This chapter explains about the the flame propagation - an its properties

1. LECTURE 7
FLAME PHENOMENA – STRUCTURE SPEED AND
THEORY
By
Er.T. AYISHA NAZIBA, Dr. D. RAMESH, Dr.S. PUGALENDHI, Dept. of REE, AEC &
RI,TNAU, Coimbatore-03

2. 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. 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. FLAMETYPES
 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 non-
luminous flame.
In kerosene lamp the fuel does not undergo complete
combustion due to the insufficient supply of oxygen.

5. FLAMETYPES

7. FLAMETHEORY

8. THERMALTHEORY

10. 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

11. 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

12. 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.

13. 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

14. TYPES OF COMBUSTION BASED ON
FLAMES
 Combustion with stationary flame
 Surface combustion / flameless combustion
 Submerged combustion
 Combustion with explosion flame
 Pulsating combustion
 Slow combustion

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. COMBUSTION OF LIQUID FUELS

23. THANKYOU