Detecting and characterizing thermoacoustic oscillations in an industrial combustor with the spectral proper orthogonal decomposition

This study demonstrates how thermoacoustic oscillations occurring at multiple frequencies in a combustor with mechanical vibrations can be detected and characterized using high-speed chemiluminescence imaging and the time-domain Spectral Proper Orthogonal Decomposition (SPOD). Three turbulent premixed methane-air flames with different morphologies are presented: an M-flame, a Bunsen flame, and a third case featuring a bistable flame that intermittently transitions between these two flame shapes. The analysis of these cases demonstrates how the SPOD can be used to (1) separate the influence of mechanical vibrations on chemiluminescence measurements from other frequency-centered phenomena, (2) characterize heat release rate oscillations in space and time with modes that describe periodic phenomena with both wide- and narrow-band spectral signatures, and (3) reveal deterministic dynamics with a low signal-to-noise ratio that were not easily detectable in the power spectral densities of either the spatially averaged ${\mathrm{OH}}^{\ast }$ chemiluminescence intensity or the pressure signal recorded upstream of the flame. In addition, this article also discusses how the SPOD enabled the identification of a correlation between the decay of a thermoacoustic mode and the state of a bistable flame.