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MAE 4261: AIR-BREATHING ENGINES
Gas Turbine Engine Combustors
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk
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COMBUSTOR LOCATION
MilitaryF119-100
CommercialPW4000
Combustor
Afterburner
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MAJOR COMBUSTOR COMPONENTSC
ompr
esso
r
Tur
bine
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MAJOR COMBUSTOR COMPONENTS
• Key Questions:
– Why is combustor configured this way?
– What sets overall length, volume and geometry of device?
Com
pres
sor
Tur
bine
Air
Fuel
Combustion Products
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COMBUSTOR EXAMPLE (F101)Henderson and Blazowski
Fuel
Com
pres
sor
Tur
bine
NG
V
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VORBIX COMBUSTOR (P&W)
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COMBUSTOR REQUIREMENTS
• Complete combustion (b → 1)
• Low pressure loss (b → 1)
• Reliable and stable ignition
• Wide stability limits
– Flame stays lit over wide range of p, u, f/a ratio)
• Freedom from combustion instabilities
• Tailored temperature distribution into turbine with no hot spots
• Low emissions
– Smoke (soot), unburnt hydrocarbons, NOx, SOx, CO
• Effective cooling of surfaces
• Low stressed structures, durability
• Small size and weight
• Design for minimum cost and maintenance
• Future – multiple fuel capability (?)
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CHEMISTRY REVIEW
OHm
nCOOm
nHC mn 222 24
478.4
1m
ns
22222 478.3
278.3
4N
mnOH
mnCONO
mnHC mn
Stoichiometric Molar fuel/air ratio Stoichiometric Mass fuel/air ratio
• General hydrocarbon, CnHm (Jet fuel H/C~2)
• Complete oxidation, hydrocarbon goes to CO2 and water
• For air-breathing applications, hydrocarbon is burned in air
• Air modeled as 20.9 % O2 and 79.1 % N2 (neglect trace species)
• Complete combustion for hydrocarbons means all C → CO2 and all H → H2O
2878.332
4
12
mn
mns
• Stoichiometric = exactly correct ratio for complete combustion
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COMMENTS ON CHALLENGES
• Based on material limits of turbine (Tt4), combustors must operate below stoichiometric values
– For most relevant hydrocarbon fuels, s~ 0.06 (based on mass)
• Comparison of actual fuel-to-air and stoichiometric ratio is called equivalence ratio
– Equivalence ratio = = stoich
– For most modern aircraft ~ 0.3
• Summary
– If = 1: Stoichiometric
– If > 1: Fuel Rich
– If < 1: Fuel Lean
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VARIATION OF FLAME TEMPERATURE WITH
Fla
me
Tem
pera
ture
Flammability LimitsStill too hotfor turbine
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WHY IS THIS RELEVANT?• Most mixtures will NOT burn so far away from
stoichiometric– Often called Flammability Limit– Highly pressure dependent
• Increased pressure, increased flammability limit
– Requirements for combustion, roughly > 0.8
• Gas turbine can NOT operate at (or even near) stoichiometric levels– Temperatures (adiabatic flame temperatures)
associated with stoichiometric combustion are way too hot for turbine
– Fixed Tt4 implies roughly < 0.5
• What do we do?– Burn (keep combustion going) near =1 with
some of compressor exit air– Then mix very hot gases with remaining air to
lower temperature for turbine
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SOLUTION: BURNING REGIONS
Air
Com
pres
sor
Tur
bine
~ 1.0T>2000 K
~0.3
PrimaryZone
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COMBUSTOR ZONES: MORE DETAILS
1. Primary Zone
– Anchors Flame
– Provides sufficient time, mixing, temperature for “complete” oxidation of fuel
– Equivalence ratio near =1
2. Intermediate (Secondary Zone)
– Low altitude operation (higher pressures in combustor)
• Recover dissociation losses (primarily CO → CO2) and Soot Oxidation
• Complete burning of anything left over from primary due to poor mixing
– High altitude operation (lower pressures in combustor)
• Low pressure implies slower rate of reaction in primary zone
• Serves basically as an extension of primary zone (increased res)
– L/D ~ 0.7
3. Dilution Zone (critical to durability of turbine)
– Mix in air to lower temperature to acceptable value for turbine
– Tailor temperature profile (low at root and tip, high in middle)
– Uses about 20-40% of total ingested core mass flow
– L/D ~ 1.5-1.8
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COMBUSTOR DESIGN
• Combustion efficiency, b = Actual Enthalpy Rise / Ideal Enthalpy Rise– h=heat of reaction (sometimes designated as QR) = 43,400 KJ/Kg
34 ttRb
P TTQ
cf
• General Observations:
1. b ↓ as p ↓ and T ↓ (because of dependency of reaction rate)
2. b ↓ as Mach number ↑ (decrease in residence time)
3. b ↓ as fuel/air ratio ↓
• Assuming that the fuel-to-air ratio is small
hm
TmTmmc
f
tatfaPb
34
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COMBUSTOR TYPES (Lefebvre)
Single Can
Tubularor Multi-Can
TuboannularCan-Annular
Annular
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COMBUSTOR TYPES (Lefebvre)
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EXAMPLES
CAN-TYPERolls-Royce Dart
ANNULAR-TYPEGeneral Electric T58
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EXAMPLES
CAN-ANNULAR-TYPERolls-Royce Tyne
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CHEMICAL EMISSIONS
• Aircraft deposit combustion products at high altitudes, into upper troposphere and lower stratosphere (25,000 to 50,000 feet)
• Combustion products deposited there have long residence times, enhancing impact
• NOx suspected to contribute to toxic ozone production
– Goal: NOx emission level to no-ozone-impact levels during cruise
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AFTERBURNER (AUGMENTER)
• Spray in more fuel to use up more oxygen
– Main combustion can not use all air
• Exit Mach number stays same (choked Mexit = 1)
– Temp ↑
– Speed of sound ↑
– Velocity = M*a ↑
– Therefore Thrust ↑
• Penalty:
– Pressure is lower so thermodynamic efficiency is poor
– Propulsive efficiency is reduced (but don’t really care in this application)
• As turbine inlet temperature keeps increasing less oxygen downstream for AB and usefulness decreases
• Requirements
– VERY lightweight
– Stable and startable
– Durable and efficient
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RELATIVE LENGTH OF AFTERBURNER
• Why is AB so much longer than primary combustor?
– Pressure is so low in AB that they need to be very long (and heavy)
– Reaction rate ~ pn (n~2 for mixed gas collision rate)
J79 (F4, F104, B58)
Combustor Afterburner
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INTRA-TURBINE BURNING
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BURNER-TURBINE-BURNER (ITB) CONCEPTS
• Improve gas turbine engine performance using an interstage turbine burner (ITB)– With a higher specific thrust engine will be smaller and lighter– Increasing payload– Reduce CO2 emissions– Reduce NOx emissions by reducing peak flame temperature
• Initially locate ITB in transition duct between high pressure turbine (HTP) and low pressure turbine (LPT)
Conventional
Intra Turbine Burner (schematic only)
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SIEMENS WESTINGHOUSE ITB CONCEPT
Tt4
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UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by
Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
Conventional Intra Turbine Burner
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2 additional burners 5 additional burners
UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by
Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
Continuous burningin turbine