MAE 5310: COMBUSTION FUNDAMENTALS

1
MAE 5310 COMBUSTION FUNDAMENTALS

• Lecture 1 Introduction and Overview
• August 18, 2009
• 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
COMBUSTION STATISTICS

• Approximately 85 of energy used in US comes from
  combustion sources
• Direct Gas- or oil-fired furnace or boiler
• Indirect Burning a fossil fuel to produce
  electricity
• More than 50 of electricity production is from
  burning coal
• Presently only 33 of electrical energy is
  nuclear or hydroelectric
• 2/3 of all petroleum imported or produced in US
  used to power transportation
• Industrial processes rely heavily on combustion
  (iron, steel, aluminum, etc.)
• Incineration of waste
• Environmental emissions and pollution
• Significant challenge in design of combustors for
  ground based and air applications, typical target
  number for NOx 5-20 PPM

4
MAE 5310 COURSE OUTLINE

• Thermochemistry and Thermodynamics
• Chemical Kinetics
• Explosive and General Oxidative Characteristics
  of Fuels
• Premixed Flames
• Detonation
• Diffusion Flames
• Ignition
• Emissions and Pollutants

5
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 gt 1,250 K
• Dissociation lowers flame temperature
• Solution technique Minimize Gibbs Free Energy,
  GH-TS
• Known P and T Equilibrium Relations ?
  Stoichiometry
• Adiabatic combustion equilibrium
• Equilibrium 1st Law ? Adiabatic Flame
  Temperature and Stoichiometry

6
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

7
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 P1 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

H2O2, y1 T500 ºC P1 atm
8
HINDENBURG MAY 6, 1937
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 T500-550 ºC Dark red T650-750
ºC Bright red T850-950 ºC Yellowish red
T1050-1150 ºC Not quite white T1250-1350
ºC White T gt 1450 ºC
11
PREMIXED FLAMES

• Fuel and oxidizer mixed at molecular level prior
  to occurrence of any significant chemical reaction

12
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)
13
6. DIFFUSION FLAMES

• 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
14
46 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

15
6 DIFFUSION FLAMES
16
5. 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

17
5. PULSE DETONATION ENGINES
18
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."

19
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.)

20
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
21
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

22
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?

23
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 132.7 inches
  
• Take-off thrust 52,000 - 62,000 pounds Bypass
  ratio 4.8 to 5.0
• Overall pressure ratio 27.5 - 32.3 Fan pressure
  ratio 1.65 - 1.80
• Planes powered Boeing 747-400, MD-11, Airbus
  A300-610, etc.

24
COMBUSTOR LOCATION
Commercial PW4000
Combustor
Military F119-100
Afterburner
25
MAJOR COMBUSTOR COMPONENTS
Turbine
Compressor
26
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?

27
COMBUSTOR EXAMPLE (F101)Henderson and Blazowski
Fuel
Turbine NGV
Compressor
28
VORBIX COMBUSTOR (PW)

• Example of vortex enhanced combustion
• Why is turbulence helpful?

29
(No Transcript)
30
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 (?)

31
CHEMISTRY REVIEW

• General hydrocarbon, CnHm (Jet fuel H/C2)
• 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 Mass fuel/air ratio
Stoichiometric Molar fuel/air ratio

• Stoichiometric exactly correct ratio for
  complete combustion

32
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 gt 1 Fuel Rich
• If f lt 1 Fuel Lean

33
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 gt 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 lt 0.5
• What do we do?
• Burn (keep combustion going) near f1 with some
  of ingested air
• Then mix very hot gases with remaining air to
  lower temperature for turbine

34
SOLUTION BURNING REGIONS
Turbine
Air
Primary Zone
f0.3
f 1.0 Tgt2000 K
Compressor
35
COMBUSTOR ZONES MORE DETAILS

• Primary Zone
• Anchors Flame
• Provides sufficient time, mixing, temperature for
  complete oxidation of fuel
• Equivalence ratio near f1
• 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 1.5-1.8

36
COMBUSTOR DESIGN

• Combustion efficiency, hb Actual Enthalpy Rise
  / Ideal Enthalpy Rise
• hheat 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

37
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 (n2 for mixed gas collision
  rate)

38
HIGH FUEL TO AIR PROBLEM / CHALLENGE

• To increase specific thrust, future engines will
  increase overall fuel-air ratios
• JSF, other commercial products affected

