Investigation on the Flame Front and Flow Field in Acoustically Excited Swirling Flames with and without Confinement

Abstract

Open and confined dimethyl ether/air swirling flames were investigated with an acoustically excited swirling burner. Simultaneous particle-image velocimetry and formaldehyde planar laser-induced fluorescence were measured using a high repetition rate burst-mode laser at 20 kHz. Time-averaged velocity field, vorticity and flame brush cloud were compared under different confinements and acoustic excitation conditions. The integrated fluorescence intensities are similar while the open flame has dramatically large fluorescence areas. Time-resolved velocity field and flame brush were measured to investigate the continuous evolution of swirling flame front and acoustically induced vortex. The flame brush was found to be much dispersed in the open flame due to the mixing effect from the surrounding air. Flame root and flame angle were extracted and analyzed statistically using the fluorescence images. Different stages of acoustically induced vortex were observed in both confined and open cases. The vortex trajectory along the outer shear layer determines that the flame tip can perceive the vortex instead of the flame base. The flame regions affected and unaffected by acoustically induced vortex indicate different perturbation mechanisms for the heat release.