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conditions as those required for the suppression of
the other pollutants, Therefore it is desirable to cool
the flame as quickly as possible and to reduce the
time available for combustion. This conflict of
conditions requires a compromise to be made, but
continuing improvements in combustor design and
performance has led to a substantially ’cleaner’
combustion process.
MATERIALS
35. The containing walls and internal parts of the
combustion chamber must be capable of resisting
the very high gas temperature in the primary zone. In
practice, this is achieved by using the best heatresisting
materials available, the use of high heat
resistant coatings and by cooling the inner wall of the
flame tube as an insulation from the flame.
36. The combustion chamber must also withstand
corrosion due to the products of the combustion,
creep failure due to temperature gradients and
fatigue due to vibrational stresses.
Combustion chambers
43
Fig. 4-10 Combustion efficiency and air/fuel
ratio.
Fig. 4-11 Combustion stability limits.
Rolls-Royce Turbomeca Adour Mk102
Rolls-Royce RB37 Derwent V
Work commenced in January 1945 on a 0.855
scale Nene, reduced to fit the engine nacelle
of a Gloster Meteor. Known as the Derwent V
the engine passed a 100 hr test at 2600 lb
thrust in June 1945 and in September went
into production with a service rating of 3500
lb. Two world speed records were set by
Meteor IV’s powered by special Derwent V’s
in November 1945 and September 1946.
5: Turbines
Contents Page
Introduction 45
Energy transfer from gas flow
to turbine 49
Construction 51
Nozzle guide vanes
Turbine discs
Turbine blades
Contra-rotating turbines
Dual alloy discs
Compressor-turbine matching 53
Materials 53
Nozzle guide vanes
Turbine discs
Turbine blades
Balancing 57
INTRODUCTION
1. The turbine has the task of providing the power to
drive the compressor and accessories and, in the
case of engines which do not make use solely of a jet
for propulsion, of providing shaft power for a
propeller or rotor. It does this by extracting energy
from the hot gases released from the combustion
system and expanding them to a lower pressure and
temperature. High stresses are involved in this
process, and for efficient operation, the turbine blade
tips may rotate at speeds over 1,500 feet per second,
The continuous flow of gas to which the turbine is
exposed may have an entry temperature between
850 and 1,700 deg. C. and may reach a velocity of
over 2,500 feet per second in parts of the turbine.
2. To produce the driving torque, the turbine may
consist of several stages each employing one row of
stationary nozzle guide vanes and one row of moving
blades (fig. 5-1). The number of stages depends
upon the relationship between the power required
45
from the gas flow, the rotational speed at which it
must be produced and the diameter of turbine
permitted.
3. The number of shafts, and therefore turbines,
varies with the type of engine; high compression ratio
engines usually have two shafts, driving high and low
pressure compressors (fig, 5-2). On high by-pass
ratio fan engines that feature an intermediate
pressure system, another turbine may be interposed
between the high and low pressure turbines, thus
forming a triple-spool system (fig, 5-3). On some
engines, driving torque is derived from a free-power
turbine (fig. 5-4). This method allows the turbine to
run at its optimum speed because it is mechanically
independent of other turbine and compressor shafts.
46
Fig. 5-1 A triple-stage turbine with single shaft system.
4. The mean blade speed of a turbine has considerable
effect on the maximum efficiency possible for
a given stage output. For a given output the gas
velocities, deflections, and hence losses, are
reduced in proportion to the square of higher mean
blade speeds. Stress in the turbine disc increases as
the square of the speed, therefore to maintain the
same stress level at higher speed the sectional
thickness, hence the weight, must be increased disproportionately.
For this reason, the final design is a
compromise between efficiency and weight. Engines
operating at higher turbine inlet temperatures are
thermally more efficient and have an improved power
to weight ratio. By-pass engines have a better
propulsive efficiency and thus can have a smaller
turbine for a given thrust.
5. The design of the nozzle guide vane and turbine
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