IMECH-IR  > 国家微重力实验室
热毛细对流与相变传热耦合传输机理研究
Alternative TitleInvestigation of the Coupling Mechanism of Thermocapillary Convection and Phase-change Heat Transfer
徐国峰
Thesis Advisor刘秋生
2019-11-22
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Keyword相变 热毛细对流 液滴蒸发 微重力 对流不稳定性 数值模拟
Abstract

本文以微重力环境中的流体界面过程以及空间两相系统基础科学和应用技术研究为背景,依托国家自然科学基金重点项目“微(变)重力环境下复杂界面流体传输特性与流动稳定性研究”、实践十号卫星(SJ-10)“蒸发与流体界面效应空间实验研究”项目和货运飞船(TZ-1)“两相系统实验平台关键技术研究”空间蒸发冷凝实验项目,分别以具有相变气液界面的水平液层和附壁蒸发液滴为主要研究对象,利用数值模拟和空间实验方法研究了微重力和重力条件下热毛细对流与相变传热耦合机理。

首先,建立了易挥发性液体在水平方向加热的封闭方腔和开放方腔内的两相系统相变热毛细对流双边模型,描述热毛细对流、浮力对流、相变界面传热传质以及蒸气对流扩散之间的相互影响。建立了考虑界面相变传热以及蒸气扩散的两相系统一维理论分析模型,对比了数值计算结果。在封闭系统相变热毛细-浮力对流中发现了四种主要流型:稳态单涡胞对流SUF、稳态多涡胞对流SMC、热液波HTW和振荡多涡胞对流OMC,并给出了随着热毛细效应和浮力效应变化的对流失稳和流型转化中性稳定性曲线。通过改变Marangoni数和动力Bond数,研究了热毛细效应和浮力效应对气液两相流动以及界面相变传热传质的影响,发现热毛细效应的增强使得原本线性分布的气液界面温度和相变质量流量出现波动,从而诱导了气液两相多涡胞结构的生成;浮力效应的增强加速了流体流动,气层出现上下分层现象,液层的多涡胞结构被抑制,对流趋向稳定。开放系统数值模拟结果表明,强蒸发与冷凝效应降低了界面温度不均匀性,从而使流动趋于稳定。

数值研究了振荡相变热毛细对流的起振过程、不稳定性特征和对流转捩途径。发现热液波不稳定性以行波的形式在气液界面(速度、温度、质量流量)以及液相流场和气相流场内(对流涡胞)从冷端向热端传播,详细阐述了热液波的生成机理与传播规律。计算了高Marangoni数下的振荡相变热毛细对流,发现了对流转捩过程中的二次稳定现象,结果表明,随着温差的增大,驻波形式的振荡逐渐增大直到热液波完全消失,经过短暂的二次稳定区间后,振荡基频突增,随后经过准周期振荡达到混沌状态。

数值分析了无界面相变效应的纯热毛细-浮力对流模型,并与考虑界面相变传热和蒸气扩散的热毛细对流进行了对比。结果表明,界面相变传热(蒸发吸热与冷凝放热)降低了界面平均温度梯度,从而使界面热毛细效应减小,阻滞了液层内多涡胞结构的生成和振荡对流的起振,起到稳定对流流动的作用。

基于实践十号卫星液滴蒸发对流实验,在地面常重力环境和空间微重力环境中开展了不同环境压力、不同底板加热温度、不同基座材料和不同初始注液量的液滴蒸发实验,利用高清CCD观测了蒸发液滴形貌变化,利用热流量传感器测量了蒸发过程中底板的热流密度,同时,开发了液滴形貌分析软件,实现对液滴图像的处理和液滴形貌参数的计算。首次在空间微重力环境下,观测到了大尺度液滴蒸发全过程,获得了空间液滴浸润特性以及蒸发速率等重要科学数据。通过对比地面实验结果,研究了重力对液滴蒸发传热与对流的影响。实验结果分析表明:空间微重力条件下的液滴呈球冠形,在蒸发过程中先后经历了接触线保持不动阶段和成膜后的快速蒸干阶段。基座的导热性能越强、环境压力越低、底板加热温度越高,液滴的蒸发速率越快。空间实验首次证实了液滴内部和外部混合气体中浮力对流的缺失严重恶化了底板对大尺度液滴的能量供给以及界面附近蒸气的扩散效率,导致蒸发速率大幅下降。

Other Abstract

Based on the fluid interface process in microgravity environment and research on fundamental science and applied technology of two-phase system in space, the horizontal liquid layer with phase-change gas-liquid interface and the sessile droplet evaporating is chosen as the main research objects, the coupling mechanism of thermocapillary convection and phase-change heat transfer under microgravity and normal gravity is numerically and experimentally investigated, which is supported by the key project of the national natural science foundation of China “investigation on complex interfacial fluid transport and flow stability in microgravity (varying gravity) environment”, “experimental research on evaporation coupling with fluid interfacial effect in space” project onboard Chinese scientific satellite SJ-10, and “key technology research of experiment platform on two-phase system” project to conduct evaporation and condensation experiments in space which is onboard Chinese first cargo ship TZ-1.

