环形液池内表面蒸发与热毛细对流耦合机理研究

Thermocapillary convection in the liquid layer with surface evaporation exists widely in industrial production and daily life. Surface evaporation and the interfacial temperature jump caused by evaporation have extremely complicated influence on thermocapillary convection in the liquid layer, and the influence mechanism is still unclear so far. Aiming at the key scientific problem ‘the coupled mechanism among surface evaporation, interfacial temperature jump and thermocapillary convection’, this project studies systematically and thoroughly thermocapillary convection when the working fluid evaporates in the low pressure steam environment in the annular pools by using experimental observations and numerical simulations. First, determine the effect of thermocapillary convection in the annular pools on the evaporation mass flux, and obtain the evaporation flux correlation based on the experimental data. Then, analyze the effects of the surface evaporation flux and thermocapillary convection on the interfacial temperature jump, and cognize its physical nature. Finally, determine emphatically critical conditions of the flow destabilization for thermocapillary convection with surface evaporation in the annular pools, explore the flow bifurcation series under the different evaporation conditions, map the flow pattern transition diagrams and clarify the physical mechanism of the coupling effect among surface evaporation, interfacial temperature jump and thermocapillary convection. This study is very important to further expand the research field on thermocapillary convection, enrich and develop the coupled theory between the interfacial heat and mass transfer and the flow, and be expected to obtain some innovative achievements on thermocapillary convection with surface evaporation in the annular pools.

具有表面蒸发的液层内热毛细对流过程广泛存在于工业生产和日常生活中，其表面蒸发及其引起的界面温度跳跃对液层内热毛细对流的影响及其复杂，机理尚不清楚。本项目以环形液池内工质在自身低压蒸汽环境下蒸发时的热毛细对流为研究对象、围绕“表面蒸发-界面温度跳跃-热毛细对流耦合机制”关键科学问题，采用实验观测和数值模拟相结合的方法开展系统而深入的研究。首先，确定热毛细对流对蒸发速率的影响规律，获取蒸发速率计算关联式；然后，分析表面蒸发速率和热毛细对流对界面温度跳跃的影响，认知温度跳跃物理本质；最后，确定具有表面蒸发液层内热毛细对流失稳临界条件及影响因素，探索不同蒸发条件下流动失稳后流型演变规律，获取流型演变图谱，揭示表面蒸发-界面温度跳跃-热毛细对流耦合的物理机制。本研究可进一步拓展热毛细对流研究领域，丰富和发展界面蒸发热质传输与流动耦合理论，并有望在考虑蒸发效应时热毛细对流稳定性方面取得创新性成果。

具有表面蒸发的液层内热对流过程涉及到多种热物理现象之间的相互耦合，特别是热毛细对流、表面蒸发及界面温度跳跃之间的相互作用及其对流动稳定性与流型演变过程的影响非常复杂。本项目采用实验测量和数值模拟相结合的方法，研究热流动对界面蒸发速率的影响规律，确定蒸发界面温度跳跃并分析主要影响因素，探索表面蒸发对环形液层内热毛细对流的稳定性及流动失稳后流型演变规律的影响，揭示表面蒸发效应与热毛细对流耦合的物理机制。结果发现，(1)工质在低压环境下蒸发时，气相侧温度始终高于液相侧温度，在蒸发界面存在明显的温度跳跃，且压力越低，蒸发速率越大，温度跳跃也越大；(2)气相侧热流密度和温度跳跃的大小呈线性关系，气相侧热流密度越大，温度跳跃越大；(3)低压环境下蒸发时液侧温度均匀层的形成条件是存在热毛细流胞和浮力流胞的上下相互叠加。在实验条件下，温度均匀层厚度一般在0到3mm之间，且随着液层深度增加，温度均匀层厚度逐渐增大并趋于定值；随着压比的减小，温度均匀层厚度稍有增加。(4)当工质为水时，在研究范围内，无论蒸发压力多低、液层多深，蒸发诱导的热对流都是稳定的；而当工质为乙醇时，随着蒸发压力的降低和液层深度的增加，流动会失稳，转变为时相关的非稳态流动，速度和温度的波动幅度在界面以下约1.5mm处最大。(5)乙醇蒸发时流动失稳的临界压比随液层深度的增加而增大，失稳的主要原因是流动的滞后效应。(6)随着蒸发压力的降低和壁面加热温度的升高，界面平均蒸发速率增大；随着离加热壁面距离的增加，局部蒸发速率迅速下降。上述研究结果进一步拓展了热对流研究领域，丰富和发展了界面蒸发热质传输与流动耦合理论，为热对流和界面蒸发相变传热研究做出了科学贡献。