液滴在固体表面的碰撞及结冰现象十分常见，在生产生活的许多领域有着大量的应用，如喷墨打印、农药喷洒、表面喷涂及食品冷冻等。同时，液滴结冰造成的危害也不容忽视，例如在热交换器、风力涡轮机、电缆和机翼表面发生结冰均会造成巨大的安全隐患和经济损失。因此，了解液滴碰撞与结冰的基本原理对防结冰问题十分关键。本文提出了一种基于耗散粒子动力学方法的液滴碰撞结冰模型，对液滴在固体表面的碰撞与结冰过程进行了研究，以“常温液滴撞击固体表面-过冷液滴静止结冰-过冷液滴撞击结冰”为研究路线，旨在阐明液滴碰撞结冰时动力学与热力学过程的发展，并揭示它们之间的耦合关系。本研究建立了三维常温液滴撞击纤维表面的多体耗散粒子动力学模型，结合理论分析，系统地探究了纤维倾斜角、液滴偏心率以及纤维润湿性对碰撞的影响。基于能量守恒的多体耗散粒子动力学（MDPDE）方法，先后研究了液滴的二维及三维静止结冰。对于二维液滴结冰MDPDE模型，假设再辉后液滴初始冰分布均匀，并对其物理性质进行了修正，提高了结冰模型的准确性。进一步地，建立了三维液滴结冰MDPDE模型，对比了再辉后初始冰均匀分布与非均匀分布这两种形式，证明了非均匀冰分布模型更加接近实际结冰过程。得到了一个平面上液滴凝固时间的预测式，能够给出受液滴过冷度影响的凝固时间的范围。改进了耗散力项，结合实验结果，将再辉速率与过冷度相关联，最终建立了三维过冷液滴碰撞结冰MDPDE模型。研究了过冷液滴在三种超疏水微结构表面的撞击，发现微柱表面的防结冰性能受固体分数和Weber数的共同影响，微坑表面的防结冰性能主要受固体分数影响，在所研究的范围内微脊表面的防结冰性能比另外两种表面更好。特别地，本文指出液滴接触时间并不能完全衡量微结构表面的防结冰性能，受液滴动力学形态的影响，在接触时间较低时，液滴的临界粘附温度可能会随接触时间的降低而升高。最后，对不同释放间隔及韦伯数下液滴在超疏水平面的连续撞击进行了数值研究，发现了五种碰撞特征形态。分析了类薄饼状回弹形态的动力学过程及产生机理，并证明了双液滴撞击时此种回弹形式有利于提升表面的防结冰性能。本研究能够为实际生活中的防结冰问题提供一定的理论参考。
The collision and icing phenomenon of droplets on solid surfaces is very common, and it has a large number of applications in many fields of production and life, such as inkjet printing, pesticide spraying, surface spraying and food freezing. At the same time, the damage caused by droplet icing cannot be ignored, such as on the surface of heat exchangers, wind turbines, cables and wings, which can cause significant safety hazards and economic losses. Therefore, it is quite important to understand the basic principle of droplet impact and freezing to droplet anti-icing. In this paper, a droplet impact freezing model based on dissipative particle dynamics method is proposed to study the collision and icing process of droplet on solid surface. The research route of "droplet impact on solid surface at normal temperature - static freezing of supercooled droplet - impact freezing of supercooled droplet" is used to illustrate the development of kinetic and thermodynamic processes during droplet impact freezing, and reveal the coupling relationship between them.In this study, a three-dimensional many-body dissipative particle dynamics model of droplet impacting fibers at room temperature was established. Combined with theoretical analysis, the effects of fiber inclination angle, droplet eccentricity and fiber wettability on the collision were studied systematically. Based on the many-body dissipative particle dynamics with energy conservation (MDPDE) method, the two-dimensional and three-dimensional static freezing of the droplet is studied. For the two-dimensional droplet freezing MDPDE model, it is assumed that the initial ice distribution of the droplet after recalescence is uniform, and its physical properties are modified to improve the accuracy of the icing model. Furthermore, a three-dimensional MDPDE model of droplet recalescence is established, and the uniform distribution of initial ice after recalescence is compared with the non-uniform distribution, which proves that the non-uniform ice distribution model is closer to the actual icing process. A prediction formula for the solidification time of droplet on the plane is obtained, which can give the range of solidification time affected by the undercooling degree of droplet.The dissipative force term is improved, and the recalescence rate is correlated with the undercooling degree based on the experimental results. Finally, a three-dimensional supercooled droplet impact freezing MDPDE model is established. The impact of supercooled droplets on the surface of three kinds of superhydrophobic microstructures is studied. It was found that the anti-icing performance of the micropillar surface was affected by the solid fraction and Weber number, and the anti-icing performance of the microcavity surface was mainly affected by the solid fraction. The anti-icing ability of the microridge surface is better than that of the other two surfaces in the study range. In particular, it is pointed out that the droplet contact time cannot completely measure the anti-icing performance of the microstructure surface, and the critical adhesion temperature of the droplet may increase with the decrease of the contact time when the contact time is low due to the influence of the droplet dynamics.Finally, the successive impact of droplets on the superhydrophobic plane at different release intervals and Weber number is numerically studied, and five kinds of characteristic morphologies are found. The dynamic process and production mechanism of the pancake-like rebound form are analyzed, and it is proved that this rebound form is beneficial to improve the anti-icing performance of the surface when the two droplets impact. This study can provide some theoretical reference for the problem of anti-icing in real life.
