超临界水非均质特性分子动力学研究

doi:10.11949/0438-1157.20230398

摘要/Abstract

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

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

中图分类号:

O 56

董明, 徐进良, 刘广林. 超临界水非均质特性分子动力学研究[J]. 化工学报, 2023, 74(7): 2836-2847.

Ming DONG, Jinliang XU, Guanglin LIU. Molecular dynamics study on heterogeneous characteristics of supercritical water[J]. CIESC Journal, 2023, 74(7): 2836-2847.

图/表 9

图1 (a) 物理模型图；(b) 氢键判定示意图；(c) 模拟工况图；(d) 热力学方法确定TPL区的起始温度Ts和终止温度Te(计算方法参考文献[18])

Fig.1 (a) Physical model of system; (b) Hydrogen bonding determination criteria; (c) Simulation points on phase diagram; (d) Ts and Te determined by thermodynamic method

图2 (a) TIP4P/2005模型的模拟密度与实验密度比较；(b) NVT系综下的模拟压力与设定压力比较

Fig.2 (a) Density from TIP4P/2005 model and experimental; (b) Simulated pressure from NVT ensemble and set pressure

图3 形成0~4个氢键的水分子比例fn 随温度变化曲线[(a)~(c)]；形成1个氢键的水分子比例f1随温度变化曲线(d)

Fig.3 The curve of fn -Tr [(a)—(c)] and f1-Tr (d) under different pressures

图4 (a) Pr=1.2时水分子平均氢键数nHB随温度变化；(b) 根据nHB划分LL、两相和GL相区域

Fig.4 (a) The curve of nHB-Trunder Pr=1.2; (b) LL, TPL, and GL regions determined by nHB

图5 (a) Pr=1.2时水分子中氧原子径向分布函数g(rc)曲线；(b) 根据f1及g(rc)确定Widom线及GL线

Fig.5 (a) The curve of g(rc)-rc under Pr=1.2; (b) Widom line and GL line determined by f1 and g(rc)

图6 (a) 根据TWF近邻法（ngas-N）划分LL、TPL和GL相区域；(b) 根据Voronoi多边形法（ngas-ρc）划分LL、TPL和GL相区域；(c) LL和GL分子的空间相分布

Fig.6 (a) LL, TPL, and GL regions determined by TWF neighborhood method (ngas-N); (b) LL, TPL, and GL regions determined by Voronoi diagram method(ngas-ρc); (c) Spatial phase distribution of LL and GL molecules

图7 (a) Pr=1.2时扩散系数D随温度变化; (b) 根据D划分LL、TPL和GL相区

Fig.7 (a) The curve of D-1/T under Pr=1.2; (b) LL, TPL, and GL regions determined by D

图8 (a) Pr=1.2时旋转与扩散系数比αrot/trans随温度变化；(b) 根据αrot/trans划分LL、TPL和GL相区

Fig.8 (a) The curve of αrot/trans-Tr under Pr=1.2; (b) LL, TPL, and GL regions determined by αrot/trans

图9 (a)不同方法预测TPL区的起始温度Ts和终止温度Te；(b) 最大误差和平均误差

Fig.9 (a) Ts andTe determined bydifferent methods; (b) Maximum errors and average errors

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