Post on 03-Jul-2020
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
PREMIXED
COMBUSTION
Professor Dr. Mazlan Abdul WahidFaculty of Mechanical EngineeringUniversiti Teknologi Malaysia
Advanced Combustion
MKMM 1443
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Outline
Introduction – combustion mode, flame type, physical
description
Principle characteristics
Flame speed:
Simplified analysis of flame speed
Meghalchi and Keck correlation
Experimental measurements
Flammability limits
Quenching
Ignition
Flame stabilization
Turbulent premixed flames
Premixed Combustion
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COMBUSTION MODES AND FLAME TYPES
• Combustion can occur in flame mode
– Premixed flames
– Diffusion (non-premixed) flames
• Combustion can occur in non-flame mode
• What is a flame?
– A flame is a self-sustaining propagation of a localized combustion zone at
subsonic velocities
• Flame must be localized: flame occupies only a small portion of
combustible mixture at any one time (in contrast to a reaction which occurs
uniformly throughout a vessel)
• A discrete combustion wave that travels subsonically is called a
deflagration
• Combustion waves may be also travel at supersonic velocities, called
detonations
• Fundamental propagation mechanism is different in deflagrations and
detonations
• Laminar vs. Turbulent Flames: both have same type of physical process and many
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Regimes of burning (Flame types)
Laminar, premixed
Laminar, nonpremixed
Turbulent, premixed
Turbulent, nonpremixed
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Laminar, premixed
An example of a laminar premixed flame is a Bunsen burner flame.
Laminar means that the flow streamlines are smooth and do not
bounce around significantly. Two photos taken a few seconds apart
will show nearly identical images. Premixed means that the fuel and
the oxidizer are mixed before the combustion zone occurs.
Laminar, nonpremixed
An example of laminar diffusion flame is a candle. The fuel comesfrom the wax vapour, while the oxidizer is air; they do not mixbefore being introduced (by diffusion) into the flame zone. A peaktemperature of around 1400°C is found in a candle flame [Gaydonand Wolfhard (1970)].
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Turbulent, premixedMost turbulent premixed flames are from engineered combustion systems: boilers, furnaces, etc. In such systems, the air and the fuel are premixed in some burner device. Since the flames are turbulent, two sequential photos would show a greatly different flame shape and location.
Turbulent, nonpremixedTurbulent nonpremixed flames are employed in the majority of practical combustion systems due to its ease of control
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LAMINAR PREMIXED FLAMES
Fuel and oxidizer mixed at molecular level prior to occurrence of any
significant chemical reaction
Air
Fuel
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DIFFUSION FLAMES• Reactants are initially separated, and reaction occurs only at interface between fuel and
oxidizer (mixing and reaction taking place)
• Diffusion applies strictly to molecular diffusion of chemical species
• In turbulent diffusion flames, turbulent convection mixes fuel and air macroscopically,
then molecular mixing completes the process so that chemical reactions can take place
Orange
Blue
Full range of φthroughout
reaction zone
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LOOK AGAIN AT BUNSEN BURNER
Fuel-rich pre-mixed
inner flame
Secondary
diffusion flame
results when
CO and H
products from
rich inner flame
encounter
ambient air
• What determines shape of flame? (velocity profile, flame speed, heat loss to tube wall)
• Under what conditions will flame remain stationary? (flame speed must equal speed of normal component of unburned gas at each location)
• What factors influence laminar flame speed and flame thickness (φ, T, P, fuel type)
• How to characterize blowoff and flashback
• Most practical devices (Diesel-engine combustion) has premixed and diffusion burning
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Our emphasis in this lecture is on
premixed flames, particularly
about premixed laminar and
turbulent combustion
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PRINCIPAL CHARACTERISTICS OF LAMINAR PREMIXED FLAMES
• Definition of flame speed, SL
• Temperature profile through flame
• Product density is less than the reactant density so that by continuity the velocity of the
burned gases is greater than the velocity of the unburned gases
– For a typical hydrocarbon-air flame at atmospheric pressure, the density ratio is about 7
• Convenient to divide the flame into two zones
1. Preheat zone: little heat is released
2. Reaction zone: most of the chemical energy is released
2.a Thin region of fast chemistry
– Destruction of fuel molecules and creation of intermediate species
– Dominated by bimolecular reactions
– At atmospheric pressure, fast zone is usually less than 1 mm
– Temperature and species concentration gradients are very large
– The large gradients provide the driving forces for the flame to be self-
sustaining, i.e. diffusion of heat and radical species from the reaction zone to
the preheat zone
2.b Wider region of slow chemistry
– Chemistry is dominated by three-body radical recombination reactions, such
as the final burn-out of CO via CO + OH → CO2 + H
– At atmospheric pressure, this zone may extend several mm
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What is Flame?
A self-sustaining propagation of a localized (*)
combustion zone at subsonic (#) velocities
(*) Flame occupies only a small portion of the combustible
mixture at any one time
(#) Combustion wave that travels sub-sonically relative 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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Premixed Combustion
• Fuel (in gaseous form) and oxidizer are homogeneously
mixed before the combustion event
• Flow is laminar or turbulent
• Turbulent premixed flames:
- combustion in gasoline engines
- lean-premixed gas turbine combustion
• Characteristics
– Reacts rapidly
– Constant pressure
– Propagates as thin zone
• Ex: Spark Engine
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• Fuel well mixed with air (O2) before burning
• Flammability limits: Mixture will only burn if concentration is within
well-defined limits
• Ignition requires sufficient energy
• Rate of combustion is high: Governed by
chemical kinetics not mixing rate
• Deflagration: Combustion propagates through
mixture as a flame
• If mixture is confined, rapid pressure rise may
cause vessel to explode
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• 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
Laminar premixed flames
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Laminar premixed flames
Reactants are completely mixed on a molecular level prior to
ignition and combustion.
