1. MAE 5310: COMBUSTION FUNDAMENTALS
Lecture 1: Introduction and Overview
January 9, 2017
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk

2. COMBUSTION FUNDAMENTALS
Thermodynamics
Energy Balance
Flame Temperature
Chemistry
Stoichiometry
Equilibrium
Kinetics
Emissions and Pollutants
Combustion
Technology
Fluid Mechanics
Flame Propagation
Laminar / Turbulent
Diffusion
Atomization
Combustor Aerodynamics
Rapid oxidation generating heat
Slow oxidation accompanied by relatively little heat and no light
Combustion transforms energy stored in chemical bonds to heat that can be utilized in a variety of ways

3. MAE 5310: COURSE OUTLINE Thermochemistry and Thermodynamics
Chemical Kinetics
Explosive and General Oxidative Characteristics of Fuels
Premixed Flames
Diffusion Flames
Ignition
Detonation
Emissions and Pollutants

4. 1. THERMOCHEMISTRY Combustion stoichiometry and thermodynamics
Balance of chemical equations
Lean, stoichiometric, and rich fuel-to-air mixtures
1st Law of Thermodynamics and enthalpy of combustion
How hot is a flame? (usually 2,000-2,500 K)
Known Stoichiometry + 1st Law → Adiabatic Flame Temperature
Chemical equilibrium: 2nd Law of Thermodynamics
Important in fuel-rich combustion
Stable species at ambient conditions begin to dissociate when T > 1,250 K
Dissociation lowers flame temperature
Solution technique: Minimize Gibbs Free Energy, G=H-TS
Known P and T + Equilibrium Relations → Stoichiometry
Adiabatic combustion equilibrium
Equilibrium + 1st Law → Adiabatic Flame Temperature and Stoichiometry

5. 2. CHEMICAL KINETICS
Equilibrium chemistry assumes that T & P are constant for a sufficiently long time for system to reach steady-state
While equilibrium chemistry lends insight into factors that control pollutant formation, greater understanding requires study of rates at which competing reactions proceed
Example:
If f ↓ then T ↓ and [NO] ↓
BUT for f ↓, hydrocarbon oxidation is slow
For finite combustor length emissions of CO and unburned hydrocarbons can ↑
Understanding developed from basic kinetic theory → Arrhenius form
Endothermic and Exothermic reactions (forward and backward)
Simplified kinetics vs. detailed mechanisms

6. 3. EXPLOSIVE AND GENERAL OXIDATIVE CHARACTERISTICS OF FUELS
Explosion: very fast reacting systems (rapid heat release or pressure rise)
In order for flames to propagate (deflagrations or detonations), reaction kinetics must be fast, i.e., mixture must be explosive
Example
At P=1 atm, NO EXPLOSION
If P is lowered to a few % of 1 atm: EXPLOSION
If P is raised to 2 atm: EXPLOSION
What are explosive limits?
Note that explosive limits are not flammability limits
Explosion limits are P & T boundaries for a specific fuel-oxidizer mixture ratio that separate regions of slow and fast reaction
Flammability limits are specify lean and rich fuel-oxidizer mixture ratio beyond which no flame will propagate
H2+O2, y=1
T=500 ºC
P=1 atm

7. HINDENBURG: MAY 6, 1937

8. ROCKET PROPELLANT

9. 4. COMBUSTION MODES AND FLAME TYPES
Combustion can occur in flame mode
Premixed flames
Diffusion (non-premixed) flames
Combustion can occur in non-flame mode
What is a flame?
A flame is a self-sustaining propagation of a localized combustion zone at subsonic velocities
Flame must be localized: flame occupies only a small portion of combustible mixture at any one time (in contrast to a reaction which occurs uniformly throughout a vessel)
A discrete combustion wave that travels subsonically is called a deflagration
Combustion waves may be also travel at supersonic velocities, which are called detonations
Fundamental propagation mechanisms are different in deflagrations and detonations

10. 4. LAMINAR PREMIXED FLAMES
Fuel and oxidizer mixed at molecular level prior to occurrence of any significant chemical reaction
Flame color gives indication of temperature
Not quite red: T~ ºC
Dark red: T~ ºC
Bright red: T~ ºC
Yellowish red: T~ ºC
Not quite white: T~ ºC
White: T > 1450 ºC

11. 4. LAMINAR PREMIXED FLAMES

12. PREMIXED FLAMES
Fuel and oxidizer mixed at molecular level prior to occurrence of any significant chemical reaction

13. APPLICATION: ENGINE KNOCK
In internal combustion engines, compressed gasoline-air mixtures have a tendency to ignite prematurely rather than burning smoothly
This creates engine knock, a characteristic rattling or pinging sound in one or more cylinders.
Octane number of gasoline is a measure of its resistance to knock (or its ability to wait for a spark to initiate a flame).
Octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane.
Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with little knock.
Heptane is given an octane rating of zero. It is an unbranched compound and knocks badly.
Flame Mode
Non-Flame Mode
(autoignition)

14. 5. DIFFUSION FLAMES Orange Blue Full range of f throughout
Reactants are initially separated, and reaction occurs only at interface between fuel and oxidizer (mixing and reaction taking place)
Diffusion applies strictly to molecular diffusion of chemical species
In turbulent diffusion flames, turbulent convection mixes fuel and air macroscopically, then molecular mixing completes process so that chemical reactions can take place
Orange
Blue
Full range of f
throughout
reaction zone

