Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125, USA.
J Phys Chem B. 2011 Jan 27;115(3):500-6. doi: 10.1021/jp109042q. Epub 2010 Dec 15.
Mass diffusion coefficients are critically related to the predictive capability of computational combustion modeling. To date, the most common approach used to determine the molecular transport of gases is the Boltzmann transport equation of the gas kinetic theory. The Chapman-Enskog (CE) solution of this transport equation, combined with Lennard-Jones potential parameters, suggests a simple analytical expression for computing self and mutual diffusion coefficients. This approach has been applied over a wide range of flame modeling conditions due to its minimal computational requirement, despite the fact that the theory was developed only for molecules that have a spherical structure. In this study, we computed the binary diffusion coefficients of linear alkanes using all-atom molecular dynamics simulations over the temperature range 500-1000 K. The effect of molecular configurations on diffusion coefficients was determined relating the radii of gyration of the molecules to their corresponding collision diameters. The comparison between diffusion coefficients determined with molecular dynamics and the values obtained from the CE theory shows significant discrepancies, especially for nonspherical molecules. This study reveals the inability of CE theory with spherical potentials to account for the effect of molecular shapes on diffusion coefficients.
质量扩散系数与计算燃烧模型的预测能力密切相关。迄今为止，用于确定气体分子输运的最常用方法是气体动理论的玻尔兹曼输运方程。该输运方程的Chapman-Enskog（CE）解，结合 Lennard-Jones 势能参数，为计算自扩散和互扩散系数提供了一个简单的分析表达式。由于其计算要求低，该方法已应用于广泛的火焰建模条件下，尽管该理论仅适用于具有球形结构的分子。在这项研究中，我们使用全原子分子动力学模拟在 500-1000 K 的温度范围内计算了线性烷烃的二元扩散系数。通过将分子的转动半径与其相应的碰撞直径相关联，确定了分子构型对扩散系数的影响。用分子动力学确定的扩散系数与从 CE 理论获得的值的比较表明存在显著差异，特别是对于非球形分子。这项研究揭示了具有球形势能的 CE 理论无法解释分子形状对扩散系数的影响。