Kinetically controlled and the rate of flame propagation, called
the burning velocity.
Dependent upon chemical composition and rates of chemical
reaction.
Lean Premixed flame
Open TipLean Premixed flame
Closed Tip
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Bunsen-burner flame is a dual flame:
A fuel rich premixed inner flame - 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)
Surrounded by a diffusion flame - The secondary diffusion
flame results when the carbon monoxide product from the
rich inner flame encounters the ambient air.
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A clear example of flame separation is achieved by the use of
Smithells separator, such as shown in the figure below that
totally separates the inner and outer cone of Bunsen flame. In
the picture the flame appeared as two-separated flame, the
inner cone resting on the top of the Bunsen, and the outer cone
continuing to burn at the top of the glass tube. The flame is as a
result of ethylene and air burning at a separator; the inner tube
had in this case an oval section, so that the inner cone is not the
typical shape.
Smithells separator that separate the
inner from the outer cone of a
Bunsen-type flame. A striking
feature for these rich mixtures reveal
by this Smithells separator is that a
hot gases between the inner cone
and the outer cone, which is referred
to as the interconal gases, are
practically non luminous
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Stoichiometry, fuel lean, fuel rich
• A premixed flame is stoichiometric if the premixed
reactants contain right amount of oxidizer to
consume
(burn) the fuel completely.
• If there is an excess of fuel: fuel-rich system
• If there is an excess of oxygen: fuel-lean system
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Structure of Premixed Flames
• Flame fixed in space but propagates against gas flow
Three Zones
• Pre-heat zone: about 0.3 mm thick. Premixed gas
heated
to ignition temperature.
• Reaction zone: 1 mm thick (hydrocarbons).
Combustion occurs; visible flame.
• Post-flame zone: High temperature / local equilibrium
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Structure of Premixed Flames
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LAMINAR FLAME STRUCTURE
Laminar flame structure. Temperature and heat-release rate profiles based on experiments of Friedman
and Burke
Reference: Turns An Introduction to Combustion
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EXAMPLE: FLAT FLAME BURNERS
Adiabatic flat-flame burner
Flame is stabilized over bundle
of small tubes through which fuel-air
mixture passes laminarly
Stable only over small range of
conditions
Non-adiabatic flat-flame burner
Utilizes a water-cooled face that
allows
heat to be extracted from the flame,
which in turn decreases SL
Stable over relatively wide
range of conditionsReference: Turns An Introduction to Combustion
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Structure of Flame
Vu = SL
ρu
Vb
ρb
[Fuel][O2]
T
Pre-heatZone
ReactionZone
Products zone
[radicals]
Diffusion of heatand radicals
Flame thickness δVisible part of the flame
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One of the most important quantities in
combustion science is the laminar burning
velocity (or flame speed), which we examine
here. It refers to the velocity at which a flame
propagates relative to the unburnt gas.
We need this quantity to design combustion
chamber i.e. design of gasoline engines, where
the duration of combustion is directly related to
how quickly the flame traverses the cylinder.
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It will also give us a general measure of the time
needed to complete the chemical reactions, and hence
it is useful for a variety of problems such as the rate
of burning in industrial burners and in gas turbines.
Finally, propagation of flames is involved in fires and
accidental explosions and hence it is important to
have a good grasp of the physical phenomena
involved.
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Laminar Premixed Flames
A flame represents an interface separating the unburned gas from
the combustion products.
A flame can propagate as in an engine application or be stationary
as in a burner application.
For a given P, T, φ and laminar conditions a flame has two basic
properties:
a) adiabatic flame temperature,Tad
b) laminar burning velocity, Sl
Note, Sl is defined in terms of the approaching unburned gas velocity
Pressure is roughly constant across the flame so ρ ~ 1/T
Vu = 0
ρu
Vb-Sl
ρb
Sl
Moving flame
Vu = SlVb
ρb
burnedunburned
Stationary flame
ρu
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Flame speed = rate of advance of flame with respect
to fixed observer
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Laminar Burning Velocity
The laminar burning velocity is measured relative to the unburned gas
ahead and the flame velocity Vf is measured relative to a fixed
observer.
If the flame is propagating in a closed-ended tube the velocity
measured is the flame velocity and can be up to 8 times the burning
velocity.
This is because the density of the products is lower than the fresh gas
so a flow is generated ahead of the flame
VuVb=0
ρb
Vf
Moving flame
ρu
- Vu= SlVb=Vf
ρb
Vf
Stationary flame
ρu
Vf
Applying the conservation of mass across the flame:
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Laminar Burning VelocityMaillard-LeChatelier theory gives:
Higher flame velocity corresponds to:1) higher unburned gas temperature2) lower pressure3) higher adiabatic flame temperature (chemical reaction)4) higher thermal diffusivity α (= kcond/ρcp)
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• A general solution of the energy and species conservation equations leads
to an equation of the form:
• Thus, the flame speed is dependent on the thermal diffusivity, the reaction
rate and the density of the reacting gases.
• The pressure dependence is
• The pressure dependence of the mass burning rate is
• The temperature dependence is
Flame Speed Calculation
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Factors affecting laminar burning velocity
1. Effect of fuel chemistry
This is evidenced by the pre-exponential factor A. The faster the
chemistry, the faster the SL. Hydrogen has a fast chemistry and hence
hydrogen flames are much faster than hydrocarbon flames.
