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blade passages is based broadly on aerodynamic
considerations, and to obtain optimum efficiency,
compatible with compressor and combustion design,
the nozzle guide vanes and turbine blades are of a
47
Fig. 5-2 A twin turbine and shaft arrangement.
48
Fig. 5-3 A triple turbine and shaft arrangement.
basic aerofoil shape. There are three types of
turbine; impulse, reaction and a combination of the
two known as impulse-reaction. In the impulse type
the total pressure drop across each stage occurs in
the fixed nozzle guide vanes which, because of their
convergent shape, increase the gas velocity whilst
reducing the pressure. The gas is directed onto the
turbine blades which experience an impulse force
caused by the impact of the gas on the blades. In the
reaction type the fixed nozzle guide vanes are
designed to alter the gas flow direction without
changing the pressure. The converging blade
passages experience a reaction force resulting from
the expansion and acceleration of the gas. Normally
gas turbine engines do not use pure impulse or pure
reaction turbine blades but the impulse-reaction
combination (fig. 5-5). The proportion of each
principle incorporated in the design of a turbine is
largely dependent on the type of engine in which the
turbine is to operate, but in general it is about 50 per
cent impulse and 50 per cent reaction. Impulse-type
turbines are used for cartridge and air starters (Part
11).
ENERGY TRANSFER FROM GAS FLOW TO
TURBINE
6. From the description contained in para. 1, it will
be seen that the turbine depends for its operation on
the transfer of energy between the combustion
49
Fig. 5-4 A typical free power turbine.
gases and the turbine. This transfer is never 100 per
cent because of thermodynamic and mechanical
losses, (para. 11).
7.when the gas is expanded by the combustion
process (Part 4), it forces its way into the discharge
nozzles of the turbine where, because of their
convergent shape, it is accelerated to about the
speed of sound which, at the gas temperature, is
about 2,500 feet per second. At the same time the
gas flow is given a ’spin’ or ’whirl’ in the direction of
rotation of the turbine blades by the nozzle guide
vanes. On impact with the blades and during the
subsequent reaction through the blades, energy is
absorbed, causing the turbine to rotate at high speed
and so provide the power for driving the turbine shaft
and compressor.
8. The torque or turning power applied to the
turbine is governed by the rate of gas flow and the
energy change of the gas between the inlet and the
outlet of the turbine blades, The design of the turbine
is such that the whirl will be removed from the gas
stream so that the flow at exit from the turbine will be
substantially ’straightened out’ to give an axial flow
into the exhaust system (Part 6). Excessive residual
whirl reduces the efficiency of the exhaust system
and also tends to produce jet pipe vibration which
has a detrimental effect on the exhaust cone
supports and struts.
9. It will be seen that the nozzle guide vanes and
blades of the turbine are ’twisted’, the blades having
a stagger angle that is greater at the tip than at the
root (fig. 5-6). The reason for the twist is to make the
gas flow from the combustion system do equal work
at all positions along the length of the blade and to
ensure that the flow enters the exhaust system with
a uniform axial velocity. This results in certain
changes in velocity, pressure and temperature
occurring through the turbine, as shown diagrammatically
in fig. 5-7.
10. The ’degree of reaction’ varies from root to tip,
being least at the root and highest at the tip, with the
mean section having the chosen value of about 50
per cent.
50
Fig. 5-5 Comparison between a pure Impulse turbine and an impulse/reaction turbine.
11. The losses which prevent the turbine from being
100 per cent efficient are due to a number of
reasons. A typical uncooled three-stage turbine
would suffer a 3.5 per cent loss because of
aerodynamic losses in the turbine blades. A further
4.5 per cent loss would be incurred by aerodynamic
losses in the nozzle guide vanes, gas leakage over
the turbine blade tips and exhaust system losses;
these losses are of approximately equal proportions.
The total losses result in an overall efficiency of
approximately 92 per cent.
CONSTRUCTION
12. The basic components of the turbine are the
combustion discharge nozzles, the nozzle guide
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