Numerical Simulation Of Liquid Film Flow On Horizontal Non-tubes

When a falling liquid flows along the tube in continuous liquid sheet caused by gravity, heat exchange will happen between the liquid and heated media. This process is called Horizontal-tube Falling Film Evaporator (HSFFE). During the complex process, flow and heat transfer occur simultaneously. This highly efficient energy conservation technology is based on the mechanism of phase transition. Heat exchangers employing this process have many advantages than conventional ones, such as higher heat transfer coefficients, smaller temperature difference, easier to be used in multi-effect distillation process and higher evaporation intensity. Thus this process is widely employed in desalination, refrigeration and chemical industries.The liquid film uniformity is the key factor to keep high heat transfer coefficients of the falling film evaporator. When the continuous film breaks, the temperature of the tube would rise dramatically. This may leads to overheat or local dryout around the tube. So the film distribution directly affects the heat exchange performance. The conventional tube of falling film evaporator is circular tube. Its film thickness fluctuates obviously and the film uniformity along the tube is bad, even occurring local dryout. Besides, the wake area of the bottom is wide and the wake area increases sharply with the increase of flow rate. All the mentioned above have a negative effect on Horizontal-tube Falling Film Evaporator. Then it is urgent to develop new tube structure for highly efficient heat exchange.In this paper, elliptic tube and egg-shaped tube model have been built for better optimization of tube structure, using the knowledge of geometry and fluid mechanics. The liquid-vapor interface is tracked using the Volume of Fluid (VOF) method. Numerical simulation was performed to study the fluid film outside a horizontal tube with different shaped tubes in different cases, such as Reynolds number, liquid feeder height and ellipticity etc. The simulation results will reveal the law of the spread time of feeding liquid along the tube, the wake area and the film distribution along the tube. Then compare the results with different shaped tubes:circular tube, elliptic tube and egg-shaped tube. Better tube structure will be chosen mainly depending on the comparative analysis of different shape. This paper would offer necessary support for further study.The results show that the circular tube、elliptic tube and egg-shaped tube both have wake area along the tube wall and the wake area increases with the increase of flow rate; The affect of liquid height to the wake area is negligible; When the liquid flow rate, tube spacing, the perimeter of the cross-section, liquid feeder height and ellipticity are fixed. The fluctuations of the film thickness increases in proper order:egg-shaped tube, elliptic tube and circular tube; The fluctuations of the film thickness decreases with the increase of ellipticity for egg-shaped tube and elliptic tube; The elliptic tube performs better than egg-shaped tube in reducing the spread time of feeding liquid and they are batter than circular tube; The spread time of feeding liquid will be decrease with the increase of ellipticity for egg-shaped tube and elliptic tube.