Transport phenomena in the Knudsen layer near an evaporating surface
Transport phenomena in the Knudsen layer near an evaporating surface
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DOI：
10.1103/physreve.100.043108
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发表时间：
2019-10-16
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
利用气体动力学理论（KTG）、玻尔兹曼输运方程（BTE）和分子动力学（MD）模拟相结合的方法，研究了平面蒸发表面附近Knudsen层中的输运现象。MD模拟首先用于验证关于蒸汽分子在努森层中的各向异性速度分布的假设。基于这一假设，我们利用KTG公式将蒸发表面的蒸汽温度和密度作为蒸发速率和质量调节系数（MAC）的函数，并将这些蒸汽性质作为边界条件，求解了Knudsen层中各向异性蒸汽流动的BTE。通过对真空蒸发过程的研究，我们发现在真空边界处，蒸汽宏观速度与蒸汽分子沿流动方向的最可几热速度之比总是达到最大值，其根为1.5。BTE解预测，在给定温度下从液体表面的最大蒸发通量取决于MAC和蒸发表面与真空边界之间的距离。从两个平行板之间的蒸发和冷凝的研究中，我们表明BTE解决方案给出了很好的预测的传输现象中的各向异性蒸汽流内的努森层和各向同性流的努森层。MD模拟结果验证了BTE的所有预测。
Using the combination of the kinetic theory of gases (KTG), Boltzmann transport equation (BTE), and molecular dynamics (MD) simulations, we study the transport phenomena in the Knudsen layer near a planar evaporating surface. The MD simulation is first used to validate the assumption regarding the anisotropic velocity distribution of vapor molecules in the Knudsen layer. Based on this assumption, we use the KTG to formulate the temperature and density of vapor at the evaporating surface as a function of the evaporation rate and the mass accommodation coefficient (MAC), and we use these vapor properties as the boundary conditions to find the solution to the BTE for the anisotropic vapor flow in the Knudsen layer. From the study of the evaporation into a vacuum, we show the ratio of the macroscopic speed of vapor to the most probable thermal speed of vapor molecules in the flow direction will always reach the maximum value of root 1.5 at the vacuum boundary. The BTE solutions predict that the maximum evaporation flux from a liquid surface at a given temperature depends on both the MAC and the distance between the evaporating surface and the vacuum boundary. From the study of the evaporation and condensation between two parallel plates, we show the BTE solutions give good predictions of transport phenomena in both the anisotropic vapor flow within the Knudsen layer and the isotropic flow out of the Knudsen layer. All the predictions from the BTE are verified by the MD simulation results.