Turbine Cooling

Prepared under
QIP-CD Cell Project
Lecture-9
Jet Propulsion
Ujjwal K Saha, Ph. D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
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Turbine Cooling
Designer’s Goal:
‰ Increase of Thrust
‰ Decrease of Weight
‰ Decrease of Fuel Consumption
Methods:
¾
Turbine IT
1100 –1200 K (Early designs)
1400-1700 K (Recent designs)
¾
Mass Flow of Air
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High TIT:
‰ Restricted by the blade material
‰ Effective cooling is required to raise the TIT
Coolant:
Air : Readily available
Bled from the Compressors
Water : Effective, High Specific Heats
Becomes complicated and Heavy
Used in Ground-based units/systems
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Air Cooling:
o Convective Cooling
o Impingement Cooling
o Film Cooling
o Transpiration Cooling
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‰ Convective Cooling:
o First method of cooling employed.
o Air is routed through the internal
passages from the base.
o Restricted by the internal passage size.
o Restricted by the quantity of coolant
mass flow.
‰
Drawback
¾
Ineffective cooling at the blade TE.
¾
Thin TE restricts the routing of coolant.
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6
‰ Impingement Cooling:
o Coolant is routed through a central core and
then turned to radial direction.
o Adapted for turbine nozzle blades.
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‰ Film Cooling:
o Injection of
secondary air into the
boundary layer of
primary air (hot Gases).
o Protects the surface
from the hot gases by
forming a cool & thin
film over the surface.
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‰ Transpiration Cooling:
o Use of porous material
through which coolant
is forced into the BL.
o For effective cooling,
pores should be small.
Chances of blockage
due to oxidation and
foreign material.
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TURBINE COOLING-OVERVIEW
Thermal Barrier Coating
The turbine blade
is coated with
ceramic, which
has a higher heat
resistance. This
coating acts as a
protective film, a
thin insulating
layer that helps
with the turbine's
maximum heat
threshold.
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Detrimental Effects
o Added cost of producing turbine blades
o Loss of turbine work due to cooling air
bypassing one or more turbine stages
o Loss due to cooling air being mixed with
the hot streams
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Noise Suppressors
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Propulsion System Noise:
Externally generated
Exhaust gases from the engine
Propeller of TP engine
Cannot be suppressed
Internally generated
Rotating machinery
Combustion process
Can be suppressed
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Sources of Noise
‰
‰
‰
Jet or exhaust noise
Fan noise
Core noise
Jet Noise:
™ Results from mixing of high velocity exhaust
gas with the ambient air.
™ A large amount of turbulence generates when
they mix at different velocities.
™ Intensity of turbulence (hence the noise) as
the velocity difference.
Magnitude of jet noise (velocity)8
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Turbofan Engine
Non-mixed type
Fan exhaust with ambient air
Core exhaust with fan exhaust
and the ambient air
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Fan Noise:
™ Major sources of noise in high by-pass ratio
turbofan engine.
Tip
Characteristics varies
Root
Tip speed
Subsonic
Supersonic
For high BPR engines, exhaust velocity
and jet velocity are reduced.
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Core Noise:
Compressor noise
Combustion noise
Turbine noise
Mainly from interaction
of Rotors and Stators
Turbulence generated
by the burning of fuel.
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Reduction of Noise:
Add weight, length and cost
Desirable features
in reducing noise
External noise reduction
Conflict with the best
aerodynamic design.
Mixed Type Engine: An exhaust gas
mixer (behind the turbine) mixes the
high velocity core exhaust with low
velocity cold stream. This reduces
maximum exhaust stream velocity
and reduces external turbulence
mixing of two streams.
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Internal noise:
Eliminated/confined.
‰ Confined by installing acoustical linear at the
inlet to the engine and in the exhaust duct.
Reduction:
¾ By decreasing fan tip speed:
Subsonic Fan: Lowest noise level.
Low speed fan
needs more compressor stages
makes a heavier & costly engine.
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¾
Increasing spacing between Rotor and Stator
Large spacing makes a lengthy engine.
Weight becomes high.
¾ Changing the number of Rotor/Stator blades
Decreases the turbomachinery performance.
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Thrust Reversers
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Larger and
modern aircraft
Higher airspeeds
&
Greater gross weight
Stopping the vehicle (after
landing) becomes a problem
Wheel brakes: May not serve the purpose as desired.
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Piston engine
Turbo-prop engine
Variable Pitch
Propeller
Solves this problem
Turbojet
Turbofan
Thrust is being reversed by thrust
reverser to reduce the landing run
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Thrust Reversers
‰ Ground-speed braking force.
‰ In-flight use (prior to landing) to slow-down,
to increase the rate of sink during descent.
Mechanical blockage type
Aerodynamic blockage type
Some form of obstruction is placed at the rear of the nozzle.
Exhaust gas is blocked and diverted at a suitable angle.
™
™
™
™
Cam shell-type deflector doors.
Retractable ejector with bucket type doors.
Blocker doors (for fan) and
Target-type deflector doors for hot streams.
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Summary
o Turbine Cooling Methods
Convective, Film, Impingement & Transpiration
o Noise Suppressors
Mixers, Acoustic Liners
o Thrust Reversers
Variable Pitch Propellers & Reversesrs
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References & Web Resources
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Hill, P.G., and Peterson, C.R., (1992), Mechanics and Thermodynamics of Propulsion,
Addison Wesley.
Saravanamuttoo, H.I.H, Rogers, G.F.C, and. Cohen, H, (2001), Gas Turbine Theory,
Pearson Education.
Oates, G.C., (1988), Aerothermodynamics of Gas Turbine and Rocket Propulsion, AIAA,
New York.
Mattingly, J.D., (1996), Elements of Gas Turbine Propulsion, McGraw Hill.
Cumpsty, N.A., (2000), Jet Propulsion, Cambridge University Press.
Bathie, W.W., (1996), Fundamentals of Gas Turbines, John Wiley.
Treager, I.E., (1997), Aircraft Gas Turbine Engine Technology, Tata McGraw Hill.
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