Multi-scale proper orthogonal decomposition analysis of instabilities in swirled and stratified flames

Abstract

This study examines the flow field dynamics of bluff-body stabilized swirling and non-swirling flames produced from the Cambridge/Sandia Stratified Swirl Burner. This burner has been used in previous studies as a benchmark for high-resolution scalar and velocity measurements and for validating numerical models. The burner was designed to create reacting flow conditions that are representative of turbulent flows in modern combustion systems, including sufficiently high turbulence levels, and to operate under both premixed and stratified conditions. High-speed stereoscopic particle image velocimetry was used to acquire time-resolved velocity data for a series of turbulent methane/air flames at both premixed and stratified conditions. We employ the multi-scale proper orthogonal decomposition (mPOD) to identify the main flow patterns in the velocity field and isolate coherent structures linked to various flow instabilities. The results show that the most energetic structures in the flow are consistent with the Bénard–von Kármán (BVK) instability due to the presence of the bluff-body and the Kelvin–Helmholtz (KH) instability caused by the shear layer between the inner and the outer flow. In both the swirling and non-swirling cases, the BVK is suppressed by the combustion, except for the most stratified swirling case. Moreover, the results show that combustion does not affect the KH instability because the shear layer does not coincide with the flame position.