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

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