IMECH-IR  > 国家微重力实验室
蒸发界面流动与相变传热特性研究
Alternative TitleInvestigation of Evaporation Interfacial Flow and Heat Transfer Characteristics of Phase Change
刘文军
Thesis Advisor刘秋生
2020-05
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Keyword蒸发,流动不稳定性,热质传输,marangoni 对流,微重力相变传热
Abstract

相变与传热一直以来是国际流体物理领域的研究热点,尤其是蒸发界面流动与相变传热互相耦合的气液两相问题,因其在散热、制冷、流体管理等多个领域具有重要的应用价值而被普遍关注,并成为微重力流体物理的前沿课题之一。本文以微重力环境下的液体蒸发对流不稳定性以及空间两相流系统中的基础科学和关键技术研究为背景,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”和TZ-1货运飞船“两相系统实验平台关键技术研究”空间蒸发冷凝实验项目,以具有蒸发界面的圆柱形浅液层和附壁蒸发液滴为主要研究对象,利用地基实验、空间实验和数值模拟相结合的方法研究了地面常重力和空间微重力条件下,易挥发液体的蒸发界面流动与相变传热特性。

在地基实验研究方面,搭建了液层/液滴蒸发实验系统,利用红外热成像技术、共聚焦测厚技术、热流密度测量技术、CCD相机等实验手段开展了FC-72液层和液滴在不同底板加热温度、液体体积、环境压力、蒸气浓度等条件下蒸发对流与相变传热的地基实验。在处于双向温度梯度下的液层蒸发实验过程中观测到了两种典型的流型:滚波与热液波相耦合的Rolls & HTWs流型(水平温度梯度主导)和多涡胞Marangoni对流流型(垂直温度梯度主导)。探究了不同流型的流动特征、流型转捩的临界条件以及蒸发过程中的热质传输规律,并且发现了Marangoni对流涡胞的波数和液层厚度之间满足的关系。在底部加热的液滴蒸发实验过程中也观测到了两种典型的流型:径向向内流动的热毛细对流和由三相线向中心发展的多涡胞Marangoni对流流型。此外,开展了准静态液层蒸发流动稳定性的实验研究,通过持续注液的方式维持液层高度恒定,从而剥离了液层厚度对流动不稳定性的影响。利用红外热像仪观测到了准静态蒸发液层内流动的3个阶段:首先为Marangoni对流涡胞的形成和分裂阶段,然后是从“源”向“汇”传播的热液波阶段,最后随着液层中心的涡胞逐渐消失,进入了无涡胞的稳定流动阶段。这与自由蒸发液层中的流动情况有所不同,进一步证明了厚度对Marangoni对流多涡胞结构有着重要的影响。

在数值模拟研究方面,基于有限体积法,开发并建立了双向温度梯度下圆柱形蒸发液层的三维数值计算模型,开展了蒸发液层对流与传热的数值模拟研究。模拟结果表明,随着Marangoni数的逐渐增大,液层内的流动将会由二维轴对称流型转捩为完全的三维流动。流型转捩的临界Marangoni数随着表征液层界面换热的Biot数的增大而减小,重力作用在一定程度上稳定了流动。首次指出了蒸发效应对这种流动不稳定性的双重作用:蒸发不仅能促进也能抑制流动失稳,取决于蒸发致冷引起的温度梯度作用的位置。若进一步增大Marangoni数,液层内会出现Marangoni对流的多涡胞结构,其涡胞中心温度低,边缘温度高,在热毛细力的驱动下,涡胞内的流体呈现由涡胞中心向外的流动。同时,发现这种多涡胞结构的演化方式是先在热壁面附近沿径向形成高温环带,然后高温环带被周向分裂为多个独立的小涡胞。此外,液层内部的温度梯度和蒸发致冷效应能明显影响这种多涡胞流型的结构。

最后,配合中国科学院力学研究所TZ-1空间实验项目,分析了部分空间蒸发实验结果。初步结果表明,由于空间微重力环境下浮力效应的缺失,使得对流传热减弱,导致液体平均蒸发速率比同工况下的地基实验减小了近34%;同时,发现了微重力环境下,液层中出现的弯液面会导致Marangoni多涡胞的结构主要发生在液层中心的区域,而地面常重力情况下的涡胞结构则覆盖了整个液层表面。此外,空间微重力环境下蒸发液滴的接触线更容易收缩,使得液滴处在定接触线(CCR)阶段的时长占总蒸发时长的比例,由地基实验的70%减小到45%左右

