FKM Combustion Processmohsin/sme4483/intro/sme4483... · A Chigier, Combustion Aerodynamics ,...

100
I I I I I I N N N N N N T T T T T T R R R R R R O O O O O O D D D D D D U U U U U U C C C C C C T T T T T T I I I I I I O O O O O O N N N N N N Combustion Process Combustion Process - - Mazlan Mazlan 2005 2005 UTM FKM FKM UNIVERSITI TEKNOLOGI MALAYSIA UNIVERSITI TEKNOLOGI MALAYSIA LECTURERS: 1. DR. MAZLAN ABDUL WAHID http://www.fkm.utm.my/~mazlan 2. EN. MOHSIN SIES http://www.fkm.utm.my/~mohsin TEXT BOOK: Introduction to Combustion Stephen R. Turns, An Introduction to Combustion, 2nd Edition, McGraw-Hill, 2000. SME 4483 SME 4483 Combustion Process Combustion Process Semester 2009/2010 - I

Transcript of FKM Combustion Processmohsin/sme4483/intro/sme4483... · A Chigier, Combustion Aerodynamics ,...

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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA

LECTURERS:1. DR. MAZLAN ABDUL WAHID

http://www.fkm.utm.my/~mazlan

2. EN. MOHSIN SIEShttp://www.fkm.utm.my/~mohsin

TEXT BOOK:

Introduction to Combustion

Stephen R. Turns, An Introduction to Combustion, 2nd Edition, McGraw-Hill, 2000.

SME 4483SME 4483

Combustion Process Combustion Process

Semester 2009/2010 - I

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Lecture HoursLecture HoursMondays: 12 – 12.50 pmWednesdays: 9 – 9.50 am

Thursdays: 1 – 1.50 pm

Prerequisites Prerequisites Undergraduate Thermodynamics

Grading Grading 25% 1st Test

25% 2nd Test10% Homework Assignments

20% Project 1

20% Project 2

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Homework:Homework:

Homework problems will be assigned periodically. You are required to solve the assigned homework problems.

Textbook:Textbook:

Stephen R. Turns, An Introduction to Combustion, 2nd Edition,

McGraw-Hill, 2000.

Suggested Reading:Suggested Reading:1. G. L. Borman, and K. W. Ragland, Combustion Engineering,

McGraw-Hill, 1998.2. A. M. Kanury, Introduction to Combustion Phenomena,

Gordon & Breach, 1975.3. J. M. Beér, and N. A Chigier, Combustion Aerodynamics,

Applied Science, 1972.4. K K Kuo, Principles of Combustion, Wiley, 2005.

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1 Introduction to Combustion2 Thermodynamics of Combustion Processes3 Fuels3 Chemical Kinetics4 Premixed Combustion 5 Non-premixed Combustion (Diffusion Flames)6 Detonation7 Pollutant Emissions

Course contentsCourse contents

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I. Introduction

Introduction to the nature and scope of combustion, definition of combustion, combustion mode and flame type

II. Thermodynamics of Combustion Processes

Treatment of first law of thermodynamics related to combustion process, enthalpies of formation, the em phasis of the importance of chemical equilibrium to combus tion. Use of software to calculate complex equilibrium for co mbustion gases.

III. Fuel

Type and classifications of fuels

Tentative details of course contents:Tentative details of course contents:

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IV. Chemical Kinetics

Deal with chemical kinetics of combustion by presen ting basic concepts, elementary and global reactions, ch emical mechanism importance to combustion and pollutant formation reactions

V. Premixed Combustion

Describe the essential characteristics of premixed flames and developed simplified analysis of these flames. To investigate factors that influence flame speed, fla me structure and flame stabilization .

VI. Nonpremixed Combustion (Diffusion Flames)

Investigate type of flame that widely used in indus tries, importance of flame geometry in combustor design, parameters that control flame size and shape.

