Evolution characteristics of 3D vortex structures in stratified swirling flames studied by dual-plane stereoscopic PIV

Stratified swirling flames are typically formed in centrally-staged combustors of modern aero engines. The pilot flame plays an indispensable role in stabilizing the stratified swirling flame. Evolutions of the primary recirculation zone (PRZ) and vortices in the central shear layer (CSL) notably affect dynamics of the pilot flame. This work employed a dual-plane stereoscopic particle image velocimetry (DP-s-PIV) to investigate the evolution characteristics of three-dimensional vortex structures in stratified swirling flows/flames at different Re numbers and working conditions. Time-resolved measurements obtained temporal evolutions of vortex structures and the scanning process resolved time-averaged volumetric flow fields under different working conditions. Results show that the flame-induced flow acceleration near the burner outlet enhances the vorticity and the strain rate of the central shear layer (CSL), where a precessing vortex core (PVC) resides and its dynamics is strongly coupled with the primary recirculation zone (PRZ). The lengths of the PRZ in swirling flames were found to be smaller than that of the swirling flow, owing to the reduced swirl intensity and the enhanced turbulent-induced momentum transport. Furthermore, statistical analyses based on the instantaneous flow fields of the reacting cases demonstrated that the averaged vorticity magnitude and averaged turbulent kinetic energy (TKE) for vortices in the CSL essentially obey the linear dependency and square dependency with the Re number, respectively. And, there exists a linear correlation between the average turbulent kinetic energy and averaged vorticity magnitude for vortices in the CSL. This implies that the enhanced vortices due to the flame-induced effect downstream of the pilot stage is the main source for local turbulence generation in stratified swirling flames.