水平及竖直基底上微小固着液滴的蒸发特性分析

doi:10.16085/j.issn.1000-6613.2020-1610

摘要/Abstract

摘要：

为揭示非水平表面上微小蒸发液滴的传热传质特性，本文在准稳态模型的假设下构造三维液滴模型，综合考虑了蒸气扩散、蒸发冷却以及气相域中的自然对流这3种传输机理，对水平以及竖直基底上液滴的蒸发过程进行数值研究。通过分析气液界面上温度分布、蒸发通量分布及总蒸发率的变化，重点探究了基底过热度以及重力的改变对液滴蒸发特性的影响。结果表明：与水平基底上温度的对称分布不同，竖直基底上气液界面温度分布表现出明显的非对称性，且非对称性随基底过热度的升高而增强，最低温度点不再位于液滴顶点，而向一侧偏移。此外，水平基底上气液界面局部蒸发通量呈对称分布，各截面分布相似，而竖直基底上局部蒸发通量分布则呈现出显著的非对称性以及各截面异性，非对称性随着基底过热度的升高而增强，这是重力改变后气相域自然对流发生改变的结果。与水平基底相比，竖直基底上蒸发率更高，总蒸发时间更少。最后，基底由水平变为竖直时，液滴内部流场由对称双涡转变为非对称单涡，单涡流速显著大于双涡流速，液滴内流速随基底过热度的上升而增大，单涡环流造成了气液界面温度分布的改变以及最低温度点的偏移。

关键词: 液滴, 汽化, 计算流体力学, 自然对流, 蒸发冷却, 传热, 扩散

Abstract:

To reveal the evaporation characteristics of small water droplets sessile on non-horizontal surfaces, this paper constructed a comprehensive 3D model, including diffusion, evaporative cooling, and natural convection in the gas domain, for the investigation of the evaporation processes of droplets evaporating on horizontal and vertical substrates under the assumption that the model was quasi-steady. By analyzing the change of temperature distribution, the local evaporation flux distribution at the gas-liquid interface, and the change of total evaporation rate, the influence of the superheat of the substrate and the change of gravity to the total evaporation performance were studied. The results showed that the temperature distribution at the interface of droplets evaporating on vertical substrates presented obvious asymmetry, which was contrary to the symmetrical distribution on horizontal substrates. The asymmetry amplified as the superheating of substrate increased and the minimum-temperature point appeared on the one side of the droplet rather than the apex. Besides, the local evaporation flux distribution on the horizontal substrate was symmetrical and the distributions of different sections were almost the same. While, the evaporation flux distribution on a vertical substrate presented apparent asymmetry and was anisotropic cross-sections due to the result of a change of natural convection in the gas domain caused by the gravity change. Compared to horizontal substrates, the total evaporation rates of droplets evaporating on vertical substrates were higher and the total evaporation times were shorter. Finally, the inner flow of the droplet turns from symmetrical double-vortex flow to the asymmetrical single-vortex circulation flow as the substrate turns from horizontal to vertical. The velocity of single-vortex flow was remarkably bigger than that of double-vortex flow and the velocity magnitude increased as the superheating of substrate rose. It was the single-vortex circulation flow inside the droplet that made the temperature distribution at the interface asymmetrical and drove the minimum-temperature point to deviate from the apex and appear on the one side of the droplet.

Key words: droplet, vaporization, CFD, natural convection, evaporative cooling, heat transfer, diffusion

中图分类号:

TK121

王宇, 潘振海. 水平及竖直基底上微小固着液滴的蒸发特性分析[J]. 化工进展, 2021, 40(7): 3632-3644.

WANG Yu, PAN Zhenhai. Analysis of evaporation characteristics of small water droplets sessile on horizontal and vertical substrates[J]. Chemical Industry and Engineering Progress, 2021, 40(7): 3632-3644.

图/表 18

图1 水平与竖直基底上液滴蒸发的三维示意图与局部剖面图

图2 计算域设置及网格划分情况

表1 流体性质

物理性质	水	气体
密度/kg·m^-3	-0.003903904T²+2.064089T+728.6815	理想气体方程［式(16)］
热导率/W·m^-1·K^-1	-0.000010512T²+0.007988T-0.840136	7.000×10^-5T+5.180×10^-3
比热容/J·kg^-1	4182	1006.8
动力黏度/kg·m^-1·s^-1	2.20867×10^-7T²-1.50835×10^-4T+2.6226×10^-2	4.897×10^-8T+3.832×10^-6
蒸气摩尔质量/kg·mol^-1	0.018	0.029（空气）
汽化潜热/J·kg^-1	-3.46T²+2.7554×10⁶

图3 液滴实际形状与不考虑重力时形状的对比

图4 液滴体积及形状随时间的变化

表2 蒸发时间对比

基底温度/℃	实验数据^［23］/s	模拟数据（误差）	扩散模型数据（误差）
45.6	295	302.3s（2.4%）	359s（21.7%）
55.6	163	159.2s（2.5%）	203s（24.5%）
65.6	98	94.5s（3.6%）	125s（27.6%）

图5 z=0平面上不同过热度下水平和竖直基底上液滴内外温度分布云图

图6 气液界面上温度的分布(z=0)

图7 气液界面与基底之间的最大温差

图8 气液界面上不同截面温度的分布

图9 气相域水蒸气质量分数云图

图10 气液界面上蒸发通量的分布(z=0)

图11 气液界面上不同截面局部蒸发通量的分布

图12 液滴总蒸发率随基底过热度的变化

图13 气相域流场图

图14 液滴体积随时间的变化

图15 水平和竖直基底上液滴内部流场分布(z=0)

图16 气液界面上液相侧平均速率随基底过热度的变化

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