Investigation of thermo-acoustic instabilities and flame structures in a partially premixed combustor
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摘要:
为探明旋流部分预混燃烧室热声不稳定特性及其与火焰结构关系，进行了自激热声实验。采用重构相图解析压力脉动、采用瑞利指数表征其与热释放率脉动关系、采用本征正交分解获得火焰相干结构。结果表明：随当量比增加，压力脉动呈现低振幅脉动、间歇振荡、极限环振荡和低振幅脉动，脉动频率受腔体1阶纯声学模态（特征频率约80.8 Hz）控制。未发生振荡，瑞利指数维持在零值；热声不稳定时，瑞利指数在零值以上。本征正交分解表明，极限环振荡，前2阶模态（模态能量占比55%以上）火焰分布沿纵向发生变明和变暗交替变化，由连续涡脱落导致，且模态时间系数频率83.2 Hz与压力脉动频率83.3 Hz一致；低当量比，模态无明显空间分布规律；间歇振荡，主导模态为火焰轴对称热释放率变化；高当量比，火焰仅外边缘沿纵向发生大尺度脉动。
Abstract:Self-excited instability experiments were performed to investigate the thermo-acoustic instabilities and their relationship with heat release rate fluctuations in swirled partially premixed combustors. The reconstructed phase space was used to figure out the pressure fluctuations, and their relationship was analyzed by using Rayleigh index. The proper orthogonal decomposition method was employed to extract flame coherent structures. Results indicated that pressure fluctuations presented evolutions of low-amplitude, intermittent, limit cycle and low-amplitude oscillations, with the increase of equivalence ratio. The fluctuation frequency was controlled by the first order pure acoustic mode of the cavity (eigenfrequency was around 80.8 Hz). The Rayleigh index was around zero value when oscillations occurred, while this parameter maintained positive at the thermo-acoustic instability state. The proper orthogonal decomposition results showed that the flame distributions reflected by the first two order modes (the total energy of these two mode was above 55%) became bright and dark alternately along the longitudinal direction at the limit cycle, resulting from continuous vortex sheddings. The frequency of time coefficients (83.2 Hz) was consistent with that of pressure pulsations (83.3 Hz). At the low equivalence ratio, the mode spatial distribution had no regular pattern. When oscillations were intermittent, the dominant mode was of an axisymmetric pattern. Only the outer edge of the flame had large-scale longitudinal motions at a high equivalence ratio.
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图 1 旋流预混燃烧室实验系统示意图（单位：mm）
Figure 1. Diagram of the swirled premixed combustion system （unit:mm）
图 3 旋流器上游及燃烧室中部位置压力脉动时域、频域及相空间重构图（Qair = 500 L/min，Φ = 0.55）
Figure 3. Pressure fluctuations in the time domain, its frequency spectrum and the corresponding reconstructed phase diagram in the upstream of swirler and the middle of combustion chamber （Qair = 500 L/min, Φ = 0.55）
图 4 旋流器上游及燃烧室中部位置压力脉动时域、频域及相空间重构图（Qair = 500 L/min，Φ = 0.60）
Figure 4. Pressure fluctuations in the time domain, its frequency spectrum and the corresponding reconstructed phase diagram in the upstream of swirler and the middle of combustion chamber （Qair = 500 L/min, Φ = 0.60）
图 5 旋流器上游及燃烧室中部位置压力脉动时域、频域及相空间重构图（Qair = 500 L/min，Φ = 0.70）
Figure 5. Pressure fluctuations in the time domain, its frequency spectrum and the corresponding reconstructed phase diagram in the upstream of swirler and the middle of combustion chamber （Qair = 500 L/min, Φ = 0.70）
图 6 旋流器上游及燃烧室中部位置压力脉动时域、频域及相空间重构图（Qair = 500 L/min，Φ = 0.80）
Figure 6. Pressure fluctuations in the time domain, its frequency spectrum and the corresponding reconstructed phase diagram in the upstream of swirler and the middle of combustion chamber （Qair = 500 L/min, Φ = 0.80）
图 7 实验段腔体1阶及2阶声学模态
Figure 7. The first and second order acoustic modes of the experimental part cavity
图 8 不同当量比下压力脉动与热释放率脉动对比（Qair=500 L/min）
Figure 8. Comparison of pressure and heat release rate fluctuations for different equivalence ratios （Qair=500 L/min）
图 9 当量比Φ=0.55，0.70时热释放率脉动频谱
Figure 9. Frequency spectrum of heat release rate fluctuations at equivalence ratios Φ=0.55，0.70
图 11 不同当量比下火焰CH*时均图像（Qair=500 L/min）
Figure 11. Time-averaged image of CH* chemiluminescence distributions （Qair=500 L/min）
图 12 当量比对模态能量分布的影响
Figure 12. Effect of equivalence ratios on mode energy distributions
图 13 火焰时均图像及前5阶POD模态分布（Φ = 0.55）
Figure 13. Time-averaged and the first 5 POD modes of flames （Φ = 0.55）
图 14 火焰时均图像及前5阶POD模态分布（Φ = 0.60）
Figure 14. Time-averaged and the first 5 POD modes of flames （Φ = 0.60）
图 15 火焰时均图像及前5阶POD模态分布（Φ = 0.70）
Figure 15. Time-averaged and the first 5 POD modes of flames （Φ = 0.70）
图 16 火焰时均图像及前5阶POD模态分布（Φ = 0.80）
Figure 16. Time-averaged and the first 5 POD modes of flames （Φ = 0.80）
图 17 前2阶POD模态时间系数时域和频谱（Φ = 0.55）
Figure 17. Time coefficient of the first 2 POD modes in the time and frequency domains （Φ = 0.55）
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