湍流非预混钝体火焰中烟炱尺寸分布演化的大涡模拟
Large Eddy Simulation of the evolution of the soot size distribution in turbulent nonpremixed bluff body flames
AI总结：
本研究通过大涡模拟结合双变量多矩分区法，研究了不同钝体直径下湍流非预混火焰中烟炱尺寸分布的演化，发现尺寸分布对钝体直径不敏感，且改进的氧化描述显著提升了烟炱体积分数的预测精度。
摘要：
采用大涡模拟（LES）研究了一系列具有不同钝体直径的湍流非预混钝体火焰中烟炱尺寸分布的演化。采用新型双变量多矩分区法（BMMSM）来表征尺寸分布。BMMSM结合了分区法和矩法的要素，凭借其联合体积-表面积公式，能够再现分形团聚体形态，且与传统分区法相比，所需输运的烟炱标量更少，计算成本相对较低。LES结果显示，烟炱体积分数分布与实验测量结果正确吻合，相比先前使用HMOM的研究有显著改进。研究了整个火焰系列中颗粒尺寸分布函数（PSDF）的演化，结果表明，尺寸分布对钝体直径的敏感性低于整体烟炱体积分数，后者随钝体直径增大而增加。与湍流非预混射流火焰相比，火焰中的PSDF表现出不同特征。回流区中的长停留时间导致尺寸分布接近双峰，最终在下游射流状区域变为双峰。进一步分析表明，需要尺寸分布模型才能正确预测烟炱演化。值得注意的是，由于BMMSM对氧化的描述优于HMOM，使用BMMSM发现回流区和射流状区域中均存在显著的成核和凝结速率，其量级与表面增长和氧化相当，从而相比HMOM改进了平均烟炱体积分数的预测。这项工作揭示了尺寸分布对于准确预测全局烟炱量和再现基本机制都至关重要。
英文摘要：
Large Eddy Simulation (LES) was used to investigate the evolution of the soot size distribution in a series of turbulent nonpremixed bluff body flames, with different bluff body diameters. The new Bivariate Multi-Moment Sectional Method (BMMSM) is employed to characterize the size distribution. BMMSM combines elements of sectional methods and methods of moments and is capable of reproducing fractal aggregate morphology, thanks to its joint volume-surface formulation, all at relatively low computational costs with fewer transported soot scalars compared to traditional sectional methods. LES results show soot volume fraction profiles agreeing correctly with the experimental measurements, exhibiting significant improvement compared to previous work using the HMOM. The evolution of the particle size distribution function (PSDF) was examined across the flame series and show that the size distribution is less sensitive to the bluff body diameter than the overall soot volume fraction, which increases with increasing bluff body diameter. The PSDF across the flame exhibit different features compared to turbulent nonpremixed jet flames. The long residence times in the recirculation zone leads to a nearly bimodal size distribution, which eventually becomes bimodal in the downstream jet-like region. Further analysis indicates that a size distribution model is needed to correctly predict the soot evolution. Remarkably, due to improved descriptions of oxidation with BMMSM compared to HMOM, significant nucleation and condensation rates in both the recirculation zone and jet-like region were found using BMMSM, on the same order of magnitude as surface growth and oxidation, leading to the improved prediction of mean soot volume fraction compared to HMOM. This work reveals that the need of size distribution is crucial to both predict global soot quantities accurately and reproduce fundamental mechanisms.
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