A partially premixed gas turbine model combustor close to an industrial design is investigated using Large Eddy Simulation (LES). Two flames, one stable and another unstable with self-excited oscillations are computed. In particular, this study addresses the previously unexplained transition of flame shape in the experiments, from V-shaped to flat when the flame becomes acoustically unstable, suggesting a notable change of the important convective delay in the thermoacoustic feedback loop. The LES results show good agreement with the measured velocities, temperature and mass fractions. The acoustic power spectral density (PSD) obtained from the LES of the unstable flame also agrees well with the measured amplitudes in the air plenum and combustion chamber, and reasonably captures the frequency with a slight under-prediction. A comparison of the stable and unstable cases shows different mixing and reaction behaviours despite similar mean velocity fields. Further detailed analysis shows that the different mixing behaviour is driven by the significantly varying air mass split between the two air passages during a thermoacoustic oscillation cycle. This variation is due to the different impedances experienced by the pressure oscillations propagating through the two swirling injector passages with different internal geometries. This causes a periodic variation of the radial momentum of the fuel jets injected between the two swirling air flows. The resulting flapping of the fuel jets creates an enhanced radial fuel–air mixing that leads to a flattened flame in the unstable case. This provides a new physical explanation for the transitions of flame shape observed in the experiments.

使用大涡模拟（LES）研究了接近工业设计的部分预混燃气轮机模型燃烧器。计算了两个火焰，一个稳定，另一个不稳定，带有自激振荡。特别是，这项研究解决了实验中以前无法解释的火焰形状过渡，即当火焰在声学上变得不稳定时，火焰形状从V形变为平坦，这表明热声反馈回路中重要的对流延迟发生了显着变化。LES结果与测得的速度，温度和质量分数显示出良好的一致性。从不稳定火焰的LES获得的声功率谱密度（PSD）也与在气室和燃烧室中测得的振幅非常吻合，并在略微预测不足的情况下合理地捕获了频率。尽管平均速度场相似，但对稳定和不稳定情况的比较显示出不同的混合和反应行为。进一步的详细分析表明，在热声振荡周期中，两个空气通道之间的空气质量分流明显不同，从而导致不同的混合行为。该变化是由于通过具有不同内部几何形状的两个涡旋喷射器通道传播的压力振荡所经历的不同阻抗所致。这导致在两个涡旋空气流之间喷射的燃料射流的径向动量的周期性变化。由此产生的燃油喷射震荡会增强燃油与空气的径向混合，在不稳定的情况下会导致火焰变平。