劳斯莱斯喷气引擎－中英(25)

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8. The torque or turning power applied to the turbine is governed by the rate of gas flow and the energy change of the gas between the inlet and the outlet of the turbine blades, The design of the turbine is such that the whirl will be removed from the gas
stream so that the flow at exit from the turbine will be per cent.
substantially 'straightened out' to give an axial flow into the exhaust system (Part 6). Excessive residual whirl reduces the efficiency of the exhaust system and also tends to produce jet pipe vibration which has a detrimental effect on the exhaust cone supports and struts.
9.
It will be seen that the nozzle guide vanes and blades of the turbine are 'twisted', the blades having a stagger angle that is greater at the tip than at the root (fig. 5-6). The reason for the twist is to make the gas flow from the combustion system do equal work at all positions along the length of the blade and to ensure that the flow enters the exhaust system with a uniform axial velocity. This results in certain changes in velocity, pressure and temperature occurring through the turbine, as shown diagram-matically in fig. 5-7.

10.
The 'degree of reaction' varies from root to tip, being least at the root and highest at the tip, with the mean section having the chosen value of about 50
10．“反力度”从叶根到叶尖是变化的，叶根处最小，叶尖处最大，在平均截面处为选定值，约50％。

结构
 
12.涡轮的基本部件有燃烧室燃气导管、导向器叶
片、涡轮盘和涡轮工作叶片。转动组件由装在涡轮
机匣中的轴承支承，涡轮轴可以和压气机轴共用一
轴，或者由自动定心的联轴器与压气机轴相接。
11．由于许多原因，损失总要发生，使涡轮不能获得100％效率。由于涡轮工作叶片的气动力损失，典型的非冷却式三级涡轮会遭受3.5％的损失。另外，由导向器叶片中的气动损失，燃气漏过涡轮工作叶片叶尖的损失以及排气系统的损失总共将产生4.5％的损失。这些损失大致上占相等的比例。计及各种损失之后，总的效率约为92％。

Fig. 5-6 A typical turbine blade showing静压
速度
导向器叶片

13．导向器叶片具有翼型截面，相邻叶片之间的通道构成了收敛的涵道。导向叶片位于(图5-8)涡轮机匣中其安装方式应能使它们发生膨胀。
twisted contour.
图5-6一种典型的涡轮工作叶片，图中示出了扭曲的外形
安装角
旋转方向
气流方向
安装角
11. The losses which prevent the turbine from being 100 per cent efficient are due to a number of reasons. A typical uncooled three-stage turbine would suffer a 3.5 per cent loss because of aerodynamic losses in the turbine blades. A further
4.5 per cent loss would be incurred by aerodynamic losses in the nozzle guide vanes, gas leakage over the turbine blade tips and exhaust system losses; these losses are of approximately equal proportions. The total losses result in an overall efficiency of approximately 92 per cent.
CONSTRUCTION
12. The basic components of the turbine are the combustion discharge nozzles, the nozzle guide vanes, the turbine discs and the turbine blades. The rotating assembly is carried on bearings mounted in the turbine casing and the turbine shaft may be common to the compressor shaft or connected to it by a self-aligning coupling.
Nozzle guide vanes
13. The nozzle guide vanes are of an aerofoil shape with the passage between adjacent vanes forming a convergent duct. The vanes are located (fig. 5-8) in the turbine casing in a manner that allows for expansion. 涡轮叶片
进入排气系统时压力和速度均匀
压力增加
(从导向叶片叶根到叶尖)
速度降低

 

图5-7 流过导向器叶片和涡轮工作叶片的燃气流的流型
Fig. 5-7 Gas flow pattern through nozzle and blade.

涡轮盘
 
15．涡轮盘通常由机械加工的锻件制成。它可以与轴制成一个整体，也可以带安装边由螺栓连接涡轮轴，而且，轮盘的外圆处还有涡轮工作叶片安装用的榫槽。
14．导向器叶片通常是空心结构，可以由压气机的
出口空气在其内部流过进行冷却，以减轻热应力和
气动负荷的影响。对于涡轮冷却的详细情况可参看
第9章。

19．工作叶片安装在涡轮盘上的方法极为重要，因为在固定部位或叶片根部周围涡轮盘的应力对于限制轮缘速度具有很重要的意义。早期的“惠特尔”(whittle)发动机的叶片曾用拉瓦尔(Laval)的球形叶根固定，但这种设计很快便由枞树形榫头所取代，这就是目前大多数燃气涡轮发动机所使用的榫头。为保证载荷能由所有齿分担，这种榫头要作非常精密的机械加工。当涡轮处于静止状态时，叶片在齿上是活动的，当涡轮旋转时，在离心载荷的作用下根部才变成刚性结合。在图5-9中示出了叶片的各种连接方法，但是德国BMW公司的空心叶片及拉瓦尔球形叶根式连接方法目前在燃气涡轮发动机上一般都不使用。
涡轮工作叶片
 
17．涡轮工作叶片设计成翼型截面，每个相邻叶片之间的通遭使气流稳定地加速到“喉部”。喉部的面积最小。速度达到了所需的出口速度来产生要求的反力度(第5章)。
16.为了限制从涡轮工作叶片向轮盘的热传导的影响，每一级轮盘的两面都通一股冷却空气(第9章)。

18.根据所用材料的许用应力及扣除为冷却(第9章)目的要求用的孔的尺寸之后，每个工作叶片横截面的实际面积是固定的。高效率要求截面的后缘薄，但是为了防止在发动机工作中由于温度的改变使叶片产生裂纹，不得已进行折衷处理。
　
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