|Alternative Title||Investigation of the Coupling Mechanism of Thermocapillary Convection and Phase-change Heat Transfer|
|Place of Conferral||北京|
|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|
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.
|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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