2. Effect of initial temperature
As T0 increases, Tf increases and hence the laminar burning velocity
increases through the exponential term. Experiment shows that for
most fuels, SL grows approximately as T02, at least for a small range
above room temperature
3. Effect of mixture strength
As the mixture moves away from stoichiometry, Tf decreases and
hence the flame speed decreases. Flame propagation becomes
impossible for very lean or very rich mixtures (see later) because
the chemistry becomes too slow.
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4. Effect of pressure
Our model predicts that the burning velocity is independent
of pressure, as the effect of pressure, through the densities
cancels out. However, experiment shows that SL decreases as
P-1/2. This discrepancy is due to our chemistry model that
predicts an overall second-order reaction.
In general, experiment shows that all hydrocarbon fuels have
approximately the same laminar burning velocity, with the
exception of acetylene (C2H2) that is significantly faster.
Hydrogen has a very high flame speed, partly due to the fast
chemistry and partly due to the very high diffusion
coefficient of the light hydrogen molecules. The experiment
also shows that SL peaks at φ slightly richer than unity,
which is due to the lower heat capacity of the products of
rich combustion that allows Tf to be higher there than at
φ=1. All in all, our theory reproduces the very important
observation that the flame speed is very sensitive to flame
temperature.
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Burning Velocity
Variation with Composition
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CH4/O2 flame speeds in N2 vs Ar vs He
• Flame speed depends on thermaldiffusivity
• And temperature
Flame Speed Dependencies
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Flame Speeds for Various Gases
Typical flame speeds for hydrocarbons are ~ 40 cm/s
• Others may vary significantly
• CO is ~ 30
• Acetylene is~140
• H2 is ~ 170
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Laminar Burning Velocity Correlation
The following is a correlation developed by Metghalchi and keck
where Ydil is the mass fraction of diluent, e.g., residual gas, and
Fuel φM BM (cm/s) B2 (cm/s)
Methanol 1.11 36.92 -140.51
Propane 1.08 34.22 -138.65
Isooctane 1.13 26.32 -84.72
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Burning Velocity Measurements
Various measurement techniques have been developed but all of
them is based on observation. The following method has been
widely used to observe the flame:
(a) Direct photograph – observing the luminous part
(b) Shadowgraph – measure the derivative of density gradient
(c) Schlieren picture – simply the density gradient
(recommended)
(d) Interferometry – measure density or temperature directly
(too sensitive and can be used only in 2D flames.
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Measurement techniques to determine burning
velocity:
Constant-Volume Bomb (Closed Spherical Bomb)
Method
Bomb methods that employ either high-speed Schlieren
photography or transient pressure measurements has been used
by some investigators to determine the burning velocities at low
concentrations and/or at elevated pressures. A double kernel
technique was developed by Raezer et al. [7] and investigated
in detail by Andrew et al.[8] and Abdel-Gayed et al.[9]
determined the laminar and turbulent burning velocities of
hydrogen-air mixtures.
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Typical schlieren photographs of the approaching
laminar flame fronts for a 50% hydrogen-air mixture.
The time zero is arbitrary chosen [10].
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The figure shows a typical sequence of photograph obtained
with the high-speed camera. The flame starts off as nearly
spherical flame fronts. As the flame fronts approach each other
their shapes becomes that of a flat flame and their measured
burning velocities approach that of a true one-dimensional
flame. Only the last few photographs were needed to calculate
the burning velocity. For a given mixture, the flame speed was
calculated by plotting the distance between the flame fronts as
a function of time and then determining the slope.
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Soap-Bubble Technique
Soap-bubble technique offering a method of measuring
the flame speed without the wall effects. In this method, a
soap bubble is blown with a combustible mixture and
ignited centrally by means of a capacitance spark. The
spherical flame spreads rapidly to the gas and since the
bubble expands as combustion proceeds, the process is
assumed to occur at constant pressure.
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Ethylene-air mixture
A 6.53 percent ethylene-air mixture; argon atmosphere; timing
light = 1000 flashes per second; standard distance, 177.88 mm at
bubble equals 23.38 mm comparator reading; initial diameter =
6.91 mm. (r2/r1)3 = (13.84/6.91)3 = 8.033; space velocity Sb (from
graph) = ∆D/2 ∆t = 12.87 x 177.88/ (23.38 x 0.03 x 1000) = 4.895
m/s; Su = 4.895/8.033 = 0.609 m/s.
(a) Bubble explosion
(b) Graph comparator reading [11].
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Flat Flame Technique
Flat flame burner technique offers the most simple flame front and
one in which the area of shadow, Schlieren, and visible flame fronts
are all the same. It is achieved by placing a porous metal disc or a
series of small tubes of 1-mm diameter or less at the exit of the
larger flow tube. This method as originally developed by Powling
[12] was applicable only to mixtures having low burning velocities
of the order of 15 cm/sec and less. At higher SL, the flame front
position itself far from the burner and forms conical shapes.
Spalding and Botha [13] extended the method to higher flame
speeds by cooling the plug. The cooling brings the flame front
closer to the plug and stabilizes it.
