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blade camber both at the blade tip and root has been
introduced. The blade extremities appear as if
formed by bending over each corner, hence the term
’end-bend’.
Stator vanes
27. The stator vanes are again of airfoil section and
are secured into the compressor casing or into stator
vane retaining rings, which are themselves secured
to the casing (fig. 3-13). The vanes are often
assembled in segments in the front stages and may
be shrouded at their inner ends to minimize the
vibrational effect of flow variations on the longer
vanes. It is also necessary to lock the stator vanes in
such a manner that they will not rotate around the
casing.
Compressors
27
Fig. 3-11 Methods of securing blades to disc.
OPERATING CONDITIONS
28. Each stage of a multi-stage compressor
possesses certain airflow characteristics that are
dissimilar from those of its neighbour; thus to design
a workable and efficient compressor, the characteristics
of each stage must be carefully matched. This is
a relatively simple process to implement for one set
of conditions (design mass flow, pressure ratio and
rotational speed), but is much more difficult when
reasonable matching is to be retained with the
compressor operating over a wide range of
conditions such as an aircraft engine encounters.
29. If the operating conditions imposed upon the
compressor blade departs too far from the design
intention, breakdown of airflow and/or aerodynamically
induced vibration will occur. These phenomena
may take one of two forms; the blades may stall
because the angle of incidence of the air relative to
the blade is too high (positive incidence stall) or too
low (negative incidence stall). The former is a front
stage problem at low speeds and the latter usually
affects the rear stages at high speed, either can lead
to blade vibration which can induce rapid destruction.
If the engine demands a pressure rise from the
compressor, which is higher than the blading can
sustain, ’surge’ occurs. In this case there is an instantaneous
breakdown of flow through the machine and
the high pressure air in the combustion system is
expelled forward through the compressor with a loud
’bang’ and a resultant loss of engine thrust.
Compressors
28
Fig. 3-12 A typical rotor blade showing
twisted contour.
Fig. 3-13 Methods of securing vanes to compressor casing.
Compressors are designed with adequate margin to
ensure that this area of instability (fig. 3-14) is
avoided.
AIRFLOW CONTROL
30. Where high pressure ratios on a single shaft are
required it becomes necessary to introduce airflow
control into the compressor design. This may take
the form of variable inlet guide vanes for the first
stage plus a number of stages incorporating variable
stator vanes for the succeeding stages as the shaft
pressure ratio is increased (fig. 3-15). As the
compressor speed is reduced from its design value
these static vanes are progressively closed in order
to maintain an acceptable air angle value onto the
following rotor blades. Additionally interstage bleed
may be provided but its use in design is now usually
limited to the provision of extra margin while the
engine is being accelerated, because use at steady
operating conditions is inefficient and wasteful of
fuel. Three types of air bleed systems are illustrated
as follows: fig. 3-16 hydraulic, fig. 3-17 pneumatic
and fig. 3-18 electronic.
MATERIALS
31. Materials are chosen to achieve the most cost
effective design for the components in question, in
practice for aero engine design this need is usually
best satisfied by the lightest design that technology
allows for the given loads and temperatures
prevailing.
Compressors
29
Fig. 3-14 Limits of stable airflow.
Fig. 3-15 Typical variable stator vanes.
Compressors
30
Fig. 3-16 A hydraulically operated bleed valve and inlet guide vane airflow control system.
32. For casing designs the need is for a light but
rigid construction enabling blade tip clearances to be
accurately maintained ensuring the highest possible
efficiency. These needs are achieved by using
aluminium at the front of the compression system
followed by .alloy steel as compression temperature
increases. Whilst for the final stages of the
compression system, where temperature requirements
possibly exceed the capability of the best
steel, nickel based alloys may be required. The use
of titanium in .preference to aluminium and steel is
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