mazlan's combustion lectures - premixed …fkm.utm.my › ~mazlan › ?download=mazlan's...

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Adv Combustion – Mazlan 2018

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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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UNIVERSITI TEKNOLOGI MALAYSIA3

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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• 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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• 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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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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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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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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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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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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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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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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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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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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Bluff bodies

Hydrodynamics

Flame holders

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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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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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Bluff body

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Flame stabilized by a different type of bluff bodies

small rodtulip shapering

hanging plate

(R.K Cheng, 2001)

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Nitrogen flame passing a wedge

Wire stabilized flame

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Schematic diagram for gas turbine

• Flame stabilization in gas turbine using flame holders

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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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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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FLAME STABILIZATION

http://liftoff.msfc.nasa.gov/shuttle/usmp4/science/elf_obj.html

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Gas turbine – flame holders

Flame

holder

Flame holder

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Hydrodynamic type of flame stabilizer

Counterflow/opposed flow burner

Schematics diagram

Counterflow flame

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Two turbulent counterflow premixed flames

Turbulent counter (or opposed) flow premixed flames

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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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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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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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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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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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Swirl burner by jet injection

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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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"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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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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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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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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(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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Premixed conical flame


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An experimental setup of fan stirred bomb facility at Leeds

University to study the premixed turbulent flame of various

mixtures

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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.

mazlan's combustion lectures - premixed …fkm.utm.my › ~mazlan › ?download=mazlan's...

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