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In burner methods the flame remains stationary. In this method it is
invariably necessary to know the area of the flame front in order to
compute the flame speed. Because of the complicated flame surface,
the different procedures used for measuring the flame cone has led to
different results. The earliest procedure of calculating flame speed by
this method was to divide the volume flow rate by the area of flame
cone:
SL = Q/A (cm/sec)
so that it is apparent then that the choice of cone will give widely
different results. Another difficulty is that the shape of the cone causes
the flow of the gases through the flame to be much less simple than in
flat flame and closed spherical vessel methods. A particular
consequence of this is that the visible, shadow and Schlieren cones
which are not coincident have areas which differ considerably. The
other difficulties in using burner method is to have a steady source of
gas supply and for rare or pure gases can be a severe problem. The
other problem encounter in the method is inability to completely
eliminate the wall effect.
Burner Methods
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LAMINAR PREMIXED FLAMES: SIMPLIFIED ANALYSIS
• Analysis couples principles of heat transfer, mass transfer, chemical kinetics, and thermodynamics to understand the factors governing:
– Flame speed, SL
– Flame thickness, δ (ANSWER, δ=2α/SL)
• Simplified approach using conservation relations
• Assumptions:
1. 1-D, constant area, steady flow
2. Neglect: kinetic and potential energy, viscous shear work, thermal radiation
3. Constant pressure (neglect small pressure difference across flame)
4. Diffusion of heat governed by Fourier’s law
5. Diffusion of mass governed by Fick’s law (binary diffusion)
6. Lewis number (Le≡α/D) unity
7. Individual specific heats are equal and constant
8. Fuel and oxidizer form products in a single-step exothermic reaction
9. Oxidizer is present in stoichiometric or excess proportions; thus, the fuel is completely consumed at the flame.
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MASS TRANSFER AND FICK’S LAW OF DIFFUSION
• Mass transfer and heat conduction in gases governed by similar physics at
molecular level
• Mass transfer can occur by molecular processes (collisions) and/or turbulent
processes
– Molecular processes are relatively slow and operate on small spatial scales
– Turbulent processes depend upon velocity and size of an eddy (or current)
carrying transported material
• Fick’s Law of Diffusion
Mass flow of species A
per unit area
(perpendicular to the
flow) Mass flow of species A
associated with bulk flow
per unit area
Mass flow of species A
associated with molecular
diffusion per unit area
DAB: Binary diffusivity and
is a property of the mixture
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SPECIES CONSERVATION
Rate of increase of
mass of A within CV
Mass flow of
A into CV
Mass flow of
A out of CV
Mass production rate of
species A by chemical
reactions= - +
Steady-flow, 1-D form of species conservation
for a binary gas mixture, assuming species
diffusion only occurs as a result of
concentration gradients
Divide by A∆x and take limit as ∆x → 0
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DETAILED ANALYSIS USING CHEMKIN: CH4-AIR PREMIXED LAMINAR FLAME
• Figure (a) shows principal C-containing species CH4, CO, and CO2.
– Note disappearance of fuel and appearance of intermediate
CO, and burn-out of CO to form CO2
– CO concentration has peak value at approx same location
where CH4 concentration goes to zero
– CO2 concentration at first lags CO concentration but then
continues to rise as CO is oxidized
• Figure (b) shows C-containing intermediate species CH3, CH2O, and
HCO, which are produced and destroyed in a narrow interval from
approximately 0.4 mm – 1.1 mm.
• Figure (d) shows same phenomena for the CH radical
• Figure (c) shows that H-intermediates, HO2 and H2O2 have
somewhat broader profiles than C-intermediates. Peak
concentrations appear slightly earlier in flame.
– H2O mole fractions reaches its 80% of equilibrium value (at
about 0.9 mm) sooner than CO2 (at about 2mm)
• All fuel has been destroyed in approx. 1 mm and most of total
temperature rise (~ 75%) occurs in same interval
– Approach to equilibrium is relatively slow beyond this point
(no equilibrium even at 3 mm)
– Slow approach toward equilibrium is a consequence of
dominance of 3-body recombinations
• Figure (d) shows NO production
– Rapid rise in NO mole fraction in same region where CH
radical is present in flame
– This is followed by a continual (almost linear) increase in NO
mole fraction. In this later region NO formation is dominated
by Zeldovich kinetics.
– Curve ultimately bends over as reverse reactions become more
important and equilibrium is approached asymptotically
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DETAILED ANALYSIS USING CHEMKIN: CH4-AIR PREMIXED LAMINAR FLAME
• Plot shows molar production / destruction rates for
various species and provides more insight into
CH4 → CO → CO2 sequence
• Peak fuel destruction rate nominally corresponds
with peak CO production rate
• CO2 production rate initially lags that of CO
• Even before location where there is no longer any
CH4 to produce additional CO, the net CO
production rate becomes negative (CO is being
destroyed)
• Maximum rate of CO destruction occurs just
downstream of peak CO2 production rate
• Bulk of chemical activity is contained in an
interval extending from about 0.5 mm to 1.5 mm
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DETAILED ANALYSIS USING CHEMKIN: CH4-AIR PREMIXED LAMINAR FLAME
• Plot shows NO production rate through flame
• Figure shows that early appearance of NO
within flame (0.5 mm – 0.8 mm see Figure
(d)) is result of passive diffusion since
production rate is essentially zero in that
region.
• First chemical activity associated with NO is
a destructive process in region approximately
0.8 mm – 0.9 mm.
• NO production reaches a maximum at an
axial location between CH and O-atom
concentrations. It is likely that both Fenimore
and Zeldovich pathways are important (see
p.168-171 or Turns.
• Beyond O-atom peak at a distance of 1.2 mm
(Figure (d)), NO production rate falls. Since
temperature continues to rise in this region,
decline in net NO production rate must be a
consequence of decaying O-atom
concentration and building strength of reverse
reactions.
