Abstract:

Understanding the heterogeneous characteristics of supercritical fluids is helpful for its efficient utilization in the fields of power cycle, waste treatment and petrochemical industry. Recently, the heterogeneous structure of supercritical fluids has been verified by experiments. Supercritical fluids can be divided into gas-like, two-phase-like and liquid-like regions. However, most studies focused on Lennard-Jones fluids in near-critical or narrow temperature and pressure parameters. As one of the most widely used supercritical fluids, the influence of hydrogen bond network in water on heterogeneous characteristics was still unclear. In this work, molecular dynamics simulation was performed to investigate the heterogeneous characteristics of supercritical water under a wide range of temperature and pressure parameters, focusing on the evolution of hydrogen bonding, physical structure and kinetic properties. The results show that the turning point of the number of dimers in supercritical water at the same pressure represents the transition from liquid-like phase to gas-like phase, and the turning point of the first peak of the radial distribution function represents the transition from two-phase to gas-like phase, which correspond to the Widom line and corresponding to gas line. According to the significant differences between the liquid-like and gas-like regions in the average number of hydrogen bonding, the physical structure such as the number of neighboring molecules, and the kinetic properties such as diffusion and rotation, the boundary locations of the two-phase-like region of supercritical water are determined. The results are in good agreement with the thermodynamic methods, with an average error of less than 5%. This work reveals the evolution of heterogeneous characteristics of supercritical water at molecular level, and provides theoretical support for relevant applications of supercritical fluids.

Key words: supercritical water, molecular simulation, heterogeneous, multiphase flow, hydrogen bonding

摘要：

为深入理解超临界流体的非均质特性，采用分子动力学模拟对宽广温压参数下超临界水的氢键结构、物理结构及动力学性质的演化规律进行分析。结果表明，同一压力下超临界水中二聚体数量的转折点代表类液相到类气相的过渡，径向分布函数第一峰峰值的转折点代表类两相到类气相的过渡，分别与Widom线和类气线相对应。根据超临界水的平均氢键数、物理结构如近邻分子数以及动力学性质如扩散和旋转运动在类液相和类气相区域的显著差异，确定了超临界水类两相区域的边界位置，与热力学方法得到结果吻合良好。研究从分子层面揭示超临界水非均质特性的演化规律，表明超临界水具有多相特征，存在类液、类两相和类气区域，为超临界流体流动及传热应用提供理论支撑。

关键词: 超临界水, 分子模拟, 非均质, 多相流, 氢键