非常规条件下液滴撞击壁面现象的微观机理与调控

Impact of droplets on surfaces often happens under off-normal conditions of temperature, pressure and gas currents. Previous studies concentrated on impact events in normal conditions and focused on the macroscale features. This project aims to study the impact of droplets on solid surfaces in off-normal conditions through experimental measurements, numerical simulations, and theoretical analyses. By overcoming the scale challenges posed by the problems in time and space, we will measure the impact process with resolutions of microseconds and micrometers. By focusing on the microscale events happening near the contact region between the droplet and the surface during the impact process, such as the generation of the gas film between the droplet and the surface and the production of microdroplets by splashing, we will build a direct connection between microscale phenomena and macroscale phenomena under the interaction between the droplet and the surface. By examining the unique behaviors of flow, heat transfer, and phase change during the impact process happening in off-normal conditions, we will uncover the law of these behaviors. By adjusting the surrounding conditions and the impact parameters in a number of applications, e.g. fuel spray in internal combustion engines, icing on aircraft surfaces, and three-dimensional printing, we will be able to control the impact outcomes and optimize the processes in these applications. This project will enrich the relevant theory of droplet impact, deepen the insights into the complex droplet behaviors, and provide theoretical bases for the numerous applications in many engineering areas.

液滴撞击壁面现象多发生在远远偏离于常规温度、压力、气流运动的条件下，以往的研究主要针对常规条件下的液滴撞壁行为并注重液滴的宏观形貌。本项目拟针对非常规条件下的液滴撞壁行为，通过实验测量、数值模拟与理论分析等技术手段，克服液滴撞壁研究中时空尺度方面的障碍，实现以微秒、微米级高时空分辨率研究液滴撞壁过程；通过关注撞壁瞬间液滴与壁面间产生的气膜和飞溅子液滴等微观行为，建立液滴与壁面相互作用下微观行为与宏观形貌的联系；通过考察液滴撞壁过程中非常规条件下所特有的流动、传热、相变行为，揭示非常规条件对液滴撞壁的作用规律；通过调节撞壁过程中的环境参数和撞击参数，针对内燃机喷雾、飞机表面结冰、三维打印等实际应用，实现对液滴撞壁的调控和实际应用过程的优化。本项目将丰富和完善液滴撞壁的相关理论，提升对复杂液滴行为的认识，并为液滴在众多工程领域的广泛应用提供重要的理论依据。

液滴撞击壁面现象多发生在远远偏离于常规温度、压力、气流运动的条件下，以往研究主要针对常规条件下的液滴撞壁行为并注重液滴的宏观形貌。本项目针对非常规条件下的液滴撞壁行为，通过实验测量、数值模拟与理论分析等技术手段，探索非常规条件对液滴撞壁的作用规律、微观行为的发生机理、撞击结果的调控手段。研究发现了高温条件下液滴撞击壁面过程中的雾化反弹现象，揭示了雾化反弹时液滴驻留时间大幅减小的流动相变机理，提出了通过调节壁面温度从而缩短液滴驻留时间的控制思路；发现了高温条件下液滴撞击壁面Leidenfrost反弹时液滴底部的界面震荡现象，揭示了该现象的发生机理为液体气化、气膜排出、表面波传播的耦合作用；发现了高压条件下液滴撞击过程中飞溅抑制，并理论分析了飞溅阈值；发现了高粘液滴撞击过程中的二次射流现象，揭示了其中表面爬升射流的产生机制；采用彩色干涉技术实现了撞击过程中气膜厚度的动态测量，建立了气膜厚度演化的简化理论模型；提出了基于改进背景纹影法的浓度场测量技术，实现了液滴撞击过程中蒸汽浓度的动态测量。本项目成果已发表SCI检索论文17篇，其中包括1篇文章被Langmuir主编选为封面文章，1篇文章被Soft Matter主编选为封底文章，1篇文章被Physics of Fluids主编选为Featured article并在期刊网站首页展示。在该项目的支持下，相关研究成果申请国家发明专利2项，在国内外学术会议上作邀请报告和主旨报告3次，以及一般口头报告3次，包括如中国工程热物理年会、美国物理学会流体力学年会APSDFD等。本项目研究结果丰富了液滴撞壁的现有相关理论，加深了复杂液滴行为的认识，可为液滴在众多工程领域的广泛应用提供重要的理论依据。