水滴闪蒸及过冷凝固的非平衡热力学过程和界面运动研究

The flash evaporation and the supercooled solidification of a water droplet during rapid depressurization are typical nonequilibrium thermodynamic processes, and have been widely used in many industrial fields. However, the nonequilibrium thermodynamics theory has not been fully applied in previous studies, and some assumptions, models and conclusions do not match with the thermodynamic principles and experiments. In addition, the internal bubble growth during droplet evaporation, as well as the dendrite development during supercooled solidification was lack of dynamic observation, the present model also ignores the internal interface movement. Therefore, this subject investigates the nonequilibrium thermodynamic process and interface motion during flash evaporation and supercooled solidification of a water droplet due to rapid depressurization. During the experiments, the bubble’s dynamic growth and the crystallization development will be observed using the high speed dynamic analysis technique. For theoretical study, the phase transition process driven by chemical potential difference will be analyzed using nonequilibrium thermodynamic theory. For model research, based on the existing droplet flash model, the motion equation for spherical bubble growth combined with the analysis of gas-liquid interface temperature will be introduced, and the bubble growth during droplet flash will be traced. Based on existing droplet solidification of Stefan model, the Dirichlet condition was introduced to describe the interface temperature during supercooled solidification, the interface tracking method will be used to capture the crystallization development.

速降压环境下水滴闪蒸及过冷凝固是典型的非平衡热力学过程，在众多工业领域具有广泛应用价值。然而过去的研究中，非平衡热力学理论并未得到充分应用，某些假设、模型、结论与热力学原理和实验不符。此外，目前对水滴闪蒸过程中内部的气泡生长，以及过冷凝固中的枝晶发展过程缺乏动态观测，模型研究方面忽略了内部界面运动。为此，本项目系统研究速降压环境下水滴闪蒸和过冷凝固的非平衡热力学过程及界面运动。实验方面，采用高速动态分析技术对气泡动态生长、以及结晶发展过程进行观测。理论研究方面，采用非平衡热力学理论，分析化学势差驱动的相变过程。模型研究方面，在现有液滴闪蒸模型基础上，引入球形气泡生长的运动方程，结合非平衡热力学理论分析获得气-液界面温度，追踪液滴闪蒸过程中球形气泡生长；在现有水滴凝固Stefan模型基础上，引入描述界面温度的Dirichlet条件，采用界面追踪法追踪结晶发展。

速降压环境下液滴闪蒸及过冷凝固过程在工业海水淡化、太空液体排放等领域具有广泛应用价值。项目系统研究速降压环境下液滴闪蒸和过冷凝固的非平衡热力学过程及界面运动，完成了如下工作：（1）首次实验获得了速降压环境下液滴内气泡生长的动态过程，捕获了气泡界面运动；（2）从液体过热以及气体过饱和两个角度分析了速降压环境下液滴内气泡生长机理，通过与实验数据比较，液滴过热造成气泡生长，悬挂液滴用热电偶的存在阻碍了气泡生长，在此基础上理论分析了影响气泡生长的主要因素，包括：压差、表面张力、以及热电偶与液体接触造成的摩擦阻力；（3）对双组分液滴在速降压环境下的蒸发过程进行了实验和理论研究，尤其考虑了两种成分浓度不断变化及其相互影响对质量蒸发率的影响；（4）对低压环境下液滴的过冷凝固过程进行了实验和理论研究，引入界面运动的动力学条件，并用坐标转换法追踪界面运动。本项目的研究成果有助于深入认识非平衡相变的热力学过程和传热传质机理。