Seoul National UniversityJune 29 - July 3, 1998
Fundamentals in the Gas Turbine Engine
1998. 6.
Lecture Notes
Prepared by
Jeong-Lak Sohn, Dong Sub Kim and Sung Tack Ro
Seoul National UniversityJune 29 - July 3, 1998
Contents
I. OverviewsI-1. History
I-2. Applications
I-3. Components
II. Basic Thermodynamics and Fluid FlowsII-1. Five Basic Principles
II-2. Some Important Formula
III. Cycle and PerformanceIII-1. Ideal cycles
III-2. Component Characteristics
IV. Aerothermodynamics in Major ComponentsIV-1. Compressor
IV-2. Turbine
IV-3. Combustor
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Contents (Continued)
V. Structure & DynamicsV-1. Blade Vibration
V-2. Stresses on Blade
VI. Materials and Failure ModesVI-1. Materials
VI-2. Failure Modes
VII. Gas Turbine Development
VII-1. Flowchart for the Gas Turbine Development
VII-2. Development Organization
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Part I. Overviews
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I-1. History
1791 John Barber (UK) World’s first patent of the gas turbine engine (British Patent No. 1833) “A method of rising inflammable air for the purpose of producing motion and
facilitating metallurgical operation”
1903 Elling (Norway) World’s first gas turbine to produce power
1904 Stolze (Germany) 1905 Armengaud & Lemale (France) 1908 Holzworth (Germany) 1937 Whittle (UK)
World’s first jet engine (British Patent No. 347,206) W2/700 Nene, Tay (Rolls-Royce) Trent
J42 (Pratt & Whitney) PW4000
J31 (General Electric) GE90
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I-2. Applications
Turbojet Engine Turboprop Engine
Turbofan Engine Turboshaft Engine
Types of Gas Turbine Engines
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I-2. Applications
Aircraft Engines Commercial / military aircrafts Helicopters Missiles
Industrial Engines Power generations Mechanical drivers Marine / ground propulsion
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I-3. Major Components
Three Major Components - Compressor - Combustor - Turbine
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I-3. Major Components
Compressor Situated at the front of the engine, Draws air in, pressurizes it, then delivers it into the combustion chamber. Two types of compressor design, centrifugal and axial flow.
Centrifugal compressor Axial Compressor
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I-3. Major Components
Combustor The air from the compressor passes into the combustion chamber where it is
mixed with the vaporized fuel sprayed from burners located in the head of the chamber.
The mixture is ignited, during the engine starting cycle, by igniter plugs located in the combustor.
Absorbs energy (heat) from fuel supplied from outside of engine Can, annular, tubular, cannular types of combustors
Can Type Combustor Cannular Type Combustor
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I-3. Major Components
Turbine Absorbs energy from the hot expanding gases leaving the combustor to keep the
compressor rotating at its most efficient speed and to produce required shaft power or thrust.
Axial and radial types of turbines
Axial Turbine
Radial Turbine
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Part II. Basic Thermodynamics and Fluid Flows
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II-1. Five Basic Principles
The First Law of Thermodynamics (Conservation of Energy)
Enthalpy
The Second Law of Thermodynamics
No engine can be more efficient than a reversible engine under the same conditions
Q
W
, , ,m h V z1 1 1 1
, , ,m h V z2 2 2 2
dS 0
h e p /
.
2
22
2
.
2
.
1
21
1
.
1 22Wz
VhmQz
Vhm
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II-2. Some Important Formulas
Adiabatic Process
Stagnation Properties
Stagnation enthalpy
Stagnation temperature
Stagnation pressure
Stagnation Properties in Adiabatic Process
p / constant
h hV
T TV
C
p pV
p
0
2
0
2
0
2
2
2
2
20
2
11 M
T
T
)1/(20
2
11
Mp
p
)1/(120
2
11
M
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Conservation of Mass
For one-dimensional flow in a pipe or duct,
Conservation of Momentum
For one-dimensional flow in a pipe or duct
Equation of State in Ideal Gas
min mout
m mout in 0
m m VAout in
p RT
0
dAnV
dAnVVF
inout
outin VmVmAAF
..
