A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is much discrepancy in the usage of phase change models and associated coefficients. First, a brief review of kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, $\alpha$) and its inconsistent definitions are discussed. The discussion focuses on the departure from equilibrium; represented as a macroscopic "drift" velocity. Then a continuous flow, phase change driven molecular dynamics setup is used to investigate steady state condensation at a flat liquid-vapor interface of argon at various phase change rates and temperatures to elucidate the effect of equilibrium departure. MAC is computed directly from the kinetic theory based Hertz-Knudsen (H-K) and Schrage (exact and approximate) expressions without the need for a priori physical definitions, ad hoc particle injection/removal or particle counting. MAC values determined from the approximate and exact Schrage expressions ($\alpha_{app}^{Schrage}$ and $\alpha_{exact}^{Schrage}$) are between 0.8 and 0.9, while MAC values from the H-K expression ($\alpha^{H-K}$) are above unity for all cases tested. $\alpha_{exact}^{Schrage}$ yields values closest to the results from transition state theory [J Chem Phys, 118, 1392-1399 (2003)]. The departure from equilibrium does not affect the value of $\alpha_{exact}^{Schrage}$ but causes $\alpha^{H-K}$ to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a non-uniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density are observed when compared with the corresponding bulk values.

中文翻译：

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漂移质量调节系数：来自稳态分子动力学设置的原位测量

对蒸发/冷凝现象的基本理解对于许多科学和工程领域至关重要，但在相变模型和相关系数的使用方面存在很大差异。首先，简要回顾了相变的动力学理论，并讨论了质量调节系数（MAC，$\alpha$）及其不一致的定义。讨论的重点是偏离均衡；表示为宏观“漂移”速度。然后使用连续流动、相变驱动的分子动力学装置来研究在不同相变速率和温度下氩气的平坦液-气界面处的稳态冷凝，以阐明平衡偏离的影响。MAC 直接根据基于 Hertz-Knudsen (HK) 和 Schrage（精确和近似）表达式的动力学理论计算，无需先验物理定义、临时粒子注入/去除或粒子计数。从近似和精确 Schrage 表达式（$\alpha_{app}^{Schrage}$ 和 $\alpha_{exact}^{Schrage}$）确定的 MAC 值介于 0.8 和 0.9 之间，而来自 HK 表达式（$ \alpha^{HK}$) 在所有测试案例中都高于统一。$\alpha_{exact}^{Schrage}$ 产生的值最接近过渡态理论的结果 [J Chem Phys, 118, 1392-1399 (2003)]。偏离平衡不会影响 $\alpha_{exact}^{Schrage}$ 的值，但会导致 $\alpha^{HK}$ 急剧变化，强调了漂移速度校正的重要性。此外，平衡偏离导致蒸气性质分布不均匀。在冷凝界面，与相应的体积值相比，可以观察到蒸汽温度的局部升高和蒸汽密度的下降。