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VII. DetonationVII. DetonationVII. DetonationVII. Detonation

The Phenomena of detonation, its mechanism and its The Phenomena of detonation, its mechanism and its The Phenomena of detonation, its mechanism and its The Phenomena of detonation, its mechanism and its characterizationcharacterizationcharacterizationcharacterization

VIII. Pollutant EmissionsVIII. Pollutant EmissionsVIII. Pollutant EmissionsVIII. Pollutant Emissions

Introduction to the quantification of emissions as well as Introduction to the quantification of emissions as well as Introduction to the quantification of emissions as well as Introduction to the quantification of emissions as well as discussing the mechanisms of pollutant formation and their discussing the mechanisms of pollutant formation and their discussing the mechanisms of pollutant formation and their discussing the mechanisms of pollutant formation and their control.control.control.control.

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Rapid oxidation of a fuel accompanied by the release of heat and/or light together with the formation of combustion products

Fuel + oxidant heat/light(thermal energy) +combustion products

Definition of combustion as quoted from Webster’s dictionary

“ rapid oxidation generating heat, or both heat and light ; also, slow oxidation accompanied by relatively little heat and no light”

What is Combustion?What is Combustion?

1. Introduction to Combustion

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• 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 our current way of life.

• Nation’s electrical needs are met mostly from combustion

• Transportation system relies almost entirely on combustion

• Spacecraft and aircraft propulsion, electric power

production, home heating, ground transportation,

and materials processing all use combustion to

convert chemical energy to thermal energy or

propulsive force.

•Energy crisis - alternative energy

•Pollution is a major issue

Some facts about combustionSome facts about combustion

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Examples of combustion applications:Examples of combustion applications:

• Gas turbines and jet engines

• Rocket propulsion

• Piston engines

• Guns and explosives

• Furnaces and boilers

• Flame synthesis of materials (fullerenes, nanomaterials)

• Chemical processing (e.g. carbon black production)

• Forming of materials

• Fire hazards and safety

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Chemical kineticsChemical kinetics

Combustion is a complex interaction ofCombustion is a complex interaction of

ThermodynamicsThermodynamics

Heat and mass transferHeat and mass transfer

Fluid dynamicsFluid dynamics

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• Physical processes

• fluid dynamics, heat and mass transfer, and thermodynamics (transport of energy, mass and momentum)

• Chemical processes

• chemical kinetics

• Fuel technology

• Conduction of thermal energy, the diffusion of chem ical species, and the flow of gases all follow from the release of chemical energy in the exothermic reaction.

• Practical applications of the combustion phenomena also involve applied sciences such as aerodynamics, fuel technology, and mechanical engineering.

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The complex interaction of various fields in The complex interaction of various fields in

combustion processes can be summarized as follows:combustion processes can be summarized as follows:

Thermodynamics: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 and Mass Transfer:

�Heat transfer by conduction

�Heat transfer by convection

�Heat transfer by radiation

�Mass transfer

Fluid Dynamics:Fluid Dynamics:

�Laminar flows

�Turbulence

�Effects of inertia and viscosity

�Combustion aerodynamics

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Chemical Kinetics: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 temperature.

�However, thermodynamics alone is not capable of telling us whether a reactive system will reach equilibrium.

�If the time scalestime scales of chemical reactionschemical reactions involved in a combustion process are larger thanlarger than the time scales of physical processesphysical processes (e.g. diffusion, fluid flow), the system the system may never reach equilibriummay never reach equilibrium .

�Then, we need the rate of chemical reactions involved in combustion.

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Fuel Fuel

OxygenOxygen IgnitionIgnition

(air) (energy)Combustion Triangle

Basic Requirements for CombustionBasic Requirements for Combustion

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Combustion modeCombustion mode

FlameFlame

Premixed

Laminar Turbulent

Non-premixed

Laminar Turbulent

NonNon--flameflame

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Primary sources of combustion research Primary sources of combustion research

literature:literature:

1. Combustion and Flame (journal)

2. Combustion Science and Technology (journal)

3. Computational and Theoretical Combustion (journal)

4. Progress in Energy and Combustion Science (review 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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•• Premixed flamesPremixed flames

– Fuel and air are mixed prior to chemical reaction

(mixing done at at molecular level)

