Analysis of the Thermoacoustic Flame Instability With Proper Orthogonal Decomposition

Abstract

Lean premixed combustion technology is a method that can inhibit the NOx emission by decreasing the flame temperature. Nevertheless, lean combustion systems are more sensitive to acoustic oscillations than rich systems. As a result, thermoacoustic instability, which is caused by the coupling between pressure and heat release rate oscillations, could causes serious device failure in the lean operating systems. A common mechanism that can trigger thermoacoustic instability is the flame-vortex interaction. The vortices forming in the shear layer can directly affect the energy exchange between combustion and the pressure field. Flame structure can experience significantly spatial change with the effect of the vortex. Therefore, this is essential to obtain the local heat release rate information to understand the global flame behavior. To capture the local information of the flame, planar laser induced fluorescence of OH radicals (OH-PLIF) was used to obtain the flame surface density (FSD), which is directly related to the local mean heat release rate. However, interruptions from the turbulent flow makes the raw local FSD data are difficult to be analyzed, especially when the acoustic perturbation level is low. To overcome this problem, the proper orthogonal decomposition (POD) method was used in the current research to analyze the FSD data to capture the dominant trend of the flame oscillation. The POD method was applied to a 5 m/s premixed low swirl flame forced by different levels of acoustic perturbation. After the dominant POD modes that contain most of the oscillation energy were obtained, they were used to reconstruct FSD results. By comparing the FSD results gained with and without POD method, it can be concluded that the dominant modes of POD can reasonably capture the key features of the heat release rate oscillation. Analysis results demonstrate that the POD method is a good candidate that can be applied to unstable combustion study to capture the dominant global and local heat release rate oscillation information, which is essential in understanding the thermoacoustic instability.