发表机构
Argonne National Laboratory（阿贡国家实验室）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究采用DNS，探究发动机相关条件下湍流稀薄预混氢-空气火焰的NOx生成，明确湍流强度等因素的竞争效应，提出预测NOx增强的概念模型，强调湍流-化学相互作用的关键作用。
AI 中文摘要
本研究采用直接数值模拟（DNS），对发动机相关条件下湍流稀薄预混氢-空气火焰中的NOx生成过程展开研究。研究考察了不同的湍流强度与分子输运模型，以分离湍流强度、刘易斯数（Lewis number）及优先扩散对NOx生成的单独影响。结果表明，相较于层流火焰，所有湍流工况下的NOx生成均显著增强，在混合物停留时间为0.15 ms时，NOx生成量约达层流工况的5倍。研究发现，提高湍流强度对NOx生成存在三种相互竞争的效应：（1）通过诱导局部超绝热热点，并在火焰刷内提升关键火焰自由基浓度，强化湍流-不稳定性相互作用，从而在局部促进NOx生成；（2）提高湍流火焰速度，缩短火焰刷停留时间，进而在全局抑制NOx生成；（3）降低火焰后温度波动，抑制火焰后区的热力型NOx增强。刘易斯数效应被确定为驱动热扩散型NOx增强的主要机制，优先扩散则起次要作用，这一点可通过单位刘易斯数湍流火焰与其层流对应物之间几乎相同的NOx反应速率得到证实。最后，研究发现峰值条件平均NOx反应速率与拉伸因子之间存在良好的相关性，并提出了一个概念模型，用于预测实际发动机模拟中的NOx增强。研究结果表明，湍流-化学相互作用对于准确预测热扩散不稳定氢火焰中的NOx生成至关重要。
英文摘要
In this study, direct numerical simulations (DNS) are employed to investigate NOx formation in turbulent lean premixed hydrogen-air flames under engine-relevant conditions. Various turbulence intensities and molecular transport models are examined to isolate the individual impacts of turbulence intensity, Lewis number, and preferential diffusion on local and global NO production. Results show that global NO production is significantly enhanced in all of the turbulent cases, reaching approximately five times the value of the 1D steady flame at a mixture residence time of 0.1 ms. Increasing turbulence intensity is found to have three competing effects on NO formation: (1) it strengthens turbulence-instability interactions by inducing local super-adiabatic hot spots and elevating the concentrations of key flame radicals within the flame brush, thereby promoting the NO reaction rate locally; (2) it accelerates the turbulent flame speed, reducing the flame-brush residence time and thus suppressing NO production globally; and (3) it reduces post-flame temperature fluctuations, suppressing thermal NO enhancement in the post-flame zone. As a result, the global NO production is slightly lower at higher turbulence intensities among all the turbulent cases considered. Lewis number effects are identified as the primary mechanism driving thermodiffusive NO enhancement, with preferential diffusion playing a secondary role, as evidenced by the nearly identical mean profiles of the NO reaction rate between unity Lewis number turbulent flames and their 1D steady flame counterparts in both the progress variable space and the residence time space. Finally, an excellent correlation between the peak conditional mean NO reaction rate and the stretch factor is identified, and a conceptual model is proposed to improve the predictions of global NO production in practical engine simulations.
