Abstract The horizontal-tube falling film evaporation is an efficient heat transfer technology for energy-saving, with good economic and environmental benefits. In the falling film evaporation technology, the thickness and uniformity of liquid film are considered as important hydrodynamic characteristics that influence the heat transfer of fluid greatly. However, through experimental methods, it is difficult to obtain the microscopic characteristics and subtle changes of the velocity and temperature distribution inside the liquid film, and hard to describe the gas-liquid interface during the evaporation process of the liquid on the tube surface. Therefore, CFD simulation is used in the researches on the falling film flow and heat transfer characteristics outside the horizontal tube. This paper mainly reviewed the CFD simulation work of falling film evaporation outside the horizontal tube in the existing literatures. The models used by researchers were made a summary. The effects and microscopic mechanisms of flow parameters such as spray density, thermodynamic parameters such as the surface tension of liquid, structural parameters such as the liquid distributor and evaporation characteristics of heat transfer tubes and tube bundle were summarized and analyzed. Based on the researches’ CFD simulation results of hydrodynamics and heat and mass transfer of falling film evaporation, it is found that these parameters have their own reasonable ranges to ensure the efficient operation of falling film evaporators under specific operation conditions. Beyond the reasonable range, the formation of liquid film on the tube surface is to be destroyed, it will result in the decrease of heat transfer performance and the dryout. In addition, by adjusting the magnitude of the surface tension, wall adhesion, inertia force and other forces, the change of these parameters leads to the generation of vortex and recirculation phenomenon, the decrease of liquid film thickness and the enhancement of liquid film fluctuation, so as to reduce the thermal boundary layer thickness and enhance the heat and mass transfer performance of the film evaporation process. It is considered that under the comprehensive action of multi-parameters, the three-dimensional distribution of flow characteristics and heat transfer coefficient of multicomponent working fluid, as well as the change of flow pattern between tube bundles, are the improvement directions of current CFD simulation. And the research directions of future CFD simulation are the microscopic hydrodynamic and heat transfer characteristics caused by wall structure reconstruction and the influence of auxiliary components on the falling film evaporation process.

中文翻译：

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水平管降膜蒸发器外液膜流动与传热特性综述：Cfd数值模拟

摘要 水平管降膜蒸发是一种高效的节能传热技术，具有良好的经济和环境效益。在降膜蒸发技术中，液膜的厚度和均匀性被认为是对流体传热影响很大的重要流体力学特性。然而，通过实验方法难以获得液膜内部速度和温度分布的微观特征和细微变化，难以描述液体在管表面蒸发过程中的气液界面。因此，CFD模拟用于水平管外降膜流动和传热特性的研究。本文主要综述了现有文献中水平管外降膜蒸发的CFD模拟工作。对研究人员使用的模型进行了总结。总结分析了喷雾密度等流动参数、液体表面张力等热力学参数、液体分布器等结构参数以及传热管和管束蒸发特性的影响及其微观机理。基于对降膜蒸发的流体力学和传热传质研究的CFD模拟结果，发现这些参数都有自己的合理范围，以保证降膜蒸发器在特定运行条件下的高效运行。超出合理范围，管表面形成的液膜被破坏，会导致传热性能下降和干涸。此外，通过调整表面张力、壁面附着力、惯性力等力的大小，这些参数的变化导致涡流和回流现象的产生，液膜厚度的减小和液膜波动的增强，从而降低热边界层厚度，提高薄膜蒸发过程的传热传质性能。认为在多参数综合作用下，多组分工质流动特性和传热系数的三维分布，以及管束间流型的变化，是目前CFD仿真的改进方向。未来CFD模拟的研究方向是壁结构重构引起的微观流体动力学和传热特性以及辅助部件对降膜蒸发过程的影响。