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采用分子动力学的方法对纳米液滴撞击高温平板壁面产生的Leidenfrost现象进行了探究, 分析了液滴撞击不同温度壁面对Leidenfrost现象的影响. 结果表明, 随着壁面温度的提升, 液滴蒸发的速度更快, 脱离壁面的时刻越早, 脱离壁面时的速度也越大, 最终悬浮时液滴体积也越大. 通过分析脱离壁面前一时刻液滴内部的密度、温度分布发现, 由于高温壁面具有更高的热通量致使蒸发过程更快进而产生更厚的蒸气层, 该蒸气层将阻碍换热, 使得液滴在壁面温度更高时液滴内部的平均温度越低, 且平均密度越小.
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关键词:
- 分子动力学 /
- 液滴 /
- Leidenfrost现象
The process of droplet impacting on a high-temperature wall is widely existent in daily life and industrial applications. Most of scholars mainly have focused on experimental and macroscopic research on this phenomenon. In this work, molecular dynamics simulation is conducted to investigate the evolution of droplet and the influence of surface temperature on its evolution, in order to explore the heat transfer mechanism of nanodroplet impacting on high-temperature surface. Droplet containing 10741 argon atoms impacts on the copper plates at temperatures of 85, 150, 200, 250 and 300 K, respectively. The number of droplet evaporation atoms is statistically obtained, the droplet barycenter displacement is analyzed, and the density distribution and temperature distribution inside the droplet are acquired. It is shown that the droplet exhibits different characteristics on the wall at different temperatures. The droplet finally stabilizes on the wall at 85 K as shown in Fig. (a), but when the temperature of the wall rises to 150 K, the droplet evaporates slowly and finally completely as shown in Fig. (b), and for the wall temperatures 200, 250 and 300 K, the Leidenfrost phenomenon is found: the droplet is suspended above the wall as displayed in Figs. (c)–(e). Fig. (f) shows the number of evaporated atoms at different wall temperatures. It also can be seen that the Leidenfrost phenomenon occurs at wall temperatures 200, 250 and 300 K, because for the three conditions there are rise steps and then the numbers of evaporated atoms almost keep constant. For the temperature conditions under which the Leidenfrost phenomenon can occur, the higher the wall temperature, the faster the droplet evaporates, the earlier the detachment occurs from the wall, the greater the droplet detaching velocity, and the larger the final suspending droplet volume. The analyses of the density distribution and temperature distribution of the droplet at the moment when it detaches from the wall show that the evaporation process is faster and a thicker vapor layer is generated due to the higher heat flux of the high-temperature wall, which will hinder the heat exchange, so that the average temperature of the droplet is lower and the average density is smaller.
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Keywords:
- molecular dynamics /
- droplets /
- Leidenfrost phenomenon
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图 1 液滴撞击高温壁面的物理模型
Fig. 1. Physical model of a droplet hitting a high-temperature wall.
图 3 接触角获得过程 (a) 液滴铺展原子快照; (b) 液滴密度云图; (c) 等值线拟合
Fig. 3. Contact angle acquisition process: (a) Droplet spreading atomic snapshot; (b) droplet density contours; (c) contour fitting.
图 5 不同壁面温度下不同时刻的原子快照 (a) 85 K; (b) 150 K; (c) 200 K; (d) 250 K; (e) 300 K
Fig. 5. Atomic snapshots with different wall temperatures at different moments: (a) 85 K; (b) 150 K; (c) 200 K; (d) 250 K; (e) 300 K.
图 8 液滴与不同温度壁面之间的热通量
Fig. 8. Heat flux between the droplet and the wall at different temperatures.
图 9 液滴脱离壁面前不同壁面温度下液滴的密度分布
Fig. 9. Density distribution of droplets at different wall temperatures before droplets detach from the wall.
图 10 液滴脱离壁面前不同壁面温度下的液滴温度分布
Fig. 10. Droplet temperature distribution at different wall temperatures before the droplet detached from the wall.
表 1 Lennard-Jones (12-6)各原子模拟参数
Table 1. Lennard-Jones (12-6) atomic simulation parameters.
ε/eV σ/Å rc/Å Cu 0.52031 2.29726 12 Ar 0.0104 3.405 12 Cu—Ar 0.07356 2.85113 12 
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表 2 物性参数对比
Table 2. Comparison of physical property parameters.
本文模拟 实验测量 误差 导热系数/(W⋅m–1·K–1) 0.128 0.132 3.03% 黏度/(10–4 Pa–1·s–1) 2.92 2.8 –4.28%
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表 3 不同壁面温度下蒸气层厚度、液滴厚度以及液滴平均密度的统计
Table 3. Statistics of vapor layer thickness, droplet thickness and average droplet density at different wall temperatures.
壁面温度/K 蒸气层厚度/Å 液滴厚度/Å 液滴密度/(g⋅cm–3) 150 0 10 0.95 200 14 50 1.15 250 19 35 1.21 300 23 24 1.29
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表 4 不同壁面温度下液滴平均温度, 上下温差以及温度梯度
Table 4. Average droplet temperature, the temperature difference between upper and lower temperatures, and the temperature gradient at different wall temperatures.
壁面温度/K 平均温度/K 液滴上下
温差/K温度梯度/
(K·Å–1)150 140 5 0.5 200 121 33 0.65 250 107 41 1.17 300 98 47 1.95
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