The Role of Hydrodynamic Instabilities On Near-LBO 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. 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) and a local hydrodynamic linear stability analysis were used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable non-reacting flow field. Furthermore, fuel Mie scattering measurements provided evidence that the FS2 spray was periodically disturbed by a helical vortex. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1. The linear stability analysis supported the hypothesis of a hydrodynamically unstable FS2 flow field and suggested that the flame shape fluctuations happen at a condition of marginal hydrodynamic stability.