what is combustion?arrow.utias.utoronto.ca/~ogulder/classnotes0.pdf · what is combustion? •...

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

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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.

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• 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:

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Thermodynamics:

• Stoichiometry

• Properties of gases and gas mixtures

• Heat of formation

• Heat of reaction

• Equilibrium

• Adiabatic flame temperature

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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.

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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.

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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)

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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

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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

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• 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

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• 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,...

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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

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Burned

Unburned

- Cross-section of a gasoline engine combustionchamber.

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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