•• NonNon--premixed (diffusion) premixed (diffusion)

flamesflames

– Fuel and air are initially separated, reaction occurs only at the interface between the fuel and the air, where mixing and reaction both take place

fuel + air

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Combustion of fossil fuelCombustion of fossil fuel

Chemical reaction between hydrogen and carbon atoms (contained in the fuel) with oxygen atoms (usually comes from the air), resulting in the heat release and the formation of combustion products(* )

(*) mainly water vapor and carbon dioxide and a certain amount combustion by-products depending on combustion process

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Simplified Main Processes of CombustionSimplified Main Processes of Combustion

Carbon + Oxygen heat + carbon dioxide

( C + O2 Heat + CO2 )

Hydrogen + Oxygen Heat + Water

( H2 + O Heat + H2O )

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Combustion Diagram

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The combining of oxygen in the air and carbon in the fuel to form carbon dioxide and generate heatis a complex process, requiring the right

•mixing turbulence,

•sufficient activation temperature

•enough time for the reactants to come into contact and combine.

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Combustion ByCombustion By--ProductsProducts

Carbon monoxide (CO)Aldehydes mainly due to incomplete Unburned Fuel combustionRadicals

Oxides of nitrogen (NOx) – reaction between O2 (in air) and nitrogen (present in air or fuel)

Oxides of sulphur (SOx) – only for Sulphur-containing fuel

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•Unless combustion is properly controlled, high concentrations of undesirable products can form. Carbon monoxide (CO) and soot, for example, result from poor fuel and air mixing or too little air.

•Other undesirable products, such as nitrogen oxides (NO, NO2), form in excessive amounts when the burner flame temperature is too high.

•If a fuel contains sulfur, sulfur dioxide (SO2) gas is formed. For solid fuels such as coal and wood, ash forms from incombustible materials in the fuel.

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Requirements for Successful CombustionRequirements for Successful Combustion

FuelAir

Ignition

Correct Amount of

Fuel and Air

MolecularlyMixed

Fuel andAir

MinimumIgnitionEnergy

(Temperature)

Residence time

LaminarTurbulent

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Categories of Combustion Process

• Stoichiometric Combustion

• Excess Air or Oxygen Combustion

(Fuel Lean Combustion)

• Excess Fuel Combustion

(Fuel Rich Combustion)

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Stoichiometric Combustion

Relative (chemically-correct) proportion of fuel and air quantities that are the theoretical minimum needed to give complete/perfect combustion (i.e., no unburned fuel and residual oxygen present in combustion products)

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Stoichiometric Combustion of Methane

CH4 + 202 (+ ignition) = C02 + 2H20 (+ heat)

means that

1 mole of methane to be proportionately (and molecularly) mixed with 2 moles of oxygen to produce 1 mole of

carbon dioxide and 2 moles of water vapor

or1 cubic meter (m3) of methane requires 2 cubic meter (m3)

of oxygen for complete combustion and will produce 1 cubic meter (m3) of carbon dioxide and 2 cubic meter

(m3) of water vapor

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Excess Air or Oxygen Combustion

Combustion of fuel-lean mixture

When oxygen or air is supplied more than the stoichiometric proportion

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Excess Air Combustion of Methane

CH4 + 302 (+ ignition) = C02 + 2H20 + 02 (+ heat)means that

1 mole of methane to be molecularly mixed with 3 moles of oxygen to produce 1 mole of carbon dioxide, 2 moles of

water vapor and 1 mole of un-reacted oxygen

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Excess Fuel Combustion

Combustion of fuel-rich mixture

When fuel is supplied more than the stoichiometric proportion

Insufficient amount of oxygen or air available to burn in the fuel-rich mixture caused incomplete combustion

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Excess Fuel Combustion of Methane

CH4 + 02 (+ ignition) = C0 + 2H20 (+ heat) + (other products of incomplete combustion )

means thatmeans that1 mole of methane to be molecularly mixed with 1 mole of

oxygen to produce 1 mole of carbon monoxide, 2 moles of water vapor and other products of incomplete combustion

such as unburned fuel, aldehydes etc.

