FKM Combustion Processmohsin/mmj1443/introduction/Mazlan's...Examples of combustion applications:...
Transcript of FKM Combustion Processmohsin/mmj1443/introduction/Mazlan's...Examples of combustion applications:...
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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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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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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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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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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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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.