Preheat Zone: Negligibly small heat release.
• 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.

Luminous Zone: Portion of flame in which temperature is high and has several radicals to emit radiation.

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.

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.


• Laminar burning velocity dictates flame quenching.

First laboratory premixed flame burner : invented by Robert Bunsen in 1855

How to get a parabolic velocity profile?
By providing sufficient tube length to ensure fully developed flow.

Angle Method

• 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 S_L V_u: average flow velocity in tube
  A_t: tube cross sectional area
  A_F: 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.

By carrying out a mass balance across the flame, V_u - average flow velocity of the unburned fuel-air mixture
A_t - cross-sectional area of the tube
A_F - conical surface area of the flame
flame surface area,