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Combustion Air Requirement

Theoretical Oxygen (air)

(Chemically-corrected amount of oxygen (air) required for complete combustion for a given quantity of a

specific fuel)Excess Air

(Sum of all primary and secondary air needed for perfectly complete the combustion of a specific fuel)

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Fossil Fuel Combustion

Gas CombustionLiquid CombustionSolid Combustion

• Successful combustion of fossil fuels requires all criteria as previously discussed

• Methods of combustible mixture preparation are of great importance not only for successful combustion but also to industrial applications

• Solid, liquid and gaseous fuels have different combustible mixture preparation mechanisms and hence combustion characteristics

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Gaseous fuel burner

• Unlike liquid burners, as the fuels are already in thegaseous form, gas burners require only good air/fuel mixing (either in a premixer or in the combustion chamber), before they can be burned

• The characteristics of the flames produced are largely dependent on both the fuel and the rate at which mixing can be achieved

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Liquid fuel burner combustible mixture preparation

atomization process

large drops of oil are broken up or atomized into small

droplets

vaporisation process

small fuel droplets are then vaporized to be in gaseous state vaporized/atomized

mixing process

Vaporized / unvaporized fuel droplets are then mixed with

combustion air to form combustible mixture

The above processes take place at very short time (order of millisecond) and may be simultaneous

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Solid fuel burner

Pulverized fuel particles(pulverization is analogous to atomization process)

Devolatilization (analogous to the vaporization process for the liquid fuel)

Gas phases fuels(CO, O2, CO2 etc.)

Solid phase fuel(char particle –

carbon)

Oxygen (Air)

Mixing / diffusion

Homogeneous reaction Heterogeneous reactionGas-gas reaction gas-solid / gas-liquid reaction

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Excess Air Requirement

Solid fuelSolid fuel highhigh

Liquid fuelLiquid fuel moderatemoderate

Gas fuelGas fuel lowlow

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Combustion calculations

Fuel properties

Mass balanceEnergy balance

Combustion productsCombustion efficiency

Thermal efficiencyExcess air requirement

Air-fuel ratioFlame temperature

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What is Flame?What is Flame?

A self-sustaining propagation of a localizedlocalized (* )

combustion zone at subsonicsubsonicor supersonicsupersonicvelocities

(*) Flame occupies only a small portion of the combustible Flame occupies only a small portion of the combustible mixture at any one timemixture at any one time

Combustion wave that travels sub-sonicallyrelative to the speed of sound in the unburned combustible mixture is known as deflagration

Combustion wave that travels super-sonically relative to the speed of sound in the unburned combustible mixture is known as detonation

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Flame stabilityFlame stability

•Flame shape: combined effects of•Velocity profile•Heat losses to the tube wall

•For the flame to remain stationary:Flame speed must equal the speed of normal component of unburned gas at each location

FLAME SPEED = SPEED OF NORMAL COMPONENT OF GAS FLOW

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Flame stabilityFlame stability

A free burning flame is said to be stable if there is no flash back or blow off , i.e.

the normal velocity of he mixture (Vu,n) is vectorically equal and opposite to the velocity of fuel-air mixture at the flame front (V u)

•The normal velocity of flame propagation , Vu,n ,depends upon the–type of fuel–composition and temperature of fuel-air mixture–burner tube diameter

•The temperature of fuel-air mixture at the burner tip depends on the heat transfer from the reaction zone, heat loss to the surrounding and size, shape and material of construction of burner wall.

•The average gas velocity, Vu, depend upon–desired flow rate–burner nozzle diameter

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Laminar premixed flamesLaminar premixed flames

�Reactants are completely mixed on a molecular level prior to ignition and combustion. �Kinetically controlled and the rate of flame propag ation, called the burning velocity.�Dependent upon chemical composition and rates of ch emical reaction. �Safety reasons: Less applied (i.e.flashback and blo w-off) and stability

Lean Premixed flame

Open Tip

Lean Premixed flame

Closed Tip

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The Bunsen flameThe Bunsen flame

Typical example of premixed flames.

