機發動機遇到的大范圍狀態下保持壓氣機工作的合理匹配就困難得多了。
29．如果加于壓氣機葉片的工作狀態偏離設計狀態過多，氣流分離和／或者空氣動力誘導的振動就會發生。這些現象通常具有下述兩種形式之一。轉子葉片可能因為空氣相對葉片的迎角太高(正迎角失速)或者太低(負迎角失速)而失速。前者是前面的級在低速下發生的問題，而后者通常在高速下影響后面的級發生問題，每一種都可以導致葉片振動，振動又會誘發迅速的破壞。如果發動機要求從壓氣機得到的壓力升高高于葉片能夠保持的壓力升高，“喘振”就出現了。在這種情況下，通過壓氣機的氣流出現瞬時分離，燃燒系統中的高壓空氣被擁推向前而穿過壓氣機，并伴有“砰”的一聲巨響和發動機推力的損失。壓氣機的設計要留有適當的裕度，以確保避免這一區域的不穩定性(圖3-14)。
Compressors
Fig. 3-12 A typical rotor blade showing圖3-12 表示扭轉外形的典型轉子葉片
twisted contour.
安裝角
旋轉方向
氣流方向
端部彎曲
安裝角
帶外環的靜子葉片
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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