flames

1. FLAMES

2. 1. Definitions.
2. Premixed and diffusion flames.
3. The structure of a laminar flame.
Presentation Outline

3. Definitions

4. Flame
 It is a thermal wave in which rapid exothermic
chemical reaction occurs and travels with
subsonic velocities.
 Subsonic flame propagation is also called;
deflagration wave.
 Luminosity in a flame is not essential but is
almost always present.

5. Combustible Mixture
 A combustible mixture is one which is capable
of propagation a flame indefinitely away
from the ignition source.

7. Types Of Flames
There are two types of flames:
1. Premixed flame.
2. Diffusion flame.

8. Premixed Flame
It is the flame in which the gaseous or atomized
fuel is mixed with the necessary amount of air
before issuing to the burner.
Fuel + Air
mixture

9. Diffusion Flame
- It is the flame in which the fuel and air are
introduced separately into the combustion and
the fuel burns as it mixes with air.

10. Flame Velocity
or burning velocity su
The flame velocity is the velocity at which the
unburned gases move through the combustion
wave in a direction normal to the wave surface.
The flame velocity depends on:
1. Initial temperature.
2. Pressure.
3. Composition of the gases, ”combustible mixture”.

13. Premixed
and Diffusion
Flames

15. Types Of Flames (cont.)

19. Bunsen Flame Structure (Cont.)
The actual visible flame differs from
Michelson model in the following respects:
- The tip of an actual Bunsen burner is rounded
instead of pointed one as given theoretically.
- The base of theoretical flame is equal to the inner
diameter of the tube while the actual flame overlaps the
burner.

21. Preheat zone :
•The mass element gains heat by conduction from the
hoter elements down stream faster than it losses heat
to cooler elements up stream.
• little heat evolved.
Reaction zone :
• increased rate of chemical reaction .
• more energy is evolved due to
chemical reaction.

22. The Structure Of A Laminar
Premixed Flame
1. Microscopic structure.
Temperature and concentration gradients
across the combustion wave.
2. Macroscopic structure
flame shape.

31. Factors Affecting The Flame
Shape
The shape of a flame is mainly governed
by two factors :
 The flow pattern of the mixture or
products.
 The quenching effect of the solid
surface.

32. Theories Of Laminar Flame
Propagation
- The factors which control the rate of
flame propagation :
1. Rate of heat transfer . (from reaction zone to the
adjucent heating zone) .
2. Diffusion of radicals or chain corriers (from the reaction
zone to unburnt gases).
3. Mechanism of reacion.
4. Chemical kinetics.(of individ reacions in the mechanism).
Current theories:
-thermal -diffusion -comprehensive

33. THERMAL THEORIES
-Mallard and le Chatelier equation (1883)
-( dT/dx) I=(Tb-Ti)/(xb-xi) .
(assuming no chemical reactions before Ti )
Where :-
-(dT/dx)I is the temperature gradient ,Tb is the
temperature at the boundary of the reaction
zone , Ti is the ignition point temp. , and ,
(xb-xi) is the reaction zone thickness.

34. MALLARD AND LE CHATELIER
EQUATION (CONT..)
The quantity of heat necessary to heat Su cm3
of fresh gas to the ignition temperature is :
q=cp(pusu)(Ti-Tu)
Where :
cp is the mean specific heat from Tu to Ti .
pu is density of the fresh gase , su the
velocity of flame propagation , Tu is the
initial temp.

35. MALLARD AND LE CHATELIER
EQUATION (CONT..)
The heat transfer by conduction from the
Reaction zone is :
- k(dT/dx)
Where : k is the thermal conductivity .
Hence : cp (pusu)(Ti-Tu)=k(Tb-Ti)/(xb-xi)
Or : su=(k(Tb-Ti)) / ((xb-xi) )(Ti-Tu)cppu)
Note : it is not easy to determine the ignition temperature
Ti ,for use in this equation.

36. JOUGUET AND NUSSELT
EQUATION (1952)
K=K0T (assuming simple linear
Relation between K,T)
Where : K is the reaction rate constant, which
differs greatly from the law of arrhenius :
K=Ae(E/RT)

37. JOUGUET EOUATION
Jouguet wrote the continuity ,momentum and
Energy equation for the two zones :
-The preheat zone before temperature
-the reaction zone from TI to Tb
(assuming a steady state ,one dimensional adiabatic
Flow with a flat stationary flame conducting heat dimensional
In the x-direction only and having a fixed ignition
Point .)
.

38. JOUGUET EOUATION
(CONT..)
The boundary conditions are :
At x= -∞ , T=Tu, dT/dX=0
and chemical variable α=0
At X=+∞ , T=Tb , dT/dx=0
And chemical variable α = 1

39. JOUGUET EQUATION
(CONT..1)
From the previous ,jouguet obtained the following
expression for the velocity of the flame propagation Su
Su= To/
Where To and po are the stagnation temperature and
pressure ,k is the thermal conductivity of the gas
mixture ,R is the gas constant and n is the number of
moles of gas mixture.
( this relaction is correct for a unimolecular reaction
when the reaction rate K is proportional to the temp)

40. CHARACTEARISTIC OF
DIFFUSION FLAME
Burning rate : is determined by the rate at
which the fuel and oxidizer are brought
together in proper proportion for the reaction.
diffusion flame are used more frequently in
industry.
in diffusion flames the reaction occurs mainly in the
maximum temperature region
Of the flame , but in the premixed flame the reaction
occurs before the maximum temperature is reached.

41. GASEOUS DIFFUSION
FLAME
Burke and solumann studied the gaseous
diffusion flame in a tube in which the fuel
stream was surrownded by an annular
stream of air , the two streams have the
same initial velocity.
Flame boundary is defined as the
surface at which combustion is complete.

42. Under – ventilated
Flame
Less oxygen
Over-ventilated
Flame
More oxygen than
necessary for complete
combustion
Air fuel Air

44. EFFECT OF GASF LOW ON
DIFFUSION FLAME SHAPE
The laminar characteristic of the diffusion
flame changes with increasing the gas flow.
Break point : is defined as the point
where
the laminar stream changes to turbulent .

45. DIFFRANCE BETWEEN
DIFFUSION FLAMES AND
PREMIXED FLAMES
1- Combustion occurs at the interface between
the fuel gas and oxidant gas .
2- The burning process depends more upon
the rate of mixing than on the rate of chemical
Reactions involved .

46. SLOW BURNING
DIFFUSION FLAMES
(such as candle flame ; main reaction zone,
c2&cH, and luminous zone)
1-fuel rises slowly and laminar flow ensures.
2-The mixing process occurs solely by
molecular diffusion.

47. INDUSTRIAL BURNERS
AND GAS TURBINES
CHARATARISTIC
1-Burning is rapid.
2-Flow speeds are high.
3-Mixing process is associated with the
turbulence of the flow.

48. Concentration profiles through a laminar
Diffusion flame

49. Any Questions