A mathematical model with phase transition was set up based on the principles of fluid dynamics and VOF multiphase flow model. The falling film evaporation procedure code was embedded in FLUENT through the user define functions( UDF). The principles in the circumferential direction of wall film thickness,velocity,temperature,mass transfer rate and local heat transfer coefficient was analyzed. Results show that the thickness of falling film was rather thin when the X was about 0. 5~0. 8,and this is good for the process of the heat transfer. The bottom and the top of the tube have the large value of local heat transfer coefficient,and the biggest value occurs at the top of the tube. In the circumferential direction,the surplus temperature increases with the distance grows,causing the biggest surplus temperature and evaporation rate at the bottom of the tube. The numerical result is in good agreement with the experimental and predicted data in literature,which means that the mathematical model with heat and mass transfer is reasonable and more comprehensive,and can be used in studying the flow,heat and mass transfer characteristics of falling water film outside horizontal tubes.%随着计算机技术的发展,计算流体力学成为深入研究流动传热传质过程的重要方法.文章基于流体动力学基本原理和VOF多相流模型,建立了水平管外降膜流动与相变传热传质过程数学模型,针对降膜蒸发复杂的相变过程,通过用户自定义函数UDF,将编制的计算程序嵌入FLUENT软件相应模块,对降膜蒸发过程进行了模拟研究,阐明了管外液膜、速度、温度分布及其局部传热传质特性沿圆周方向的变化规律.结果表明:在X=0.5~0.8时,液膜厚度较薄,有利于过程传热,圆管底部与顶部局部传热系数较大,并在管顶部出现传热系数最大值;相界面过余温度沿周向距离的增大而增加,圆管下方具有最大的相界面换热温差和最高的蒸发速率;此数值模拟方法和传热传质模型可以较为合理地进行水平管外降膜蒸发过程的理论分析.
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