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The typical Bunsen-burner flame is a dual flame:

•Inner Flame - a fuel rich premixed inner flame•Outer Flame – surrounds inner flame – also called

secondary diffusion flameThe secondary diffusion flame results when the carbon monoxide product from the rich inner flame encounters the ambient air. Luminous zone is that portion where most of the reaction takes place and therefore it’s the hottest. The temperature at the tip of the primary flame can reach about 1,500ºC (2,700º F)

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•With an excess of air, the reaction zone appears blue. This blue radiation results from excited CH radicals in the high-temperature zone. When the air is decreased to less than stoichiometric proportions, the flame zones appears blue-green, now as a result of radiation from excited C2. OH radicals also contribute to the visible radiation

•If the flame is made much richer, soot will form. The flame can be seen as bright yellow to dull orangeemission, depending on the flame temperature.

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Nonpremixed flames

•Reactants mix by diffusion into a thin flame zone, and reaction rates are diffusion controlled. •They are preferred in industrial practice, gas turbines, internal combustion engines. Safer since the fuel and oxidant are kept separate and flexibility in controlling flame size and shape and combustion intensity

Laminar nonpremixed (diffusion) flamesLaminar nonpremixed (diffusion) flames

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Nonpremixed (diffusion) flame Nonpremixed (diffusion) flame -- Candle flameCandle flame

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Laminar nonpremixed (diffusion) flamesLaminar nonpremixed (diffusion) flames

•Diffusion flames (either laminar or turbulent) are characterized as combustion state controlled by mixing phenomena, i.e. molecular or turbulent diffusion of fuel into oxidizer (i.e. air) or vice versa until some flammable mixture ratio is reached

•Mixing is slow compared with reaction rates…so mixing controls the burning rate

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•Shape of laminar jet flame depends on the mixture strength, i.e.quantity of air supplied

•If fuel is admitted into a large volume of quiescent air, over-ventilated type of diffusion flame is formed

•If excess fuel or air supply is reduced below an initial mixture strength of stoichiometric, a bell or fan shaped under-ventilated flame is formed.

Laminar nonpremixed (diffusion) flamesLaminar nonpremixed (diffusion) flames

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Almost all Almost all flamesflames used in used in practical combustionpractical combustion devices are devices are turbulentturbulent because because turbulent mixing increases burning ratesturbulent mixing increases burning rates , , allowing more power/volumeallowing more power/volume

Even without forced flow, if the Even without forced flow, if the GrashofGrashof number number is larger than is larger than about 10about 10 66 turbulent flow will exist due to buoyancyturbulent flow will exist due to buoyancy

Examples

��Premixed turbulent flamesPremixed turbulent flames

»»GasolineGasoline --type (spark ignition, premixedtype (spark ignition, premixed --charge) internal charge) internal combustion enginescombustion engines

»»Stationary gas turbines (used for power generation, not Stationary gas turbines (used for power generation, not propulsion)propulsion)

��Nonpremixed turbulent flamesNonpremixed turbulent flames

»»DieselDiesel --type (compression ignition, nonpremixedtype (compression ignition, nonpremixed --charge) charge) internal combustion enginesinternal combustion engines

»»Gas turbinesGas turbines

»»Most industrial boilers and furnacesMost industrial boilers and furnaces

Turbulent CombustionTurbulent Combustion

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Turbulent premixed flamesTurbulent premixed flames

(b) Schlieren image of (a) reveling its turbulent nature

(a) Premixed flames

Stiochiometric mixture of natural gas and air , Re = 3000

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Premixed conical flame

Turbulent premixed flamesTurbulent premixed flames

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Turbulent premixed flamesTurbulent premixed flames

An experimental setup of fan stirred bomb facility at LeedsUniversity to study the premixed turbulent flame of various mixtures

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Glassman (1996)

Turbulent nonpremixed flamesTurbulent nonpremixed flames

The transition from laminar to turbulent diffusion (nonpremixed) flames

Laminar diffusion flame is converted into the turbulent type by increasing the gas velocity beyond a critical value of cold Reynolds number depending on fuel and quantity of primary air.

