Nozzle guide vanes | Turbine blades | BALANCING

Fig. 5-12 Section through a dual alloy disc.

Nozzle guide vanes
25. Due to their static condition. the nozzle guide vanes do not endure the same rotational stresses as the turbine blades. Therefore, heat resistance is the property most required. Nickel alloys are used, although cooling is required to prevent melting. Ceramic coatings can enhance the heat resisting properties and, for the same set of conditions, reduce the amount of cooling air required, thus improving engine efficiency.
Turbine discs

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.

INTRODUCTION - An early combustion chamber.

INTRODUCTION

Fig. 4-1 An early combustion chamber.

1. The combustion chamber (fig. 4-1) has the difficult task of burning large quantities of fuel, supplied through the fuel spray nozzles (Part 10), with extensive volumes of air, supplied by the compressor (Part 3), and releasing the heat in such a manner that the air is expanded and accelerated to give a smooth stream of uniformly heated gas at all conditions required by the turbine (Part 5). This task must be accomplished with the minimum loss in pressure and with the maximum heat release for the limited space available.