A flame (flamma) is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone

1. z
Flames –
Formation, Types,
Structures

2. z
FLAMES
 A flame (from Latin flamma) is the visible, gaseous part of a fire
 It is caused by a highly exothermic reaction taking place in a thin zone
 Flame is a luminous zone of the rapid exothermic reaction in combustion of
vapour with the formation of light and heat energy
 A non-luminous region is appeared just after the flame where the
temperature is slightly reduced.
 A flame is bounded between the ignition zone and a non-luminous gaseous
zone.

3. z
FORMATION OF FLAME
 If the combustible substances produce vapour during burning process, a
flame is produced.
 The combustion of gaseous fuels in a flame need the intimate contact of fuels
with an oxidant, either oxygen or air prior to the reaction.
 The ranges of flammability and the point at which the mixture spontaneously
ignites must be known. They must be heated to the combustion temperature
and the flame produced will be at a high temperature.
 Then the reaction take place in within a narrow zone or region in the flame.
This combustion zone is called the flame front with this mixture is often
several thousand degrees.

4. z
FLAME TYPES
 Depending on the amount of oxygen available for burning the flames can
be of two types
Nonluminous or blue flame Luminous flame
When the supply of oxygen is
sufficiently large, the combustion is
complete and fuel burns with a blue
flame.
When the supply the oxygen is insufficient,
the combustion is not complete and in the
flame some unburnt carbon particles are
formed.
For example, the flame in a pressure
stove.
These carbon particles become hot and glow in flame.
As a result, the flame emits yellow light. This type of
flame is, therefore called luminous flame. For example,
the flame of a kerosene lamp.
This type of flame does
not give much light and is called
nonluminous flame.
In kerosene lamp the fuel does not undergo complete
combustion due to the insufficient supply of oxygen.

5. z
FLAME TYPES
 Flames may be of different types depending on the extent of mixing of fuel
and oxidizer or how the mixture reach the reaction zone
 The flow patterns in the reaction vessel, such as well mixed and plug flows
are the major tools to classify the flames in different types
 The flame may be turbulent and laminar types depending on the flow
behaviour of the combustion gases
 The flames are mainly classified as
 Non -premixed or Diffusion flames and
 Premixed flames

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FLAME TYPES

7. z
NON-PREMIXED FLAMES
 In many combustion processes, the fuel and air are often initially not mixed.
 The fuel and oxidizer are kept on either side of the reaction zone and moved
to the reaction zone. The resultant flame is termed as the diffusion or non-
premixed flame.
 In some cases, the gas and air are injected in a coaxial parallel tubes and
ignited.
 The flow behavior of the non-premixed flame is laminar type.
 Molecular or turbulent diffusion is responsible for the mixing of the gases in
non-premixed flames.
 In a laminar flow region, they reach the reaction front by diffusion, before the
reaction takes place.

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 The products of combustion also diffuse to come out from the reaction zone.
The flame is also called Laminar Non-Premixed Flames or Diffusion Flame.
 The diffusion flame occurs at the interface of the gaseous fuel and air.
 With the progress of time as the flame propagates, the thickness of the
reaction zone increases.
 In diffusion flame the combustion rate will solely controlled by the diffusion
rate, not by the kinetic rate of reaction. The mechanism of this flame is very
complicated.
 The fuel approaches gradually towards the flame zone, there is a deficiency of
oxygen and it is pyrolyed to smaller molecules or radicals. Then, there will be
the formation of carbon soot and fuel burns with a bright luminous yellow
flame.

9. z
 As the flame propagates, the oxygen concentration in the zone increases and
the product of pyrolyzed products are further reacted
 Gradually the reaction occurs under the stoichiometric proportion of oxygen
and combustible
 Under this circumstances, the total enthalpy of the reactants will be equal to
the total enthalpy of the products and the energy losses
 The non premixed flames at stoichiometric conditions optimize the flame
temperature with a definite fuel-air ratio

10. z
PREMIXED FLAMES
 In this type, the fuel and oxidant gas are mixed together at ambient condition
before being delivered to the flame zone.
 As the mixture approaches the flame front, it is heated by conduction and
radiation.
 A reaction takes place before reaching the flame front. Gradually the mixture
is sufficiently heated at the reaction front and the chemical reaction takes
place.
 If the fuel and the oxidant gas are thoroughly mixed prior to reaching the
flame front, the location of the flame front does not depend on the diffusion of
reactants.

