Decomposition of fuel takes place, leading to intermediate radical formation.
Certain chemical reactions take place in this zone.
Reaction Zone: Most of the chemical energy is released in the form of heat.
Reaction zone is very thin as compared to the preheat zone.
Temperature gradient and concentration gradient are high.
Diffusion of heat and radicals from this zone to preheat zone sustains combustion.
Recombination Zone: CO2 and H2O are formed.
Negligible heat release in this zone.
Hydrocarbon flames � characterized by the emitted visible radiation.
Dark zone: unburnt gases are heated to the critical temperature.
Luminous zone: region where much of the chemical reactions take place.
Highest temperature prevails in this zone.
Color of luminous zone depends on the fuel-air ratio.
Fuel lean mixture: blue colored flame due to excited CH radical.
Fuel rich mixture: green colored flame due to excitation of C2 molecule.
Highly fuel rich mixture: yellow colored flame due to soot formation.
Diffusion Zone: outer cone above luminous zone.
Observed especially in rich flames.
Characterization of Premixed Flame
Importance of burning velocity
Premixed flame can be characterized by laminar burning velocity.
Laminar burning velocity influences the flame shape.
Laminar burning velocity dictates the stability characteristics of the flame.
Uniform velocity profile has to be maintained at the tube exit.
Nozzle is employed to maintain uniform velocity profile.
Infinitely thin perfectly conical flame front is established.
For flame to be stationary, the local burning velocity must be equal to the local flow velocity.
Area Method
The gas burns at the exit of the tube and a conical flame with a tip and base is established .
For flame to be stationary, the local burning velocity must be equal to the local flow velocity.
Flame shape will be influenced by the exit velocity profile and heat loss to the tube wall.
Lengthy tube ensures fully developed flow.
For a stationary flame, mass balance provides expression for SL
Vu: average flow velocity in tube, At: tube cross sectional area, AF: conical surface area of flame
This method is known as area method.
Disadvantages:
Heat loss to the wall cannot be avoided completely.
Burning velocity does not remain constant along its surface.
Flame stabilization for large diameter is difficult due to flash back.
Procedure for Area Method
By carrying out a mass balance across the flame,
Vu - average flow velocity of the unburned fuel-air mixture
At - cross-sectional area of the tube
AF - conical surface area of the flame
flame surface area,