Flow Direction
JSF
Compressor
Combustor
Turbine
F119
39
TURBINE COOLING TRENDS

• Thrust and performance increases monotonically
  with turbine inlet temperature, qt
• Isp and hthermal also increase
• Because of associated increase in pc
• STRONG INCENTIVE TO INCREASE qt
• Turbine efficiency decreases
• Blade materials oxidation-resistant, high s,
  such as Nickel and Cobalt based alloys
• Introduction of directionally-solidified and
  single-crystal blade materials

Increase limited by metallurgical progress
Most current advancement due to air-cooling
40
WHERE DOES COOLING AIR COME FROM?
Turbine blades cooled with compressor discharge
air
Other components (burner, liners, disks,
etc.) also cooled with compressor air
41
FILM COOLING BEHAVIOR
42
COOLING STRATEGIES FILM COOLING
43
COOLING STRATEGIES INTERNAL COOLING

• Cooling air is pumped through inside of blades
• Air is pumped in at root and makes multiple
  passes before exiting at root
• Material is cooled by forced convection on inside
  surface and by conduction through blade
• Different regions of blades can have different
  cooling profiles
• Large surface area on inside
• Many designs employ roughened internal microfin
  structure

44
PHENOMENOLOGICAL OVERVIEW
PW229
EMISSIONS INTO TURBINE
EXHAUST MIGRATION
SURFACE HEAT FLUX IMPACT
45
F119-100 1st ROTOR
46
F119-100 1st ROTOR
47
F119-100 1st ROTOR
48
BOAS BLADE OUTER AIR SEAL
49
BLADE OUTER AIR SEAL (BOAS) POST EVENT
50
DETAIL BOAS
51
TURBINE ROTOR BLADE FAILURE (ROLLS-ROYCE)
52
RESEARCH QUESTIONS

• What is impact to turbine surfaces due to
  secondary reactions?
• What is change in surface heat flux due to a
  local reaction over a range of operating
  conditions
• What is influence of blowing ratio, B?
• What is influence of the total fuel content, E?
• What is influence of flow and chemical time
  scales, Da tflow/tchem?
• Etc
• What if you knew answers?
• How do you use this information?
• How to incorporate into a design system
  framework?

53
EXPERIMENTAL INVESTIGATION
Fuel rich air flow
Air-Side Injection
Heat Flux Gauges
Nitrogen-Side Injection
54
EFFECT OF LOCAL REACTIONB 1.0, Da 13, CO
65,000 ppm (Moderate Energy Content)
Downstream
Upstream
25 augmentation over inert side Cooled side
injection agrees to within 10 of literature
values and correlation
55
CFD STUDY B 0.5 (ATTACHED JET) TOTAL
TEMPERATURE CONTOURS Tflame 1840 K
Da lt 1 Maximum Temperature 1200 K, 0 of
potential (cold flow)
A-A
A-A x/D 10
Da gt 1 Maximum Temperature 1715 K, 80 of
potential
Note maximum wall heat release at z/D /- 0.5
x/D 10
56
CFD STUDY B 2.0 (LIFTED JET) TOTAL
TEMPERATURE CONTOURS Tflame 1840 K
Da lt 1 Maximum Temperature 1200 K, 0 of
potential (cold flow)
x/D 10
Da gt 1 Maximum Temperature 1683 K, 75 of
potential
x/D 10
Note maximum wall heat release at z/D 0.0
57
IN-LINE AND STAGGERED HOLE GEOMETRIES
Numerical studies extended to engine conditions
B 1.0, Da 0.3, H 0.54, Qs 0
B 1.0, Da 0.3, H 0.54, Qs 70
Staggered hole (z/D3) at low B (0.5-1.0)
provides good surface protection burning is
kept off-surface, h gt 0.15
58
PERSONAL OBSERVATIONS

• Considering importance of combustion in society,
  it is somewhat surprising that very few engineers
  have more than a cursory knowledge of combustion
  phenomena
• MAE curriculum already packed at undergraduate
  level
• Engineers with some background in combustion may
  find many opportunities to use expertise
• Aside from purely practical motivations for
  studying combustion, subject is intellectually
  stimulating in that it integrates all of thermal
  sciences nicely and brings chemistry into the
  practical realm of engineering

59
RESEARCH EXAMPLES
60
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
  (IVV) of liquid rocket combustion process
• Delta II, Delta IV, and Atlas Rockets

61
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
  (IVV) of solid rocket combustion process

http//utias.utoronto.ca/groth/research_rockets.h
tml
http//monsoon.colorado.edu/toohey/latest.html
62
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.
63
COMBUSTION RESEARCH AT FLORIDA TECH

• Reproduce engine geometries (flow-box, row 1
  vanes via VSS).

64
COMBUSTION RESEARCH AT SIEMENS