11. z
 The flame speed or the velocity of the reactants in the reaction zone is
important
 In another case, the fuel is injected into the oxidant gas flow in the upstream
side of the flame front but the mixing is improper
 The turbulent pre-mixed flame plays an important role in the various
practical applications because, it increases the ignition of fuel with the
reduction in the emission of gases
 Both the laminar premixed or laminar non-premixed flames (diffusion flame)
are slow processes and not economic
 Turbulence actually results in a reduction in flame length. The turbulence
can be increased by recirculation of the fuel-air mixture

12. LAMINAR PREMIXED FLAMES
A premixed flame is a self-sustaining propagation of a localized combustion zone
at subsonic velocities (deflagration regime)
The classical device to generate a laminar premixed flame is the Bunsen burner:
Typical Bunsen burner flame

13. Inner cone (dark
zone):
fuel rich flame
Preheating region
containing fuel and
air
Outer cone (luminous
zone):
reaction and heat transfer
Outer diffusion
flame
Typical Bunsen-burner flame is a dual flame
• a fuel-rich premixed inner flame
• a diffusion outer flame: CO and H2 from inner flame encounter ambient air
TYPICAL BUNSEN-BURNER CH4/AIR
FLAME

14. Experimental evidence for the presence of a cool inner preheating region
A wire to reveal the presence of a cool preheating region containing unburned
CH4 and O2
A match in preheating region does not ignite until it is moved to the inner cone

15. Fuel/Air Ratio
Flame colour, i.e.
colour of the outer
cone
Fuel lean Stochiometric Fuel rich Very fuel rich
Deep Violet
due to large
concentrations of
excited CH
radicals
Blue Green
due to large
concentrations of
C2 species
Yellow
due to carbon
particles
High-T burned
gases usually
show a reddish
glow due to
radiation from
CO2 and H2O
BASIC FEATURES OF LAMINAR
PREMIXED FLAMES
Flame characteristics for hydrocarbon-air stochiometric mixtures
• The flame is ~1 mm thick and moves at ~0.5 m/s
• Pressure drop through the flame is very small: ~1 Pa
• Temperature in reaction zone is ~2200-2600 K
• Density ratio of reactants to products is ~7
• 2 sub regions exist: a fast chemistry zone, dominated by bimolecular
reactions, and a slow chemistry zone (CO+OH=CO2+H)

16. TURBULENT FLAMES
• Most of combustion devices operate in
turbulent flow regime, i.e. internal
combustion or aircraft engines, industrial
burners and furnaces. Laminar combustion
applications are almost limited to candles,
lighters and some domestic furnaces.
Turbulence increases the mixing processes
thus enhancing combustion.
• Also combustion influences turbulence.
Heat release due to combustion causes very
strong flow accelerations through the flame
front (flame-generated turbulence).
Moreover, huge changes in kinematic
viscosity associated with temperature
changes may damp turbulence leading to
flow re-laminarization

17. PREMIXED FLAMES IGNITION AND
STABILIZATION
Two ways exist to cause ignition
• Self-ignition. Reactants’ temperature and pressure are such that combustion
is self-sustained
• Induced ignition. The reactants’ mixture is ignited locally by means of sparks,
piloted flames, hot wires
Ignition criteria
• Chemical-diffusive theory (Tanford and Pease). Ignition is caused by radicals
recirculation in the preheating region
• Thermal theory. The heat provided to the fluid in the preheating region is
enough to initiate oxidation processes
 Ignition occurs when the heat generated in the reaction zone equals heat
losses to surrounding (Jost)
 Ignition occurs when the cooling time of reactant’s mixture, heated up to
adiabatic flame temperature, is greater than chemical reaction time
(Zeldovich)
 Ignition occurs when a portion of fluid as thick as a laminar propagating
flame is heated up to adiabatic flame temperature (Lewis e Von Elbe)

18. z
PREMIXED VS DIFFUSION FLAMES

19. z
LAMINAR DIFFUSION FLAMES

20. DIFFUSION FLAMES
• In a diffusion flame combustion occurs at the interface
between the fuel and the oxidizer. Fuel burning rate
depends more on reactants’ diffusion than on chemical
reaction rates.
• It is more difficult to give a general treatment of diffusion
flames, largely because no simple, measurable parameter,
analogous to laminar burning velocity (SL) in premixed
flames, can be defined.
Typical concentration profiles for
diffusion flames:
• Diffusion flames equivalence ratio
varies locally (Φ>1 and Φ<1
regions)
• Real flame front is no zero-
thickness layer

21. z
FLAME STRUCTURE
 In the premixed type, the laminar flame is the most simple type. The
structure of the flame may be analysed by a flame in the burner.
 The flame consists of four distinct regions
1. Zone containing unburnt gases
2. Reaction zone
3. Incomplete combustion zone, and
4. Complete combustion zone.
 The idealized shape of the reaction zone of a laminar premixed flame is a
cone.
 The height of the cone represents the flame length, and depends on the
velocity at the burner outlet.

