Nozzle guide vanes | Contra-rotating turbine | COMPRESSOR-TURBINE MATCHING

Fig. 5-8 Typical nozzle guide vanes showing their shape and location.

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.
14. The nozzle guide vanes are usually of hollow form and may be cooled by passing compressor delivery air through them to reduce the effects of high thermal stresses and gas loads. For details of turbine cooling, reference should be made to Part 9.

Fig. 5-9 Various methods of attaching blades to turbine discs.

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 The blades on the early Whittle engine were attached by the de Laval bulb root fixing, but this design was soon superseded by the ’fir-tree’ fixing that is now used in the majority of gas turbine engines.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 allthe 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 isrotating. 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 nowgenerally used on gas turbine engines.

Fig. 5-10 Free power contra-rotating turbine.

20. A gap exists between the blade tips and casing,which varies in size due to the different rates ofexpansion 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. ActivClearance Control (A.C.C.) is a more effectiv method of maintaining minimum tip clearancethroughout the flight cycle. Air from the compressor isused to cool the turbine casing and when used witshroudless turbine blades, enables higher temperatures 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, i effect, turbine blades attached to a rotating casingwhich revolves in the opposite direction to a rotatindrum. Since all but one aerofoil row extracts energ from the gas stream, contra-rotating turbines ar 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 bladeroot fixing of high work rate turbines, which makeconventional methods of blade attachmentimpractical. A dual alloy disc, or ’blisk’ as shown infig. 5-11, has a ring of cast turbine blades bonded tothe disc. This type of turbine is suitable for small highpower helicopter engines.
COMPRESSOR-TURBINE MATCHING

Fig. 5-11 Section through a dual alloy disc.

23. The flow characteristics of the turbine must bevery carefully matched with those of the compressor to obtain the maximum efficiency and performance of the engine. If, for example, the nozzle guide vanes allowed too low a maximum flow, then a back pressure would build up causing the compressor to surge (Part 3); too high a flow would cause the compressor to choke. In either condition a loss ofefficiency would very rapidly occur.
MATERIALS
24. Among the obstacles in the way of using higher turbine entry temperatures have always been the effects of these temperatures on the nozzle guide vanes and turbine blades, The high speed of rotation which imparts tensile stress to the turbine disc an blades is also a limiting factor.