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STOICHIOMETRIC METHANOL-AIR FLAME
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FACTORS INFLUENCING SL AND δ• Scaling relation developed on p. 274-275
• Laminar flame speed has a strong temperature
dependence
– Global reaction orders for HC ~ 2
– EA ~ 1.67x108 J/kmol
• Example: CASE A vs. CASE B
– SL increases by a factor of 3.64 when the
unburned gas temperature is increased from
300 K to 600 K
– Increasing unburned gas temperature will
also increase the burned gas temperature by
the same amount (neglect dissociation and
variable specific heats)
• Example: CASE A vs. CASE C
– Case C forces a lower Tb
– Captures the effect of heat transfer of
changing equivalence ratio, either rich or
lean, from the maximum-temperature
condition.
CASE A B C
Tu 300 600 300
Tb 2,000 2,300 1,700
SL/SL,A 1 3.64 0.46
δ/δ,A 1 0.65 1.95
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LAMINAR FLAME SPEED (T&P) SCALING: USEFUL
DATA
Experimental measurements
generally show a negative
pressure dependence
Plot is for CH4 - Air
SL (cm/s) = 43P-0.5 (atm)
Plot is for CH4 – Air
φ=1.0
P=1 atm
Primary effect of φ is through flame temperature
Max slightly rich of φ=1.0
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LAMINAR FLAME SPEED FOR VARIOUS FUELS
• Comment on H2
– Thermal diffusivity of H2 is many times greater than HC fuels
– Mass diffusivity of H2 is much greater than HC fuels
– Reaction kinetics for H2 are very rapid (no slow CO → CO2 step)
Laminar flame speeds for pure
Fuels burning in air at φ = 1.0
P = 1 atm, Tu = 300K
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FLAME SPEED CORRELATIONS FOR SELECTED FUELS• One of most useful correlations for laminar flame speed, SL, given by Metghalchi and Keck
– Determined experimentally over a range of temperatures and pressures typical of those found in
reciprocating IC engines and gas-turbine combustors
• EXAMPLE: Employ correlation of Metghalchi and Keck to compare laminar flame speed gasoline
(RMFD-303)-air mixtures with φ = 0.8 for 3 cases:
1. At reference conditions of T = 298 K and P = 1 atm
2. At conditions typical of a spark ignition engine operating at T = 685 K and P = 18.38 atm
3. At same conditions as (2) but with 15 percent (by mass) exhaust gas recirculation
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TRANSIENT BEHAVIOR• 3 important aspects to consider
1. Quenching distance
• Critical diameter of a circular tube where a flame extinguishes, rather
than propagates
2. Flammability limits
• Lower limit: leanest mixture (φ<1) that will allow steady flame
propagation
• Upper limit: richest mixture (φ>1) that will allow steady flame
propagation
3. Minimum ignition energy
• In each of these, heat loss is the controlling phenomena
• Ignition and Quenching Criteria (also called Williams’ criteria):
1. Ignition will only occur if enough energy is added to the gas to heat a slab
about as thick as a steadily propagating laminar flame to the adiabatic flame
temperature
2. The rate of liberation of heat by chemical reactions inside the slab must
approximately balance the rate of heat loss from the slab by thermal
conduction
• Keep in mind that (1) and (2) are just rules-of-thumb
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EXAMPLE: FLAME ARRESTERS
Flame arresters are used to prevent
propagation of flame fronts in process piping
Flame arresters on boat motor Davey Miner’s
Safety Lamp
The screen's ability to dissipate heat and prevent
combustion while allowing flammable mixtures of gases to
pass through has been used in practical applications. Sir
Humphrey Davy used this principle in his invention of the
miner's safety lamp in 1815. Flammable gases from the
mine could pass through the screen and burn in the
enclosed flame with a 'colored haze' while the screen
prevented the open flame from causing a mine explosion
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Flame Thickness and Quenching Distance
A rough estimate of the laminar flame thickness δ can be obtained by:
As a flame propagates through a duct heat is lost from the flame to the
wall
It is found experimentally that if the duct diameter is smaller than
some critical value then the flame will extinguish. This critical value is
referred to as the quenching distance dmin and is close in magnitude
to the flame thickness.
dLocal quenching
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Quenching Distance
A flame cannot propagate inside a tube with very small diameter, if
the walls of the tube are kept at a low temperature. The reason is
that for flame propagation, the heat generated must be allowed to
diffuse towards the reactants. If heat is removed by other means
(e.g. by conduction to the wall), then the flame propagation
mechanism fails.
The minimum pipe diameter that allows flame propagation is called
the quenching distance.
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CH4-Air at 1 atm
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Mixture will only burn if concentration (fuel in air) is within
well-defined limits
L = lower flammability limit (LFL)
= lowest concentration that is flammable
U = upper flammability limit (UFL)
= highest concentration that is flammable
Example: Methane CH4 at T=25ºC & P=1 atm
L = 5% (by volume) and U = 15% (by volume)
[Note: Flammability limits = explosive limit]
Flammability Limits
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UNIVERSITI TEKNOLOGI MALAYSIA65
FLAMMABILITY LIMITS
• Experiments show that a flame will propagate only within a range of mixture compositions
(sometimes called mixture strengths in this context) between lower and upper limits of
flammability
– Lower limit is leanest mixture (φ < 1) that will allow steady flame propagation
– Upper limit is richest mixture (φ > 1) that will allow steady flame propagation
– Flammability limits are frequently quoted as percent fuel by volume in mixture, or as a
percentage of the stoichiometric fuel requirement
• Experimental determination: Tube Method
– Determine whether or not a flame initiated at the bottom of a vertical tube
(approximately 50 mm diameter and 1.2 m long) propagates the length of tube
• A mixture that sustains the flame is said to be flammable and by adjusting the
mixture strength, flammability limit can be ascertained
• In addition to mixture properties, experimental flammability limits are related to
heat losses from the system, and hence, are generally apparatus dependent
• Example: A full propane cylinder from a stove leaks
contents of 1.02 lb (0.464 kg) into a 12’ x 14’ x 8’ (3.66 m x
4.27 m x 2.44 m) room at 20 ºC and 1 atm. After a long time,
the fuel gas and the room air are well mixed. Is mixture in
room flammable?
