电场力作用下液滴撞击过冷环境中发热体表面凝固相变传热机理及特性

Ice accretion on power lines is paramount for the safety of long-distance electricity transmission in harsh weather condition in China. A good understanding of freezing mechanism is of considerable significance for scientific and engineering applications. The project aims at multiphase flow characteristics and heat transfer performance of droplets which impact on the self-heating cylinder under electric field in sub-cooled environment. Special emphasis has been placed on the intricate influence of electrical field, velocity field and temperature field on the droplet-impact-based flow and solidification. In the scheme of this project, the research works include: (i) droplet impacting on cylinder in electrical field, (ii) influences of electric-induced-heating of power lines and sub-cooled environment on droplet solidification and ice formation, (iii) the relationship between ice formation and electrical field distortion and (iv) the freezing mechanism and characteristics governed by the hierarchy of coupled extrinsic fields. This project intends to reveal the underlying mechanism of phase interface movement and phase-changed solidification during the droplet impacting on the electricity-induced-heating surfaces. Implementation of this research can advance the common understanding of electrical field distribution variation, droplet impacting and movement, solidification behavior on self-heating surfaces and so on. Also, a mathematical model will be proposed to predict the droplet behaviors under the interaction of multi-fields when impacting on the hot solid surfaces in super cooling circumstances. By virtue of this model, the influence of different factors on ice formation can be clearly illustrated. Moreover, another model is expectedly proposed to predict the icing accretion. All these works paved the way to a safer operation of electricity grids and promoted the development of anti-ice technologies.

输电线覆冰对我国电力输运安全有着重大影响，对覆冰机理的认识有重要的学术意义和工程价值。本项目以电场力作用下液滴撞击过冷环境中圆形发热体表面的多相流动与凝固相变传热机理及特性为研究对象，以重点解决电场、流场、温度场等多物理场交互影响的多相流动和界面凝固相变传热科学问题为目标，分别开展电场力对液滴运动及撞击、导线发热及与外界过冷环境换热对液滴撞击表面凝固传热、表面覆冰生长对电场畸变及电场力的相互影响，以及多场耦合作用下的覆冰机理与特性等不同层次且相互关联的研究工作。揭示电场力作用下液滴撞击发热体表面的相界面迁移和凝固相变传热的机理，明晰电场分布、液滴运动及撞击行为、发热体表面凝固相变传热的相互作用关系及规律，建立多物理场耦合作用下的液滴撞击过冷环境中发热体表面的凝固相变传热理论模型，揭示各参数对覆冰生长特性的影响规律，提出覆冰生长的预测模型。促进我国电网安全运行及防结冰技术的发展。

覆冰是一种常见的自然现象，但是对于输电导线，覆冰却是一种威胁，轻则导致电路出现闪络、放电、短路等现象，造成大量的电力损失；重则造成覆冰过重，使得线路舞动以至于线路断裂或是电塔倒塔事故，进而引发大面积电网断电甚至电网崩溃。因此，导线覆冰问题成为了影响电网安全的最大隐患。本项目旨在研究输电导线的覆冰机理，探索外场下液滴与固壁间的多相流动与相界面迁移特性，解决电场、流场、温度场等多物理场交互影响的多相流动和界面凝固相变传热等关键科学问题。项目实施至今，已全部完成研究内容，获得以下重要结果：获得了液滴在无电场下的凝固相变传热及相界面迁移特性，探明了壁面性状及微纳结构参数对液固间的动力学行为及换热特性的影响规律，实验发现超疏水壁面在-20℃条件下依然具有良好的自清洁效应，同时疏水效应也可有效避免雨滴残留在输电绞线的凹槽中，显著减缓或避免后续的覆冰过程；研究了液滴在电场下的动态特性和荷电机制，获得电场强度与分布、液滴大小、撞击速度、壁面性状等因素对液滴撞击输电导线的动力学特性和荷电规律的影响，实验发现，电场下雨滴撞击壁面的动态行为有三种不同模式，数值模拟和可视化实验研究发现，电场力、液滴毛细力及惯性力是导致不同撞击动态行为的主要原因；研究了液滴在电场下的凝固成核机制，获得电场强度和方向以及表面浸润性等因素对液滴电场下非均相成核的影响规律，实验发现在约6kV/cm电场时，液滴的结冰延迟时间将会显著降低80%，研究表明了电场的极化作用和液固界面强双电子层作用是促进液滴非均相成核的主要机制；此外，课题组已经成功制备了光热超疏水表面，该表面既可具备超疏水延迟结冰的效果，-15℃条件下，结冰延迟的时长可达1h，同时也具有优良的光热除冰能力，在一个太阳光下，150s以内即可完全除冰，研究结果为输电线研发高效防除冰技术提供技术指导。