竖壁降膜流动特性与稳定性的数值研究

Numerically Investigating Flow Characteristics and Stability of Film Falling down a Vertical Plate at High Reynolds Number

采用流体体积法(VOF)对于高雷诺数竖壁降膜流场进行了数值模拟,分析了液膜速度分布、平均液膜厚度、基液膜厚度、波动振幅与波长等特性参数的变化规律,讨论了波动特性对于降膜传质及稳定性的影响。计算结果表明,随着流动向下游发展,平均液膜厚度先是迅速减小,然后减小速度趋缓;波动振幅和波长随着流动距离的增加持续增大,这将导致波动传质能力增强,同时也使得基液膜厚度不断减小,液膜的稳定性变差。采用VOF方法的模拟结果与高速摄影实验数据符合良好,说明VOF方法准确描述了绝大部分液膜流动特性,在总体上对于竖壁降膜流场进行了准确的预测。

We have come to believe that falling film will be widely applied to various industries in the near future to obtain high heat and mass transfer performance. We now report our exploratory study results on the flow field of the falling film down a vertical plate at high Reynolds number. In the full paper, we explain in some detail our explora- tion; in this abstract, we just add some pertinent remarks to two topics of our explanation： （ 1 ） computational method and model; Fig. 1 shows the computational model and boundary conditions, and Fig. 2 shows the physical structure of a wavy liquid film based on experimental and numerical observations; （2） computational results and their analysis ; we use the volume of fluid （VOF） method and k-e turbulence model to carry out the numerical sim- ulation of the flow field of the falling film and then compare the simulation results with experimental data. We ana- lyze the variation laws of some important characteristic parameters such as velocity distribution, average thickness and base thickness of the falling film and the amplitude and the length of surface wave and then discuss the effects of wave characteristics on mass transfer and stability of falling film. The computational results indicate that the flow fields have very complicated structures as shown in Figs. 3 and 5. The average thickness of falling film decreases first quickly, then slowly and finally tends to be constant as the flow goes downstream, as shown in Fig. 4. As the film falls downstream, the amplitude and length of surface wave increase gradually, but the thickness of base film decreases gradually ; the mass transfer is enhanced at the same time, however, the stability of falling film becomes worse, we compare the VOF-computational results with the high speed photography measurements. Fig. 7 indicates that the VOF method accurately captures most of flow characteristics and predicts better the flow field of the film falling down a vertical plate at high Reynolds number.