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UNIVERSITI TEKNOLOGI MALAYSIA
Lean and rich flammability limits
As we have just seen, if the flame temperature drops, the laminar
burning velocity will drop due to the Arrhenius term. We have additional
chemical effects if the temperature becomes too low.
Below about 1500K, the chain-branching reactions that produce the
radicals necessary to achieve combustion are slow and the chain-
terminating reactions dominate. This means that self-sustaining
combustion becomes impossible. At very lean mixtures therefore we
cannot expect flame propagation. For very rich combustion, the
chemistry has additional complications in that the deficient reactant
(oxygen) simply is not enough to “trigger” the chain-reactions. Hence,
there is a lean and a rich limit for flame propagation, which are called
flammability limits.
Knowledge of the flammability limits is obtained through experiment and
extensive tabulated values exist (e.g. in Glassman). They are reported
usually in terms of % by vol. of fuel in the mixture or in terms of the
equivalence ratio. We have successful flame propagation only between the
two limits.
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Flammability Limits
As we have just seen for burning velocity eqn, if the flame temperature
drops, the laminar burning velocity will drop due to the Arrhenius
term. We have additional chemical effects if the temperature becomes
too low. Below about 1500K, the chain-branching reactions that
produce the radicals necessary to achieve combustion are slow and the
chain-terminating reactions dominate. This means that self-sustaining
combustion becomes impossible. At very lean mixtures therefore we
cannot expect flame propagation. For very rich combustion, the
chemistry has additional complications in that the deficient reactant
(oxygen) simply is not enough to “trigger” the chain-reactions. Hence,
there is a lean and a rich limit for flame propagation, which are called
flammability limits. Knowledge of the flammability limits is obtained
through experiment and extensive tabulated values exist (e.g. in
Glassman). They are reported usually in terms of % by vol. of fuel in
the mixture or in terms of the equivalence ratio. We have successful
flame propagation only between the two limits.
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UNIVERSITI TEKNOLOGI MALAYSIA
Flammability Limits - Significance
The flammability limits are very important for safety and
for the proper design of combustion equipment. In
industry, accidental releases of fuel vapors are not
dangerous only if sufficient ventilation makes the fuel-air
mixture well below the lean limit. In gasoline engines, the
lean and rich limits guide the fuel preparation and the
engine design. but the flame may become too slow and
hence prone to extinction.
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Flammability limit test: Example by U.S. Bureau of Mines
Apparatus
• Flame tube: 1.5 m long // 0.05 m ID
• Filled with premixed fuel-air mixture
• Cover plate is removed
• Ignition source is activated
• Mixture is deemed flammable if flame propagates upwards at least 0.75 m
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An experimental study of flammability limits of LPG/air mixtures
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A schematic of this experimental rig is shown in previous
figure, which consists of flammability tubes, mixing chamber,
fuel and air supply lines with controlling valves, etc.
Flammability tube is made by borosilicate glass having inner
diameter of 50 mm and length of 1200 mm. This is chosen
according to the standard of US Bureau of Mines [1].
The flammability limits are determined by visual inspection of
flame propagation. An arrangement is also made to ignite the
flame at the top of the flammability tube such that the flame will
propagate downwardly. In this way, the flammability limits are
determined for down propagation of the flame.
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Flammability limit
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Equation for complete combustion of CH4 in air
CH4 + 2O2 + 2 x 3.76 N2 CO2 + 2H2O + 7.52
N2
For complete combustion:
1 mol of CH4 requires 9.52 mol of air
Cst = Stoichiometric concentration
Cst = 1 / (1 + 9.52) x 100% = 9.5%
L (5%) < Cst (9.5%) < U (15%)
Stoichiometric Concentration - CH4
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Stoichiometric Concentration - CH4
L / Cst = 5 / 9.5 = 0.53
U / Cst = 15 / 9.5 = 1.6
In general, for the alkanes
L / Cst = 0.55
L = 0.55 Cst
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UNIVERSITI TEKNOLOGI MALAYSIA75
IGNITION
• GOAL: Estimate minimum ignition energy, Eign, as a function of T & P
• CRITERIA: Volume of gaseous reactants heated during ignition must be
large enough so that when ignition source is removed, heat loss to the
surroundings will not exceed the chemical energy release rate.
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Ignition Energy
Methane / air mixtures at 26°C and 1 atm
• Very small amount of energy required for ignition
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Ignition Energy
• Energy required for ignition varies with composition
• Minimum ignition energy (MIE) corresponds to most
reactive mixture: Just on fuel rich side of stoichiometry
• No mixture can be ignited with less than MIE. Design
electrical equipment for potentially flammable or
explosive atmosphere so faults do not exceed MIE.
