Fig. 3-10 Rotors of drum and disc 圖3-10 鼓筒和盤式轉(zhuǎn)子結(jié)構(gòu)
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(zhuǎn)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(yīng)級(jí)的具體要求。一般而言，目的是設(shè)計(jì)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(shù)壓氣機(jī)設(shè)計(jì)具有分開的葉片，以滿足制造和可維護(hù)性要求，但是。在最小發(fā)動(dòng)機(jī)上設(shè)計(jì)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(chǎn)和盤為一整體的葉片，即所謂的 “整體式葉盤”來(lái)克服。
轉(zhuǎn)子葉片
26．轉(zhuǎn)子葉片呈翼型截面形狀(圖3-12)，通常設(shè)計(jì)成沿其長(zhǎng)度有一壓力梯度，以保證空氣維持一個(gè)比較均勻的軸向速度。向葉尖方向遂漸變高的壓力抵消轉(zhuǎn)子作用在氣流上的離心作用。為了獲得這些狀態(tài)，必須將葉片從葉根向尖部“扭轉(zhuǎn)”，以便在每一點(diǎn)都具有正確的迎角。流過壓氣機(jī)的空氣在其內(nèi)外壁面處產(chǎn)生二個(gè)邊界層，一直將氣流減慢到滯止的程度。為了補(bǔ)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(lái)象是將其每個(gè)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(jī)機(jī)匣中，或者固定到靜子葉片保持環(huán)中，再將這些環(huán)本身固定到機(jī)匣上(圖3-13)。在前幾級(jí)中，靜子葉片常常成組地裝配，并在其小半徑一端加有凸臺(tái)，以盡量減輕氣流變化對(duì)較長(zhǎng)葉片產(chǎn)生的振動(dòng)影響。對(duì)靜子葉片還必須鎖定,不讓它們沿機(jī)匣轉(zhuǎn)動(dòng)。
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(tài)
28．多級(jí)壓氣機(jī)的每一級(jí)部具有一定的流量特性，而且與其相鄰級(jí)的各不相同。因此，要設(shè)計(jì)一臺(tái)有效工作的壓氣機(jī)，每級(jí)的特性都必須經(jīng)過精心的匹配。這對(duì)于一套狀態(tài)(設(shè)計(jì)質(zhì)量流量、增壓比和旋轉(zhuǎn)速度)執(zhí)行起來(lái)是比較簡(jiǎn)單的過程，但是，要在飛機(jī)發(fā)動(dòng)機(jī)遇到的大范圍狀態(tài)下保持壓氣機(jī)工作的合理匹配就困難得多了。
29．如果加于壓氣機(jī)葉片的工作狀態(tài)偏離設(shè)計(jì)狀態(tài)過多，氣流分離和／或者空氣動(dòng)力誘導(dǎo)的振動(dòng)就會(huì)發(fā)生。這些現(xiàn)象通常具有下述兩種形式之一。轉(zhuǎn)子葉片可能因?yàn)榭諝庀鄬?duì)葉片的迎角太高(正迎角失速)或者太低(負(fù)迎角失速)而失速。前者是前面的級(jí)在低速下發(fā)生的問題，而后者通常在高速下影響后面的級(jí)發(fā)生問題，每一種都可以導(dǎo)致葉片振動(dòng)，振動(dòng)又會(huì)誘發(fā)迅速的破壞。如果發(fā)動(dòng)機(jī)要求從壓氣機(jī)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(xiàn)了。在這種情況下，通過壓氣機(jī)的氣流出現(xiàn)瞬時(shí)分離，燃燒系統(tǒng)中的高壓空氣被擁推向前而穿過壓氣機(jī)，并伴有“砰”的一聲巨響和發(fā)動(dòng)機(jī)推力的損失。壓氣機(jī)的設(shè)計(jì)要留有適當(dāng)?shù)脑６龋源_保避免這一區(qū)域的不穩(wěn)定性(圖3-14)。
Compressors
Fig. 3-12 A typical rotor blade showing圖3-12 表示扭轉(zhuǎn)外形的典型轉(zhuǎn)子葉片
twisted contour.
安裝角
旋轉(zhuǎn)方向
氣流方向
端部彎曲
安裝角
帶外環(huán)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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