• Flow visualization is carried out by illuminating the jet with a laser sheet (532 nm) passing through the center of the jet.


• The jet issued from the burner exit initially has sufficient momentum and hence the jet is undisrupted.


• Subsequently, this jet interacts with the ambient air to form vortices at the shear layer.
  W_Cr : minimum width after flow gradually starts spreading in radial direction.
  L_Cr : minimum length above which first vortex formation is observed.

• The neck region got widened with increase in swirl number.


• Increase in jet width is due to the enhanced entrainment and mixing of ambient air with the jet.


• At S = 0, first vortex formation is observed at L_Cr/D = 2.8 and W_Cr = d.


• At S = 2, L_Cr/D is decreased by 39 % and W_Cr/d increases by 36 %.


• L_Cr/D and W_Cr/D are decreased almost by 87% and 90% for S = 0 to 9.

Visible flame length, L_v: Total height of visible flame region.

Lift-off height, L_f: The vertical distance from burner exit to the base of lifted off flame.

Flame blow-out limits: Maximum air flow rate beyond which flame cannot be stabilized.

Momentum Flux Ratio, MFR: Ratio of tangential air jet momentum to the axial air jet momentum

• The luminous zone height increases with increase in fuel flow rate.


• With increase in swirl number, the luminous zone of the flame reduces.


• The lift- off height (L_f) decreases with increase in swirl number.

• Visible flame length, L_v increases with momentum flux ratio, MFR = 3–30 and remains almost constant beyond MFR = 30.


• Almost 25 % reduction in flame length is achieved at S = 9 and MFR.


• In buoyancy dominant flow, kinetic energy of flow is not sufficient, so reaction zone is distributed, leading to an increase in flame length.


• In momentum dominant regime, enough kinetic energy leads to better mixing of reactants and thus results in negligible change in L_v.


• Swirling improves mixing in radial direction (as discussed in cold flow), therefore length of combustion zone and thereby L_v decreases.

• Flame blowout is examined by varying the air flow rate until the flame blows out.


• Blowout limit gets enhanced with increase in S = 0–9 for fuel flow rate of 3–5 lpm.


• For 3 lpm fuel flow rate and S = 0, overall F near blow out is ~ l.2.


• A minimum Φ_LBO (= 0.44) is observed for 5 lpm fuel flow rate and S = 9.