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Turbulent nonpremixed flamesTurbulent nonpremixed flames

Reaction zone

TemperatureFuel

concentrationProduct

concentration

2000K

300K

Distance from reaction zone Convection-diffusion zone

Oxygen concentration

300K

Nonpremixed flames establish themselves at the interface betweenfuel and oxidizer; the flame is sustained by diffusion on each side. The flame does not propagate and moves only as fuel and air are convected by, sometimes turbulent, fluid motion.

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Pool FiresPool Fires

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Wild forest firesWild forest fires

Alaska forest fires Canada forest fires

US wild forest fire

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Pyrolysis Flaming fire

Yellowstone Park forest fire (USA)

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Solid combustionSolid combustion

Paper combustion

Flame of burning tree bark

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Solid coal particles on flameSolid coal particles on flame

Flat flame burner – propane gas with coal particles

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Flame instabilityFlame instability

Cellular flame

Cellular structure of flames caused by instability

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Polyhedral flamesPolyhedral flames

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Spray combustionSpray combustion

Schematic of diesel simulation facility

Diesel spray ignition in the combustion facility

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Flame of liquid heptanes spray NASA spray jet

NASA spray flames – luminous top NASA spray flames – highly turbulent

Spray flamesSpray flames

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Spray combustion main terminologySpray combustion main terminology

Primary atomization/break-up - Break-up of the liquid core into liquid ligaments.

Secondary atomization/break-up - Break-up of liquid droplets into smaller droplets.

Droplet evaporation - Evolutions of droplet diameters and temperatures due to due to mass- and heat exchange in the course of phase transition.

Turbulent diffusion/dispersion - Random motion of droplets.

Turbulent modulation - Generation and change, modulation, of turbulent properties due to liquid/air interaction.

Turbulent combustion – Heat release due to chemical reactions complicated by small scale flow-field fluctuations.

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Spray AtomizationSpray Atomization

Normal air at 25 C Pre-heated air at 185 C Steam at 195 C

Observed Features of Spray with Three Different Atomization Gases

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Drop Collision/CoalesceDrop Collision/Coalesce

Binary Collision of Droplets

Drop collision and coalescence phenomena become important in dense sprays. In the event of particle collision, the time for a particle to respond to the local aerodynamic field is important.

Stretching separation collision of two unequalStretching separation collision of two unequal--size droplets at size droplets at WeWe = 52= 52..

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Possible outcomes of a binary collision as We number is increased:

a) coalescence,

b) collision followed by break-up,

c) shattering,

Droplet collisions may result in a) droplet coalescence, b) grazing, and c) shattering, depending on the relative velocity of the colliding droplets. In grazing collisions, a droplet formed immediately breaks up into two big droplets and many small ones.

Possible outcomes of a binary droplets collisionPossible outcomes of a binary droplets collision

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Droplet combustionDroplet combustion

Spray combustion lead to the study of individual droplet, or called – droplet combustion

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�In combustion systems swirling flows help to increase burning intensity through enhance mixing and higher residence time.�With strong swirl, the centrifugal forces and induced pressure gradients generate a toroidal vortex type of recirculation zone help in stabilizing the high intensity combustion process

��In reacting (combustion system): ExamplesIn reacting (combustion system): ExamplesInternal combustion engines, gas turbines and industrial furnaInternal combustion engines, gas turbines and industrial furna cesces

Swirl combustionSwirl combustion

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Swirling flameSwirling flame

Injector burner with a swirling

propane flame

Swirling (lifted) flame

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Rotating FlamesRotating Flames

Premixed flames: φφφφ = 0.59, V = 43 cm/s, ωωωω = 3240 rpm

LOW VELOCITY JET FLAME

(b) Rotating with the speed of 3240 rpmFlame buckles – forming Cusp Flame Shape

(a) StationaryOpen tip flame

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Rotating flameRotating flame

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Detonation and deflagrationDetonation and deflagration

A detonation is defined as a combustion wavepropagating at supersonic velocity relative to the unburned gas immediately ahead of the flame, i.e., the detonation velocity, D, is larger than the speed of sound, C, in the unburned gas.

In simple terms, a detonation wave can be described as a shock wave immediately followed by a flame(ZND theory). The shock compression heats the gas and triggers the combustion. However, an actual detonation wave is a three-dimensional shock wave followed by the reaction zone.