Other Abstract

Phase change and heat transfer have always been hot research topics in the field of fluid physics, especially the gas-liquid two-phase problem with the coupling of evaporative interface flow and phase change heat transfer. Because of its important application value in many fields such as heat dissipation, refrigeration, and fluid management, it has been widely concerned and become one of the frontier topics of microgravity fluid physics. Based on the flow instability with evaporation under microgravity environment and the research of basic science and key technologies in space two-phase flow system, the cylindrical shallow liquid layer with an evaporation interface and sessile evaporation droplets are chosen as the main research objects. The characteristic of evaporation interface flow and phase-change heat transfer of volatile liquid under ground gravity and space microgravity is experimentally and numerically 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” and by evaporation and condensation experiments on TZ-1 cargo ship of “key technology research of experiment platform on two-phase system” project.

Experimentally, the liquid layer/droplet evaporation experimental system has been set up. The ground-based experiments on evaporation and heat transfer process of the FC-72 liquid layer/droplet under different conditions of substrate heating temperature, liquid volume, ambient pressure, and vapor concentration have been carried out by using infrared thermal photography, confocal thickness meter, heat flux density sensor, CCD camera, and other experimental methods. Two typical flow patterns are observed in the evaporation layer under a bidirectional temperature gradient: Rolls & HTWs flow patterns (dominated by horizontal temperature gradients), and multicellular Marangoni convection patterns (dominated by vertical temperature gradients). The flow characteristics of different flow patterns, the critical conditions of flow transition, and the heat and mass transfer laws during evaporation are studied. And the relationship between the wave number of Marangoni convective cells and the layer height is found to satisfy . Two typical flow patterns are also observed during the droplet evaporation experiment: inward thermocapillary flow and multicellular Marangoni convection developing from the triple line to the center. In addition, an experimental study on the stability of the quasi-static liquid layer evaporation flow is also carried out. The constant liquid layer height is maintained by continuous injection, so the effect of liquid layer thickness on flow instability is stripped. Three periods of the flow in the quasi-static evaporative liquid layer are investigated using an infrared camera: first, the formation and splitting of Marangoni convection cells, then the hydrothermal wave propagating from "source" to "sink", and finally the stable flow stage without vortex cells. This is different from the flow in the freely evaporating liquid layer, which further proves the important effect of layer thickness on the structure of multicellular Marangoni convection.

Numerically, based on the finite volume method, a 3D numerical calculation model of a cylindrical evaporation layer under a bidirectional temperature gradient is developed and a numerical simulation investigation of convection and heat transfer of the evaporation layer is carried out. The simulation results show that as the Marangoni number gradually increases, the flow in the layer will change from a 2D axisymmetric flow pattern to a fully 3D flow. The critical Marangoni number of the flow transition decreases with the increase of the Biot number, which characterizes the heat transfer on layer surface. And the gravity stabilizes the flow to a certain extent. More importantly, we elucidate the twofold role of the latent heat of evaporation in the stability: evaporation not only destabilizes the flow but also stabilizes it, depending upon the place where the evaporation-induced thermal gradients come into play. If the Marangoni number is further increased, a multicellular structure of Marangoni convection will appear. The temperature of the vortex cell center is low and the edge is high. Driven by thermocapillary forces, the fluid in the vortex cell flows outward from the center of the vortex cell. It is also indicated that the evolution of this multicellular structure is to first form a high-temperature annular zone in the radial direction near the hot sidewall, and then the annular zone is split into multiple independent small vortex cells in the circumferential direction. What’s more, the temperature gradient and the evaporation cooling effect can obviously affect the structure of this multicellular pattern.

Finally, in conjunction with the TZ-1 space experiment project of Institute of Mechanics, CAS, the results of some space evaporation experiments have been analyzed. The preliminary results indicate that the lack of buoyancy effect weakens the heat transfer in the space microgravity environment, resulting in an average liquid evaporation rate reduced by nearly 34% compared to the corresponding ground experiment. It has been found that the meniscus appearing in the liquid layer under the microgravity environment will cause the Marangoni multicellular structure to occur mainly in the center of the liquid layer, while the ground pattern covers the entire liquid layer surface under normal gravity. In addition, the contact line of the evaporative droplet is easier to shrink under microgravity, which reduces the proportion of the duration of Constant Contact Radius (CCR) stage to total evaporation lifetime from nearly 70% for the ground-based experiment to about 45% for microgravity experiment.

Call NumberPhd2020-015
Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/81917
Collection国家微重力实验室
Recommended Citation
GB/T 7714
刘文军. 蒸发界面流动与相变传热特性研究[D]. 北京. 中国科学院大学,2020.
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