Zhou Dongdong, Zhang Yuhui, Hou Yu, Zhong Xin, Jin Jian, Sun Lidong
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Shaanxi 710049, China.
State Key Laboratory of Mechanical Transmission, School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China.
Phys Rev E. 2020 Oct;102(4-1):043108. doi: 10.1103/PhysRevE.102.043108.
Influences of surface nanotubes at high temperatures are investigated on droplet impact dynamics and Leidenfrost effect. Five distinct regimes of impact droplets are found on the nanotube surface, including contact boiling, film levitation, central jet levitation, central jet, and Leidenfrost phenomenon. The regimes of film levitation, central jet levitation, and central jet are characterized by either film levitation and/or liquid central jet. The regime of Leidenfrost phenomenon is characterized by droplet bounce-off behavior free of any liquid jets. Film levitation is driven by the vaporization of two parts of the droplet, with one as the droplet bottom layer over the contact area above the nanotube structure, and the other as the hemiwicking liquid in nanotubes. Both the vaporization is impaired by increasing the surface temperature, which is attributed to the reduced contact time and less extent of spread of the droplet at a higher surface temperature. The central jet phenomenon is driven by the vapor stream produced by hemiwicking liquid in the central area upon impact. It is enhanced and then suppressed by elevating the surface temperature, resulting from the collective effects of the vapor pressure in nanotubes which increases with the surface temperature, and the cross-sectional area of the vapor stream, which increases and then decreases with the surface temperature. At a high Weber number, the Leidenfrost temperature can be increased by 125^{∘}C on the nanotube surface, implying a great potential in heat transfer enhancement for droplet-based applications.
研究了高温下表面纳米管对液滴冲击动力学和莱顿弗罗斯特效应的影响。在纳米管表面发现了五种不同的冲击液滴状态，包括接触沸腾、膜悬浮、中心射流悬浮、中心射流和莱顿弗罗斯特现象。膜悬浮、中心射流悬浮和中心射流状态的特征是存在膜悬浮和/或液体中心射流。莱顿弗罗斯特现象状态的特征是液滴反弹行为，且无任何液体射流。膜悬浮是由液滴两部分的汽化驱动的，一部分是纳米管结构上方接触区域上的液滴底层，另一部分是纳米管中的半芯吸液体。表面温度升高会削弱这两种汽化现象，这归因于在较高表面温度下接触时间缩短以及液滴的铺展程度减小。中心射流现象是由冲击时中心区域半芯吸液体产生的蒸气流驱动的。表面温度升高会先增强然后抑制这种现象，这是由纳米管中蒸气压随表面温度升高以及蒸气流横截面积随表面温度先增大后减小的综合作用导致的。在高韦伯数下，纳米管表面的莱顿弗罗斯特温度可提高125℃，这意味着在基于液滴的应用中增强传热具有巨大潜力。
