what is combustion?arrow.utias.utoronto.ca/~ogulder/classnotes0.pdf · what is combustion? •...
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What is Combustion?• Combustion is a key element of many of modern
society’s critical technologies.• Combustion accounts for approximately 85 percent
of the world’s energy usage and is vital to ourcurrent way of life.
• Spacecraft and aircraft propulsion, electric powerproduction, home heating, ground transportation,and materials processing all use combustion toconvert chemical energy to thermal energy orpropulsive force.
0.Introduction 1 AER 1304–OLG
Examples of combustion applications:• Gas turbines and jet engines• Rocket propulsion• Piston engines• Guns and explosives• Furnaces and boilers• Flame synthesis of materials (fullerenes, nano-
materials)• Chemical processing (e.g. carbon black produc-
tion)• Forming of materials• Fire hazards and safety
0.Introduction 2 AER 1304–OLG
Combustion is a complex interaction of:
• physical processes
- fluid dynamics, heat and mass transfer
• chemical processes
- thermodynamics, and chemical kinetics
Practical applications of the combustion phenomenaalso involve applied sciences such as aerodynamics,fuel technology, and mechanical engineering.
0.Introduction 3 AER 1304–OLG
• The transport of energy, mass, and momentum arethe physical processes involved in combustion.
• The conduction of thermal energy, the diffusion ofchemical species, and the flow of gases all followfrom the release of chemical energy in the exother-mic reaction.
• The subject areas most relevant to combustion inthe fields of thermodynamics, transport phenom-ena, and chemical kinetics can be summarized asfollows:
0.Introduction 4 AER 1304–OLG
Thermodynamics:
• Stoichiometry
• Properties of gases and gas mixtures
• Heat of formation
• Heat of reaction
• Equilibrium
• Adiabatic flame temperature
0.Introduction 5 AER 1304–OLG
Heat and Mass Transfer:• Heat transfer by conduction• Heat transfer by convection• Heat transfer by radiation• Mass transfer
Fluid Dynamics:• Laminar flows• Turbulence• Effects of inertia and viscosity• Combustion aerodynamics
0.Introduction 6 AER 1304–OLG
Chemical Kinetics:• Application of thermodynamics to a reacting system
gives us- equilibrium composition of the combustion
products, and- maximum temperature corresponding to this
composition, i.e. the adiabatic flame tempera-ture.
• However, thermodynamics alone is not capable oftelling us whether a reactive system will reach equi-librium.
0.Introduction 7 AER 1304–OLG
Chemical Kinetics (cont’d):• If the time scales of chemical reactions involved in
a combustion process are comparable to the timescales of physical processes (e.g. diffusion, fluidflow) taking place simultaneously, the system maynever reach equilibrium.
• Then, we need the rate of chemical reactions in-volved in combustion.
0.Introduction 8 AER 1304–OLG
Primary sources of combustion research literature:1 Combustion and Flame (journal)2 Combustion Science and Technology (journal)3 Combustion Theory and Modelling (journal)4 Progress in Energy and Combustion Science (re-
view journal)5 Proceedings of the Combustion Institute (Biennial
Combustion Symposia (International) proceedings).6 Combustion, Explosions and Shock Waves (journal
translated from Russian)
0.Introduction 9 AER 1304–OLG
Fundamental DefinitionsChemical Reaction:• exchange and/or rearrangement of atoms between
colliding molecules
CO+H2O→ CO2 +H2
Reactants → Products• The atoms are conserved (C, H, O)• On the other hand, molecules are not conserved.
H2 + 0.5(O2 + 3.76N2)→ H2O+ 1.88N2Reactants → Products
0.Introduction 10 AER 1304–OLG
Amount of substance or mole numbers (mol):• 1 mol of a compound corresponds to 6.023 · 1023
particles (atoms, molecules, or any chemicalspecies).
• Avogadro’s constant = 6.023 · 1023• Mole fraction χi of species i with mole number ofNi is
χi =NiSj=1Nj
0.Introduction 11 AER 1304–OLG
• Mass fraction Yi of species i with mass of mi is
Yi =mi
Sj=1mj
• Molar or Molecular Mass, Mi (molecular weightis misleading and should not be used)
- MCH4 = 16 g/mol
- MH2 = 2 g/mol
- MO2 = 32 g/mol
0.Introduction 12 AER 1304–OLG
• Mean molar mass, M , of a mixture of species de-notes an average molar mass:
M = χiMi
• S = number of species in the system
Yi =MiNiSj=1MjNj
=MiχiSj=1Mjχj
χi =Yi
MiM=
Yi/Mi
Sj=1 Yj/Mj
0.Introduction 13 AER 1304–OLG
For a system of volume, V :
• Mass density (density), ρ = m/V (kg/m3)
• Molar density (concentration), c = N/V (kmol/m3)
• Mean molar mass is given by:ρ
c=m
N=M
Chemical kinetics convention: concentrations c ofchemical species are usually shown by species symbolin square brackets.
cCO2 = [CO2]
0.Introduction 14 AER 1304–OLG
For most conditions involved in combustion, it issatisfactory to use the perfect gas equation of statefor the gas phase.