22. z
STRUCTURE OF A CANDLE FLAME
Outer zone (blue)
Non luminous zone
(Complete combustion)
Middle zone (yellow)
Luminous zone
(Partial combustion)
Inner zone (black)
Unburnt wax vapours
Hottest part
Moderately hot
Least hot

23. z
BURNER ADJUSTMENTS
a. Correct flame
b. Over-aerated
c. Under-aerated

24. z
FLAME PROPOGATION
Laminar premixed flame in a Bunsen Burner

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FLAME COLOUR
 Flame color depends on several factors, the most important typically being black-
body radiation and spectral band emission, with both spectral line emission and
spectral line absorption playing smaller roles.
 In the most common type of flame, hydrocarbon flames, the
most important factor determining colour is oxygen supply and the extent of fuel
oxygen premixing, which determines the rate of combustion and thus the
temperature and reaction paths, thereby producing different color hues.
 In a laboratory under normal gravity conditions and with a closed oxygen valve, a
bunsen burner burns with yellow flame (also called a safety flame) at around
1,000°C (1,800°F).
 This is due to incandescence of very fine soot particles that are produced in the
flame.
 With increasing oxygen supply, less black body radiating
soot is produced due to a more complete combustion and reaction creates enough
energy to excite and ionize gas molecules in the flame, leading to a blue
appearance.
 The spectrum of a premixed (complete combustion) butane flame shows that the
blue colour arises specifically due to emission of excited molecular radicals in the
flame, which emit most of their light well below ~565 nanometers in the blue and
green regions of the visible spectrum.

26. z
 The colder part of a diffusion (incomplete combustion) flame will be red,
transitioning to orange, yellow and white as the temperature increases
as evidenced by changes in the blackbody radiation spectrum.
 For a given flame's region, the closer to white on this scale, the hotter that
section of the flame is.
 The transitions are often apparent in fires, in which the color emitted closest
to the fuel is white with an orange section above it and reddish flames the
highest of all.
 A blue colored flame only emerges when the amount of soot decreases and the
blue emissions from the excited molecular radicals become dominant though
the blue can often be seen near the base of the candles where airborne soot is
less concentrated.
 Specific colors can be imparted to the flame by introduction of excitable
species with bright emission spectrum lines.

27. z
 Different flame types of a Bunsen burner depend on oxygen supply. On the left a
rich fuel with no premixed oxygen produces a yellow sooty diffusion flame; on the
right a lean fully oxygen premixed flame produces no soot and the flame color is
produced by molecular radicals, especially CH and CO band emission

28. z
FLAME TEMPERATURE
 When looking at a flame's temperature there are many factors which can
change or apply. An important one is that a flame's color does not necessarily
determine a temperature comparison because blackbody radiation is not the
only thing that produces or determines the color seen; therefore it is only an
estimation of temperature.
Other factors that determine its temperature:
 adiabatic flame; i.e., no loss of heat to the atmosphere (may differ in
certain parts);
 atmospheric pressure
 percentage oxygen content of the atmosphere;
 the fuel being burned (i.e., depends on how quickly the
process occurs; how violent the combustion is.)
 Any oxidation of fuel
 Temperature of atmosphere links to adiabatic flame temperature (i.e., heat
will transfer to a cooler atmosphere more quickly).

29. z
 How stoichiometric the combustion process is (a 1:1 stoichiometricity)
assuming no dissociation will have the highest flame temperature. Excess
air/oxygen will lower it and likewise not enough air/oxygen.
 In fires (particularly house fires), the cooler flames are often red
and produce the most smoke. Here the red color compared
to typical yellow color of the flames suggests that the temperature is lower.
 This is because there is a lack of oxygen in the room and therefore there is
incomplete combustion and the flame temperature is low, often just 600–
850°C (1,112–1,562°F). This means that a lot of carbon monoxide is formed
(which is a flammable gas)
 When this occurs combustible gasses, already at or above flashpoint of
spontaneous combustion, are exposed to oxygen,
carbon monoxide and superheated hydrocarbons combust and temporary
temperatures of up to 2,000°C (3,632°F) occur.
 Flame temperatures of common items include a candle at 1,400°C (2,600°F),
a blow torch – at around 1,600°C (2,900°F), a propane torch at 1,995°C
(3,620°F), or a much hotter oxyacetylene combustion at 3,000°C (5,400°F).

