Insight into the flow dynamics of a high shear injector equipped with center-body: Suppression of precessing vortex core oscillations

Abstract

A precessing vortex core (PVC) is a self-excited helical instability that results from the precession of the vortex core around the flow axis in the upstream region of a vortex breakdown bubble. PVC oscillation in a swirl flow-based combustor aids the thermoacoustic instability that results in hardware damage and poor emission characteristics of the engine. The PVC oscillation can be suppressed intermittently or absolutely in the high shear injector-based combustor with proper design and placement of the fuel nozzle in the injector. A high shear injector is an arrangement of two radial swirlers in general, namely, primary and secondary swirlers, equipped with a fuel nozzle at its center to deliver the fuel. In this study, we examine the impact of the placement of the fuel nozzle/center-body and its design over the dynamics of PVC oscillations in a non-reacting flow in a counter-rotating swirler/high shear injector. Time-resolved high-speed (@ 5 kHz) stereoscopic particle image velocimetry measurements are conducted to elucidate the dynamics of PVC and other coherent structures. Spectral proper orthogonal decomposition of the velocity field data shows that fuel nozzle flushing with the base of the primary swirler has the most robust PVC oscillation that subsequently gets intermittent or suppressed by placing the center-body of diameters, Dc = 7, 9, and 11 mm at constant upstream mass flow rate. The results show that the center-body with the end face flushing to the exit plane would be helpful to avoid PVC with proper selection of the center-body diameter.