Study of combustion characteristics and flame stabilization mechanism of hydrogen-containing micromix jet flames
HU Xuechao1,2, BI Xiaotian1,2, LIU Ce1,2, SHAO Weiwei1,2,3
1. Key Laboratory of Advances Energy and Power, Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing 100190, China; 2. University of Chinese Academy of Science, Beijing 100049, China; 3. Jiangsu Zhongke Research Center for Clean Energy and Power, Lianyungang 222069, China
Abstract：[Objective] Micromix combustion is an excellent low-pollution combustion technology. However, the instability of micromix combustion based on multiple small flames, especially high-frequency oscillation under high hydrogen content, is still unclear. [Methods] Herein, the emission performance and oscillation characteristics of micromix combustion under different hydrogen enrichments were studied. Furthermore, an experimental study on the combustion instability of hydrogen-rich fuel was conducted using a novel micromix burner under atmospheric pressure and preheated air at 673 K, which provided a reference for practical engineering applications. Power spectral density was used for spectral analysis. Phase-space reconstruction was applied to analyze the developmental changes in the dynamical system and determine the limit cycle oscillations. Proper orthogonal decomposition (POD) was used to analyze flame dynamics under oscillating conditions, and the time coefficients and spatial distribution characteristics of the modes were extracted. Dynamic pressure sensors were arranged in the air inlet and exhaust outlet contraction sections to measure pressure fluctuations. A high-speed camera system was used to realize the fast acquisition of chemiluminescence signals. The NOx emission, dynamic pressure, flame structure, and other combustion characteristics were studied under different hydrogen contents, from pure methane to pure hydrogen. [Results] The results showed that: 1) The micromix burner had an excellent low-emission performance for pure hydrogen with< 5 μmol/mol NOx at 15% O2 and could adapt to a wide hydrogen content to achieve stable combustion. These characteristics indicated that this micromix burner could be directly applied to designing hydrogen turbine combustion chambers. 2) The oscillatory combustion phenomenon occurred when the hydrogen content was between 10% and 20%. Under those conditions, the phase-space reconstruction trajectory manifested as limit cycle oscillation, and the root mean square values of pressure fluctuation were >1%, representing strong correlation structures. High-order harmonics were also found. Heat release was shown as a periodic overall increase and decrease, and the periodic formation and axial propagation of flame vortices could be observed. The flames with high hydrogen contents fluctuated at a high frequency of >900 Hz, but the amplitude of these flames was low. 3) Time-average images were used to characterize the flame structure under different conditions. The decreasing flame height with increasing hydrogen content contributed to the changes in the heat release concentration position. On the one hand, it affected the coupling relationship between the heat release fluctuation and pressure fluctuation, on the other hand, it shortened the period of pressure fluctuation, corresponding to the increase in main frequency. 4) Between 10% and 20% hydrogen content, the first-order mode was a volume oscillation, which was identical to the main frequency of the whole oscillation, and the second-order mode was an axial oscillation, which was twice the main frequency of the oscillation. With the increase of hydrogen content, the main POD modes switched from the axial mode to flame interaction. [Conclusions] The oscillation conditions and the instability characteristics of the hydrogen-containing fuel were obtained via data analysis. The experimental results could be used to master the mechanism of combustion instability and provide a reference for developing control technology for combustion instability.
扈学超, 毕笑天, 刘策, 邵卫卫. 氢燃料微预混火焰燃烧不稳定性实验研究[J]. 清华大学学报（自然科学版）, 2023, 63(4): 572-584.
HU Xuechao, BI Xiaotian, LIU Ce, SHAO Weiwei. Study of combustion characteristics and flame stabilization mechanism of hydrogen-containing micromix jet flames. Journal of Tsinghua University(Science and Technology), 2023, 63(4): 572-584.
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