Numerical simulation of the behavior of falling films on horizontal plain tubes

In this paper, a numerical simulation is performed in order to reveal the evaporation heating transfer mechanism of the falling-film of water on the outer wall of horizontal tubes, and the influence of gas-water-solid contact angles on the film distribution on the tube is mainly addressed. The simulation is performed by solving the motion equation of the fluid on fixed three-dimensional (3D) grid systems, and the so-called volume of fluid (VOF) method is employed to handle the movement of interface between the liquid and the vapor phase. The numerical results are compared with that of a simplified model and also with the experimental recordings obtained in visualization experiments conducted with tubes made of different materials, and it is shown that the numerical results are in very good agreement with the experimental observation of the falling films, indicating that the numerical methods applied in the present study is correct and accurate. Based on the numerical results, effects of the contact angle, the flow rate, the tube diameter and the liquid feeder height on the falling film behavior are discussed. Generally, the film width increases along with the increase of flow rate or the liquid feeder height, while variation of the film thickness exhibits different features at different conditions, and the contact angle is one of the major factors governing the behavior of the falling films.