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 The blades on the early Whittle engine were attached by the de Laval bulb root fixing, but this design was soon superseded by the 'fir-tree' fixing that is now used in the majority of gas turbine engines.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.在葉尖和機匣之間存在間隙，由于膨脹和收縮率的不同，間隙的尺寸是變化的。為了減少燃氣漏過葉片頂部時的效率損失，通常裝有葉冠，如圖5-1所示。這是由每個葉片的葉尖處加一個小片構成的，這些小片在葉尖的周圍形成一個圓環。如第9章所述，機匣中還可以采用一條易摩帶，用來減少燃氣漏氣。主動間隙控制是在整個飛行循環中保持葉尖間隙最小的更有效的方法。來自壓氣機的空氣用于對渦輪機匣冷卻，當與不帶葉冠的渦輪工作葉片一起使用時，可以使用更高的溫度和轉速。
對轉渦輪
21．圖5-10所示為驅動一個對轉后風扇的一臺12級對轉自由動力渦輪。這種設計只有一排靜止的導向器葉片。實際上其余的導向器葉片是固定在旋轉機匣上的渦輪工作葉片，它與旋轉鼓筒的旋轉方向相反。由于只有一排葉片不從燃氣流吸收能量，所以對轉渦輪能夠在比常規渦輪高得多的級載荷下工作，對于直接驅動的用途，它非常有吸引力。
雙合金輪盤
22．在高功率渦輪的葉片根部固定處承受著非常高的應力，這使得葉片的常規連接方法變得不實用。如圖5-11所示，一種雙合金輪盤，即“整體葉盤”上有一圈鑄造的渦輪葉片焊接在輪盤上。這種渦輪適用于小型的高功率直升機發動機。
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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