The Role of Hydrodynamic Instabilities on Near-Lean Blowout Flame Shapes in a Swirl-Stabilized Spray Combustor

This study investigates the role of hydrodynamic instabilities on near-lean blowout (LBO) flame shapes in a swirl-stabilized spray combustor. Hydrodynamic instabilities often manifest themselves in swirling flows as a helical vortex that winds around the vortex breakdown bubble. However, the heat released from combustion tends to suppress coherent vortex structures, which can limit the helical vortex to certain combustor geometries and operating conditions. Flame shape changes often accompany changes in hydrodynamic stability because they reposition the heat release and consequently modify the degree of coherent vortex suppression. In this study, laser diagnostics measurements were used to characterize the flow fields and spray patterns corresponding to different flame shapes that were observed in the Air Force Research Laboratory (AFRL) referee combustor. In particular, the flame fluctuated between its original shape, FS1, and a new flame shape, FS2, when the combustor operated on the threshold of LBO. Proper orthogonal decomposition (POD) was used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable nonreacting flow field. Furthermore, fuel Mie scattering measurements and phase-averages of the velocity field provided evidence that the FS2 spray was periodically disturbed by a helical vortex. Near the swirler exit, this helical vortex structure involved both outer and inner shear layer vortices that appeared to be synchronized with each other. However, the inner shear layer vortices decayed as the flow progressed downstream and only the outer shear layer vortices remained throughout the measurements' field of view. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1.