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

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

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

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

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

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

HINDENBURG: MAY 6, 1937

ROCKET PROPELLANT

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

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~500-550 ºC Dark red: T~650-750 ºC Bright red: T~850-950 ºC Yellowish red: T~1050-1150 ºC Not quite white: T~1250-1350 ºC White: T > 1450 ºC

4. LAMINAR PREMIXED FLAMES

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

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)

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

DIFFUSION FLAME: EARTH vs. SPACE

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

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

5: DIFFUSION FLAMES

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

7. PULSE DETONATION ENGINES

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."

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.)

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

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

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?

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.

MAJOR COMBUSTOR COMPONENTS Turbine Compressor

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?

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

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

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 (?)

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

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

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

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

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 ~ 1.5-1.8

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:

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

RESEARCH EXAMPLES

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

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 http://utias.utoronto.ca/~groth/research_rockets.html http://monsoon.colorado.edu/~toohey/latest.html

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.

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).

COMBUSTION RESEARCH AT SIEMENS