A joint experimental and computational study of thermoacoustic instabilities in a model swirl-stabilised combustor is presented. This paper aims to deliver a better characterisation of such instabilities through the examination of measurements in conjunction with the numerical results obtained via Large Eddy Simulation (LES). The experimental configuration features a cylindrical combustion chamber where the lean premixed methane/air flame experiences self-sustained thermoacoustic oscillations. The nonlinear behaviour of the acoustically excited flame is experimentally investigated by broadband chemiluminescence and dynamic pressure measurements. In LES, the Eulerian stochastic field method is employed for the unknown turbulence/chemistry interaction of the gas-phase. Comparisons of the predicted frequency spectrum and phase-resolved flame structure with measurements are found to be in good agreement, confirming the predictive capabilities of the LES methodology. The presence of Period 2 thermoacoustic oscillations, as a consequence of period-doubling bifurcation, is also confirmed in LES. Through the application of nonlinear analysis both to the experimental and numerical acoustic fluctuations, it is highlighted that the nonlinear behaviour of combustion instabilities in the burner under investigation follows a pattern typical of Period 2 oscillations. Furthermore, the current work demonstrates a useful approach, through the use of dynamic mode decomposition (DMD), for the investigation of unstable flame modes at a specific frequency of interest. The experimental and numerical DMD reconstructions suggest that hot combustion products are convected azimuthally at a rate dictated by the subharmonic frequency.

提出了在模型旋流稳定的燃烧室中热声不稳定性的联合实验和计算研究。本文旨在通过对测量结果以及通过大涡模拟（LES）获得的数值结果进行检验，从而更好地表征此类不稳定性。实验配置的特点是一个圆柱形燃烧室，稀薄的预混合甲烷/空气火焰在燃烧室中经历自我维持的热声振荡。通过宽带化学发光和动态压力测量实验研究了声激发火焰的非线性行为。在LES中，欧拉随机场方法用于未知的气相湍流/化学相互作用。发现预测频谱和相分辨火焰结构与测量值的比较吻合良好，从而证实了LES方法的预测能力。在LES中也证实了由于倍增分叉而导致的2期热声振荡的存在。通过将非线性分析应用于实验和数值声学波动，我们强调指出，所研究燃烧器中燃烧不稳定性的非线性行为遵循典型的周期2振荡模式。此外，当前的工作通过使用动态模式分解（DMD）展示了一种有用的方法，用于研究特定感兴趣频率下的不稳定火焰模式。