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INTRODUCTION TO CONCEPT
OF TURBINE COOLING AND
BLADE MATERIALTECHNOLOGY
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COOLING OF TURBINE BLADES
The efficiency of gas cycle depends upon the rangeof maximum temperature of the gas
If the maximum temperature increases, efficiency of thecycle also increases
Any increase in maximum temperature of the gas,results in an increase in blade temperature, therebyinducing more thermal stresses in the blade material
This limits the capacity of the turbine
To achieve greater power, the blades are cooledthrough the hollow passages
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DIFFERENT METHODS OF BLADE COOLING
Internal Air Coo l ing It is done by supplying cold air throughhollow blade It implies an internal baffle or deflector to direct the
flow over the hotter portions on internal surface
Film Coo l ing It involves in supplying of thin film of cooled
compressed air through a narrow slit on the blade surface to form
boundary layer over the blades
Water Cooling Circulation of water is maintained through
hollow section of the blade from root towards tip
Disc Cool ing Disc is cooled by circulating cooling fluid andthus blade temperature is reduced by conduction
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ADVANTAGES OF COOLING
The specific thrust of the jet engine is increased by
32 % for gas temperature increased from 800C to 1100C
at the expense of 7 % specific fuel consumption
If the maximum temperature is increased to 1600C
in place of present practice 900C then the specific fuel
consumption will be increased by 50 % for increase in a
specific power by 200 %
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TURBINE COOLING
Because the limiting factor in most turbine designs is themaximum temperature that can be tolerated at the turbine inlet,
design engineers use every method at their command to
increase the allowable inlet temperature
On practically all large engines, one such method is to cool
the inlet guide-vanes of the first-stage turbine and the first-
stage rotor blades
The cooling is accomplished by directing compressor bleedair through as sage inside the engine to the turbine area where
the air (or coolant) is led to longitudinal holes, tubes, or
cavities in the vanes
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TURBINE COOLING
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TURBINE COOLING
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TURBINE COOLING
After entering the passages in the vanes and blades, the air
(coolant) is distributed through holes at the leading and trailing
edges of the vanes and blades
The air impinges along the vane, blade surfaces, and then
passes out of the engine with the engine exhaust
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THE INTERNAL COOLING SYSTEM OF AN
ADVANCED TURBINE
The PW4000 turbofan, an advanced engine, has acooling
system for the high-pressure turbine vanes and blades,
outer air seal, transition duct between the high-pressureand low-pressure turbines, and the high-pressure turbine
disks
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THE INTERNAL COOLING SYSTEM OF AN
ADVANCED TURBINE
This system must not be confused with the system thatuses fan air to cool the turbine cases to control blade tipclearance
The cooling system optimizes engine performance bybleeding and controlling twelfth-stage compressor air to thehigh-pressure turbine
The twelfth-stage cooling air is bled from the high-Pressure
compressor through four ducts
Two of the ducts continually inject cooling air
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THE INTERNAL COOLING SYSTEM OF AN
ADVANCED TURBINE
The air from the two remaining ducts is controlled by
valves; thus, fifty percent of the cooling air for the turbine
vanes can be shut off at lower power to increase engine
efficiency
The two valves also control the twelfth-stage cooling air
that is used to supplement fifteenth-stage air
The twelfth stage air is supplied from the valves to thediffuser case via two ducts
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THE INTERNAL COOLING SYSTEM OF AN
ADVANCED TURBINE
It flows through diffuser case struts and joins the modified
fifteenth-stage air flowing around the No 3 bearing
compartment
This combined coolant flows to and cook the high-pressureturbine disks
The electronic engine control controls the valves according
to a schedule determined by altitude and high-pressurerotor speed
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Blade-Tip Clearance Control
Some modern jet engines, particularly those of Transports,
have a system that controls the clearance between theblade-tips and the outer case of the turbine,
It improves fuel efficiency and increases the life of the cases
Blade-tip clearance is controlled by scooping cool air from
the fan stream and distributing it through ducts
To spray over the turbine cases the hot cases are cooled by
the air, shrink, and thus lessen the gap around the rotating
turbine blades
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BLADE-TIP CLEARANCE CONTROL
The reduced leakage of air past the tightly running blade
tips increases fuel efficiency
On the PW2000 and PW4000-series engines, both the low-
pressure and high-pressure turbine cases are cooled duringthe climb and cruise portions of flight
The FADEC commands the operation of the system
according to a schedule determined by altitude and high-pressure rotor speed
DIRECTIONAL SOLIDIFICATION PROCESS
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DIRECTIONAL SOLIDIFICATION PROCESSDS-200
Close examination of a conventional turbine blade revealsa myriad of crystals that lie in all directions (equip-axed)
Improved service life can be obtained by aligning the
crystals to form columns along the blade length, produced
by a method known as DirectionalSolidification
Further advance of this technique is to make the blade out
of a single crystal
Each method extends the useful creep life of the blade, in
the case of the single crystal blade, the operating
temperature can be substantially increased
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TURBINE MATERIALS
SINGLE CRYSTALS
Use of single crystals allows the raising of engine operating
temperatures, yielding increased power and fuel efficiency
The durability of single crystals in blades and vanes are
advances on directionally solidified, or columnar grain,
airfoils, which were themselves stronger than airfoils cast
by traditional means
The metal of conventional airfoils comprises crystals joinedat boundaries
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TURBINE MATERIALS
SINGLE CRYSTALS
When high performance gas-turbine blades fail in service,
one of two modes of failure, rupture or fatigue, usually
predominates
The mechanism usually is failure along crystal boundaries,
with subsequent propagation of a crack
The inter crystal line cracking which leads to failure isinitiated principally at grain boundaries that are oriented
normal to the stress axis
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TURBINE MATERIALS
SINGLE CRYSTALS
In the directional solidification process introduced by
Pratt& Whitney in 1969 grain boundaries are aligned in
columns parallel to the airfoil axis, providing improved high-
temperature performance
Single crystal material goes even further by eliminating all
grain boundaries,
It allowing maximum use of the natural strength of the
metal
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COMPRESSOR-TURBINE
MATCHING
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.
Nozzle guide vanes allowed too low a maximum flow, then
a back pressure would build up causing the compressor to
surge, too high a flow would cause the compressor to
choke.
In either condition a loss of efficiency would very rapidly
occur.
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CONCEPT OF BLISK WITH REFERENCE
TO COMPRESSOR (DUAL ALLOY DISCS)
Very high stresses are imposed on the blade root fixing of
high work rate turbines, which make conventional methods
of blade attachment impractical
A dual alloy disc, or blisk has a ring of cast turbine blades
bonded to the disc
This type of turbine is suitable for small high power
helicopter engines
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