Maximum evaporating flux of molecular fluids from a planar liquid surface
Maximum evaporating flux of molecular fluids from a planar liquid surface
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DOI：
10.1103/physreve.102.043102
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发表时间：
2020-10-05
影响因子：
2.4
通讯作者：
Liang, Zhi
中科院分区：
文献类型：
--
作者：
Bird, Eric;Liang, Zhi
在这项工作中，我们利用气体动力学理论 (KTG) 开发了一个理论模型，以了解分子内部运动对平面液体表面最大蒸发通量的作用。应用动力学理论研究分子流体在真空中的蒸发，并预测无量纲最大蒸发通量（J(R,)(ma)(x)，即最大蒸发通量与从液体表面发射的摩尔通量之比）。关于蒸发表面附近高度非平衡蒸气中多原子分子的速度分布函数 (VDF) 的关键假设通过直接从分子动力学 (MD) 模拟获得的 VDF 进行了验证。我们基于 KTG 的分析表明，J(R,)(ma)(x) 受到与流体分子内部自由度相关的比热 (c(V,)(int)) 的影响。当达到最大蒸发通量时，远离液体表面的各向同性蒸发蒸气无论是单原子蒸气还是多原子蒸气都以其声速运动。为了从根本上理解分子流体蒸发到真空中，我们求解玻尔兹曼输运方程（BTE）以获得高度非平衡蒸发蒸气流中的温度、密度和流速分布。我们的 BTE 解决方案表明，当出现最大蒸发通量时，蒸发蒸气有几个普遍特征。特别是，我们发现蒸发蒸气流动速度在真空边界处达到最大值，为蒸气流动方向上最可能热速度的1.5倍，并且该最大值与流体性质无关。本工作中的所有理论预测都通过模型液体Ar和模型液体正十二烷蒸发到真空中的MD模拟结果以及现有的实验数据得到验证。
In this work, we use the kinetic theory of gases (KTG) to develop a theoretical model to understand the role of internal motions of molecules on the maximum evaporation flux from a planar liquid surface. The kinetic theory is applied to study the evaporation of molecular fluids into a vacuum and predict the dimensionless maximum evaporation flux (J(R,)(ma)(x), i.e., the ratio of the maximum evaporation flux to the molar flux emitted from a liquid surface). The key assumptions regarding the velocity distribution function (VDF) of polyatomic molecules in the highly nonequilibrium vapor near the evaporating surface are validated by the VDF obtained directly from molecular dynamics (MD) simulations. Our KTG-based analysis shows that J(R,)(ma)(x) is affected by the specific heat (c(V,)(int)) associated with internal degrees of freedom of fluid molecules. When the maximum evaporation flux is reached, the isotropic evaporating vapor far from the liquid surface moves at its speed of sound regardless of whether it is a monatomic vapor or polyatomic vapor. To fundamentally understand the evaporation of a molecular fluid into a vacuum, we solve the Boltzmann transport equation (BTE) to obtain the temperature, density, and flow speed distributions in the highly nonequilibrium evaporating vapor flow. Our BTE solutions indicate that there are several universal features of the evaporating vapor when the maximum evaporation flux occurs. In particular, we find that the evaporating vapor flow speed reaches the maximum value of 1.5 times the most probable thermal speed in the vapor flow direction at the vacuum boundary, and this maximum value is independent of fluid properties. All theoretical predictions in this work are verified by the MD simulation results of the evaporation of the model liquid Ar and the model liquid n-dodecane into a vacuum, and existing experimental data.