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Part III. Cycle and Performance
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III-1. Ideal Cycle
Gas Turbine Basic Cycle : Brayton Cycle Simple Shaft Power Cycle
Efficiency
Specific Work
/)1(
23
1243 11
s
rTTC
TTCTTC
uppliedheat
outputworknet
p
pp
4312 // pppprasratiopressureisrwhere
11
1 /)1(/)1(
1
rr
tTC
W
p
13 / TTTasratioetemperaturisTwhere
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Regeneration Cycle Efficiency
if T5=T4,
C T T C T T
C T Tp p
p
( ) ( )
( )3 4 2 1
3 5
11r
t
( )/
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Reheat Cycle
Efficiency
Specific Work
2 1 2
2
t c t c
t c t c
/
/
W
C Tt c
t
c
where c r
p 1
1
2 12
( ) /
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Reheat Cycle Influence of temperature ratio to efficiency and specific ratio
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IPC CombustorHPC
HPT LPT PowerTurbine
Regenerator
Exhaust Gas
Intercooler
Intake Air
WaterFuel
OutputShaft
GAS-TURBINE WITHINTERCOOLER & REGENERATOR
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III-2. Real Cycle
Ideal vs. Real Brayton Cycle
2 2’ : Aerodynamic losses in compressor 3 3’ : Pressure drop in combustor 4 4’ : Aerodynamic losses in turbine
1
2
3
4
T
S
1
2
3
4
T
S
3’
2’
4’
Ideal Brayton Cycle Real Brayton Cycle
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Isentropic Efficiencies of Compressor and Turbine
Compressor isentropic efficiency
Turbine isentropic efficiency
c
W
W
h
h
T T
T T
0
0
02 01
02 01
t
W
W
h
h
T T
T T
0
0
03 04
03 04
1/)1(
01
02010102
p
pTTT
c
/)1(
0403030403 /
11
ppTTT t
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Combustor Efficiency
Combustor Efficiency
Fuel-Air Ratio
Specific Fuel Consumption [kg/kwh]
Thermal Efficiency
Heat Rate [kJ/kWh]
where WN is net work produced by the whole engine per unit mass of air [kW/kg],
Qnet,p is heat value, i.e., heat rate supplied by unit mass of fuel
at constant pressure combustion process [kW/kg].
bf
ff f
m
mwhere m mtheoretical
actual
theoretical actual
,
fm
mf
a
SFCf
WN
Work produced by the engine
Heat to the engine
W
fQN
net psupplied ,
SFC Qnet px ,
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Cycle Performance Curves
Simple cycle Regeneration Cycle
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Part IV. Aerothermodynamics of Major Components
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IV-1. Axial Compressor
Elementary Theory Comparison of typical forms of turbine and compressor rotor blades
T-s Diagram
W mc T T mc T Tp p ( ) ( )03 01 02 01
s
T T
T T
03 01
03 01
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Velocity Diagram
To obtain high temperature rise
in a stage ;
(1) high blade speed
(2) high axial velocity
(3) high fluid deflection in the rotor blade
Assuming that Ca1=Ca2=Ca
Pressure ratio per stage
)tan(tan
)tan(tan)tan(tan
tantantantan
21
010201030
21
.
12
.
12
.
2211
p
a
s
aa
ww
a
C
UC
TTTTT
UCmUCm
CCUmW
C
U
)1/(
01
0
01
03 1
T
T
p
pR ss
s
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Design Process of an axial compressor(1) Choice of rotational speed at design point and annulus dimensions
(2) Determination of number of stages, using an assumed efficiency at design point
(3) Calculation of the air angles for each stage at the mean line
(4) Determination of the variation of the air angles from root to tip
(5) Selection of compressor blades using experimentally obtained cascade data
(6) Check on efficiency previously assumed using the cascade data
(7) Estimation on off-design performance
(8) Rig testing
Blade profile
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Performance Curves
(a) Mass flow rate vs. pressure ratio (b) Mass flow rate vs. isentropic efficiency
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IV-2. Axial Turbine
Elementary Theory
'0301
0301
)1/(
0301010
32
.
32
.
0301
.
0
.
