15.
Turbine discs are usually manufactured from a machined forging with an integral shaft or with a flange onto which the shaft may be bolted. The disc also has, around its perimeter, provision for the attachment of the turbine blades.
16.
To limit the effect of heat conduction from the turbine blades to the disc a flow of cooling air is passed across both sides of each disc (Part 9).
Turbine blades
17. The turbine blades are of an aerofoil shape, designed to provide passages between adjacent blades that give a steady acceleration of the flow up to the 'throat', where the area is smallest and the
velocity reaches that required at exit to produce the required degree of reaction (para. 5).
18.
The actual area of each blade cross-section is fixed by the permitted stress in the material used and by the size of any holes which may be required for cooling purposes (Part 9). High efficiency demands thin trailing edges to the sections, but a compromise has to be made so as to prevent the blades cracking due to the temperature changes during engine operation.
19.
The method of attaching the blades to the turbine disc is of considerable importance, since the stress in the disc around the fixing or in the blade root has an important bearing on the limiting rim speed. The blades on the early Whittle engine were attached by the de Laval bulb root fixing, but this design was soon superseded by the 'fir-tree' fixing that is now used in the majority of gas turbine engines. This type of fixing involves very accurate machining to ensure that the loading is shared by all
20.在叶尖和机匣之间存在间隙，由于膨胀和收缩率的不同，间隙的尺寸是变化的。为了减少燃气漏过叶片顶部时的效率损失，通常装有叶冠，如图5-1所示。这是由每个叶片的叶尖处加一个小片构成的，这些小片在叶尖的周围形成一个圆环。如第9章所述，机匣中还可以采用一条易摩带，用来减少燃气漏气。主动间隙控制是在整个飞行循环中保持叶尖间隙最小的更有效的方法。来自压气机的空气用于对涡轮机匣冷却，当与不带叶冠的涡轮工作叶片一起使用时，可以使用更高的温度和转速。
对转涡轮
21．图5-10所示为驱动一个对转后风扇的一台12级对转自由动力涡轮。这种设计只有一排静止的导向器叶片。实际上其余的导向器叶片是固定在旋转机匣上的涡轮工作叶片，它与旋转鼓筒的旋转方向相反。由于只有一排叶片不从燃气流吸收能量，所以对转涡轮能够在比常规涡轮高得多的级载荷下工作，对于直接驱动的用途，它非常有吸引力。
双合金轮盘
22．在高功率涡轮的叶片根部固定处承受着非常高的应力，这使得叶片的常规连接方法变得不实用。如图5-11所示，一种双合金轮盘，即“整体叶盘”上有一圈铸造的涡轮叶片焊接在轮盘上。这种涡轮适用于小型的高功率直升机发动机。
the serrations. The blade is free in the serrations when the turbine is stationary and is stiffened in the root by centrifugal loading when the turbine is rotating. Various methods of blade attachment are shown in fig. 5-9; however, the B.M.W. hollow blade and the de Laval bulb root types are not now generally used on gas turbine engines.
20. A gap exists between the blade tips and casing, which varies in size due to the different rates of expansion and contraction. To reduce the loss of efficiency through gas leakage across the blade tips, a shroud is often fitted as shown in fig. 5-1. This is made up by a small segment at the tip of each blade which forms a peripheral ring around the blade tips. An abradable lining in the casing may also be used to reduce gas leakage as discussed in Part 9. Active Clearance Control (A.C.C.) is a more effective method of maintaining minimum tip clearance throughout the flight cycle. Air from the compressor is used to cool the turbine casing and when used with shroudless turbine blades, enables higher tempera-tures and speeds to be used.
Contra-rotating turbine
21. Fig. 5-10 shows a twelve stage contra-rotating free power turbine driving a contra-rotating rear fan. This design has only one row of static nozzle guide vanes. The remaining nozzle guide vanes are, in effect, turbine blades attached to a rotating casing which revolves in the opposite direction to a rotating drum. Since all but one aerofoil row extracts energy from the gas stream, contra-rotating turbines are capable of operating at much higher stage loadings than conventional turbines, making them attractive for direct drive applications.
Dual alloy discs
22. Very high stresses are imposed on the blade root fixing of high work rate turbines, which make conventional methods of blade attachment impractical. A dual alloy disc, or 'blisk' as shown in fig. 5-11, has a ring of cast turbine blades bonded to the disc. This type of turbine is suitable for small high power helicopter engines.
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