Droplet/ligament modulation of local small-scale turbulence and scalar mixing in a dense fuel spray
Droplet/ligament modulation of local small-scale turbulence and scalar mixing in a dense fuel spray
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DOI：
10.1016/j.proci.2014.06.088
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
J. Shinjo;J. Xia;A. Umemura
中科院分区：
文献类型：
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作者：
J. Shinjo;J. Xia;A. Umemura
在这项研究中，有限大小的液滴/韧带在稠密燃油喷雾中对湍流和标量混合的调制使用了直接数值模拟(直接数值模拟)数据集。从高速喷嘴喷出的液体燃料射流变形，燃料喷雾雾化成许多韧带和液滴。在这些过程中，由于液滴/韧带动力学，气体流动变得湍急。同时，液滴的蒸发和与环境空气的混合也受到小尺度气体湍流的影响。了解浓雾区的混合特性对于建立喷雾燃烧模型具有重要意义。在液滴数密度相对较低的区域，发现了各向同性湍流的普遍特征，尽管液滴的存在对应变特征向量与涡度和混合分数梯度的排列有轻微的调制，这是颗粒流动的特征。在接近液滴表面的气相区，湍流动能的耗散率大大增加，排列受到更多的调制，特别是具有应变特征向量的标量梯度排列。这也可以从液滴/韧带表面近场的能量耗散、拟能和标量耗散之间的拓扑相似性中看出。首次发现，在真实的湍流喷雾中，尺寸与湍流尺度相当的液滴确实影响混合特性。这一发现为蒸发和雾化燃油喷雾中的流动湍流和标量混合的建模提供了新的线索。
In this study, the modulation of turbulence and scalar mixing by finite-size droplets/ligaments in a dense fuel spray is investigated using a DNS (Direct Numerical Simulation) dataset. Ejected from a spray nozzle with a high speed, a liquid-fuel jet deforms and the fuel spray is atomized into many ligaments and droplets. During these processes, the gas flow becomes turbulent due to droplet/ligament dynamics. At the same time, droplet evaporation and mixing with ambient air are affected by the small-scale gas turbulence. An understanding of the mixing characteristics in the dense spray zone is important for modeling spray combustion. In a region where the droplet number density is relatively low, a universal feature of isotropic turbulence was found, although the alignments of strain eigenvectors with vorticity and the mixture fraction gradient are slightly modulated by the presence of droplets, which is a characteristic of particle-laden flows. In gas-phase regions close to droplet surfaces, where the dissipation rate of turbulent kinetic energy is strongly increased, the alignments are more modulated, especially those of the scalar gradient with strain eigenvectors. This can also be seen in the topology similarity among energy dissipation, enstrophy and scalar dissipation in the near field of droplet/ligament surfaces. For the first time, it is found that droplets whose size is comparable to turbulence scales do affect the mixing characteristics in a realistic turbulent spray. This finding has shed new light upon the modeling of flow turbulence and scalar mixing in an evaporating and atomizing fuel spray.