Combustion 2009 Lecture 3

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Combustion(Lecture 3)

Lecture prepared for course in laserbased combustion diagnostics by Per-Erik Bengtsson and Joakim Bood

What is combustion?

Joakim Bood

Combustion takes place in a flame characterized by:

• Exothermic reactions – Reactants → Products + Energy

• Oxidation processes– Oxygen in air is usually the oxidizer

• High temperatures of the products– Typically above 2000 K

• Radiation – Chemiluminiscence, Planck radiation

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Types of flames

Joakim Bood

Flat flameBunsen flame (followed by a nonpremixed candle for φ>1)

Wood fireCandle

Radiant burners for heatingLaminar

LaminarDiesel engineAircraft turbine

H2/O2 rocket engineTurbulentNonpremixed

(Diffusion)

Spark-Ignited gasoline enginesLow-NOx stationary gas turbineTurbulent

Premixed

ExamplesFluid motionFuel/oxidizer mixing

Premixed flames - Diffusion flames

Per-Erik Bengtsson and Joakim Bood

Fuel + air

Reaction zonePre-heat zone

Product zone

Porous-plug burnerUnburned gas zone

FuelAir Air

Fuel and air is mixed before combustion

Fuel and air burn when they meet

Premixed flames Nonpremixed flames(Diffusion flames)

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Premixed flames

Joakim Bood

• Gaseous fuel and oxidizer are mixed on a molecular level prior to combustion

• Hydrocarbon/air flames have burning velocities around 0.5 m/s

Example: Spark-Ignition Engine

Nonpremixed flames (Diffusion flames)

Joakim Bood

• Fuel and oxidizer are introduced separately and mix during combustion

• Energy release rate limited by mixing process

• Reaction zone between oxidant and fuel zoneExample: Diesel Engine

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Laminar flames

Joakim Bood

• Premixed– e.g. Bunsen flame– Rather low flame velocity

• Nonpremixed (Diffusion)– e.g. candle flame– Fuel: wax, Oxidizer: air– Reaction zone between

wax vapors and airPhoto: Per-ErikBengtsson

Photo: Per-Erik Bengtsson

v cold flow velocityα half the cone angleSL laminar flame speed

(the velocity of a reaction zoneorthogonal to its surface)

Bunsen flame structure

The flame is stationary, thus the following relation is valid:SL =v sinα

Reactionzone

SL is a property of a fuel/oxidant mixture at certain T and p, and it is around 0.5 m/s for hydrocarbon/air mixtures.

α

SL

v

x

© Per-Erik Bengtsson


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The reaction zone moves towards the unburned gas at velocity SL, mainly because H atoms diffuse towards the unburned gas and react with unburned oxygen

H + O2 OH + O

Zones of a premixed flame

Unburned gas zone Preheat zone Reaction zone Product zone

Temperature profile along x

300 K

~2000 K

x v sinα SL Radicals, such as H, OH, and O,are formedhere!



Turbulent flames

Joakim Bood

• Premixed– Fast heat release– Increased flame propagation

rate– e.g. Spark-Ignition Engine

• Diffusion– Can obtain high rates of energy

release per unit volume– Modeling is very complex, no

well established approach– e.g. Diesel Engine


Turbulent diffusion flame

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Spark plug

Flamepropagation

Adiabatic assumptions

Joakim Bood

• No heat losses to the surroundings

• All heat produced by the combustion is available to heat the product gas

• Adiabatic flame temperature may be calculated

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Adiabatic flame temperature

Joakim Bood

• Highest possible temperature that a flame can attain

• Never achieved in practice– No realistic combustion chamber is adiabatic– Dissociation of product lowers temperature

• Useful design parameter– Sets the upper temperature limit of the exhaust

Fuel Laminar flame speed Adiabatic flame[m/s] temperature [K]

AlkanesMethane/air 0.45 2225 Ethane/air 0.47 2260 Propane/air 0.46 2267

AlkenesEthene/air 0.75 2370Propene/air 0.72 2334

AlkynesEthyne/air 1.58 2539

Maximum laminar flame speed


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Fuel + Air

Preheat zone

Reaction zone

Product zone

Sintered porous-plug

Flat flame on porous-plug burner

The flat premixed flame on a porous-plug burner is a proper research flame.

Each height represents a certain time in the combustion process.

The flame in the picture is very fuel-rich and soot is formed in the product zone.



mixturetricstoichiomeinoxygenmolesfuelmoles

mixturerealinoxygenmolesfuelmoles

)#/(#

)/#(#=Φ

StoichiometryStoichiometry expresses the ratio between the fuel and oxidant concentration in a mixture.