• Limits of ignitability: vary with strength of ignition
source (small ignition sources)
• Limits of flammability: Larger ignition source ignites
mixtures near limits
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Minimum Ignition Energy and Flammability Limits
A flame is spark-ignited in a flammable mixture only if the spark energy is larger than some critical value known as the minimum ignition energy Eign
It is found experimentally that the ignition energy is inversely proportional to the square of the mixture pressure.
Experiments show that a flame will only propagate in a fuel-air mixturewithin a range of mixture compositions known as the flammability limits.
The fuel-lean limit is known as the lower (or lean) flammability limit and the fuel-rich limit is known as the upper (or rich) flammability limit.
The flammability limit is affected by both the mixture initial pressure and
temperature.
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Flame Stabilization
In a practical device, a stable flame is one
that is anchored at a desired location and
is resistant to flashback, liftoff, and
blowoff over device the device’s operating
range.
Stable flame: resistant to
• Flashback
• Liftoff
• Blowoff Attached/stable flame
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Stability limit for premixed laminar flame
Flashback - flame enters and propagates through
the burner tube without quenching.
Liftoff - the flame is not attached the burner tube
- stabilize at some distance from the burner port
Blow off - no location across the flow which is the
local flame speed is match with the flow velocity
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
The importance of flame stabilization
To prevent from;
Blow off - unburned gas will escape to
the surrounding
Flashback - flame will get sucked inside to
the tube
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Flame stabilizer
One of the basic requirements of the combustor is to
support combustion over a wide range of operating
conditions. This could be achieved by an appropriately
designed flame stabilizer. Flame stabilization could be
achieved if a sheltered region is provided to entrain and
recirculates some of the hot combustion burnt gases with the
fresh incoming fuel air mixture. There are several types of
flame stabilizer such as:
Bluff bodies
Hydrodynamics: > Opposed jets
> Jet mix
> Swirl
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Bluff bodies
Hydrodynamics
Flame holders
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Flame stabilization by bluff body
The principal:
The creation of a recirculation zone using a solid obstruction anchors the flame
Bluff body
A solid object; when it is suspended in a fluid stream a re-
circulatory flow is formed in its immediate wake
A flame holder such as a bluffbody is necessary in
order to generate a recirculation zone in which reactantsthoroughly mix and react.
Then, the aerodynamic wake provides a sufficientresidence time of burnt gases to ensure flamestabilization.
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
A flame holder such as a bluffbody isnecessary in order to generate a recirculation zone inwhich the two streams thoroughly mix and react.
Then, the aerodynamic wake provides a
sufficient residence time of burnt gases to ensureflame stabilization.
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Bluff body
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Flame stabilized by a different type of bluff bodies
small rodtulip shapering
hanging plate
(R.K Cheng, 2001)
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Nitrogen flame passing a wedge
Wire stabilized flame
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Schematic diagram for gas turbine
• Flame stabilization in gas turbine using flame holders
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA91
FLAME STABILIZATION COMMENTS
• Both Flashback and Liftoff are related to matching local laminar flame speed to local flow
velocity
• Flashback occurs when the flame enters and propagates through the burner tube without
quenching
– Can be dangerous and can lead to explosions
– Can be useful as a ‘flash tube’ from pilot flame to a burner
– Occurs when local flame speed exceeds local flow velocity (when fuel flow is being
decreased or turned off – transient event)
– Controlling parameters: fuel type, equivalence ratio, flow velocity, and burner geometry
(same parameters that control quenching)
• Liftoff is the condition where the flame is not attached to the burner tube but is stabilized at
some distance from the port
– Can lead to escape or loss of unburned gases
– Can lead to incomplete combustion
– Ignition is often difficult above lifting limit
– Tough to accurately control position of flame
– Poor heat transfer
– Flame can be noisy
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA92
FLAME STABILIZATION COMMENTS
• Liftoff depends on local flame and flow properties near the edges of the burner port
• Liftoff and blowoff can be explained by the countervailing effects of decreased heat and radical
loss to burner and increased dilution with ambient air, both occur when flow velocity is increased
• Consider a flame that is stabilized close to burner rim
1. Local flow velocity at stabilization location is small because of boundary layer (Vwall=0)
2. Because flame is close to cold wall, both heat and reactive species diffuse to wall, which
leads to small SL
– With SL and flow velocities small and equal, flame edge lies close to burner tube
– When flow velocity is increased, flame anchor point moves downstream
• SL increases since heat/radical losses are less because flame is now not as close to cold
wall
• Increase in SL results in only a small downstream adjustment
• Flame remains attached
– Now increase flow velocity further
• New effect is important: dilution of mixture with ambient air as a result of diffusion
• Dilution tends to offset effects of heat loss and flame lifts
• With further increases in flow velocity, a point is reached at which there is no location
across the flow at which the SL matches the flow velocity, and the flame blows off the
tube
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA93
FLAME STABILIZATION
http://liftoff.msfc.nasa.gov/shuttle/usmp4/science/elf_obj.html
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Adv Combustion – Mazlan 2018
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Gas turbine – flame holders
Flame
holder
Flame holder
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Hydrodynamic type of flame stabilizer
Counterflow/opposed flow burner
Schematics diagram
Counterflow flame
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Two turbulent counterflow premixed flames
Turbulent counter (or opposed) flow premixed flames
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Adv Combustion – Mazlan 2018
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Recirculation zone can also be created by introducing swirl
Swirl can be created by:
• Vanes
• Tangential jet
• Rotating tube
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Advantages of Swirl
Swirling motion is help to increase the burning intensity
through enhanced mixing and higher residence time.
Toroidal vortex type of recirculation help in stabilizing
the combustion/flame.