PV = NRoT
(Pa)(m3) = (mol)(J/molK)(K)
Ro = 8.314 J / mol K, universal gas constant
P = pressure, Pa
T = temperature, K
0.Introduction 15 AER 1304–OLG
When the gas phase temperatures are near or lessthan the critical temperatures, or when pressures arenear or above the critical pressures, the density or con-centration is not correctly predicted by the perfect gasrelationship. Real gas equations should be used.
- van der Waals
- Peng-Robinson
0.Introduction 16 AER 1304–OLG
Basic Flame Types:• Premixed Flames
- Laminar- Turbulent
• Non-Premixed (Diffusion) Flames- Laminar- Turbulent
• Partially Premixed Flames- Laminar- Turbulent
♠ triple flames, edge flames,...
0.Introduction 17 AER 1304–OLG
Laminar (Turbulent) Premixed Flames:
• Fuel (in gaseous form) and oxidizer are homoge-neously mixed before the combustion event
• Flow is laminar (turbulent)
• Turbulent premixed flames:
- combustion in gasoline engines
- lean-premixed gas turbine combustion
0.Introduction 18 AER 1304–OLG
Burned
Unburned
- Cross-section of a gasoline engine combustionchamber.
0.Introduction 19 AER 1304–OLG
Stoichiometry:
• A premixed flame is stoichiometric if the premixedreactants contain right amount of oxidizer to con-sume (burn) the fuel completely.
• If there is an excess of fuel: fuel-rich system
• If there is an excess of oxygen: fuel-lean system
• Standard air composition commonly used for com-bustion calculations:
O2 + 3.762N2
0.Introduction 20 AER 1304–OLG
Stoichiometry (cont’d):
C3H8 + 5(O2 + 3.762N2)→4H2O+ 3CO2 + 18.81N2
• (A/F )stoich=air-to-fuel ratio (mass)= (mass ofair)/(mass of fuel)
• (A/F )stoich=[5(32+3.762*28)]/(44) = 15.6
• Φ = (A/F )stoich/(A/F )actual = Fuel EquivalenceRatio
0.Introduction 21 AER 1304–OLG
Stoichiometry (cont’d)::
• Φ = 1: stoichiometric combustion
• Φ < 1: lean mixture, lean combustion
• Φ > 1: rich mixture, rich combustion
• European convention (and to a certain extentJapanese) is to use Air equivalence ratio, λ:
λ = 1/Φ
• In certain industries, excess air ratio, excess oxygen,and similar terminologies are also used.
0.Introduction 22 AER 1304–OLG
Laminar (Turbulent) Non- Premixed Flames:
• Fuel (in gaseous form) and oxidizer are mixed/comein to contact during the combustion process
• A candle flame is a typical laminar non-premixed(diffusion) flame
• Turbulent non-premixed flames:- hydrogen rocket engine- current aero gas turbines- diesel engines
0.Introduction 23 AER 1304–OLG
A candle flame.
0.Introduction 24 AER 1304–OLG
EXHAUSTEMISSIONS
INJECTION AND SPRAYCHARACTERISTICS
FUEL-AIRMIXING
PROCESSIGNITION
Air InletInlet Port DesignChamber Design
Turbocharge
AIR MOTION / TURBULENCEIN THE
COMBUSTION CHAMBER
Fuel Properties
MOSTLYNON-PREMIXEDCOMBUSTION
PARTIALLY"PREMIXED"
COMBUSTION
Injection TimingInjection System Design
Injection DurationInjection Rate
EGR
HEAT RELEASERADIATION EXCHANGE BETWEENHOT AND COLD POCKETSNOX & SOOT FORMATIONSOOT OXIDATION
Processes in the diesel engine combustion.
0.Introduction 25 AER 1304–OLG
Spark-ignited gasoline engineLow-NOx stationary gas turbine
Flat flameBunsen flame
Aircraft turbineHydrogen-oxygen rocket motorDiesel enginePulverized coal combustion
Candle flameRadiant burners for heatingWood fire
PREMIXED
NON-PREMIXED(DIFFUSION)
TURBULENT
TURBULENT
LAMINAR
LAMINAR
FUEL/OXIDIZERMIXING
FLUIDMOTION EXAMPLES
Examples of combustion systems.
0.Introduction 26 AER 1304–OLG