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.
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机译：稀薄的预混燃烧技术是一种可以通过降低火焰温度来抑制NOx排放的方法。然而，稀薄燃烧系统比浓密燃烧系统对声振动更敏感。结果，由压力和放热速率振荡之间的耦合引起的热声不稳定性可能会在精益操作系统中导致严重的设备故障。可以引发热声不稳定性的常见机制是火焰-涡旋相互作用。在剪切层中形成的涡流可以直接影响燃烧与压力场之间的能量交换。火焰结构在旋涡的作用下会经历明显的空间变化。因此，这对于获取局部放热率信息以了解整体火焰行为至关重要。为了捕获火焰的局部信息，使用平面激光诱导的OH自由基荧光（OH-PLIF）获得了火焰表面密度（FSD），该密度直接与局部平均放热率相关。然而，湍流的中断使得原始局部FSD数据难以分析，尤其是在声学扰动水平较低的情况下。为了克服这个问题，目前的研究中使用了适当的正交分解（POD）方法来分析FSD数据，以捕获火焰振荡的主要趋势。 POD方法应用于由不同水平的声扰动强迫的5 m / s预混合低涡流火焰。在获得包含大部分振荡能量的主导POD模式后，将其用于重建FSD结果。通过比较使用和不使用POD方法获得的FSD结果，可以得出结论，POD的主导模式可以合理地捕捉放热速率振荡的关键特征。分析结果表明，POD方法是一种很好的候选方法，可用于不稳定燃烧研究，以捕获主要的全局和局部放热速率振荡信息，这对于理解热声不稳定性至关重要。
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