微尺度流动沸腾的不稳定性形成机理与非稳态液膜相变动力学

The heat transfer performance of flow boiling is strictly limited by the instability and CHF phenomena, which are essentially influenced by the phase change and fluid dynamics of the thin liquid film underneath boiling bubbles. By the study on the phase change and fluid flow of the thin liquid film, the current project aims to clarify the mechanisms associated with the occurrence of instability and CHF of the flow boiling inside micro-scale tunnels (e.g. mcirochannels and micropipes) and to make quantitative assessments. The thin liquid film located between the micro tunnel wall and gas phase is multiscale, which has usually the transitions from nano, to micro and to mini scales. First, in this study a 3D transient model is to be developed for the thin liquid film. The solutions by a certain multiscale numerical simulation can track accurately the film formation and movement, and the development of dryout spot. Second, we will conduct visualization experiments to observe the simultaneous distribution of the boiling bubble and the wall temperature underneath the thin liquid film, so as to capture the relations between the thin liquid film movement and the boiling instability. By the combination of both the experimental and numerical results, correlations will be proposed for the quantitative evaluation of the heat transfer performance, and the occurrence of instability and CHF. It is worthy of note that, to our knowledge, neigher 3D transient model nor multiscale transient simulation is reported for the thin liquid film inside micro tunnels. The study seems to have non trivial significance for the enhancement and structural optimization of the microscale phase change heat exchangers and heat management equipments.

沸腾气泡底层液膜的相变动力学特性与沸腾不稳定性和临界热流密度(CHF)现象密切相关，后者严重制约着微尺度流动沸腾的换热性能。本项目以研究沸腾液膜的传热和流动机理为途径,旨在阐明微通道(微槽、微管)内的流动沸腾中普遍存在的不稳定性现象和临界热流密度的发生机理、量化相关规律。微通道内的沸腾液膜通常具有从分子尺度蒸干区到微尺度区再到小尺度区的过渡特性, 本研究将建立底层液膜的三维非稳态数理模型，用多尺度数值计算方法高精度地模拟液膜的形成、时间变化和蒸干区域的出现。并将使用高速可视化技术，同时观测沸腾气泡和底层液膜温度的时间变化，弄清底层液膜变化与沸腾不稳定性的联系。结合数值模拟和实验结果，拟合定量计算沸腾换热系数、不稳定性存在条件和CHF值的关联式。国际上还没有微通道内沸腾液膜的三维非稳态相变动力学方程和相应的数值模拟研究先例，本研究对于微尺度相变换热器和热管理装置的换热强化具有重要意义。

本项目开展了微通道沸腾液膜的传热和流动机理研究，利用激光共聚焦方法实现了沸腾微液膜厚度的定量测量，通过微液膜与传热同步测量，阐明了微通道内的流动沸腾不稳定性现象和临界热流密度的发生机理，提出了更为精确的沸腾换热计算模型。针对微通道内的非稳态沸腾液膜流动传热问题, 建立了非稳态数理模型，利用特有的数值计算方法高精度地模拟液膜的形成、时间变化和蒸干区域的出现。利用Micro-PIV 进行了微通道内液膜弯月面附近的表面张力-重力驱动微对流分布研究，测量了动态接触角随运动速度变化的规律。通过记忆合金可变形微结构表面，浸润性随温度变化的表面对沸腾换热进行智能调控，开辟了相变传热强化研究的新领域。本研究加深了对微尺度流动沸腾和微液膜传热传质物理机制的认识，对于微尺度相变换热器和热管理技术的研究具有重要意义。