Fundamentals of Combustion Processes by Sara McAllister l Jyh-Yuan Chen A. Carlos Fernandez-Pello, p 140
COMBUSTION PHENOMENA Selected Mechanisms of Flame Formation, Propagation, and Extinction JOZEF JAROSINSKI BERNARD VEYSSIERE Pp 172
• Fuel (solid, liquid, gaseous) must be present in certain concentrations.
• Other sources of fuel source: if there is a leak, during filling operations, transfer
operations, or excessive dusts.
• Although we often cannot always eliminate these sources we can help by having
good ventilation to keep vapors from building up.
• Out door operations
• Oxygen is the most common oxidizer, especially that found in ambient air.
• For oxygen, we often use “inerting” with nitrogen, helium blankets over
flammable materials to reduce O2 content below that where you can have
• Heat is a common ignition
• “Ignition sources are free!!!”
• Although we can eliminate
ignition sources, it is almost
inevitable that an ignition source
will be available if there is a large
release of flammable material
that cannot be diluted quickly.
• The fire tetrahedron or fire pyramid adds
a fourth component—chemical chain
reaction—as a necessity in the prevention
and control of fires.
• The free radicals formed during
combustion are important intermediates
in the initiation and propagation of the
3 T’s for Combustion
Combustion efficiency can explained in terms of the three T’s;
1. Temperature: high enough to ignite and maintain ignition of the fuel (flash
2. Turbulence: or intimate mixing of the fuel and oxygen
3. Time: sufficient for complete combustion
What is a flame?
A flame is a self-sustaining propagation of a localized combustion zone at subsonic
• Flame must be confined: 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 wave may be also travel at supersonic velocities, called detonation
• Fundamental propagation mechanism is different in deflagrations and detonations
Laminar vs. Turbulent Flames: both have same type of physical process and many
turbulent flame theories are based on an underlying laminar flame structure
Description of a Candle Flame
The flame surface is where vaporized fuel and oxygen
mix, forming a stoichiometric mixture. At the flame
surface, combustion leads to high temperatures that
sustain the flame. The elements of the process are:
• Heat from the flame melts wax at the base of the
• Liquid wax moves upward by capillary action, through
the wick towards the flame.
• Heat from the flame vaporizes the liquid wax.
• Wax vapors migrate toward the flame surface,
breaking down into smaller hydrocarbons.
• Ambient oxygen migrates toward the flame surface by
diffusion and convection.
Selected Mechanisms of Flame Formation, Propagation, and Extinction, pp17
JOZEF JAROSINSKI BERNARD VEYSSIERE
Structure of a Candle Flame
• In the outer zone complete combustion of
the fuel takes place and the color of the
flame is blue and is the hottest part of the
flame. It is the non luminous part of the
• In the middle zone partial combustion of
the fuel takes place and the color of the
flame is yellow and is moderately hot part of
the flame. It is the luminous part of the
• In the inner zone there are un-burnt vapors
of the fuel and the color is black and is least
Radical Mechanism of
The combustion can be explained by radical
mechanism usually consisting of initiation,
propagation, branching and termination.
M is a third body in the system, which is non-reacting.
General Fuel Terms
• Combustion – a chemical reaction in which a substance combines with an
oxidizer and releases energy.
• Explosion – rapid expansion of gases resulting in a rapid moving pressure or
• Mechanical Explosion – due to failure of vessel with high pressure non reactive
• Detonation – explosion (chemical reaction) with shock wave greater than speed
• Deflagration – explosion (chemical reaction) with shock wave less than speed of
• Flash Point (FP) – a property of material used to determine the fire and explosive
hazard. The lowest temperature of a liquid at which it gives off enough vapor to
form an ignitable mixture with air.
• Needs to be determined experimentally.
• Different methods to determine, open cup and closed cup. Open cup is usually a
few degrees higher
• Flammability limits are specify lean
and rich fuel-oxidizer mixture ratio
beyond which no flame will
• There is usually a range of
compositions of a flammable vapor
and air where combustion occurs.
• Too little fuel (lean mixture) not
enough fuel to burn.
• Too much fuel (rich mixture) not
enough oxygen to burn
Mechanism of Combustion
• mechanism of combustion follows the complex phenomena with several
elementary steps. ‘
• In the preliminary study, the possible steps on the overall reaction are to be
• Large number of elementary reaction steps involving large number of
intermediates are possible.
• For example, a large number of species are involved in the combustion of simple
hydrocarbons like CH4.