Swirl reduces flame height.
Disadvantages of Swirl
• Increase heat transfer to surroundings
• Inducing buoyancy forces which alter the configuration of the burning zone
• Effect on mechanical vibration
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Swirl Number
[Beer, J.M. and Chigier, N.A., 1983]
Swirl intensity or swirl number, S
Nondimensional number representing:
Axial flux of swirl momentum
Axial flux of axial momentum x Exit radius
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Degree of swirl
Low swirl: S < 0.4 - Significant lateral pressure gradients only
High swirl: S > 0.6 - Radial and axial pressure gradients large
enough to cause an axial recirculation to form of a recirculation zone
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) have
invented a unique, clean-burning combustion technology known as the low-swirl
burner (LSB), which was honored with a 2007 R&D100 Award (Figures 11 and
12). The basic LSB principle is fundamentally different than the conventional high-
swirl combustion method and defies many established notions of turbulent flame
properties and burner engineering concepts. The new technology not only burns
efficiently and cleanly, producing a very low level of nitrogen oxides, it is also
more economical to manufacture and operate than many conventional burners. The
LSB has been scaled for devices ranging in size from home furnaces to industrial
boilers and power plants.
LSB Case Study
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Adv Combustion – Mazlan 2018
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Swirl burner by jet injection
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
The Maxon MPAKT Ultra
Low NOx Burner is the first
product using ultraclean,
low-swirl combustion
technology.
A 12.7-centimeter
UCLSB designed for
water and steam
boilers. In this design
the flame is highly
lifted to optimize
performance in boiler
tubes.
Key components of a UCLSB are, at top,
the vane swirler, with an open center
channel and a screen. Shown at bottom,
left to right, are UCLSBs from 2.54
centimeters to 12.7 centimeters in
diameter. Variations in the number of
swirl vanes and center-body sizes show
this to be a robust and easily adaptable
technology.
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
"Currently, natural-gas industrial equipment emits on the order of 100 parts per million of
NOx. Ultraclean, low-swirl combustion for industrial processes can reduce the average
emission to well below 10 parts per million NOx," Cheng says. "In the U.S. alone, this would
remove 340,000 tons of NOx per year from our atmosphere." That is equivalent to the NOx
emissions of 45 thousand-megawatt coal-fired power plants. Adapting this technology to
power generation, and to residential and commercial applications as well, could remove an
additional 400,000 tons per year of NOx.
A unique type of clean-burning combustion technology called ultraclean, low-swirl combustion
(UCLSC), developed by Berkeley Lab combustion researcher Robert Cheng, is now entering the
marketplace after years of research and development. Burners using this technology produce 10
to 100 times lower emissions of nitrogen oxides than conventional burners, making it easier and
more economical for industries to meet clean air requirements.
A laboratory prototype of an ultraclean,
low-swirl burner (UCLSB) has an internal
diameter of five centimeters, shown firing at
a rate of 15 kilowatts. This burner is made
entirely out of plastic components to
showcase its unique lifted-flame feature.
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Effect of tube rotation on stability of premixed flames
Lean – closed tip flame
Richer flame – open tip
flame
Direct photographs of lean
butane-air Bunsen flames with
(a) ω = 0,
(b) ω = 51 rps,
(c) ω = 69 rps, and
(d) ω = 69 rps. [26]
Direct photographs of rich
butane-air Bunsen flames(a) ΩF = 5.6, ω = 0;
(b) ΩF = 5.6, ω = 44 rps;
(c) ΩF = 5.6, ω = 50 rps;
(d) ΩF = 7.12, ω = 67 rps. [26]
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Turbulent flames
Unlike the laminar burning velocity, the turbulent
flame velocity is not a property of the gas but instead
it depends on the details of the flow.
Structure of turbulent flames is much more complex
than simple premixed flames, which are essentially
1-Dimensional.
Buoyancy can be a significant factor, as well as small
scale and large scale turbulence
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Turbulent flames
Under turbulent conditions flame is said to display a structure known as
wrinkled laminar flame.
A wrinkled laminar flame is characterized by a continuous flame sheet
that is distorted by the eddies passing through the flame.
The turbulent burning velocity depends on the turbulent intensity ut and
can be up to 30 times the laminar burning velocity
Laminar flame Turbulent flame
Sl St
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Adv Combustion – Mazlan 2018
FKM
UNIVERSITI TEKNOLOGI MALAYSIA
Turbulent premixed flames
(b) Schlieren image of (a)
reveling its turbulent nature(a) Premixed flames
Stoichiometric mixture of natural gas and air , Re = 3000
wrinkled
flame
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Premixed conical flame
Turbulent premixed flames
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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
Turbulent 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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Adv Combustion – Mazlan 2018
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UNIVERSITI TEKNOLOGI MALAYSIA
The experimental study is carried out
with a modular, swirl stabilized
burner with center body. The swirl
number can be changed by partially
blocking of the tangential slits of the
swirler. The inner diameter of the
nozzle is D=40mm, the diameter of
the center body is d=16mm. Fuel is
natural gas with a methane content of
98% and/or hydrogen. The air-fuel
mixture is externally premixed to
avoid any mixture fraction
fluctuations in plenum and burner.
The thermal power can be adjusted in
the range from 10kW to 120kW, the
equivalence ratio can be varied
between 0.5 and 1.25. The air-fuel
mixture can be preheated.
Experimental investigation of the premixed turbulent flames by Martin Lauer. A way to increase
efficiency and to reduce emissions of modern combustion systems is to expand the classic range
of stable flames.