3322
,/
11
)tan(tan)tan(tan
tantantantan
TT
TTwhere
ppTT
UCmUCmTTCmTCmW
C
U
sss
aapsp
a
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Blade Profile Performance Curves
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Blade Cooling
(a) Nozzle
(b) Rotor Blade
Impingement cooling
Convective cooling
Film cooling
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IV-3. Combustor
Typical Combustion chamber
Pressure Loss
Pressure Loss Factor
Pressure Loss in the Combustor
1
2/ 01
0221
21
.2
0
T
TKK
Am
pPLF
m
2
01
01
.
01
0
2
pA
TmRPLF
p
p
m
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Combustion Stability Loop
Methods of Flame Stabilization
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Gas Turbine Emission
Effect of flame temperature on NOx emission
Dependence of emission on fuel/air ratio Diffusion vs. pre-mix burning
Pre-mixed combustor
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V. Structure & Dynamics
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V-1. Blade Vibration
Blade Vibrations Forced Vibration
Arises from the movement of the rotor through stationary disturbances such as upstream stator wakes, support struts, inlet distortions, or by forcing functions such as rotating stall.
Leads to high stresses and failure when the excitation frequency coincides with blade natural frequency.
Almost all the sources must be harmonics of the rotating speed of engine.
Flutter Arises by aerodynamic effects in the axial compressor. Occurs at frequencies that are not multiples of engine order and at different locations
on the compressor operating map.
Vibration Modes Natural Modes
Occur at characteristic frequencies determined by the distribution of mass and stiffness resulting from the variable thickness of the blade area.
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Typical Vibration Modes Flap Modes
Torsional Modes
Disk Modes
The natural frequency or rotor vibration Reduced with increasing temperature Because of reduction in Young’s Modulus Increased at high speed Because of centrifugal stiffening
Rotor blade with 1F vibration mode
Rotor blade with 1T vibration mode
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Typical vibration mode of a rotor in holographic image
Campbell Diagram
A design tool to estimate whether engine operates in resonance condition or not. Engine order : Excitation frequency Resonance condition : Coincidence of a natural frequency with exciting frequency
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V-2. Stresses on Blade
Centrifugal Stress Centrifugal Tensile Stress
Limiting rotor tip speed and hub-tip ratio (i.e., blade length)
A factor in the hot section of gas turbines in conjunction with creep effects The maximum centrifugal stress occurs at the blade root.
Centrifugal Bending Stress Generated if the centers of gravity of shroud, foil, root are not located on the common radial axis.
Gas Flow Induced Steady State Stress Bending stress superimposed on the centrifugal stress Proportional to the aerodynamic loading on blade
Gas Flow Induced Alternating Stress Caused by stator vane wakes and wakes from support struts, etc...
2
222
max 12
2t
rt
bb
t
rr
bct r
rUANardr
a
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Thermal Stress Blades are subjected to severe thermal stresses during transient conditions such
as startup and shutdowns. Typical thermal-mechanical cycle for a first stage turbine blade
Blade Failure due to Overspeed 25% overspeed 56% increase in resulting stress
Tension
Compression
Strain Metal Temperature
Warm-upAcceleration
LoadBase Load
Unload
Shut-down
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VI. Materials and Failure Modes
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VI-1. Materials
Typical Materials
Compressor- Blades/Vanes : Cr-Alloy, Titanium (Forging, Fabrication) - Discs : Ni-Alloy (Forging)- Cylinders : Cast iron, Titanium (Forging, Casting, Fabrication)
Combustor- Liner/Transition : Ni-Alloy, Hestalloy (Fabrication) - Casing : Steel (Fabrication, Casting)
Turbine- Blades/Vanes : Ni-Alloy (Casting) - Discs : Ni-Alloy (Forging)
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Issues Related to the Material Selection High temperature material
100oF 10% increase in power
2.4% increase in thermal efficiency Historically, 30oF/year (1939-1979)
Resistance to the material selection : Fatigue(HCF/LCF), Creep, Corrosion
Requirements & Considerations Mechanical strength
Under 600oF : Yield and endurance for low temperature Above 600oF: Creep and endurance for high temperature
Corrosion resistance Low temperature Hot corrosion
Workability / Availability Casting/Forging Machining
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VI-2. Failure Modes in Gas Turbine Blading
42% of engine failures are related to the blade-problems Failure Modes in Gas Turbine Blading
Low Cycle Fatigue - Compressor and turbine discs High Cycle Fatigue - Compressor/turbine blades & discs, compressor vanes Thermal Fatigue - Turbine vanes, combustor components Environmental Attack (Oxidation, Sulphidation, Hot Corrosion, Standby
Corrosion) - Hot section blades & vanes, transition pieces, combustors Creep Damage - Hot section blades & vanes Erosion & Wear Impact Overload Damage (Due to FOD, DOD or Compressor surge) Thermal Aging Combined Failure Mechanisms - Creep/fatigue, Corrosion/fatigue,
Oxidation/erosion, etc.
