Liquid alkane droplets transit to supercritical fluids under supercritical conditions. However, the subsequent thermodynamic state evolution, phase transition, and mixing in this case, which directly influences combustion in actual diesel engines, need further investigation. We studied the thermodynamic state evolution, phase transition, and mixing of n-heptane droplets in supercritical laminar nitrogen streams through a set of conservation equations. Non-ideal thermodynamic and transport properties were calculated to incorporate the dramatic variation of physical properties in the supercritical regime. We focus on the effects of Reynolds number, ambient pressure, and droplet size on the thermodynamic state evolution, phase transition and mixing of droplets. Results showed that increasing the Reynolds number can accelerate the evolution of thermodynamic states, which leads to the faster transition of droplets from the liquid phase and supercritical fluids to gas-phase mixtures. With the increased Reynolds number, the phase transition rate is increased and the mixing homogeneity is improved due to the enhanced mass transfer. Caused by the negative correlation between mass transfer and ambient pressure, in higher pressure, droplets after the supercritical transition require more time to transition from supercritical fluids to vapor-phase mixtures. Increasing ambient pressure inhibited the droplet phase transition process and deteriorated the mixing homogeneity, but no significant influence was shown on the droplet dynamic evolution process. Smaller droplets transition faster from liquid phase and supercritical fluids to vapor-phase mixtures. The faster thermodynamic state evolution and faster dynamic evolution process of the smaller droplet leads to its more rapid homogeneous mixing with ambient gasses, but the mode of dynamics is independent of the droplet size.

中文翻译：

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对流超临界环境中碳氢化合物液滴的热力学状态演化、相变和混合研究

液态烷烃液滴在超临界条件下转变为超临界流体。然而，这种情况下随后的热力学状态演化、相变和混合直接影响实际柴油机的燃烧，需要进一步研究。我们通过一组守恒方程研究了超临界层流氮气流中正庚烷液滴的热力学状态演化、相变和混合。计算非理想热力学和输运性质，以纳入超临界状态下物理性质的巨大变化。我们重点研究雷诺数、环境压力和液滴尺寸对液滴热力学状态演化、相变和混合的影响。结果表明，增加雷诺数可以加速热力学状态的演化，从而导致液滴从液相和超临界流体更快地转变为气相混合物。随着雷诺数的增加，相变速率增加，并且由于传质增强而改善混合均匀性。由于传质与环境压力之间的负相关性，在较高的压力下，超临界转变后的液滴需要更多​​的时间从超临界流体转变为气相混合物。环境压力的增加抑制了液滴相变过程并恶化了混合均匀性，但对液滴动态演化过程没有表现出显着影响。较小的液滴从液相和超临界流体转变为气相混合物的速度更快。较小液滴更快的热力学状态演化和更快的动态演化过程导致其与环境气体更快速的均匀混合，但动力学模式与液滴尺寸无关。