In order to design the vertical falling film evaporators applied in the sewage source heat pump systems, a numerical study employing the VOF (Volume of Fluid) method has been carried out to determine the thickness and the wave behavior of falling liquid film outside a vertical tube. The effects of the annular gap, tube diameter, tube length, spray density, viscosity and surface tension are discussed. The simulation results demonstrate that spray density and the fluid viscosity are the two most influential factors affecting the film thickness. Besides, when spray density is larger than 0.100 kg/(m∙s), the film thickness decreases slightly with the increase of the flow distance. So the influence of tube length on film thickness cannot be ignored and it also should be considered in the film thickness prediction model. On the other hand, the tube diameter and the surface tension have little effect on the film thickness. It mainly have a significant impact on the wave amplitude of the liquid film at low flowrate. Besides, the annular gap provides a limited effect on the outlet section of the falling film. On the basis of the simulation results, a new correlation incorporating the tube length is developed to effectively predict the falling film thickness. Compared with the experimental data, it is found that the prediction accuracy of the correlation can be controlled within ±15%. The present research provides a theoretical basis for designing falling film evaporators.

为了设计在污水源热泵系统中使用的垂直降膜蒸发器，已经进行了使用VOF（流体体积）方法的数值研究，以确定垂直管外部降液膜的厚度和波动特性。 。讨论了环形间隙，管径，管长，喷雾密度，粘度和表面张力的影响。仿真结果表明，喷雾密度和流体粘度是影响膜厚的两个最有影响力的因素。此外，当喷涂密度大于0.100 kg /（m∙s）时，膜厚随流动距离的增加而略有减小。因此，管长对膜厚的影响不容忽视，在膜厚预测模型中也应予以考虑。另一方面，管的直径和表面张力对膜厚几乎没有影响。在低流量下，它主要对液膜的波幅产生重大影响。此外，环形间隙对降膜的出口部分的作用有限。根据模拟结果，开发了一种结合管长的新相关性，以有效预测降膜厚度。与实验数据相比，发现相关性的预测精度可以控制在±15％以内。本研究为设计降膜蒸发器提供了理论依据。环形间隙对降膜出口部分的作用有限。根据模拟结果，开发了一种结合管长的新相关性，以有效预测降膜厚度。与实验数据相比，发现相关性的预测精度可以控制在±15％以内。本研究为设计降膜蒸发器提供了理论依据。环形间隙对降膜出口部分的作用有限。根据模拟结果，开发了一种结合管长的新相关性，以有效预测降膜厚度。与实验数据相比，发现相关性的预测精度可以控制在±15％以内。本研究为设计降膜蒸发器提供了理论依据。