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Fatigue High Cycle Fatigue (HCF)
Caused by aerodynamic excitations (Blade passing frequency) or by self-excited vibration and flutter.
Whereas fluctuating stresses may not be very high, the maximum stress at resonance can increase dramatically.
S-N (Stress vs. Number of cycles) curve
Low Cycle Fatigue (LCF) Occurs as a result of machine start/stop cycles. Associated with machines that have been in operation for several years. Minute flaws grow into crack which result in rupture. Predominant in the bores and bolt hoe areas of compressor and turbine disks which
operate under centrifugal stresses.
Thermo-Mechanical Fatigue (TMF) Associated with thermal stresses, e.g., differential expansion of hot section
components during startup & shutdown. Temperature variation in hot section blading : 200oC/minute
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Environmental Problems High temperature oxidation
Occurs when nickel based superalloys are exposed to temperature greater than 1000oF (538oC). Nickel-oxide layer on the airfoil surface
When subjected to vibration and start/stop thermal cycles during operation, nickel-oxide layer tends to crack and spall.
Sulphidation A reaction which occurs when sulpher (in fuel) reacts with oxygen and attacks the base
metal. Particular concern when it is found in the blade root region or along the leading or
trailing edges, or under the blade shroud.
Hot corrosion Combined oxidation-sulphidation phenomena of hot section parts.
Standby corrosion Occurs during a turbine shutdown and as the result of air moisture and corrosive being
present in the machine. Blade fatigue strength is significantly reduced by corrosion.
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Creep Occurs when components operates over time under high stresses and
temperature. Creep Curve
Creep-sensitive parts in engine Hot section parts and the final stages of high pressure ratio compressors. Mid span region of the airfoil which experiences the highest temperature. Disk rim region where high stresses and temperature can cause time dependent plastic
deformation.
15oC increase in blade metal temperaturecuts creep life by 50%.
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Erosion/Wear Particulate Erosion
Compressor Particle size causing erosion : 5~10 microns Reduction in the surge margin can occur if the tips get severely eroded.
Hot Gas Erosion Turbine
Occurs when the cooling boundary layer on the blade surface breaks down even for short periods of time or cooling effectiveness drops.
The surface roughness of the blade contacted by the hot gas are subjected to high thermal stress cycles.
After several cycles, damage takes places and the increased roughness (erosion) worsens the problems.
First stage turbine vane
Combined Mechanism Corrosion reduce blade section size and drop the fatigue strength Erosion in the blade attachment regions reduce damping causing increased
vibration amplitudes and alternating stresses
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VII. Gas Turbine Development
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VII-1. Flowchart for the Gas Turbine Development
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VII-2. Development Organization
Work Components
Engineering Organization
설 계 제 작 시 험
개념설계
기본설계
상세설계
치공구 제작
부품 제작
조 립
부품 시험
구성품 시험
엔진 시험
형상관리
개발총괄 설계 시험 /소재 공정
엔지니어링
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형상관리
A 사업
B 사업
C 사업
개발 총괄
상세설계 /성능 제어 /공력 열전달 연소 /구조 동역학 /전기 전자
설계 시험 /소재 공정
엔지니어링
형상관리
개발 총괄 설계
설비기술 측정기술 시험기술
시험 /소재 공정
엔지니어링
Q C
생산관리
구매관리
제작관리
생산계획
치공구설계
생산기술
제작기술
선반
M illin g
판금
연마
용접
C o atin g
D eb urin g
가공 주조공장
제작
Design Organization
Test Organization
Manufacturing Organization