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• Condensed (or high) explosives generate very high pressures, ~106 psi

• Extremely high pressures generate extremely destructive shock waves

• Detonation velocities ~8,000 to 9,000 m/sec - that’s 20,000 mph!

• Detonation velocity and pressure maintained in the HE only

• Shock velocity degrades after leaving HE

Detonation initiation Expanding fireball Dissipating shock wave

Condensed Phase or Condensed Phase or ““High High

ExplosivesExplosives””

“High Explosives” normally refer to condensed phase materials (solids, liquids and mixtures)

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Buoyancy effect on flamesBuoyancy effect on flames

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Buoyancy effect on flamesBuoyancy effect on flames

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Buoyancy effect on flamesBuoyancy effect on flames

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External effects on flamesExternal effects on flames

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Pollutant emissionsPollutant emissions

��Description of pollutantsDescription of pollutants

��NONOxx

��SootSoot

��COCO

��Unburned hydrocarbons (UHC)Unburned hydrocarbons (UHC)

��Emissions are a Emissions are a NONNON--EQUILIBRIUM PROCESSEQUILIBRIUM PROCESS ””

��If we follow two simple rules:If we follow two simple rules:

��Use lean or stoichiometric mixturesUse lean or stoichiometric mixtures

��Allow enough time for chemical equilibrium to occurAllow enough time for chemical equilibrium to occur as the as the products cool downproducts cool down

��…… then NO, CO, UHC and C(s) (soot) are then NO, CO, UHC and C(s) (soot) are practically zeropractically zero

��So the problem is that we are So the problem is that we are not patient enoughnot patient enough (or unable to (or unable to allow the products to cool down slowly enough)!allow the products to cool down slowly enough)!

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Soot formation Soot formation -- what is soot?what is soot?•Soot is good and bad news

–Good: increases radiation in furnaces

–Bad: radiation & abrasion in gas turbines, particles in atmosphere

•Typically C8H1 (mostly C)

•Structure mostly independent of fuel & environment

–Quasi-spherical particles, 105 - 106 atoms (100 - 500 Å), strung together like a “fractal pearl necklace”

–Each quasi-spherical particle composed of many (~104) slabs of graphite (chicken wire) carbon sheets, randomly oriented

•Quantity of soot produced highly dependent on fuel & environment

•Formation dependent on

–Pyrolysis vs. oxidation of fuel

–Formation of gas-phase soot precursors

–Nucleation of particles

–Growth of particles

–Agglomeration of particles

–Oxidation of final particles

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Soot photographsSoot photographs

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Combustion DiagnosticsCombustion Diagnostics

Spectroscopic methods in combustion researchSpectroscopic methods in combustion research

1. LIF: Laser induced fluorescence

2. Raman spectroscopy

3. CARS: Coherent anti-Stokes Raman spectroscopy

4. Quantitative aspects of LIF

5. Observation of sound generating flames in a gas turbine burner

6. Fuel oil concentration measurements in gas turbine burners

High temperatures and pressures in the combustion chamber make it a most hostile environment.

Laser diagnosticsLaser diagnostics have been used in a number of measurements, such as:

- air/fuel flows and velocities

- pollutant formation

- combustion processes; - droplet and particle sizes

- identification of molecular components, etc.

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First, why at all are optical and spectroscopic methods used in combustion research?

The methods have a couple of striking advantages ov er conventional techniques

Overview of methods most commonly used for spectros copicOverview of methods most commonly used for spectros copiccombustion diagnosticscombustion diagnostics

The advantages:The advantages:+ The methods are nonnon --intrusiveintrusive , nonnon --contactcontact measuring methods.

+ In general, optical methods allow for fast observationfast observation .

+ Often a 22--dimensional informationdimensional information (an image) is intrinsically available

+ The methods can be applied to situations inaccessible by conven tional methodcan be applied to situations inaccessible by conven tional method ss, e.g. Harsh and hostile environments.

+ Minority or trace species as pollutants are detectabletrace species as pollutants are detectable .

