Abstract:

The high-speed imaging technique was used to observe the evaporation and nucleation process of the droplets of deionized water after impacting the surface of the micro-pillar structure. The results show that the evaporation time of droplet decreased with the increase of wall temperature. Compared with smooth surface, the heat transfer coefficient (HTC) of micro-pillar surface can be only enhanced under certain conditions. In this paper the temperature is 50, 60, 70, 80 and 120℃,and the strengthening effect is best at 120℃. The evaporation of droplet can be divided into two stages. In the first stage, the diameter of the droplets is constant, and the height changes. In the second stage, the thickness of the droplet is close to the height of micro-pillar structure and the droplet dry in a very short time. As the wall temperature increases, the period of the first stage decreases obviously, and the nucleation density and the bubble diameter increase accordingly. It is worth to note that nucleation bubbles show a radial distribution on the micro-pillar surface because of the structure of array micro-pillars and the impact of dropping down.

Key words: droplet impacting, micro-pillar structure, evaporation, nucleation site, bubble diameter, bubble distribution

摘要：

利用高速摄像技术对去离子水液滴撞击微柱结构表面后的蒸发及核化过程进行观测。实验测得不同壁面温度下液滴蒸干时间，获得液滴沸腾曲线；发现相对光滑表面，微柱表面在50、60、70、80、120℃强化相变换热，120℃时强化比例最大，达到35.71%；壁温为90、100、110℃时，微柱表面无强化作用。从液滴直径和厚度的变化可知微柱表面液滴蒸发分为两个阶段：第一阶段，液滴直径不变，厚度变化；第二阶段，液滴厚度接近微柱高度，直径减小。随壁温升高，第一阶段时长显著缩短。液滴内部核化点密度和气泡平均直径随壁面温度的升高均有明显增大的趋势。需指出的是，液滴冲击对微柱表面液滴内部核化点分布有重要影响，受微柱结构及滴落冲击作用液滴内部成核气泡沿液滴半径呈辐射状分布。

关键词: 液滴撞击, 微柱结构, 蒸发, 汽化核心, 气泡直径, 气泡分布