|Alternative Title||Investigation of Evaporation Interfacial Flow and Heat Transfer Characteristics of Phase Change|
|Place of Conferral||北京|
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。
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.
|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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