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substantial whirl velocity to assist efficient entry of
the air into the rotating cooling passages.
8. Cooling air for the turbine discs enters the
annular spaces between the discs and flows
outwards over the disc faces. Flow is controlled by
interstage seals and, on completion of the cooling
function, the air is expelled into the main gas stream
(fig. 9-5); see para. 23., Hot gas ingestion.
Bearing chamber cooling
9. Air cooling of the engine bearing chambers is not
normally necessary since the lubrication system
(Part 8) is adequate for cooling purposes.
Additionally, bearing chambers are located, where
possible, in the cooler regions of the engine. In
instances where additional cooling is required, it is
good practice to have a double skinned bearing
housing with cooling air fed into the intermediate
space.
Internal air system
88
Fig. 9-3 Development of high pressure turbine blade cooling.
Accessory cooling
10. A considerable amount of heat is produced by
some of the engine accessories, of which the
electrical generator is an example, and these may
often require their own cooling circuit. When air is
used for cooling, the source may be the compressor
or atmospheric air ducted from intake louvres in the
engine cowlings.
11. When an accessory unit is cooled during flight
by atmospheric air it is usually necessary to provide
an induced circuit for use during static ground
running when there would be no external airflow. This
is achieved by allowing compressor delivery air to
pass through nozzles situated in the cooling air outlet
duct of the accessory. The air velocity through the
nozzles create a low pressure area which forms an
ejector, so inducing a flow of atmospheric air through
the intake louvres. To ensure that the ejector system
only operates during ground running, the flow of air
from the compressor is controlled by a valve. A
generator cooling system with an ejector is shown in
fig. 9-6.
SEALING
12. Seals are used to prevent oil leakage from the
engine bearing chambers, to control cooling airflows
and to prevent ingress of the mainstream gas into the
turbine disc cavities.
13. Various sealing methods are used on gas
turbine engines. The choice of which method is
dependent upon the surrounding temperature and
pressure, wearability, heat generation, weight, space
available, ease of manufacture and ease of installation
and removal. Some of the sealing methods are
described in the following paragraphs. A hypothetical
turbine showing the usage of these seals is shown in
fig. 9-5.
Labyrinth seals
14. This type of seal is widely used to retain oil in
bearing chambers and as a metering device to
control internal airflows. Several variations of
labyrinth seal design are shown in fig. 9-7.
15. A labyrinth seal comprises a finned rotating
member with a static bore which is lined with a soft
abradable material, or a high temperature
honeycomb structure. On initial running of the engine
the fins lightly rub against the lining, cutting into it to
give a minimum clearance. The clearance varies
throughout the flight cycle, dependent upon the
thermal growth of the parts and the natural flexing of
the rotating members. Across each seal fin there is a
pressure drop which results in a restricted flow of
Internal air system
89
Fig. 9-4 High pressure nozzle guide vane construction and cooling.
Internal air system
90
Fig. 9-5 A hypothetical turbine cooling and sealing arrangement.
sealing air from one side of the seal to the other.
When this seal is used for bearing chamber sealing,
it prevents oil leakage by allowing the air to flow from
the outside to the inside of the chamber. This flow
also induces a positive pressure which assists the oil
return system.
16. Seals between two rotating shafts are more
likely to be subject to rubs between the fins and
abradable material due to the two shafts deflecting
simultaneously. This will create excessive heat which
may result in shaft failure. To prevent this, a non-heat
producing seal is used where the abradable lining is
replaced by a rotating annulus of oil. When the shafts
deflect, the fins enter the oil and maintain the seal
without generating heat (fig. 9-7).
Ring seals
17. A ring seal (fig. 9-7) comprises a metal ring
which is housed in a close fitting groove in the static
housing. The normal running clearance between the
ring and rotating shaft is smaller than that which can
be obtained with the labyrinth seal. This is because
　
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