This study explores flame structures and combustion dynamics in high-pressure n-dodecane fuel sprays, focusing on the formation and consumption of formaldehyde (CH2O) during autoignition and the development of poly-aromatic hydrocarbons (PAH) as soot precursors. These processes are crucial for optimizing combustion efficiency and reducing emissions. However, traditional approaches, which rely on single-shot measurements or ensemble-averaged visualizations, often overlook critical early-stage processes during low-temperature ignition. To overcome these challenges, we employed an innovative high-speed planar laser-induced fluorescence (PLIF) technique at 50 kHz using a pulse-burst Nd:YAG laser system with an excitation wavelength of 355 nm. This approach, applied for the first time to Engine Combustion Network (ECN) Spray D flames, provides unprecedented insights into the combustion processes at varying ambient temperatures and oxygen concentrations. Additionally, simultaneous high-speed schlieren imaging at 100 kHz was used to visualize spray penetration, first-stage ignition, and thermal expansion zones. Our findings reveal that, similar to Spray A flames, CH2O forms in cold, fuel-rich zones well upstream of the combustion zone. However, in Spray D flames, the schlieren signal softening observed in the jet's head does not lead to complete disappearance, and the CH2O signal is absent from the full head of the spray. During the second-stage ignition, CH2O consumption accelerates due to high-temperature reactions, leading to a significant reduction in its signal. Unlike the mushroom-shaped structure seen in Spray A flames, Spray D flames exhibit a quasi-steady PAH phase structure, with lean peripheral mixtures insufficient for soot precursor formation. Notably, reducing ambient oxygen concentration to 13 % while maintaining or increasing temperature prolongs the presence of CH2O, highlighting its influence on ignition dynamics and oxidation processes in dodecane spray flames. This study provides new insights into the combustion mechanisms of high-pressure sprays and offers valuable data for developing next-generation combustion technologies, including models, aimed at improving efficiency and reducing emissions.

中文翻译：

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发动机燃烧网络喷雾 D 火焰中两级点火和燃烧的高时空分辨率光学测量


本研究探讨了高压正十二烷燃料喷雾中的火焰结构和燃烧动力学，重点关注自燃过程中甲醛（CH2O）的形成和消耗以及作为烟灰前体的多芳烃（PAH）的开发。这些过程对于优化燃烧效率和减少排放至关重要。然而，依赖于单次测量或整体平均可视化的传统方法常常忽视低温点火过程中关键的早期过程。为了克服这些挑战，我们采用了创新的高速平面激光诱导荧光 (PLIF) 技术，频率为 50 kHz，使用激发波长为 355 nm 的脉冲突发 Nd:YAG 激光系统。这种方法首次应用于发动机燃烧网络 (ECN) Spray D 火焰，为不同环境温度和氧气浓度下的燃烧过程提供了前所未有的见解。此外，还使用 ​​100 kHz 的同步高速纹影成像来可视化喷雾穿透、第一阶段点火和热膨胀区域。我们的研究结果表明，与喷射 A 火焰类似，CH2O 在燃烧区上游的寒冷、富含燃料的区域形成。然而，在喷雾 D 火焰中，在喷射头中观察到的纹影信号软化不会导致完全消失，并且整个喷雾头中不存在 CH2O 信号。在第二阶段点火过程中，由于高温反应，CH2O消耗加速，导致其信号显着减弱。与喷雾 A 火焰中看到的蘑菇形结构不同，喷雾 D 火焰表现出准稳态 PAH 相结构，稀薄的外围混合物不足以形成烟灰前体。 值得注意的是，在维持或升高温度的同时将环境氧气浓度降低至 13%，可以延长 CH2O 的存在时间，突出显示其对十二烷喷雾火焰中的点火动力学和氧化过程的影响。这项研究提供了对高压喷雾燃烧机制的新见解，并为开发下一代燃烧技术（包括旨在提高效率和减少排放的模型）提供了宝贵的数据。