11/28/2017 1 P R E M I X E D C O M B U S T I O N Adv Combustion – Mazlan 2018 FKM UNIVERSITI TEKNOLOGI MALAYSIA PREMIXED COMBUSTION Professor Dr. Mazlan Abdul Wahid Faculty of Mechanical Engineering Universiti Teknologi Malaysia Advanced Combustion MKMM 1443 P R E M I X E D C O M B U S T I O N Adv Combustion – Mazlan 2018 FKM UNIVERSITI TEKNOLOGI MALAYSIA 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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Adv Combustion – Mazlan 2018
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PREMIXED
COMBUSTION
Professor Dr. Mazlan Abdul WahidFaculty of Mechanical EngineeringUniversiti Teknologi Malaysia
– 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: