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In this paper, deep learning is involved to comprehend thermoacoustic instability more deeply and achieve early warning more reliably. Flame images and pressure series are acquired in model combustors. A total of seven data domains are obtained by changing the combustor structural parameters. Then, the pre-trained model TIPE (Thermoacoustic Image-Pressure Encoder), containing an image encoder with ResNet architecture and a pressure encoder with transformer architecture, is trained through the contrastive self-supervised task of aligning the image and pressure signals in the embedding space. Furthermore, transfer learning in thermoacoustic instability prediction is performed based on k-nearest neighbors. Results show that the pre-trained model can better resist the negative effect caused by class imbalance. The weighted F1 score of the pre-trained model is 6.72% and 2.61% larger than supervised models in zero-shot transfer and few-shot transfer, respectively. It is inferred that the more generic features encoded by TIPE result in superior generalization in comparison with traditional supervised methods. Moreover, our proposed method is insensitive to the thresholds of determining thermoacoustic states. Principal component analysis reveals the physical interpretability preliminarily through the connection between feature principal components and pressure fluctuation amplitudes. Finally, the key spatial region of flame images and temporal interval of pressure series are visualized by class activation map and global attention scores. [ABSTRACT FROM AUTHOR]

The present study investigates the effect of the exit geometry of the main stage on thermoacoustic instabilities in a model combustor with a dual-swirl layout. In practical combustors, the main stage has a much larger airflow than the pilot stage, therefore dominating the aerodynamics of the global flame. In this paper, six different geometries of the main-stage exit are designed to cover the common styles in engineering applications. Their effects on thermoacoustic instability and flame shape are examined experimentally. Sudden transitions from the attached flame to the blurred flame are found in most of the exit geometries, excluding the outer-rough case. This transition of flame shape triggers the onset of strong thermoacoustic oscillation, leading to a jump in the pressure fluctuation amplitude. Frequency drop and the acoustic mode shift are also found at the transition point. Flame dynamics are further analyzed using the dynamic mode decomposition method. It is concluded that the outer-rough case has the best thermoacoustic stability within the tested range. The mechanisms of the observed phenomena are analyzed by measuring the reacting flow fields. It is found that the increased roughness of the outer wall can prevent the flame shape transition and the onset of thermoacoustic instability, possibly due to the weakened vortex structures and strain rate in the outer shear layer. [ABSTRACT FROM AUTHOR]

The centrally staged combustor is an effective way to reduce NOx emissions from combustors. However, combustion instability caused by the mutual coupling between flames and acoustics during the combustion process is almost unavoidable. To better understand this problem, the effect of the swirl rotational direction is investigated in this paper using two different schemes with co-swirl and counter-swirl configurations. Pressure fluctuations and flame dynamics are investigated under self-excited combustion oscillation conditions. The CH* chemiluminescence distribution captured by a high-speed camera is utilized to characterize the flame macrostructure and heat release fluctuations. Furthermore, non-oscillating reaction velocity fields are acquired using particle image velocimetry (PIV) technology. The results indicate that the amplitude and frequency of the counter-swirl scheme are higher than those of the co-swirl scheme at varying main stage equivalence ratios. Combining the results from dynamic mode decomposition and the local Rayleigh index, it is found that the heat release regions of the counter-swirl scheme are mainly concentrated in the shear layer. Higher velocity gradients, vorticities, and strain rates in the inner shear layer (ISL) and outer shear layer (OSL) for the counter-swirl scheme are verified using PIV technology. The driving sources of thermoacoustic oscillations are located in the regions of the ISL, OSL, and the area where the flame impinges on the sidewall of the liner. Additionally, the counter-swirl scheme exhibits larger vorticities and strain rates in the ISL and OSL, facilitating the development of thermoacoustic oscillations. [ABSTRACT FROM AUTHOR]