A two-sided phase-change thermocapillary convection model of the volatile liquid layer in a two-phase cavity, which is subjected to horizontal temperature gradient and open or enclosed for top and bottom walls, is proposed. The coupling effect of thermocapillary convection, buoyancy convection, heat and mass transfer on the interface and convective diffusion of vapor is investigated. The one-dimensional theoretical model accounting for vapor diffusion and interfacial heat and mass transfer due to phase change is proposed and compared with the simulation results. Four fundamental flow patterns are found in the enclosed model, which are steady unicellular flow, steady multicellular flow, hydrothermal waves and oscillating multicellular flow, and the neutral stability curve and transition map between four flow patterns are given. By changing Marangoni number and dynamic Bond number, the effects of thermocapillary effect and buoyancy effect on gas-liquid two-phase flow and interfacial phase change heat and mass transfer were studied. It is revealed that the enhancement of thermocapillary effect makes the interfacial temperature and phase-change mass flux fluctuate, which induces the generation of gas-liquid two-phase multicellular structure; the enhancement of buoyancy effect accelerates the fluid flow, the gas layer appears stratification, the multicellular structure of the liquid layer is inhibited, and the convection tends to be stable. It is shown that strong evaporative cooling and condensation heating effects reduce the interfacial temperature, so that the flow tends to be stable.

The process of oscillation initiation, instability characteristics and transition paths of oscillating phase-change thermocapillary convection are numerically studied. It is found that the hydrothermal wave instability propagates from the cold end to the hot end in the form of traveling wave in gas-liquid interface (velocity, temperature, mass flux) and flow field in liquid and gas phase (convective cells). The formation mechanism and propagation law of hydrothermal waves are described in detail. Oscillating phase-change thermocapillary convection was calculated under high Marangoni number, secondary stability in the process of flow transformation is found. The results show that, with the increase of temperature difference, an oscillation in the form of standing wave increased gradually until hydrothermal wave completely disappear, after a short secondary stable interval, the main oscillating frequency increases sharply, and then the flow reaches the chaotic state after the quasi-periodic oscillation.

The pure thermocapillary-buoyancy convection model without interfacial phase-change heat transfer and vapor diffusion in gas phase is numerically calculated and compared with phase-change thermocapillary convection which accounts for the interfacial heat transfer and vapor convective diffusion. The pure thermal capillary-buoyancy convection model without considering interfacial phase change heat transfer and vapor diffusion is numerically calculated and compared with phase change thermal capillarity convection. It is shown that the interfacial phase change heat transfer (heat absorption from evaporation and heat release from condensation) reduces the interfacial average temperature gradient, which reduces the thermocapillary effect and delays the generation of multicellular structure and the process of oscillation initiation, finally the flow is stabilized.

Based on the droplet evaporation and convection experiment onboard SJ-10 satellite, sessile droplet evaporation experiments with different ambient pressure, heating temperature beneath the substrate, substrates materials and initial droplet volume were carried out in the terrestrial normal gravity environment and microgravity environment in space. High definition CCD is used to observe the change of droplet morphology during evaporation lifetime, and the heat flux sensor is used to measure the heat flux beneath the substrate. At the same time, the droplet shape analysis software is developed to process the droplet image and calculate the parameters of droplet shape. For the first time, the whole process of large scale droplet evaporation was observed in the microgravity environment in space, and crucial scientific data, such as droplet spreading wetting characteristics and evaporation rate, are obtained. The influence of gravity on evaporation heat transfer and convection of droplets is studied by comparing the experimental results on the ground, it is revealed that the droplets in the microgravity condition are in perfect spherical shape, and undergo the stage of constant contact radius and the stage of rapid drying out when liquid film occurs. Droplets evaporates faster for higher substrate thermal conductivity, lower ambient pressure, and the higher heating temperature of the substrate. For the first time, it is confirmed by the space experiment that the absence of buoyancy convection inside and outside the droplet seriously aggravates the energy supply of the large scale droplet from the bottom substrate and the diffusion efficiency of the vapor near the interface, which results in a significant decrease in evaporation rate.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/80726
Collection国家微重力实验室
Recommended Citation
GB/T 7714
徐国峰. 热毛细对流与相变传热耦合传输机理研究[D]. 北京. 中国科学院大学,2019.
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