+ In situ measurementsIn situ measurements are possible.

The drawbacks:The drawbacks:- Optical methods need, of course, optical accessoptical access . The implementation of windows

may be difficultdifficult , especially at IC engines and alike.

- The high laser intensitieslaser intensities used do change the mediumdo change the medium

- Quantitative measurements are more difficult as gen erally believQuantitative measurements are more difficult as gen erally believ eded.

- The problem of reasonable temperature field measurements is still not solvedreasonable temperature field measurements is still not solved in a satisfactory way.

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The underlying process is almost the same for all methods discussed. Electromagnetic radiation infrared, visible, or ultraviolet light - impinges on the molecule under investigation and is scattered. The methods differ in scattering angle, in the energetics of the scattering process, the polarization of the scattered light, the temporal evolution of the signal, and the efficiency ("cross section") of the process.

Optical versus spectroscopic methodsOptical versus spectroscopic methods

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Optical versus spectroscopic methodsOptical versus spectroscopic methods

In the optical methodsoptical methods the exact value of the wavelength of the light used is of minorimportance.

The spectroscopic methodsspectroscopic methods on the other hand use specific wavelengths for the light source or they analyze the emerging signal with res pect to its spectral composition, or both. These wavelengths closely relate to the mo lecules under investigation .

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Laser induced fluorescence (LIF)Laser induced fluorescence (LIF)

Of the spectroscopic techniques applied to combustion processes, LIF is among the most often used. Mostly LIF of the OH radical is used, for flame form /position analysis.

Experimental setup for 2Experimental setup for 2--D LIFD LIF

Spectroscopic TechniqueSpectroscopic Technique

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Example of the results of using LIFExample of the results of using LIF

Summary:Summary:

LIF is well suited for combustion diagnostics. A co mparatively simple and straightforward experimental setup readily yiel ds 2-D images . However, the fun ends with trials to extract quanti tative concentration data or to map species with really low concentration (CH, C N, C2, ...).So LIF is mostly used to visualize OH radicals indi cating flame positions and flame forms. The equipment is not che ap and the lasers use are a source of many frustrations.

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Example of OH concentration map detected by LIF

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A short look back at laser induced fluorescence

Which kind of species can be observed by the LIF te chniqueWhich kind of species can be observed by the LIF te chnique ? Comparing the species leads to a more general answer:

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•Because of its insensitivity to quenching (the lifetime of the virtual state is ~10-14s), Raman spectroscopy is of considerable interest for quantitative measurements on combustion processes.

•Further, important flame species such as O2, N2 and H2 that do not exhibit IR transitions can be readily studied with the Raman technique.

•However, because of the inherent weakness of the Raman scattering process only non-luminous (non-soothing) flames can be studied.

Raman SpectroscopyRaman Spectroscopy

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Coherent antiCoherent anti--stokes Raman scattering (CARS)stokes Raman scattering (CARS)

•CARS spectroscopy is of particular interest for combustion diagnostics because of the strong signal availablestrong signal available as a new laser beam emerging from the irradiated gas sample.

•Thus CARS is largely insensitive to the strong background light that

characterizes practical combustion systemscharacterizes practical combustion systems such as industrial flame and internal combustion engines.

Coherent anti-Stokes Raman scattering is a nonlinea r four wave mixing process.

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The above picture shows image of a two-dimensional Bunsen flame using methane at equivalence ratio of ϖϖϖϖ = 1.2. Blue flame luminosity due to emission from CH and C2 radicals, green particle tracks due to scattering from MgO particles under 6kHz excitation of a Cu-Vp laser. FUJI 1600 ASA film (Echekki and Mungal, 1990).

Reference: Tarek Echekki & M. G. Mungal (1990), "Flame Speed Measurements at the Tip of a Slot Burner: Effects of Flame Curvature and Hydrodynamic Stretch," Twenty-Third Symposium (Int.) on Combustion, The Combustion Institute, 455-461.

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Combustion in a spark-ignition engine

Thin zone of intense chemical reaction , known as flame,

propagating through the unburned fuel-air mixture, leaving behind hot

products of combustion