Internal air system
pressure and temperature. Therefore, to reduce
engine performance losses, the air is taken as early
as possible from the compressor commensurate with
the requirement of each particular function. The
cooling air is expelled overboard via a vent system or
into the engine main gas stream, at the highest
possible pressure, where a small performance
recovery is achieved.
COOLING
3. An important consideration at the design stage of
a gas turbine engine is the need to ensure that
certain parts of the engine, and in some instances
certain accessories, do not absorb heat to the extent
that is detrimental to their safe operation. The
principal areas which require air cooling are the
combustor and turbine. Refer to Part 4 for combustor
cooling techniques.
4. Cooling air is used to control the temperature of
the compressor shafts and discs by either cooling or
heating them. This ensures an even temperature distribution
and therefore improves engine efficiency by
controlling thermal growth and thus maintaining
minimum blade tip and seal clearances. Typical
cooling and sealing airflows are shown in fig. 9-1.
Turbine cooling
5. High thermal efficiency is dependent upon high
turbine entry temperature, which is limited by the
turbine blade and nozzle guide vane materials.
Continuous cooling of these components allows their
environmental operating temperature to exceed the
material’s melting point without affecting the blade
and vane integrity. Heat conduction from the turbine
blades to the turbine disc requires the discs to be
cooled and thus prevent thermal fatigue and uncontrolled
expansion and contraction rates.
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