15. DIFFUSION FLAME: EARTH vs. SPACE

16. 4+5: LOOK AGAIN AT BUNSEN BURNER
Secondary diffusion flame
Results when CO and H
products from rich inner flame
encounter ambient air
Fuel-rich pre-mixed
inner flame
What determines shape of flame? (ANS: velocity profile and heat loss to tube wall)
Under what conditions will flame remain stationary? (ANS: flame speed must equal speed of normal component of unburned gas at each location)
Most practical devices (Diesel-engine combustion) has premixed and diffusion burning

17. PROPULSION SYSTEMS Gas Turbine Engine for Propulsion
X-37B Orbital Test Vehicle
Gas Turbine Engine for Power Generation

18. 5: DIFFUSION FLAMES

19. 7. DETONATION Pure Explosion vs. Detonation (not same)
Explosion requires rapid energy release
An explosion does not necessarily require passage of a combustion wave through exploding medium
Both deflagrations and detonations require rapid energy release and presence of a waveform
To have either a deflagration or a detonation, an explosive gas mixture must exist
Recall:
Deflagration: a subsonic wave sustained by a chemical reaction
Detonation: a supersonic wave sustained by a chemical reaction

20. 7. PULSE DETONATION ENGINES

21. PULSE DETONATION WAVE ENGINES
Liquid methane or liquid hydrogen is ejected onto fuselage
Fuel mist is ignited, possibly by surface heating
The PDWE works by creating a liquid hydrogen detonation inside a specially designed chamber when aircraft is traveling beyond speed of sound
When traveling at such speeds, a thrust wall is created in front of the aircraft
When detonation takes place, airplane's thrust wall is pushed forward
This process is continually repeated to propel aircraft
"...use a shock wave created in a detonation - an explosion that propagates supersonically- to compress a fuel-oxidizer mixture prior to combustion, similar to supersonic inlets that make use of external and internal shock wave for pressurization."

22. 8. EMISSIONS AND POLLUTANTS
Major pollutants produced by combustion are:
Unburned and partially burned hydrocarbons, CnHm
Nitrogen oxides (NOx, NO and NO2)
Carbon monoxide (CO)
Sulfur oxides (SOx, SO2 and SO3)
Subjected to legislated controls (smog, acid rain, global warming, ozone depletion, health hazards, etc.)

23. EXAMPLES OF EMISSIONS (FIGURES 1.1 – 1.5)
Organic Compounds and
Unburned hydrocarbons
CO emissions
Note that Clean Air Act of 1970
can be clearly seen in figures

24. 8. EMISSIONS AND POLLUTANTS
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

25. DOES COMBUSTION SCALE?
What are limiting effects on combustion system size?
Can you burn at any scale?
Do any non-dimensional numbers exist to predict combustion scaling?

26. DETAILED EXAMPLE: DIFFUSION FLAMES
Reactants are initially separated, and reaction occurs only at interface between fuel and oxidizer (mixing and reaction taking place)
PW4000 Fan Engine Cutaway
Characteristics
Fan tip diameter: 94 inches; Length: inches
Take-off thrust: 52, ,000 pounds; Bypass ratio: 4.8 to 5.0
Overall pressure ratio: ; Fan pressure ratio: ;
Planes powered: Boeing , MD-11, Airbus A , etc.

27. MAJOR COMBUSTOR COMPONENTS
Turbine
Compressor

28. 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?

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

30. VORBIX COMBUSTOR (P&W)
Example of vortex enhanced combustion
Why is turbulence helpful?

31. 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 (?)

32. 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

33. 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

34. 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 ingested air
Then mix very hot gases with remaining air to lower temperature for turbine

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

36. 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 ~

37. 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 fuel-to-air ratio is small:

38. COMBUSTOR LOCATION Commercial PW4000 Combustor Military F119-100
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)
Afterburner

39. RESEARCH EXAMPLES

40. COMBUSTION RESEARCH AT FLORIDA TECH
Phase 1 Development of a Combustion Prediction Capability for Sinda/Fluint
Work with NASA KSC Launch Services Program
Develop Independent Verification and Validation (IV&V) of liquid rocket combustion process
Delta II, Delta IV, and Atlas Rockets

41. COMBUSTION RESEARCH AT FLORIDA TECH
Solid Rocket Motor Propellant Combustion and Plume Characterization
Work with NASA KSC Launch Services Program
Develop Independent Verification and Validation (IV&V) of solid rocket combustion process

42. COMBUSTION RESEARCH AT FLORIDA TECH
2007 Florida Centers of Excellent Proposal
$50 M proposal to bring elevated combustion testing capability to Florida
Primary partners Siemens and Florida Turbine Technologies
Area of Interest for Combustion Testing
Reproduce the same conditions that is expected in the engine in
terms of air, fuel, temperature, geometry, equipment.
Best data that can be obtained prior to testing in the engine.

43. COMBUSTION RESEARCH AT FLORIDA TECH
Inlet support piece.
BullHorn
Measuring section duct area (inside)
Manhole cover on measuring section
Plate 1 area coverage
Plate 2 area coverage
Plate 3 area coverage
Plate 4 area coverage
Note: Traversing cylinder not installed in this picture
Reproduce engine geometries (flow-box, row 1 vanes via VSS).

44. COMBUSTION RESEARCH AT SIEMENS