气流条件对水平管外降膜相变界面形态演变及蒸发传热影响机理

The falling film evaporation over horizontal tube surface is a complex flow and heat transfer process of two-phase flow, and its complexity is further enhanced under the air-flow condition because the shear stress at the gas-liquid interface affects the change rule of physical features involved in the liquid film flow and evaporation heat transfer process, including the liquid drop impact, liquid film spreading, liquid circulating flow, liquid film rupture, liquid film dry up, liquid aggregation, suspension drafting flow and falling of the liquid film. Exploring the corresponding mechanism is an extension and deepening of this very field, and is very valuable for us to acquire the nature of this complex heat and mass transfer phenomenon. The objective of this project is to investigate the key scientific issues, such as the evolution laws of falling film interface patterns, the characterization criterion to judge the transition of liquid film flow regimes and the flow patterns between tubes, as well as its effects on evaporation heat transfer. By combining the visual experimental study, parameter measurement experimental investigation, theoretical analysis and numerical simulation, liquid film pattern map and inter-tube flow transition criterion, film thickness prediction correlation, heat and mass transfer coefficient correlation are developed for falling film evaporation outside horizontal tubes under airflow condition. Establishment of internal connection between airflow condition and the law of heat and mass transfer process, liquid film pattern map and inter-tube flow transition criterion forms the research feature and innovation to reveal accurately the interaction laws among the shear stress, surface tension, inertial force, viscous force and gravity force along with their effects on the falling film flow and evaporation heat transfer process. Ultimately, it is expected to provide theoretical support for the development of falling film evaporation techniques.

气流条件下的气-液界面剪切力使得水平管外降膜蒸发过程中液膜铺展、绕流、断裂、脱落、撞击等流动形态演变及降膜蒸发热质传递变得更加复杂。本项目以发展和完善气流条件下的水平管外降膜蒸发机理揭示为研究目标，综合可视化实验、参数测量实验、理论分析和数值模拟研究，深入剖析气流条件对相变界面演变、液膜流态与管间流型、液膜厚度及传热温差分布等的影响机制及规律，构建适用于气流条件下的水平管外降膜蒸发过程中液膜流态与管间流型相图、管外液膜厚度及热质传递系数表征关系式。本项目预期在气流条件下液膜流态与管间流型甄别及转捩判据表征、气流条件与热质传递规律的内在关联等方面，形成研究特色和创新，旨在揭示气流剪切力与表面张力、惯性力、粘性力、重力等力势的耦合作用规律及其对流动形态演变与降膜蒸发热质传递的影响机理，为降膜蒸发技术发展提供理论支持。

气流条件下的气-液界面剪切力使得水平管外降膜蒸发过程中液膜铺展、绕流、断裂、脱落、撞击等流动形态演变及降膜蒸发热质传递变得更加复杂。本项目设计搭建了可视化的逆流气流条件下水平圆管外降膜流动实验系统，运用高速摄像仪背光拍摄法捕捉水平管表面液膜运动表现形式和水平管间液膜流型的变化。对管间降膜流型进行甄别，构建不同气流速度、管间距和降膜喷淋密度下的液膜流态、管间流型相图和液柱波长关系式。开发了恒温恒湿条件下气流对水平椭圆管外降膜蒸发热质传递影响的实验系统，量化分析气流速度、喷淋密度、空气湿球温度和空气相对湿度对气液传热传质的影响，构建了传热传质经验关系式。采用相变蒸发冷凝、结合VOF（Volume of Fluid）多相流模型，对实验工况下的水平管外降膜流动与传热过程进行数值模拟。通过可视化实验观察到过大的气流（大于3.59m/s）导致降膜流型不稳定，降膜发生破碎、飞溅、甚至逆流，气流速度的增加阻碍了其管间流型的正向转换，管间流型转换Re数也随之增加。总传热系数、水膜换热系数和空气对流传质系数随气流速度的增加呈现先增加后平稳的趋势，最佳风速为3.91m/s。越靠近气流速度的入口，其受到影响越大。当Re=574时，静止环境下的液膜厚度沿着轴向方向一致，沿着圆周方向先减小后增加，范围为0.37 mm至0.71 mm，周向角为120°时液膜厚度最小。随着气流速度的增加，第一根管上半周液膜厚度不变，第一根管下半周的液膜厚度由于气液剪切力的影响，轴向不同位置处液膜厚度产生了差异。当气流速度为0.5m/s时，膜厚差异出现在圆周角120°之后。随着气流速度的增加，影响区域随之增加。当气流速度增加到3m/s时，液膜厚度受影响区域也由圆周角120°上移至圆周角85°。第二根管表面液膜受剪切力的影响较大，液膜波动较为剧烈导致其厚度分布杂乱无章，毫无规律，最高的液膜厚度远远超过1mm。为降膜蒸发技术发展提供理论支持。