Combustion in Premixed & Diffusion Flames

8/6/2019 Combustion in Premixed & Diffusion Flames

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Rahul Singh Tomar

Roll No. 03

COMBUSTION IN PREMIXED & DIFFUSION

FLAMES



COMBUSTION

Whats combustion?

� A chemical reaction between substances, usually including oxygen

and usually accompanied by the generation of heat and light in the

form of flame.



COMBUSTION PROCESS

� The combustion spreads from the ignition source to the adjacent

layer of gas mixture; in turn, each point of the burning layer serves

as an ignition source for the next adjacent layer, and so on.

� Combustion terminates when equilibrium is achieved between the

total heat energies of the reactants and the total heat energies of the products.

� Most reactions terminate when what is called thermal equilibrium

has been attainedi.e., when the energy of the reactants equals the

energy of the products.



FLAMES & FLAME FRONT

Whats a flame ?

� Its a rapidly reacting body of gas, commonly a mixture of air and a combustible gas, that gives off heat and usuallylight and is self-propagating.

Whats its Front?

� The leading edge of a flame propagating through a

gaseous mixture or across the surface of a solid or liquid.



FLAME

Flame front



TYPES OF FLAMES

� Diffusion

� Premixed

� Reducing / oxidizing



TYPES OF FLAMES

1.Diffusion flame

� These are the flames whose ingredients are not mixed

prior to entering the burning zone.

� Molecular or turbulent diffusion is responsible for the

mixing of the gases in such flames.

� E.g.: Flame of a candle , or burning wood etc.



TYPES OF FLAMES

1.Diffusion flame

� E.g. Solid fuel burning

in open air



DIFFUSION FLAME CHARACTERISTICS

� The distribution of the combustible material and of oxygen over

various flame cross sections is regular in laminar flames but becomes

much more complicated in turbulent gas flows.

� The flame speed is limited by the rate of diffusion, and hence

Diffusion flames tend to burn slower.

� More amount of soot is produced in the burning process

� The soot typically produced in a diffusion flame becomes

incandescent from the heat of the flame and lends the flame its

readily identifiable orange-yellow color



DIFFUSION FLAME CHARACTERISTICS

� This type of flame causes inefficient burning of the fuel

� The flame is of the reducing type.

� Diffusion flames tend to have a less-localized flame front

than premixed flames



TYPES OF FLAMES:

2. Premixed :

� A premixed Flame combustion is the one where fuels

have been premixed with an oxidant, ie either oxygen or a

compound that provides oxygen, for the reaction.

� The temperature of flames with this mixture is often

several thousand degrees.



TYPES OF FLAMES:

2. Premixed :

� E.g. : Bunsen burner flames



TYPES OF FLAMES:

2. Premixed :

� E.g. : Gas cutters



PREMIXED FLAMES CHARACTERISTICS

� Oxidizer has been mixed with the fuel before it reaches

the flame front.

� Have relatively higher temperatures than diffusive flames.

� Well defined and sharp flame front.

� Less amount of soot generated

� More efficient combustion and rapid burning of fuel



PREMIXED FLAMES

� Effect of premixed

oxygen/ oxidant

More air- oxidizing flame

Less air- reducing flame

If air level is reduced

below a certain level

mixed flame or diffusion

flame



DIFFUSION FLAMES

Combustion in Premixed & Diffusion Flames

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