1. MAE 4261: AIR-BREATHING ENGINES
Gas Turbine Engine Combustors
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk

2. COMBUSTOR LOCATION Commercial PW4000 Combustor Military F119-100
Afterburner

3. MAJOR COMBUSTOR COMPONENTS
Turbine
Compressor

4. MAJOR COMBUSTOR COMPONENTS
Fuel
Combustion Products
Turbine
Air
Compressor
Key Questions:
Why is combustor configured this way?
What sets overall length, volume and geometry of device?

5. COMBUSTOR EXAMPLE (F101) Henderson and Blazowski
Fuel
Turbine
NGV
Compressor

6. VORBIX COMBUSTOR (P&W)

8. COMBUSTOR REQUIREMENTS
Complete combustion (hb → 1)
Low pressure loss (pb → 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 (?)

9. CHEMISTRY REVIEW 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
Stoichiometric Molar fuel/air ratio
Stoichiometric Mass fuel/air ratio
Stoichiometric = exactly correct ratio for complete combustion

10. COMMENTS ON CHALLENGES
Based on material limits of turbine (Tt4), combustors must operate below stoichiometric values
For most relevant hydrocarbon fuels, ys ~ 0.06 (based on mass)
Comparison of actual fuel-to-air and stoichiometric ratio is called equivalence ratio
Equivalence ratio = f = y/ystoich
For most modern aircraft f ~ 0.3
Summary
If f = 1: Stoichiometric
If f > 1: Fuel Rich
If f < 1: Fuel Lean

11. VARIATION OF FLAME TEMPERATURE WITH f
Still too hot
for turbine
Flammability Limits

12. 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 f > 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 f < 0.5
What do we do?
Burn (keep combustion going) near f=1 with some of compressor exit air
Then mix very hot gases with remaining air to lower temperature for turbine

13. SOLUTION: BURNING REGIONS
Turbine
Air
Primary
Zone
f~0.3
f ~ 1.0
T>2000 K
Compressor

14. COMBUSTOR ZONES: MORE DETAILS
Primary Zone
Anchors Flame
Provides sufficient time, mixing, temperature for “complete” oxidation of fuel
Equivalence ratio near f=1
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 tres)
L/D ~ 0.7
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 ~

15. COMBUSTOR DESIGN
Combustion efficiency, hb = Actual Enthalpy Rise / Ideal Enthalpy Rise
h=heat of reaction (sometimes designated as QR) = 43,400 KJ/Kg
General Observations:
hb ↓ as p ↓ and T ↓ (because of dependency of reaction rate)
hb ↓ as Mach number ↑ (decrease in residence time)
hb ↓ as fuel/air ratio ↓
Assuming that the fuel-to-air ratio is small

17. COMBUSTOR TYPES (Lefebvre)
Single Can
Tubular
or Multi-Can
Tuboannular
Can-Annular
Annular

18. COMBUSTOR TYPES (Lefebvre)

19. EXAMPLES
CAN-TYPE
Rolls-Royce Dart
ANNULAR-TYPE
General Electric T58

20. EXAMPLES
CAN-ANNULAR-TYPE
Rolls-Royce Tyne

21. 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

22. 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

23. RELATIVE LENGTH OF AFTERBURNER
J79 (F4, F104, B58)
Combustor
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)

24. INTRA-TURBINE BURNING

25. BURNER-TURBINE-BURNER (ITB) CONCEPTS
Conventional
Intra Turbine Burner (schematic only)
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)

26. SIEMENS WESTINGHOUSE ITB CONCEPT
Tt4

27. UNDERSTANDING BENEFIT FROM CYCLE ANALYSIS From “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

28. UNDERSTANDING BENEFIT FROM CYCLE ANALYSIS From “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
Continuous burning
in turbine
2 additional burners
5 additional burners