在核工业领域，高效的冷凝换热技术可以压缩换热设备体积，提高发电效率，增强系统安全性。液滴聚并弹跳现象是超疏水冷凝表面上一种自发的高效的液滴脱湿现象，强化液滴聚并弹跳以加速液滴在冷凝壁面的更新速度对提高冷凝换热效率具有重要意义。目前，学界在平坦超疏水表面上的液滴聚并弹跳领域开展了大量的研究，发现改变流体及固体参数无法克服起跳速度低、能量利用效率低、起跳角度不可控等缺点。而宏观结构表面克服了平坦表面的缺点，其液滴起跳速度大，能量利用效率大幅上升，起跳偏转角度可控。首先，本文搭建了液滴聚并弹跳可视化实验平台，开展了阶梯结构表面上的液滴聚并弹跳实验研究。实验表明阶梯结构上的最大无量纲液滴聚并起跳速度v* = 0.65，是平坦表面上的2.8倍，能量利用效率η = 34.9%，是平坦表面的8.2倍，起跳角θj = 6.2° - 90°，实现了已有宏观结构的最大起跳偏转。其次，本文基于三维相场法数值模拟了平坦表面、侧墙结构、棒状结构、阶梯结构、脊型结构和蛋形结构上的液滴聚并弹跳过程。在液滴形变过程中，宏观结构对液滴形变的影响主要发生在接触阶段，而在合并阶段的影响较小。对应地，在合并阶段，液滴合并释放的剩余表面能主要以振荡动能形式存在，平动动能基本为零，在接触阶段，振荡动能转化为平动动能，因此，液滴仅在接触阶段获得质心速度。从能量分布上看，宏观结构不仅大幅提高了液滴的平动动能还提高了液滴的剩余表面能，而液滴内部的振荡动能和粘性耗散相应减小。最后，本文建立了基于能量利用效率的液滴聚并弹跳能量演化模型。模型将总能量利用效率分解为三个子能量利用效率η = εs · εm · εk，分别表征宏观结构对液滴聚并弹跳过程中粘性耗散、重力势能和速度场一致性的影响。动能利用效率εk是大部分结构提高液滴起跳速度的最关键因素，结构重构了液滴内部压力场，阻碍了液滴合并的进程，延长了液滴与壁面的接触时间，因此液滴内部速度场一致性大大提高；表面能利用效率εs是结构提高液滴起跳速度的重要原因，宏观结构主要影响合并阶段粘性耗散，且合并阶段粘性耗散随液滴合并阶段的持续时间ti的减小而减小；机械能利用效率εm略有下降，其负面影响相对εk和εs的大幅提高可以忽略。
Coalescence-induced droplet jumping is a spontaneous and efficient droplet dewetting phenomenon on superhydrophobic condensing surface. It is of great significance to accelerate droplet renewal rate on the condensing wall to improve the heat transfer efficiency. Currently, a lot of researches have been carried out in the field of coalescence-induced droplet jumping on flat superhydrophobic surfaces. However, it is found that parameters, including surface wettability, fluid properties, droplet size and number, have little influence on jumping enhancement and transport control. Interestingly, macrostructure overcomes the shortcomings of flat surface, with which departure velocity is enhanced, the energy efficiency is greatly increased, and the departure direction is controllable.Firstly, this paper built an experimental platform for coalescence-induced droplet jumping, designed and constructed superhydrophobic flat surface and step structure. The experimental results showed that the maximum dimensionless departure velocity on step structure v* = 0.65 is 2.8 times as that on flat surface, the energy efficiency η = 34.9% is 8.2 times as that on flat surface, and the departure direction is θj = 6.2° - 90°, which is the largest deflection at present.Secondly, based on the three-dimensional phase-field method, this article numerically studied coalescence-induced droplet jumping process on the flat surface, sidewall structure, string structure, step structure, ridge structure and egg structure. The simulations obtained the physical field data of droplet morphology evolution characteristics and momentum/ energy evolution process, which are hard to measure in the experiment. The influence of macrostructures on the physical parameters of coalescence-induced droplet jumping was compared and analyzed. Data showed that the energy distribution of droplet on macrostructure cases is much different from that on flat surface. Hence, the former theoretical model of coalescence-induced droplet jumping on flat surface is no longer applicable.Finally, an analysis of coalescence-induced droplet jumping based on energy efficiency was established. In the analysis, three sub-energy efficiencies were proposed to represent the influence of macrostructure on viscous dissipation, gravitational potential energy and velocity consistency during coalescence-induced droplet jumping, so as to evaluate the effect of macrostructure on droplet jumping enhancement. Increase of excess surface energy and of energy efficiency are two paths to droplet jumping enhancement. The increase of droplet energy efficiency on the macrostructure is the main factor that improves droplet departure velocity. The enhancement mechanism is that the macrostructure promotes the velocity consistency and greatly reduces the viscous dissipation. The egg structure allows droplet to gain a large amount of excess surface energy by forcing the droplet to deviate from sphere-shape before merge, thus achieving the maximum departure velocity found so far in the study.This article studied the physical phenomenon of coalescence-induced droplet jumping on macrostructure, designed and constructed the step structure with excellent performance, revealed the physical mechanism of droplet jumping enhancement on macrostructure, and established mechanism analysis of coalescence-induced droplet jumping on macrostructure. It provides theoretical support for the optimization of condensation equipment of various nuclear safety systems such as residual heat removal system and emergency core cooling system.