30. z

31. z
HOTTEST Vs COOLEST FLAME
HOTTEST FLAME COOLEST FLAME
• Dicyanoacetylene, a compound of
carbon and nitrogen with chemical formula
C4N2 burns in oxygen with a bright blue-
white flame at a temperature of 5260K
(4986.85°C, 9008.33°F), & at up to 6000 K
in ozone
• This high flame temperature is partially due
to the absence of hydrogen in the fuel
(dicyanoacetylene is not a hydrocarbon)
thus there is no water among the
combustion products
• At temperatures as low as 120°C, fuel- air
mixtures can react chemically and produce
very weak flames called cool flames.
• The phenomenon was discovered by Humphry
Davy in 1817.
• The process depends on a fine balance of
temperature and concentration of the reacting
mixture, and if conditions are right it can
initiate without any external ignition source.
• Cyclical variations in the balance of chemicals,
particularly of intermediate products in the
reaction, give oscillations in the flame,
with a typical temperature variation of about
100 K, or between "cool" and full ignition. Some
times the variation can lead to explosion.

32. z
TYPES OF COMBUSTION BASED ON
FLAMES
 Combustion with stationary flame
 Surface combustion / flameless combustion
 Submerged combustion
 Combustion with explosion flame
 Pulsating combustion
 Slow combustion

33. z
COMBUSTION WITH STATIONARY
FLAME
 Normal combustion process as practiced in ovens/furnaces.
 The resultant flame front is more or less stationary in space. A stationary
flame may be premixed or diffusion flame.
 In the premixed type, fuel and oxidant are premixed before they enter the
burning zone.
 When the fuel & air are separately supplied to the burning zone, the flame is
called a diffusion flame.
 In practice, a part of the total air may be premixed with the fuel and the
remaining may be directly supplied to the combustion area.
 The premixed air is known as primary air and the rest is called secondary air.
While solid, liquid and gaseous fuels can all give premixed flame, a truly
diffusion flame is obtained only with gaseous fuels.

34. z
SURFACE COMBUSTION OR
FLAMELESS COMBUSTION
 All refractory solid surfaces at high temperatures accelerate the rate of
combustion of fuel gas and air.
 Some solids e.g. platinum can accelerate the process even at low
temperatures.
 Combustion with a stationary flame is limited by a range of velocity and
concentration conditions of the gas and air.
 Stable combustion is possible even outside this range, if the reaction proceeds
in contact with solid surfaces. This is termed as surface combustion.
APPLICATION
 Its industrial application is to achieve rapid combustion of a large quantity of
fuel in a comparatively small space with the production of high temperature
and high heat transfer rate.
 Tunnel burner is a type of gas burner operating on the surface combustion
principle in which normal flames are absent.

35. z
SUBMERGED COMBUSTION
 It is a special case of application of surface combustion process in which, the
burner is partly or fully submerged in a liquid and the hot combustion
products bubble through it in an agitated condition.
 High heat transfer rate results from direct contact of hot gases with the liquid
leading to high evaporation rate.
 Up to 95% of the potential heat of the fuel may be useful heat for the process.
 Submerged combustion finds application in the evaporation of severely
scaling/ corrosive solution/liquids.

36. z
COMBUSTION WITH EXPLOSION
FLAME
 It occurs in a homogenous mixture of fuel and air and is characterized by the
flame front progressing rapidly through the mixture.
 The process may be either constant pressure e.g. mine explosion, or constant
volume e.g. combustion in a gasoline engine.
 Detonation is a special type of explosion, where the extremely high reaction
rate generates high velocity pressure waves (1 to 4 km/sec) and an abnormal
rate of pressure rise.

37. z
PULSATING COMBUSTION
 Pulsating combustion occurs when one end of a long tube is open and the
other is closed by non-return valve, and the air & fuel are introduced at the
closed end.
 On ignition, the pressure in the system rises sharply at near-constant volume
and it prevents the flow of the air and fuel momentarily; when the exhaust
gases leave the tube through the open end, a fresh supply of air and fuel
arrives at the hot zone and combustion is repeated in the form of pulsations
whose frequency corresponds to the resonant frequency of the combustion
unit.
 There is no stationary flame in the system. Pulsating combustion is a specific
type of explosion flame.

38. z
SLOW COMBUSTION
 Slow combustion takes place at sub-flame temperatures (<400ºC) at slow but
determinable rates.
 Slow combustion of higher hydrocarbons is useful in determining the chain-
reaction rates.
 In a premixed system of fuel vapour and air, slow combustion proceeds at a
number of points simultaneously in the whole system.
 No reaction zone or flame-front is visible. This process is called homogeneous
combustion which is often characterized by the appearance of cool flames in
succession which emit small quantities of heat and pale bluish light usually
seen only in the dark.
 Slow combustion has no direct industrial application. However, it is indirectly
useful in the study of mechanism of combustion.