Numerical Study On Flow Behavior And Film Distribution Outside Horizontal Tube

Efficient energy conservation technology has attracted more and more attention in practical processes recently. The falling film evaporator with high efficiency has been utilized in many fields, such as seawater desalination, refrigeration and thermodynamic systems. Cooling liquid sprays from a supply system and falls over the horizontal tube bundles, then the cooling liquid will spread both along the circumferential and axial direction. The falling film becomes the main carrier of heat transfer. So the flow behavior and film distribution around tube bundles have a crucial role in the process. Flow behaviors of liquid column formation, elongation, convergence and detachment decide the condition of heat transfer. Depending on VOF (Volume of Fluid) method, the falling film flow outside horizontal tube was simulated applying the software of Fluent. The 2D and 3D models were used in this paper to simulate the flow behavior and film distribution, respectively. Parameters that would affect falling film flow process were investigated and the main ones were chosen out for deeper research.Early work in 2D simulation focused on the parameters that affected the film distribution outside horizontal tube, such as spray density, tube spacing, the Reynolds number and diameter size of horizontal tube. Based on assumptions and limitations in 2D model, effects of parameters on liquid flow and film thickness had been presented. Results showed that some parameters listed before had little effect on the results while some had great impact. Then main factors that affected flow characteristics would be chosen out and studied deeply in 3D simulating work.Falling film flow characteristics both in circumferential and axial directions were investigated in 3D simulating work. Emphasis was mainly laid on the column pattern outside tube bundle. The falling process of liquid, the impact role, the liquid film waves and film elongation outside horizontal tube played an important role in heat transfer. In addition, the collision and overlapping roles of two adjacent columns made the flow characteristics quit different. The distribution of film thickness would be studied deeply in this 3D simulation. Main factors that affected the film flow were also studied, such Reynolds number and tube spacing. Besides, the drop-column pattern was also discussed in this study. Its flow behaviors were not the same as column pattern. Instabilities and ripples of liquid film would occur in the falling film process and caused obvious fluctuations.Numerical results show that the liquid film outside horizontal tube varies due to many kinds of force effects. Gravity force, surface tension, viscous force and internal force of liquid govern the flow behavior together.3D simulating results are reliable and can visually present the elongation of liquid film. The elongating distance of advancing liquid in axial direction reaches a maximum and keeps stable. Finally a saddle-shaped spreading wave will appear when liquid sprays from an individual feeding hole. The film thickness in circular direction decreases first because ofthe impact of falling liquid. Then film thickness changes little as the circular angle varies from 20°-150°. Liquid accumulation and detachment cause a sharply increase of film thickness in the range of 150°-180°. The film thickness was considered to be infinity when circular angle is 18°. New flow behaviors will happen when liquid falls from two adjacent spray holes. Collision and overlap of two adjacent waves along the axial direction change the flow velocity and result in stagnating effect, which would cause crests and valleys of liquid film outside horizontal tube. A new column is formed in the middle of two adjacent columns and detaches from the tube. The crests appear in the middle of two columns and these regions that are closer to crests will have larger film thickness. In order to fully investigate the flow characteristics outside horizontal tube, variations of film thickness with circumferential angle and axial position are given in detail. Compared with 3D simulation, the 2D simulation mainly considered the sheet flow and presented the flow characteristics in circumferential direction. The flow behavior of spreading liquid in axial direction could not be observed in 2D model. But the 2D model requires much less calculating resources than 3D model. What's more, the 2D simulating results can provide the guidance in analyzing large amount of parameters that may affect the flow characteristics.