Numerical Investigation on Flow Characteristics of Falling Film outside Horizontal Tubes in MED Evaporator

Horizontal-tube falling film evaporation technology is widely used in multi-effect desalination (MED) evaporators because it employs the advantages of the high heat transfer capability with small temperature differences and mass transfer potentials at low liquid flow rates. This technology is a kind of energy-saving technology. However, the flow characteristics and uniform distribution of liquid along the tubes are the key factors for heat transfer. Based on the finite volume solution of Navier-Stoke equations coupled with volume of fluid method (VOF) for the liquid-vapor interface, the flow characteristics of falling film outside horizontal tubes in MED evaporator were simulated. The effects of liquid flow rate, arrangements of tubes, tube spacing, and liquid feeder height on the flow behavior of falling film and film distribution around the horizontal tubes had been studied. Furthermore, the effect of liquid flow rate on flow transitions and the characteristics of droplets during their formation, detachment, fall and impact had been investigated. The results show that the film thickness decreases with the increases of liquid feeder height, and the film thickness increases with the increases of liquid flow rate, but when the fluid flow density decreases to a certain value, the local dryout spot on the surface of the tube would occur. The minimal values of the film thickness tend to locate at different angular position of 90º-120º under different conditions, and the angular position of the thinner film may be the main position for dryout detection. Tube arrangements and tube spacing have little effect on the top tube, but for the lower tubes, the film thickness of staggered mode is significantly less than that of aligned mode. The flow pattern changes first to a column mode, and then to a liquid sheet when the flow rate increases. In addition, the liquid flow over a bank of tubes exhibits some interesting patterns such as droplet formation, growth, detachment, fall and impact and the resulting satellite droplets, which are governed by force of gravity and surface tension. These results would provide a basis and guidance for further developing efficient horizontal-tube falling film evaporator.