Effects of shear inhomogeneities on the structure of turbulent premixed flames

This paper studies the effects of shear inhomogeneities on the structure and stability characteristics of turbulent piloted premixed flames. The shear is introduced by splitting the fuel-air mixture in two stream and feeding each stream at different velocities through two annular concentric tubes residing within the pilot annulus. The shear is varied by varying the proportion of fluid injected through the streams and also by changing the recess distance of the inner tube with respect to the jet exit plane. It is noted that with increasing rates of shear, the Bunsen-like flames become shorter with the flame tip opening downstream of the pilot stream. Measurements of flow fields reveal useful information about the level and location of the shear layers. Joint imaging of Laser-Induced Fluorescence (LIF) of OH and CH show that, with increasing rates of shear, the degree of flame front fragmentation increases implying the occurrence of local extinction even in regions extremely close to the pilot. The flame surface area, marked by LIF-CH is taken as an indicator of consumption rates of reactants. For all flames examined here, the peak flame surface area increases with increasing shear and subsequently decreases as the flame shortens and approaches blow-off. The peak flame surface density decreases with increasing shear. Regime diagrams show that the flames investigated here bridge the regions of broadened preheat thin reactions as well as the distributed reaction zone. While the thickness of the CH layer remain somewhat uniform, the number of flame fragments, as indicated by breaks in the CH profiles, increases with increasing shear before stabilizing at some peak value. The observed mechanism of local extinction in highly sheared Bunsen-like, turbulent piloted premixed flames complements existing results for bluff-body stabilised flames.