【作者】 兰忠；

【导师】 马学虎；

【作者基本信息】 大连理工大学 ， 化学工程， 2006， 博士

【摘要】 界面效应对蒸气冷凝传热过程有显著的影响，而对于该影响的机理及量化关系的研究却比较缺乏。本文首先实验研究了液固表面自由能差对蒸气冷凝传热过程的影响。通过两种方法改变液固表面自由能差：（1）在相同的实验条件下，以不同的处理表面改变冷凝壁面的表面自由能；（2）相同的冷凝表面，改变操作条件或以混合蒸气冷凝来改变冷凝液的表面自由能。通过实验观察发现一定条件下，冷凝形态发生膜状到过渡状态和滴状的变化，并相应地伴随着传热性能的变化。以界面效应强化冷凝传热，蒸气冷凝传热系数随表面自由能差变化的机理，对这一现象作了解释，并得到了乙醇—水混合蒸气的冷凝形态变化条件，阐述了乙醇—水混合蒸气冷凝过程中，热通量和传热系数与过冷度变化趋势。表面自由能差小于14±1mJ·m^-2表现为膜状冷凝，表面自由能差大于21±1mJ·m^-2表现为滴状冷凝，表面自由能差介于二者之间为过渡区域，从一个新的角度探索了液相共溶混合蒸气的冷凝传热机制。 对于界面效应影响冷凝液的运动过程和特征，本文研究了重力场中竖壁上液滴的脱落直径和三相接触线形态。实验观察到液滴的三相接触线形状并非是圆或椭圆形，而是非规则的形状。根据液滴脱落时的形态，本文提出了接触线形状为两个半椭圆的组合，并由重力和表面张力的平衡推导出脱落直径的数学表达式。并进行了实验验证和分析计算。结果表明，对于一定的液-固-气系统，液滴的脱落直径是由液体表面张力和前进角、后退角以及平衡接触角共同决定的。实验测定脱落直径与模型计算结果比较吻合，表明接触角越大或（及）接触角滞后越小，则脱落直径越小。 本文分析了竖壁沟流流动，基于最小能量原理，当壁面为存在接触角滞后的非理想表面时，其控制方程的边界条件与滞后接触角有关，分析了流率存在波动时沟流厚度的波动原因和范围。实验研究了不同液体在不同表面上的沟流流动，结果表明了沟流波动与界面效应有关，沟流的实际厚度介于由滞后角限定的某一范围内，并且与沟流平均流率有关。若流率波动未超出由滞后接触角及沟流基础流率限定的范围时，沟流只在厚度方向发生波动；若流率波动超出这一范围，则沟流在宽度方向也开始伸缩。进而可得出，接触角滞后越大，则沟流在壁面上越不易消失，即后退角越小的表面，可能出现沟流（过渡冷凝形态）的可能性也越大。 本文实验研究和分析了滴状冷凝过程中，液滴的随机运动行为以及液滴分布的分形特征，发现液滴的运动和分布与接触角及接触角滞后有关。并分析得到液滴分形分布模型中相邻两代液滴半径比与滞后接触角的关系式，该式计算结果与实验结果十分吻合。实验结果表明，在存在接触角滞后的表面上，液滴能以脉动形式长大，造成液滴分布呈更多还原

【Abstract】 The condensation heat transfer process of binary mixture of steam and ethanol vapor as well as pure steam were studied experimentally with various vapor concentrations and surface subcooling at the pressure around atmospheric. With changing of the surface free energy difference through different solid surfaces or vapor component, it is found that the condensation modes altered from filmwise, to transition and finally to dropwise with respect to the operation conditions. The concept of surface-free-energy-difference is introduced to explain the mechanism of the conversion of the condensation modes and the enhancement of heat transfer due to the interfacial interaction. From this viewpoint, the condensation characteristic curves of binary mixture of steam and ethanol vapor can be explained reasonably. Based on the data of the present paper and those reported in literatures, the conversion criterion of the condensation modes is proposed. The condensation state is filmwise for the surface-free-energy-difference less than 14±1 mJ.m^-2 and dropwise for more than 21±1 mJ.m^-2 and transition for between the above two. From the experimental results, it can be simply concluded that the interfacial interaction affects the moving of the condensates and and thus affects the heat transfer.By studying the moving of the condensates with the effect of the interfacial interaction, the departure droplets, the rivulets, and the droplet size distribution were analyzed. Firstly, this paper deals with the relationship of the departing diameter of a liquid drop and the shape of contact line on a vertical solid plate in the gravitational field. The experimental photoes showed that the contact line is not circular or elliptical, but in an irregular shape. The combination of two ellipses is proposed to model the shape of the contact line. Simultaneously, equations for the departing diameter of droplet are derived in terms of the force balance between gravity and surface tension. For a specified liquid and substrate, the departure diameter is related to the equilibrium, advancing and receding contact angles. The effects of these factors on departing diameter have been modeled in this paper. Secondly, based on the principle of the minimum total energy （MTE）, the analysis of the laminar liquid rivulet flowing downward along a vertical plate is conducted theoretically and experimentally. The contact angle hysteresis of the solid surface is taken into account in the boundary conditions for the governing equations. The mechanism and possibility of the wave occurrence at the free surface of the rivulet are discussed. In addition, the maximum liquid film thickness of the rivulet influenced by the contact angle hysteresis is experimentally measured. The analytical results and the visual photos show that the wave at the free surface更多还原