Page 38 - The Jet Engine

Compressors

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 characteris-
tics 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 aerodynami-
cally 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 instan-
Fig. 3-12 A typical rotor blade showing taneous breakdown of flow through the machine and
twisted contour. 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.

Fig. 3-13 Methods of securing vanes to compressor casing.

28