Effect of air ratio on unsteady partially premixed flames

The burning characteristics of unsteady partially premixed flames have been investigated in this paper. Two reaction zones namely rich inner premixed zone and outer diffusion wing were observed for all the equivalence ratios considered. This kind of structure is called double flame structure. Flickering of flames were observed due to the formation of toroidal vortices around the flame surface. These toroidal vortices are convected up as a result of buoyant acceleration. The flames are observed to oscillate in two different modes namely, bulk flickering and tip flickering. In the bulk flickering mode, the flame tip is chopped off from the flame surface and in the tip flickering mode, the flame surface is observed to oscillate up and down without any breakage of flame tip. Flames with higher equivalence ratio (greater than 6) exhibit bulk flickering and flames with lower equivalence ratio (less than 6) experience tip flickering. As the equivalence ratio is lowered by partial premixing of the fuel with more air, the flame height is reduced and the strength of inner premixed flame is increased. The amplitude of oscillation of the flame height also decreases with reduction in the equivalence ratio due to the formation of smaller toroidal vortices but the frequency of oscillation is found to increase slightly. A generalised Strouhal number - Froude number correlation has been obtained in the form St αFr-β, by varying the flow rates of air or fuel systematically, keeping the other parameters constant. The effect of increasing air flow rate is to increase the flickering frequency and increasing fuel flow rate is to decrease the flickering frequency.