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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.
51
Fig. 5-6 A typical turbine blade showing
twisted contour.
Fig. 5-7 Gas flow pattern through nozzle and blade.
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.
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design was soon superseded by the ’fir-tree’ fixing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
52
Fig. 5-8 Typical nozzle guide vanes showing their shape and location.
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 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, 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
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