The equivalence ratio, Φ, is used to specify this relationship:

The stoichiometric relation for propane combustion:

1 C3H8 + 5 O2 + 18.8 N2 → 3 CO2 + 4 H2O + 18.8 N2

Example: Calculate the equivalence ratio for a mixture with the molar ratio 1:4 between propane and oxygen:

2.15/1

4/1==Φ


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Stoichiometry (2)

The mole fraction of propane:

Xpropane= 0.040 the mixture is stoichiometric

Xpropane< 0.040 the mixture is fuel-lean ⇒ O2 in the exhaust

Xpropane> 0.040 the mixture is fuel-rich ⇒ CO and H2 in the exhaust

040.08.1851

1=

++=propaneX

A stoichiometric hydrocarbon mixture gives a flame that ideally gives the products CO2 and H2O only. For such a flame Φ=1.

The stoichiometric relation for propane combustion:

1 C3H8 + 5 O2 + 18.8 N2 → 3 CO2 + 4 H2O + 18.8 N2


0% 20% 40% 60% 80% 100%

H2 + O2

H2 + air

CH4 + O2

CH4 + air

C2H6 + air

C3H8 + air

n-C4H10 + air

C2H2 + air

Fuel concentration in mixture

Flammability limits

It must be remembered that combustion is always a competition between heat-generating reactions and cooling processes!


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Temperatures in flames

The highest temperature for a premixed hydrocarbon/air flameoften obtained at the slightly rich side of stoichiometric.

Temperature decreases when Φdecreases from around 1, since the heat released also must be used to heat up “surviving”oxygen and increasing amounts of nitrogen.

Temperature in ethane-air flames

0

500

1000

1500

2000

2500

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Equivalence ratio

Tem

pera

ture

/ K


Major species concentrations in product gases

Concentrations in ethane-air flames

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Equivalence ratio

Mol

e fr

actio

n

CO2

H2OWater and carbon dioxide have high concentrations over a large range of equivalence ratios.


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Concentrations in ethane-air flame

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Equivalence ratio

Mol

e fr

actio

n

O2

H2

CO

• Concentrations of CO and H2 increase when Φ is raised above 1.

• Concentration of O2increases when Φ is lowered below 1.

• At Φ =1 CO, H2, and O2have mole fraction above zero due to equilibrium considerations.

Species concentrations in product gases


Concentrations profiles across reaction zone (1)

Per-Erik Bengtsson

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Concentrations across the reaction zone (2)

Per-Erik Bengtsson

Concentration distributions in diffusion flames

The figure illustrates the concentration distribution of major species in a diffusion flame on methane and air.

The figure illustrates the concentration distribution of additional species in a diffusion flame on methane and air.

Per-Erik Bengtsson

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Combustion Chemistry (1)A global reaction is a reaction that shows the reactants and products. Howevernothing is said about how the reactionsoccur on a molecular level. An example is 2 H2 + 1 O2 2 H2O

The question is now how does the reactionbetween hydrogen and oxygen start?

H2+ M H + H + M

The hydrogen molecule and the other molecule must both have a very high energy, i.e. a high velocity, to create the first radicals.

The reaction starts from two molecules colliding and breaking apart. For example, H2 collides with another molecule in the gas, arbitrarily called M.


Combustion Chemistry (2)

Joakim Bood

Rates of chemical reactions

xAA + xBB + …. → xPP + xQQ + ….

Rate law:[ ] [ ] [ ] [ ] [ ] [ ]ba

QPBA

BAkdtQd

xdtPd

xdtBd

xdtAd

x===

−=

− 1111

Rate constanta: reaction order with respect to species Ab: reaction order with respect to species Ba + b: overall order of reaction

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Combustion Chemistry (3)

Joakim Bood

Temperature dependence of the rate constant

RTEAk /exp−=

( )RTEAk /exp −= Arrhenius equation (two-parameter repr.)

A: Pre-exponential factor, E: Activation energy, R: ideal gas const, T: Temp

ExperimentCH4 + OH

Three-parameter Arrhenius expressionfitted to the measured data:

( )RTETAk n /exp' −=

Combustion chemistry (4)

A detailed chemical mechanism for hydrogen combustion contains 19 reactions.

Per-Erik Bengtsson & Joakim Bood

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Combustion of methane:

1 CH4 + 2 O2 → 1 CO2 + 2 H2O

How does combustion proceed?

Methane oxidation mechanism:

Per-Erik Bengtsson

Full methane mechanism

149 reactions

Per-Erik Bengtsson

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Summary: Premixed flames

Per-Erik Bengtsson

0

5E+10

1E+11

1.5E+11

2E+11

2.5E+11

3E+11

3.5E+11

4E+11

4.5E+11

400 800 1200 1600 2000 2400 2800

Wavelength (nm)

Sign

al in

tens

ity (W

/m3 )

T=1600KT=2000K

Visible spectral range

Planck radiation



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The blue-green emission from flames

The blue-green emission from the reaction zone has its origin in radicals that have been produced in an excited electronic state from chemical reactions, so-called chemi-luminescence.

CH contributes in the blue spectral region, and C2contributes in the blue and green spectral regions.

UV Visible


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Combustion 2009 Lecture 3