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power helicopter engines.
COMPRESSOR-TURBINE MATCHING
23. The flow characteristics of the turbine must be
very 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 of
efficiency 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 and
blades is also a limiting factor.
53
Fig. 5-9 Various methods of attaching blades to turbine discs.
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
26. A turbine disc has to rotate at high speed in a
relatively cool environment and is subjected to large
rotational stresses. The limiting factor which affects
the useful disc life is its resistance to fatigue
cracking.
54
Fig. 5-10 Free power contra-rotating turbine.
Fig. 5-11 Section through a dual alloy disc.
55
Fig. 5-11 Section through a dual alloy disc.
27. In the past, turbine discs have been made in
ferritic and austenitic steels but nickel based alloys
are currently used. Increasing the alloying elements
in nickel extend the life limits of a disc by increasing
fatigue resistance. Alternatively, expensive powder
metallurgy discs, which offer an additional 10% in
strength, allow faster rotational speeds to be
achieved.
Turbine blades
28. A brief mention of some of the points to be
considered in connection with turbine blade design
will give an idea of the importance of the correct
choice of blade material. The blades, while glowing
red-hot, must be strong enough to carry the
centrifugal loads due to rotation at high speed. A
small turbine blade weighing only two ounces may
exert a load of over two tons at top speed and it must
withstand the high bending loads applied by the gas
to produce the many thousands of turbine horsepower
necessary to drive the compressor. Turbine
blades must also be resistant to fatigue and thermal
shock, so that they will not fail under the influence of
high frequency fluctuations in the gas conditions, and
they must also be resistant to corrosion and
oxidization. In spite of all these demands, the blades
must be made in a material that can be accurately
formed and machined by current manufacturing
methods.
29. From the foregoing, it follows that for a
particular blade material and an acceptable safe life
there is an associated maximum permissible turbine
entry temperature and a corresponding maximum
engine power. It is not surprising, therefore, that metallurgists
and designers are constantly searching for
better turbine blade materials and improved methods
of blade cooling.
30. Over a period of operational time the turbine
blades slowly grow in length. This phenomenon is
known as ’creep’ and there is a finite useful life limit
before failure occurs.
31. The early materials used were high temperature
steel forgings, but these were rapidly replaced by
cast nickel base alloys which give better creep and
fatigue properties.
32. Close examination of a conventional turbine
blade reveals a myriad of crystals that lie in all
directions (equi-axed). Improved service life can be
obtained by aligning the crystals to form columns
along the blade length, produced by a method known
as ’Directional Solidification’. A further advance of
this technique is to make the blade out of a single
56
Fig. 5-13 Comparison of turbine blade life
properties.
Fig. 5-14 Ceramic turbine blades.
crystal, Examples of these structures are shown in
fig. 5-12. Each method extends the useful creep life
of the blade (fig. 5-13) and in the case of the single
crystal blade, the operating temperature can be substantially
increased.
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