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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
LECTURER:
DR. MAZLAN ABDUL WAHIDhttp://www.fkm.utm.my/~mazlan
MMJ 1443MMJ 1443
Combustion Process Combustion Process
Semester 2009/2010 - I
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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
Lecture Hours
Prerequisites Undergraduate Thermodynamics
Grading 20% 1st Test
20% 2nd Test20% Assignments
40% Projects and Presentations
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References:
1. Stephen R. Turns, An Introduction to Combustion, 2nd
Edition,
McGraw-Hill, 2000.
2. Warnatz, Maas, Dibble, Combustion, Springer Verlag.
3. Kenneth K. Kuo, Principles of Combustion, Wiley.
4. C.K. Law, Combustion Fundamentals.
5. Glassman, Combustion, Academic Press.
6. G. L. Borman, and K. W. Ragland, Combustion Engineering,
McGraw-Hill, 1998.
7. A. M. Kanury, Introduction to Combustion Phenomena, Gordon
& Breach, 1975.
8. J. M. Beér, and N. A Chigier, Combustion Aerodynamics,
Applied Science, 1972.
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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
1 Introduction 2 Thermodynamics of Combustion Processes3 Fuels4
Chemical Kinetics5 Conservation Equations6 Premixed Combustion 7
Non-premixed Combustion (Diffusion Flames)8 Detonation9 Pollutant
Emissions 10 Combustion Applications
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.
VII. Detonation
The Phenomena of detonation, its mechanism and its
characterization
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VIII. Pollutant Emissions
Introduction to the quantification of emissions as well as
discussing the mechanisms of pollutant formation an d their
control.
IX. Combustion Applications
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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?
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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
• 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.
• 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.
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
• Chemical processes
- thermodynamics, and chemical kinetics
Practical applications of the combustion phenomena also involve
applied sciences such as aerodynamics, fuel technology, and
mechanical engineering.
• Transport of energy, mass, and momentum are the physical
processes involved in combustion.
• 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.
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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
Oxygen Ignition (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
7. Shock
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• Premixed flames– Fuel and air are molecularly
mixed prior to chemical reaction
• Non-premixed (diffusion) flames– 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
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
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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 and enough time for the reactants to come into contact
and combine.
•Unless combustion is properly controlled, high concentrations
ofundesirable 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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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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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 metre (m3) of methane requires 2 cubic metre (m3)
of oxygen for complete combustion and will produce 1 cubic metre
(m3) of carbon dioxide and 2 cubic metre
(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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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
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?
•Laminar premixed flames•Laminar diffusion flames
•Turbulent premixed flames
•Turbulent nonpremixed flames•Wild forest fires
•Flame stability
•Flame instabilities•Spray combustion
•Droplet combustion
•Solid propellant combustion•Detonation and deflagration
•External effects on flames
•Buoyancy effect on flames•Pollutant emissions
•Soot formation
•Combustion laser diagnostics
Combustion flames visualization, emissions and
diagnosticsCombustion flames visualization, emissions and
diagnostics
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What is Flame?What is Flame?
A self-sustaining propagation of a localizedlocalized (* )
combustion zone at subsonicsubsonic(#) velocities
(*) 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 subCombustion wave that travels
sub--sonically relative to the sonically relative to the speed of
sound in the unburned combustible mixture is speed of sound in the
unburned combustible mixture is known as deflagrationknown 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 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 stabilityA 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 Laminar premixed flamesflames
�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 blow -off) and
stability
Lean Premixed flame
Open TipLean Premixed flame
Closed Tip
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Schematic diagram of Bunsen burner showing the typical flame
configuration (left) and the photograph of Bunsen flame
(right).
The typical Bunsen-burner flame is a dual flame: a fuel rich
premixed inner flame surrounded by a diffusion flame. The dark zone
is consists of unburned premixed gases before they enter the area
of the luminous zone where reaction and heat release takes place.
The 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 1,500º C (2,700º F) [ ].
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, and to a lesser degree, chemiluminescence from
the reaction CO + O →→→→ CO2 + hv. If the flame is made richer
still, soot will form. The flame can be seen as bright yellow to
dull orange emission, depending on the flame temperature [1].
The Bunsen flame is a common example of premixed flames. The
mixed gas burns with a pale blue flame, the primary flame, seen as
a small inner cone, and a secondary, almost colorless flame, seen
as a larger, outer cone, which results when the remaining gas is
completely oxidized by the surrounding air.
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Nonpremixedflames•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 Laminar nonpremixednonpremixed (diffusion)
flames(diffusion) flames
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NonpremixedNonpremixed (diffusion) flame (diffusion) flame --
Candle flameCandle flame
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Laminar Laminar nonpremixednonpremixed (diffusion)
flames(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 Laminar nonpremixednonpremixed (diffusion)
flames(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 with forced turbulence, if the Even with forced turbulence,
if the GrashofGrashof number gdnumber gd 33//νννννννν22 is is
larger than about 10larger than about 10 66 (g = 10(g = 1033
cm/scm/s 22, , νννννννν ≈≈ 1 cm1 cm 22/s /s ⇒⇒⇒⇒⇒⇒⇒⇒ d > 10 cm),
d > 10 cm), 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)
��NonpremixedNonpremixed turbulent flamesturbulent flames
»»DieselDiesel --type (compression ignition, type (compression
ignition, nonpremixednonpremixed --charge) charge) internal
combustion enginesinternal combustion engines
»»Gas turbinesGas turbines
»»Most industrial boilers and furnacesMost industrial boilers
and furnaces
Turbulent 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 Leeds
University to study the premixed turbulent flame of various
mixtures
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Glassman (1996)
Turbulent Turbulent nonpremixednonpremixedflamesflames
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 Turbulent nonpremixednonpremixedflamesflames
Reaction zone
TemperatureFuel
concentrationProduct
concentration
2000K
300K
Distance from reaction zone Convection-diffusion zone
Oxygen concentration
300K
Nonpremixed flames establish themselves at the interface between
fuel 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
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Spray combustionSpray combustion
Schematic of diesel simulation facility, DSF,
Diesel spray ignition in the DSF.
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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
terminologyPrimary 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 smallscale 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/COALESCENCEDROP COLLISION/COALESCENCEBinary
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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Rotating FlamesPremixed 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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Swirling flameSwirling flame
Injector burner with a swirling
propane flame
Swirling (lifted) 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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Summary of High Explosives
• 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 “High Explosives”
“High Explosives” normally refer to condensed phase materials
(solids, liquids and mixtures)
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Buoyancy effect on flames
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Buoyancy effect on flames
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Buoyancy effect on flames
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External effects on flamesExternal effects on flames
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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
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 Use lean or stoichiometricstoichiometric
mixturesmixtures
��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 then NO, CO, UHC and C(sC(s) (soot)
are ) (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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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
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 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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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA
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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Combustion Diagnostics Combustion Diagnostics -- LIFLIF
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 for m/position analysis.
Experimental setup for 2Experimental setup for 2--D LIFD LIF
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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 yields 2 -D
images. However, the fun ends with trials to extract quantitative
concentration data o r to map species with really low concentration
(CH, CN, C2, ...). So LIF is mostly used to visualize OH radicals
indi cating flame positions and flame forms. The equipment is n ot
cheap and the lasers use are a source of many frustrations.
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A short look back at laser induced fluorescence
Which 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 stokes Raman
scattering
(CARS)(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. A general
formulation of the reaction of matter on electric f ields has
been proposed by Bloembergen in 1965:
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Magnetic support flames
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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.