Lean Flame Stabilization by Means of a Coaxial Synthetic Jet

In the present paper the effect of a synthetic jet, coaxial with an annular type combustor, has been investigated in order to infer aerodynamic interaction between the two gaseous flows. The experimental investigation has the purpose to find the existence of synthetic jet excitation frequency able to extend the stability limits of a lean air/propane mixture flame. Lean flame is widely adopted in low thermal NOx emission combustors. As it is well known for gaseous premixed flame increasing the air/fuel ratio (lean flame) the flame propagation velocity is decreasing until it is getting comparable to the velocity of supplied fresh gaseous mixture. At this point, the flame is flushed away from the combustor mouth and the combustion process is quenched (blow off). In order to avoid those effects, for very low air/combustible ratio the modern premixed industrial combustors control the flame stability by means of the so called “pilot flame”, a small secondary relatively rich flame. The pilot flame, fed by a few percent of the total amount of supplied combustible, represent a relatively high source of thermal NOx. The experimental investigations have been performed by means of a Particle Image Velocimetry (PIV), in order to obtain instantaneous two dimensional velocity distribution overall the entire gaseous reacting jet, in order to stretch the influences of coherent structures generated in the jet, by means of natural aerodynamic effect and by means of perturbation forced into the flow by means of the synthetic coaxial jet under combustion conditions.