Transcript Slide 1

Laminar Flame Theory
By
Eng. Mohamad Okour
UINVERSITY OF JORDAN
MECHANICAL ENGINEERING
DEPARTEMENT
Laminar Flames

Premixed flames

Partial premixed
flames

Diffusion flames
Premixed flames are characterized
by
 Laminar
flame speed
 Flame
structure
 Flame
thickness
THE LAMINAR FLAME SPEED
The flame velocity, which is also called
the burning velocity, normal combustion
velocity, or laminar flam speed
 Defined as:
The velocity at which unburned gases
move through the combustion wave in the
direction normal to the wave surface.

The theoretical approaches to
calculate the laminar flame speed
can be divided into three categories
(a) Thermal theories
 (b) Diffusion theories
 (c) Comprehensive theories
In thermal theories The flame zone
according to Mallard and Le Chatelier
consist of two zones broken up by the
point where the next layer ignites, as
shown in the Figure.


In thermal theories The flame zone according to
Mallard and Le Chatelier consist of two zones
broken up by the point where the next layer
ignites, as shown in the Figure.
The Theory of Mallard and Le
Chatelier

Mallard and Le Chatelier stated that the heat
conducted from zone II in Fig.1 equaled that
necessary to bring the unburned gases to the
ignition temperature (the boundary between
zone I and II).
The enthalpy balance then becomes
 cP Ti  T0    T f  Ti / 
m
 is the
Where  is the thermal conductivity and m
(1)
mass rate into the combustion wave.
m  Au  S L A
(2)
Where  is the density, A the cross-sectional
area, u the velocity of the gases, and SL
the laminar flame speed. Because
unburned gases enter normal to the wave,
by definition S  u
L
Equation (1) then becomes
 SLcP Ti  T0    Tf  Ti / 
Or
SL 
 (T f Ti ) 1
cP (Ti  T0 ) 
(3)
Since
Thus
 is the reaction zone thickness.
  S L   S L
1
(4)
d dt
Where  is the reaction time and (d  dt ) is
the reaction rate.
Substituting this expression into Equation
(4), obtains
  (T f To ) d 

S L  
 c P (Ti  To ) dt 
12
SL ~ (RR)
Where (   / cp  is the thermal diffusivity and

RR is the reaction rate.
 In
order to calculate the laminar flame
speed
one must know the thermophysical
properties of a mixture at high temperatures
and have accurate reaction rate data.
Flame Speed Measurements

Different measuring techniques can be used to measure the laminar
flame speed such as:
1- Bunsen burner method
2- Flame in tubes
3- Soap bubble method
4- Flat flame methods
These methods are arranged in order of decreasing complexity of flame
surface and correspond to an increasing complexity of experimental
arrangement.
In this amount we will discuses the Bunsen Burner Method only.
Bunsen burner method
 In
this method premixed gases flow up a
cylindrical jacketed tube long enough to
insure streamline flow at the mouth .The
gas burns at the mouth of the tube and the
shape of the Bunsen cone is recorded and
measured by various ways.
 The
burning velocity is not constant over
the cone .The velocity near the tube wall is
lower because of cooling by the walls.
Thus there are lower temperature;
therefore, lower flame speeds.
 Actually,
if one measures SL at each point
he will see that it varies along every point
for each velocity vector and it is not really
constant. This is the major disadvantage
of this method.
 The
earliest procedure of calculating flame
speed by this method was to divide the
volume flow rate by the area of flame
cone.
V cm3 / sec
SL 
 cm / sec
2
A cm
 Two
disadvantages of the Bunsen method
are:
1- One can never completely eliminate
wall effects.
2- One needs a steady source of supply
of gas, which for rare or pure gases can
be a severe problem.
Effects of Chemical Variables on
Flame Speed
 Effect
of Mixture ratio
* The vibration of laminar flame speed with
fuel-oxidant ratio is governed by variation
of the temperature with the mixture ratio.
* The mixture with maximum flame
temperature is a mixture with maximum
flame speed
 Effect
of Fuel Molecular
* As the fuel molecular weight increases,
the range of flammability becomes narrow
* The difference in SL for fuels containing
different numbers of carbon is due to the
changes in the thermal diffusivity which is
function of fuel molecular weight.
 Effect
of additives
* The purpose of using additives is to raise
the ignition temperature and reduce the
tendency to preignition as knocking.
* Additives such as acetone, benzene and
so on which studies for oxidation
intermediates in low–temperature
expected to decrease the induction period
and thus increase the flame velocity .
Effects of physical Variables on
Flame Speed
 Effect
o Pressure
* The pressure effect is straightforward ;
the flame speed varies as
( n 2) / 2
P
 Effect
of initial temperature
* The effect of the initial temperature on
the flame propagation rate appears upon
the effect on the final or flame
temperature.
* Generally, small changes of initial
temperature have little effect on the flame
temperature
* The flame propagation expression contains
the flame temperature in an exponential
term; thus small changes in flame
temperature can give noticeable changes
in flame propagation rates.
 Effect
of Thermal Diffusivity
* The flame speed in helium (He) mixture is
higher than that in Argon (Ar) mixture that
is due to the thermal Diffusivity of helium
He which is much larger than Ar because
the molecular weight of He is much
smaller.
Diffusion Flames

Diffusion flame defined as:
* Flame in which the fuel and oxidizer are
initially separated (non-premixed).
* Systems in which mixing is slow compared
with reaction rate so that mixing controls the
burning rate, these systems are called diffusion
flames.

For example a pan of oil burning in air and a
lighted candle produced diffusion flames
Types of Diffusion Flames
 Gaseous
fuel jets
 Burning of condensed phase (liquid or
solid)
 Catalytic combustion
Gaseous Fuel Jets
In combustion field gaseous diffusion
flames have received less attention than
premixed flame, on the other hand
diffusion flames have greater practical
application.
The difficulty with gaseous diffusion
flames is that there’s no fundamental
characteristic like flame velocity which can
be measured.
Appearance of gaseous fuel jet flames
 Appearance
the shape of a laminar jet of fuel depends
on the mixture strength (the quantity of air
supplied ). If an excess of air is present
the flame is a closed, elongated figure.
Such flames occur when two coaxial jet
are used, the inner contain fuel and the
outer contain an excess of air. As shown in
the figure.
 Structure
The diffusion flame has a wide region
over which the composition of the gases
changes. Where the actual reaction take
place in a narrow zone.
It’s most important to release that the
diffusion establishes a bulk velocity in the
direction x (or r).
Burning of condensed phases
When liquids or solids projected to
atmosphere combustible mixture formed
and when this mixture is ignited ,a flame
surrounded the liquid or solid phase.
The diffusion flame happened when the
mixture projected to very lowest pressures
around 10-6 Torr.
 Condensed
phase occurrence
If considered a liquid fuel and gaseous
oxidizer as oxygen, then fuel is evaporated
from the liquid interface and diffuses to the
flame front as the oxygen moves from the
surroundings to the burning front.
Conclusion

Premixed flame:
• Mixing before combustion
• Reacts rapidly
• Propagates as thin zone

Diffusion flame
• Mixing during combustion
• Reaction occurs at Fuel / Air interface
• Controlled by the mixing of the reactants
 References:
1- IRVIN GLASSMAN, “Combustion”,1977.
2- Roger A. Strehlow, “Combustion
Fundamentals” 1984.
3- Internet sites.