Large-Eddy Simulation of Acoustic Flame Response to High-Frequency Transverse Excitations

Abstract

The objective of this numerical study is to analyze the excitation mechanisms of high-frequency thermoacoustic instabilities. Two distinct acoustic wave/flame interaction scenarios are considered, which can arise in a technical combustion system. First, an acoustic oscillation is enforced with the pressure antinode at the burner centerline, termed as pressure forcing. Second, a transverse acoustic oscillation is enforced with the velocity antinode at the burner centerline, termed as velocity forcing. The excitations are created by the interaction of two incident acoustic waves for a range of excitation frequencies of practical relevance. The susceptibility of the flame to acoustic oscillations is quantified using the Rayleigh index. Detailed postprocessing is performed to investigate the underlying thermoacoustic mechanisms and to quantify the contributions of each possible mechanism to the total Rayleigh index of the system as a function of different excitation frequencies. It is observed that the main driving mechanism with pressure forcing can be related to the density oscillation and that the main driving mechanism with velocity forcing can be related to the flame surface oscillation relative to the position of the acoustic pressure node. For both forcing cases, the laminar flame speed is varied to change the length of the flame. With pressure forcing, the highest Rayleigh index is found for a specific flame length, associated to a specific laminar flame speed. With velocity forcing, the Rayleigh index increases when the flame speed is reduced and the flame length is increased, respectively.