Numerical Investigation On Flow Characteristics Of Falling Film Outside Horizontal Tubes

Falling-film heat exchanger with horizontal tube bundles have been widely used in refrigeration, desalination, chemical and petroleum refining industries because they employ 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. The heat consumption is small, and the heat resource required is low, which helps to utilize low-grade waste heat. It is extremely significant to research and develop this highly efficient energy conservation technology, especially in today's increasingly urgent energy situation.However, liquid mal-distribution on the tube array or too little liquid supply may favor the occurrence of surface dryout, resulting in a drastic reduction in heat exchanger performance. On the other hand, liquid overfeeding to avoid such a condition may cause flooding of the evaporator which may also result in the decline of overall heat transfer performance. And it could cause a waste of energy to pump back the unevaporated liquid to the distributor. Obviously, when the liquid flows around the horizontal tube surface, the flow uniformity is the key factor for heat transfer. And uniform distribution of liquid along the tubes is one condition of normal operation in horizontal tube falling film evaporator.The present study attempted to numerically model the film thickness and film distribution of falling film flowing around the horizontal tube, and presented the effects of liquid flow rate, tube diameter, different position along the tube, arrangements of tubes, tube spacing, liquid feeder height on the flow behavior of falling film and film distribution around the horizontal tubes. Physical and mathematical models for numerical simulation were developed. These flows were numerically analyzed using the volume of fluid method for the liquid-vapor interface. 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.By means of post-processing of the calculating results, this work would represent intuitive and vividly image of the falling film flow process outside horizontal tubes, which made up for the lack of experimental phenomena. The results would provide a basis and guidance for further developing efficient horizontal-tube falling film evaporator.The results show that wave behavior generates on the interface of a thin liquid film falling down an circumferential wall and fluctuations change irregularly over time; The film thickness decreases with the increases of liquid distribution height. 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 film thickness decreases from the top of the tube firstly and then increases around the perimeter of the cylinder; The minimal values of the film thickness tend to locate at different angular position of 90°-120°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 tube below the top tube, the film thickness of staggered mode is significantly less than that of aligned mode. The film thickness of the second tube reduced with the increase of tube spacing. 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 flow patterns have a significant impact on the heat and mass transfer process.