液池中粘弹性热毛细流的流动特性及稳定性机理研究

Thermocapillary flow is induced along a free surface with a temperature gradient if the surface tension depends on temperature. It arises in a number of technical applications, such as material welding and crystal growth. The flow stability is very important for the quality of products and therefore has received much attention for many years. In former studies, Newtonian fluid is often used. However, in some processes, for example plastic welding, the plastic melt is polymer liquid, whose viscoelasticity has great impact on the flow. This project will study the flow characteristic and stability mechanism for viscoelastic thermocapillary flow in liquid pools. Rectangular and cylindrical pools are used as the geometrical conditions. Two fluid models are considered which are UCM and Oldroy-B fluids. The thermal boundary conditions are constant temperature or constant heat-flux. The basic flow will be computed. The numerical modeling and stability analysis methods will be developed. We will determine the physical mechanism of flow instability based on the most unsteady modes and the form of energy transfer. The impact of elasticity on the flow stability will be illustrated. This project with the background in plastic welding, will be studied by theoretical analysis and numerical modeling. It will expand the research of thermocapillary flow in liquid pools to viscoelastic fluids which have strong application background. The basic law of the impact of viscoelasticity on thermocapillary flows will be determined.

热毛细流是由液体表面温度分布不均匀形成表面张力梯度而驱动的流体运动，广泛存在于材料焊接、晶体生长等工业领域。其流动稳定性对于产品质量极为重要，长期受人们关注。以往研究中主要采用的是牛顿流体模型。但是某些工艺如塑料焊接中，融化状态下的塑料是聚合物流体，其粘弹性对于流动特性具有重要影响。本项目研究液池中粘弹性热毛细流的流动特性及稳定性。几何条件采用矩形和圆柱形液池模型，流体采用UCM、Oldroy-B两种粘弹性流体模型，传热条件采用热流边界或温度边界；计算基本流场，发展粘弹性热毛细流的数值模拟和稳定性分析方法，并根据最不稳定模态和能量传输关系确定流动失稳的物理机制，说明弹性对稳定性的影响。 本项目以塑料焊接等工艺为背景，基于理论分析和数值模拟方法，将在热毛细效应下液池流动失稳的研究开拓到具有重要应用背景的粘弹性流体范畴，揭示并分析弹性效应对热毛细流动特性及稳定性产生影响的基本规律。

聚合物流体的热毛细对流广泛存在于涂膜、液膜干燥、光刻、喷墨打印和空间站聚合物加工等工程应用中。本项目以聚合物流体的热毛细对流为研究对象，通过数值计算和线性稳定性分析，发现聚合物的热毛细液层存在三种失稳模态，分别是斜波、流向波和展向稳态模态。. 对于粘弹性热毛细液层，其失稳的第一种模态的临界Marangoni数随着弹性数增长而升高；第二种模态的扰动应力做功在竖直方向上有多次震荡；当弹性数足够大时，第三种模态成为最不稳定的模态，它的扰动能量来自表面热毛细力的做功，而由扰动应力耗散。当弹性效应很强时，流体在极低雷诺数下还会出现弹性失稳。. 对于剪切稀化流体，线性流的稳定性在中低普朗特数时下降而在大普朗特数时略有上升；回流的稳定性增强，其扰动能量集中在液面，在大普朗特数时出现流向波，而中普朗特数时温度振荡最大点出现在液层底部。. 对于屈服应力流体，流场分成三层，其中间层为屈服区，速度扰动仅出现在最上层而温度扰动可以在小普朗特数下出现在整个流场，上屈服面会出现扰动，其幅度随普朗特数增大而迅速减小。. 本项研究的成果对于理解非牛顿流体热毛细的对流失稳机理具有重要的理论价值，对于空间站等微重力条件下聚合物流体液层的流动控制具有重要的应用价值。